BEARING

Basic Rolling Bearing Technology

Bearings to achieve Trouble-Free Operation and optimum rolling bearing life. There are five basic elements, which are essential for achieving maximum machine reliability and performance:1. Cleanliness2. Correctly designated high quality bearing3. Damage free installation4. Correct lubrication5. Efficient sealing

Why Rolling Bearings ?A bearing's function is to support and accurately locate a load, allowing the load to move with the minimum of friction. For rolling bearings, the load is carried on a rotating shaft or housing. Plain bearings were popular and used extensively until the 20th century. The main drawback of plain bearings, the relatively high friction and lubrication problems at low speeds and start up, resulted in the need for a more effective and efficient solution. It had long been realized that rolling friction was much more efficient than sliding friction. With the development of improved methods of manufacture and steels, the rolling element bearing came into its own.

Ball and Roller BearingsTwo distinct families of rolling bearings have been developed: the ball and the roller bearings. The ball bearing is normally used in applications where the loads are relatively light and the speed is high. The contact ellipse between the rolling element and raceway in the load-carrying zone is very small, resulting in extremely high contact pressure. In roller bearings the load is carried over a significantly larger area due to the line contact between roller and raceway. These bearings are generally capable of carrying heavier loads but are restricted in the speed they can operate at.

Ball Bearings

Single row deep groove ball bearingsThis bearing type forms the largest group because of its ability to carry both axial and radial loads, the very low friction and low noise and the low maintenance required. It is suitable for high speed and light loads. Many of these bearings are sealed or shielded for life. Most are designated and dimensioned to the ISO standards.

Self-aligning ball bearingsThis bearing has a double row of balls. Its outer ring raceway is spherical allowing the rings to be dynamically misaligned, making it suitable for use in applications where the misalignment of bearings on a common shaft can occur. It can carry radial and axial loads but is not suitable for heavy or arduous operation. The number of balls and the contact area each ball makes with the raceways restrict the capacity or load rating of the bearing. Since this contact area is so small, contact stresses can be as high as 4,000 MPa, which equates to 400 kg/mm or approximately 90 tones on an area the size of a thumbnail. This capacity to carry such enormous loads makes the bearing the most highly stressed component in the

equipment.

Single row angular contact ball bearingsThis bearing is capable of carrying high combined radial and axial loads in one direction only. It must be used in conjunction with a similar bearing acting in the opposite direction. This pairing of the bearings is done in a back-to-back, load lines diverging towards the bearing axis, or face-to-face configuration, load lines converging towards the bearing axis, It is extremely important that these bearings are mounted together in the correct orientation and with the correct degree of pre-load or clearance.

Y-bearing unitsThe Y-bearing unit is a complete unit consisting of a bearing mounted in a bearing housing. The bearings are based on the deep groove ball bearing but have an extended inner ring and a spherical outer ring diameter, which allows for static misalignment of the bearings. The bearings can be fitted in various types of housing; cast iron being the most common but pressed steel and plastic housings is also used. The Y-bearing units are very robust and simple to use but do not allow for axial displacement that may take place due to thermally induced dimensional changes. For this reason the bearing centres should be short or the housing supported on a resilient material or structure, such as sheet metal.  

Roller BearingsSingle row cylindrical roller bearingsThe rollers of these bearings are guided between internal flanges usually present on only one of the bearing rings The bearing ring with the flanges also carries the roller and cage assembly, which allows the other ring of the bearing to be removed and mounted or dismounted separately. The NU type bearing has two integral flanges on the outer ring and an inner ring without flanges. The N type has two integral flanges on the inner ring and an outer ring without flanges. Other less common variations are also available. This bearing is capable of carrying very high radial loads and will allow axial displacement of the shaft within the bearing. They are therefore suitable for use as non-locating bearings.

Taper roller bearingsTaper roller bearings have tapered inner and outer ring raceways between which tapered rollers are arranged. The tapered surfaces, if extended, would converge towards a single point on the shaft axis. Their design makes them very suitable for carrying combined radial and axial loads. The axial load carrying capacity is determined by the contact angle, which corresponds to the angle of the outer ring raceway. Like the single row angular contact ball bearing, the taper roller bearing can only carry loads in one direction. A second bearing must be used in either back-to-back or face-to-face configuration. Since the roller ends are in sliding contact with the flange face' good lubrication is of utmost importance.

Spherical roller bearingsThe spherical roller bearing consists of an inner ring with a double raceway on which two rows of spherical rollers are attached by the cages. A central guide ring is present separating the roller ends. The outer ring has a spherical raceway. These bearings are capable of carrying large axial and radial loads. They also have the capability to accept misalignment.The spherical roller bearing is extremely rugged and is used in applications where heavy and arduous loads are present. The bearing can be found in two basic configurations; taper and cylindrical bore. Taper bore allows for easier replacement of bearings on the shaft and greater control of the residual clearance in the bearing during fitting.

Bearing LifeThe theoretical life of a bearing can be calculated using

the ISO standards. These calculations are usually based on the L10 life. This life is defined as the point at which

10% of a representative sample of identical bearings in a given application would have failed. Failure is defined as

the stage when the first signs of fatigue occur on either the rolling element or raceway. Research shows that 50% of

the total sample bearings would achieve five times of their calculated life and some continue to perform satisfactorily

over even longer periods. With these vast differences in life expectancy of rolling

bearings, many other factors influence the bearing service life. Most important factors are the effectiveness of the oil film, bearing quality and contamination. SKF takes these factors into account when calculating the life expectancy

of its bearings. SKF has developed the infinite life theory, which states that under good operating conditions and provided the fatigue load limit is not exceeded; the bearing life will

exceed that of the machine. It must be recognized that handling and contamination damage can dramatically

reduce the bearings life. This is referred to as the bearing service life.

An important factor in the anticipated life of a roller bearing is that a doubling of the load on a significantly loaded

roller bearing results in a life reduction of around 90%.

Fatigue, spalling or flakingWhen rolling bearings are in operation the action of the rolling elements carrying the load through the load zone of the bearing results in cyclic stresses occurring in the sub-surface of the rolling element and raceway immediately below the load carrying surfaces. In normal operation with correctly sized and well-maintained bearings, the effect of these stress cycles is minimal. However, if the loads are excessive or damage occurs to the race way or rolling elements, these forces can increase the formation of sub-surface cracks which will progressively extend to the surface. As the rolling elements continually pass over the cracks, fragments of the material break away. This effect is called flaking or spalling and is progressive, eventually rendering the bearing unserviceable.

Minimum Loads Rolling element bearings require minimum loads to

function satisfactorily. This is because the balls or rollers must rotate when carrying the load. 

Skidding will occur if the load is not sufficient to overcome cage friction and the resistance from the lubricant,

especially during periods of low temperature operation. The consequent is damage to both raceways and rolling

elements. Usually the load from the shaft assembly is sufficient to prevent this from happening. In cases where the load is not sufficient, the bearings can be pre-loaded by the use of springs. In some instances reduced bearing internal clearance may help but great caution should be

taken to avoid a high pre-load condition, which can result in catastrophic failure

Bearing ArrangementThe bearing arrangement of a rotating shaft requires two

bearings to locate and support the shaft radially and axially. In most applications one of the bearings locates the shaft both radially and axially. The second bearing

provides purely radial support. These are known as locating and non-locating or "fixed" and "free" bearings.

Locating (fixed) bearingsThe locating bearing must be capable of carrying

combined loads in both directions and must also be secured to both the shaft and housing to prevent lateral movement. Suitable bearings for this purpose are deep

groove ball, spherical roller, double row or paired angular contact ball or paired taper roller bearings. In some

instances a purely radial bearing such as a cylindrical roller bearing without flanges can be used if paired with a

thrust bearing having radial clearance in the housing. Non-locating bearings

Non-locating bearings provide radial support only and must allow the shaft relative movement to the locating or

fixed bearing to allow thermally induced dimensional changes. This can take place within the bearing itself, as

in a cylindrical roller bearing, or between one of the bearing rings and its seating.

Cross-located bearingsIn some applications both bearings are used to locate the

shaft axially. All types of radial ball and roller bearings, which can accept axial loads in at least one direction, can be included in this category. In these instances the effect

of thermal expansion within the bearings and shaft assembly must be provided for when fitting the bearings.

Radial Location of BearingsFor a rolling bearing to fulfill its load carrying potential and to function correctly, the rings must be adequately supported. Additionally, the rings must be secured to their seating to prevent them from turning. Satisfactory radial location and

adequate support for the rotating ring is usually only obtained when the rings are mounted with an appropriate degree of interference. For a bearing ring supporting the non-rotating load, a slight clearance fit is acceptable and

may be essential when axial displacement is required for a non-locating bearing.

Conditions of rotation - cylindrical boreA rotating load exists if the direction of load is stationary

relative to the rotating ring, or if the ring is stationary and the load rotates, subjecting all points around the raceway to the load in the course of one revolution. Under these conditions

the bearing ring will turn or "creep" on its seating unless secured to the shaft or housing by an interference fit. The

operating conditions and bearing type dictate the degree of interference fit.

A stationary load exists when both the bearing ring and load are stationary, or if the ring and load rotate at the same speed so that the load is always directed to the same point on the ring. These rings should normally have a clearance

fit. An indeterminate load exists when the direction of load can

rotate in respect of both rings or cannot be accurately determined. Vibrating applications and applications where out-of-balance loads are present are some examples. In these cases both rings should have an interference fit.

Magnitude of loadThe interference fit of a bearing inner ring on its seating can

be affected by increasing load and relative temperature changes. The greater the load, the greater the interference

required. Bearing internal clearance

All bearing applications have to take into account the fact that significant dimensional changes occur within a bearing

which has an interference fit on the shaft and/or in the housing. Additionally, temperature differences can also be present between the bearing components. To prevent the

bearings going into pre-load under these conditions, bearings with varying internal clearances are available and specified with suffixes C1 and C2 for less internal clearance than normal. Suffixes C3, C4 and C5 indicate an increased internal clearance. For "normal" applications where the fits

used for the bearing rings and the load and operational speed are not excessive, the SKF standard clearance range

should be used. Standard clearance bearings are not identified by special marking or numbering.

Temperature conditionsService rings often have a higher temperature than the

mating components. This can result in an easing of the fit on the inner ring and reduced internal bearing clearance or an

increased interference between the outer ring and its seating.

Running accuracyWhere high demands are placed on running accuracy,

clearance fits should not be used. To reduce run-out and vibration, bearing seating should be machined to tolerances

to least grade 5 for the shaft and at least grade 6 for the housing as recommended by SKF.

Shaft and housing designThe fit of the bearing on its seating must not lead to uneven

load distribution or distortion of the bearing rings. Split housings are not suitable for applications where the outer

ring requires an interference fit. To ensure adequate support of the bearing rings when mounted on hollow shafts, thin

walled or light alloy housings, heavier fits are required than those specified for steel or cast iron housing or solid steel

shafts.

Ease of mounting and dismountingBearings with clearance fits are easier to dismount than

those with interference fits. In cases where interference fits are necessary and it is essential that mounting can be easily

accomplished, separable bearings or bearings with taper bore and adapter or withdrawal sleeves may be used.

Displacement of non-locating bearingsWhen a non-separable bearing is used as the non-locating bearing, one of the rings must be free to move axially at all

times. A clearance fit should be used for the ring, which carries the stationary load. In the case of stationary outer ring where the axial movement takes place within the housing, it is sometimes necessary to insert a hardened bush to reduce wear and possible locking of the bearings movement. This would apply especially to light alloy housings. Cylindrical

roller bearing types N and NU can be fitted with an interference fit on both rings, as the axial displacement can

take place within the bearing.

Bearings with taper boreBearings with taper bore can be mounted directly onto a

taper seating machined on the shaft, or with an adapter or withdrawal sleeve on a cylindrical shaft. Taper bore bearings

can be identified by the suffix K in their designation. This identifies a standard 1:12 taper. The wide spherical roller bearings of the 240 and 241 series with a taper bore have

the suffix K30 to identify a 1:30 taper. When mounting the inner ring, the distance the ring is driven up the taper determines the fit to the shaft. The reduction in clearance or drive-up distance must be carefully controlled when mounting these bearings. The outer ring fit should be

determined as for bearings having a cylindrical bore. Shaft tolerances of h9 or h10 may be applied with the use of adapter or withdrawal sleeves. The cylindricity of the sleeve-seating diameter should be to IT5 (for h9) and IT7 (for H10).

Housing and Shaft ToleranceTo ensure that the correct fit is achieved between the bearing rings, shaft and housing seating, it is necessary to determine the type of application and bearing selection. To do this the following steps must be considered:

1.  Determine whether the rotating load is inner ring, outer-ring or indeterminate (rotating load conditions applying to both rings). In each case the ring carrying the rotating load i.e. any ring that carries the load, the total circumference of the raceway each revolution must have an interference fit with the housing or shaft. In general practice a bearing ring carrying a stationary load has a transition or free fit.

2. Determine the need for axial displacement of the bearing.

3. The magnitude and type of load and the requirements regarding accuracy of running.

The appropriate fit of the bearing for the shaft and housing can then be determined by using the tables. These refer to solid steel shafts and cast iron or steel housings and are valid for a wide range of applications

Bearing mounting  Prevent 16% of premature bearing failure

 Around 16% of all premature bearing failures are caused by poor fitting, usually using brute force, and being unaware of the availability of the correct mounting tools and methods. Individual installations may require mechanical, heat or hydraulic application methods for correct and efficient mounting. Professional fitting, using specialised tools and techniques, is another positive step towards achieving maximum machine uptime.

Jawpuller

Bearingseparator

Hydraulicpuller

Fittingtool

Hookspanner

Impactspanner

Hydraulicnut andpump

Oilinjectionmethod

Hot plateInductionheater

AluminiunringEAZheater

 Bearing arrangements Mounting tools 

Mechanical

Hydraulic

Oil injection

Heaters

 Cylindrical seating

 Small bearings                                          

 Medium bearings                     

 Large bearings                     

 Cylindrical roller

 bearing types  NU, NJ, NUP,  all sizes

 Tapered seating

 Small bearings

 Medium bearings

 Large bearings                                         

 Adapter sleeve

 Small bearings

 Medium bearings

 Large bearings                                         

Withdrawal sleeve

 Small bearings                                          

 Medium bearings                                                               

 Large bearings                                                              

Small bearings: bore diameter < 80mm

 Medium bearings: bore diameter 80 - 200 mm

 Large bearings: bore diameter > 200 mm

Storage and handling Rolling bearings should be stored in a cool, clean, low

humidity environment free of dust, shocks and vibrations. Storing bearings directly on the floor should therefore be avoided. If stocks of bearings are kept, it is important to impose the practice of stock rotation. SKF bearings are preserved and packaged to ensure that when complying to these storage circumstances the storage life will be

several years.The simplest and most effective bearing maintenance is to keep the bearing clean. Contamination will shorten the life of any bearing and the importance of cleanliness cannot

be overstressed.

The following considerations are important for bearing maintenance:

All components should be clean and dry.

Cleaning material should be readily available.

Cotton waste should not be used to clean or dry bearings.

Clean hands and tools when mounting are a must.

Before mounting the bearing the following should always be observed:

Make sure that housing and shaft are clean and not damaged.

Make sure that the new bearing is identical to the one being replaced

Ensure that the lubricant to be used is clean and of the correct specification.

Make sure the necessary tools and equipment are at hand.

Keep the work area clean.

When mounting the bearing the following should always be observed:

Do not remove the bearing from its wrapping until the last moment.

Do not try to wash the bearing. The preservative used is compatible with mineral based oils and greases and should only be removed from the bore and the outside diameter of the bearing's outer ring. A lint-free cloth dampened with a cleaning agent is suitable for this purpose.

Ensure that the mounting forces are only applied to the bearing ring with the interference fit.

Use a minimum force with maximum control method.

Lightly coat the mating surfaces with oil.

Mechanical bearing mounting Mechanical mounting is generally suitable for small bearings.

Mounting force can be applied to the bearing by placing a fitting tool impact ring and sleeve against the inner ring and using a press or hammer to advance the bearing to its proper location on the shaft. Be sure the correct size impact ring and sleeve are selected from a bearing fitting tool kit, such as the SKF TMFT 33 fitting tool kit. The

bearing should be exactly at right angles to the shaft before beginning, and the shaft lightly lubricated.

Do not apply a sleeve to the outer raceway when mounting on a shaft, or to the inner raceway when mounting in a housing- and NEVER mount by striking the bearing directly with a hammer.

Raceway damage incurred when a bearing is incorrectly mounted can result in premature bearing failure. Typical problems, which can cause premature failure, are:

   Damage caused during the fitting procedure

   Incorrect sized shafts and housings i.e. too loose or too tight

   Retaining lock nuts becoming loose during operation

   Burred and damaged shaft and housing seats and shoulders

   Incorrectly mounted bearings

Cylindrical shaftsinterference fits

Most bearings are fitted to their shaft or housing with one component having an interference fit or in some cases both.

For determining the correct fit, refer to the SKF General Catalogue, the SKF Maintenance Handbook or consult an

SKF application engineer.

Incorrect mountingWhen bearings are mounted cold, care must be taken to

ensure that the drive-up forces are applied to the ring with the interference fit. Damage and a resulting bearing failure can occur if the mounting force is transmitted through the

rolling elements causing damage to the raceways.

Correct mountingThe correct way to minimise raceway damage is to use the specifically designed tools from SKF - the TMFT fitting tools

series. These tools ensure that the drive-up forces are applied effectively and evenly to the ring with the

interference fit avoiding raceway damage

 A  Shaft interference fit

B Housing interference fit

C Uneven distribution of forces can result in raceway damage

C With the correct tools raceway damage is avoided

Tapered shafts interference fits

Bearings mounted on tapered seatings, adapter sleeve or withdrawal sleeve achieve their interference fit by being

driven up the tapered shaft. Care should be taken to ensure that the bearing is not driven up too far as all the internal clearance may be removed, which can result in bearing

damage.

Spherical roller bearings Method:

Correct adjustment of spherical roller bearings is determined by measuring the residual internal clearance in the bearing

or by the amount of axial drive-up.

A 

Correctly mounted: Bearing driven up the correct distance and the right clearance is achieved

B Incorrectly mounted: Bearing is driven up too far and all clearance removed: damage possible

C Before adjustment

D After adjustment

Self-aligning ball bearings  Method:Adjustment of double row, self-aligning ball bearings to obtain a correct interference fit is more difficult to achieve than spherical roller bearings because the feeler gauge method cannot be used. The most effective method to adjust this type of bearing correctly is to use the SKF TMHN 7 lock not spanner set.

A 

Correctly mounted: Bearing driven up the correct distance and the right clearance is achieved 

B Incorrectly mounted: Bearing is driven up too far and all clearance removed: damage possible

Mounting bearings using heat The force needed to mount a bearing increases rapidly with bearing size. Because of the mounting force required, larger

bearings cannot easily be pressed onto a shaft or into a housing. Therefore the bearing, or the housing, is heated

before mounting. The temperature difference between the bearing and seating

depends on the magnitude of the interference fit and the bearing size. Normally a bearing temperature of 80 to 90 °C

(144 to 162 °F) above that of the shaft is sufficient for mounting. Never heat a bearing to a temperature greater

than 125 °C (257 °F), unless otherwise specified. Extreme heat can cause the material to change metallurgically and

produce alterations in diameter or hardness. Local overheating must be avoided and in particular never heat a

bearing using an open flame.

 Hot mountingNever heat a bearing using an open

flame

Wear clean protective gloves when mounting a hot bearing. Lifting (hoisting) gear can facilitate mounting. Push the bearing along the

shaft as far as the abutment and hold the bearing in position, pressing until a tight fit is obtained. SKF supplies a full range of

heating equipment, which covers most common mounting needs. Specials are also available on request.

 Lifting gear SKF Bearing Handling Tool

Principle of induction heating

An induction heater can be compared to a transformer using the principle of a primary coil with a large number of windings, and a secondary coil with a few windings, on a mutual iron core. The input/output voltage ratio is equal to the ratio of the windings, while the energy remains the same. Consequently, the secondary coil will provide a low voltage at high amperage. In the case of an SKF induction heater, the bearing is a short circuited, single turn, secondary coil through which a low A.C. voltage flows at high amperage, thus generating high heat. The heater itself, including the yoke, remains at ambient temperature.

As this type of heating induces an electric current, the bearing will become magnetized. It is important to ensure that the bearing is then demagnetized so that it will not attract metal particles during operation. All SKF induction heaters are equipped with automatic demagnetizing cycles.

Hydraulic bearing mounting SKF Oil PowerMounting bearings with tapered bore

The inner rings of bearings with a tapered bore are always mounted with an interference fit. The degree of

interference in this case is not determined by the chosen shaft tolerance, as with bearings having a cylindrical bore, but by how far the bearing is driven up onto the tapered

seating. The original radial internal clearance is reduced in the process and this reduction gives an indication on the

interference fit obtained.For larger bearings, considerably more force is required to drive them up a tapered seating. The practical solution is to

use an SKF hydraulic nut, which uses hydraulic power to provide the drive-up force. If the oil injection method is also employed, then the force required can be further reduced.

The hydraulic nut is screwed onto a threaded section of the journal or sleeve thread, so that its annular piston abuts the

inner ring of the bearing, a nut on the shaft, or a disc attached to the end of the shaft.

With the oil injection method, high-pressure oil is injected between the mating surfaces. The oil film formed separates the mating surfaces and appreciably reduces the friction between them. The method is mainly used when mounting bearings directly on tapered journals, but is also used to mount bearings on adapter and withdrawal sleeves, which have been prepared for the oil injection method. A pump or oil injector produces the required pressure, the oil being supplied to the mating surfaces via ducts and distribution grooves in the shaft or sleeve. The necessary ducts and grooves in the shaft must be considered when designing the bearing arrangement

A = Hydraulic nut for driving the bearing onto a tapered seating

B = Hydraulic nut screwed onto the shaft for driving in a withdrawal sleeve

C = Hydraulic nut for driving the bearing onto an adapter sleeve

D = Hydraulic nut and special stop nut for diving in a withdrawal sleeve

Degree of interferenceA certain degree of interference is needed when mounting

bearings with a tapered bore. Different methods can be applied to measure the degree of interference fit.

Measurement of clearance reduction with feeler gaugesFor small and medium size bearings the reduction in radial internal clearance can be measured. Before mounting the bearing, the radial internal clearance is measured. During

drive-up the reduction in radial internal clearance is checked, until the requisite value is obtained. To measure the

clearance feeler gauges having blades with a thickness of 0,03 mm and above should be used.

SKF Oil Injection Method Developed by SKF in the 1940s, the Oil Injection Method

allows bearings and other components with an interference fit to be fitted and removed in a safe, controllable and rapid

manner. The method does not require keyways to be machined on the shaft, saving valuable time and money in materials and production. Interference fits have been long recognised for their reliability in transmitting large torsional

loads. Very often interference fits offer the only solution when connecting hub to shafts with intermittent or fluctuating

loads.When using the SKF Oil Injection Method the mating

surfaces are separated by a thin film of oil injected under high pressure, thereby virtually eliminating the friction

between them. This enables interference fits on cylindrical shafts to be overcome and the fitted component removed

quickly and effortlessly. The method is even more versatile when used on tapered shafts, as it can be used for both mounting and dismounting the fitted components. The

method, which is used for many bearing applications, can also be found in other applications such as coupling, gear

wheels, railway wheels, propellers and built-up crankshafts.

SKF Oil Injection Method

The conceptInjecting the oil between two tapered surfaces will create a reaction force, which could be quite substantial as the oil will also act as a “hydraulic cylinder” which can push the outer component off. 

The preparationDuring manufacture the shafts are prepared with oil ducts and grooves. For technical information on how to prepare the shafts, consult an SKF application engineer. 

The actionBearings are mounted by pushing them up the shaft with the aid of an SKF HMV E nut. 

Bearing dismounting 

Reduce the risk of personal injury and bearing & shaft damage

 Dismounting bearings can be hazardous and demanding task. Selecting the correct tool is therefore of utmost importance for reducing the risk of personal injuries. One reason for dismounting an "old" bearing is to replace it by a new one. When dismounting such a bearing, care must be taken not to damage the shaft while dismounting, which can result in compromising the machine's efficiency. Another reason for dismounting bearings is for maintenance or replacement of other machine's components. These dismounted bearings are mounted again, unless they are damaged during dismounting. To enable the reuse of the same bearing, care must e taken when selecting dismounting methods and tools. Individual applications may require mechanical, heat or hydraulic dismounting methods for safe, correct and efficient dismounting. Professional dismounting, using specialised tools and techniques, is another positive step towards achieving maximum machine uptime.

 Bearing arrangements

 Dismounting tools 

Mechanical Hydraulic Oil injection Heaters

 Cylindrical seating

 Small bearings                     

 Medium bearings                                                               

 Large bearings                                          

 Cylindrical roller

 bearing types  NU, NJ, NUP,  all sizes

Tapered seating

 Small bearings                                          

 Medium bearings                                          

 Large bearings                     

Adapter sleeve

 Small bearings                                          

 Medium bearings                     

 Large bearings                                         

Withdrawal sleeve

 Small bearings                                          

 Medium bearings                                          

 Large bearings                                         

Small bearings: bore diameter < 80mm

 Medium bearings: bore diameter 80 - 200 mm

 Large bearings: bore diameter > 200 mm

Mechanical bearing dismounting Choosing the right puller for the job is critical. Not only the puller type, but also its maximum withdrawal force

(kN) is crucial for completing any dismounting job safely and easily. Make sure you avoid puller overload, as it

can result in breakage of the puller’s arms and/or beam. This breakage can cause damage to the puller, shaft

and bearing as well as personal injury. It is recommended to use a three-armed puller instead of a two-armed puller. It is preferred to grip the ring with the

interference fit whenever possible.

Dismounting bearings with cylindrical bore

Interference fit on the shaft: External pullSmall and medium-size bearings mounted with an interference fit on the shaft can be dismounted using a puller. If possible let the puller grip the inner ring, then remove the bearing with a steady force until the bearing bore completely clears the entire length of the cylindrical seating. 

Apply the puller to the outer ring if it is not possible to grip the inner ring. However, this presents a risk of damaging the bearing. If the bearing has to be used again, the outer ring must be rotated during dismounting. This can be done by locking the spindle and turning the puller continuously until the bearing comes free. Another way to minimize this risk is to use a strong back puller.The puller should be accurately centered during dismounting in order to help prevent seating damage. To eliminate that risk, use self-centering pullers.

SKF Bearing pullers

Interference fit in the housing: Internal pullThe use of a slide hammer assisted puller is recommended for easy and quick removal of bearings from housings. When using a puller to dismount self-aligning ball bearings, swivel the inner ring assembly so the puller can be applied to the outer ring. For other bearing types, when the puller has to be applied to the inner ring and the bearings are to be re-used, the inner ring should be rotated during dismounting to minimize the risk of damage the bearing. 

Interference fit on shaft and in the housing: Blind pullFor non-separable bearings with an interference fit both in the housing and on the shaft, the best method is to allow the bearing to be pressed out of the housing with the shaft. This technique helps ensure that no dismounting force is transmitted to the rolling elements. The opposite procedure, allowing the bearing to come off the shaft with the housing, can also be used.Ball bearings can be removed with a special blind housing puller. This puller’s finger-shaped extensions grip between the rolling elements and on to the ring, allowing the bearing to be easily removed. 

Hydraulically assisted pullersMedium-size bearings with an interference fit on the shaft often require considerable dismounting force. In these cases a hydraulically assisted puller facilitates quick and effortless dismounting. A self-centring hydraulic puller can be used for forces up to 500 kN (112 500 lbf). 

Dismounting bearings using heat

Induction heatersSKF has developed special induction heaters, the EAZ series, for dismounting the inner rings of cylindrical roller bearings having no flanges or only one flange. They heat the inner ring rapidly without heating the shaft to any degree, so that the expanded ring can easily be removed. These electrical induction heaters have one or more coils energised by alternating current. The coils are so arranged that the ring to be withdrawn is placed in an alternating magnetic field. The eddy currents thus produced cause rapid heating and expansion of the ring. Large heaters are provided with temperature cutouts and are connected to the mains via a time relay to prevent excessive heating of the induction coils or inner rings. It is necessary to demagnetise the inner rings after heating and removal. This can be done using these special SKF induction heaters themselves. The use of these types of induction heaters becomes economic when this type of bearings are frequently mounted and dismounted, e.g.

axle box bearings or rolling mill bearings.

Heating ringsWhen flangeless inner rings of cylindrical roller bearings, or those with only one flange have to be removed infrequently, or if larger sizes of inner ring have to be dismounted, it is less costly to use heating rings such as the SKF TMBR series. The inside diameter of the ring is the same as the raceway diameter of the inner ring and it is machined to tolerance Z9. The ring is heated using a hot plate or naked flame to approximately 280 °C and then placed over the inner ring and clamped using the handles. To ensure satisfactory heat transfer from the ring, the inner ring should first be coated with a viscous oil which is resistant to oxidation.

Hydraulic bearing dismounting With the oil injection method, oil under high pressure is

injected between the mating surfaces. An oil film is formed, which separates the mating surfaces and appreciably

reduces the friction between them. The method is mainly used when dismounting bearings fitted directly on shafts, but is also used to dismount bearings on adapter and withdrawal sleeves that have been prepared for the oil injection method. A pump or oil injector produces the requisite pressure, the oil

being supplied to the mating surfaces via ducts and distributor grooves in the shaft or sleeve. The necessary ducts and grooves in the shaft must be considered when

designing the bearing arrangement. 

SKF Oil Power

 A = Hydraulic nut and stop ring in position to press an adapter sleeve free

B = Hydraulic nut used to free a withdrawal sleeve

The dismounting of large bearings from tapered journals, adapter or withdrawal sleeves is greatly eased if both a hydraulic nut and the

oil injection method are used. After injecting pressurized oil between the mating surfaces, the bearing will separate suddenly from its seating and therefore some form of stop must be provided, for

example a shaft nut or end plate to limit the axial movement of the bearing.

The oil injection method can also be used to help dismount bearings from medium and large cylindrical shafts. After injecting pressurized oil between the mating surfaces, the bearing will be separated by an oil film. This oil film greatly reduces the friction between the bearing

and the seating, thus reducing the force required to dismount it using a puller.   

SKF Oil Injection Method

Developed by SKF in the 1940s, the Oil Injection Method allows bearings and other components with an interference fit to be fitted and removed in a safe, controllable and rapid manner. The method does not require keyways to be machined on the shaft, saving valuable time and money in materials and production. Interference fits have been long recognised for their reliability in transmitting large torsional loads. Very often interference fits offer the only solution when connecting hub to shafts with intermittent or fluctuating loads.When using the SKF Oil Injection Method the mating surfaces are separated by a thin film of oil injected under high pressure, thereby virtually eliminating the friction between them. This enables interference fits on cylindrical shafts to be overcome and the fitted component removed quickly and effortlessly. The method is even more versatile when used on tapered shafts, as it can be used for both mounting and dismounting the fitted components. The method, which is used for many bearing applications, can also be found in other applications such as coupling, gear wheels, railway wheels, propellers and built-up crankshafts.

SKF Oil Injection Method

Cylindrical shafts

The concept:By injecting oil of a certain viscosity between two shrink fitted surfaces, the mating surfaces will be separated by a thin oil film. If the surfaces are cylindrical, the fitted components can easily be removed.

The preparation:During manufacture the shafts are prepared with oil ducts and grooves. For technical information on how to prepare the shafts, consult an SKF application engineer.

The action:Dismounting the bearing is made easy by pumping oil under pressure between the mating surfaces. Once the oil pressure has built up, the component can be removed from the shaft with a minimum of effort.

Tapered shafts

The concept:Injecting the oil between two tapered surfaces will create a reaction force, which could be quite substantial as the oil will also act as a “hydraulic cylinder” which can push the outer component off.

The preparation:During manufacture the shafts are prepared with oil ducts and grooves. For technical information on how to prepare the shafts, consult an SKF application engineer.

The action:Bearings are dismounted by injecting oil between the mating surfaces and when sufficient pressure is reached, the bearing will be pushed off. A nut is required to keep the bearing from sliding off the shaft.

Lubrication Correct bearing lubrication is essential for your

bearings' lifeOvercoming friction has challenged man ever since he invented the wheel. Friction occurs between contacting

surfaces that are in relative motion. As loads with relative movement increase without lubrication the surfaces will fuse

together and breakdown will occur. The main purpose of lubrication is to provide a separating film to prevent this from

happening and ensuring Trouble-Free Operation.A lubricant not only reduces friction, but it also prevents

wear and corrosion and helps guard against contamination. Rolling bearings are designed to carry very heavy loads at high speeds and it is essential to have a film of lubricant

between the contact surfaces. Mineral oil is generally used to create this oil film, which ranges from one tenth of a micron up to one micron thick in the contact area. Very smooth surfaces and clean conditions are required to prevent the film being penetrated and wear started.

Lubrication requirementsTo achieve adequate oil film thickness within the bearing a number of variables must be considered. The most important is the viscosity (the ease at which fluids flow) of the oil, which influences the film thickness. The viscosity is specified as cSt (centistokes) or mm squares per second (mm/s). As viscosity changes with temperature, it is usually quoted at a reference temperature of 40 °C. Viscosities used for bearing lubrication are ranging from 2 cSt, which is very fluid like water, up to 500 cSt, which is comparable to syrup and even 1000 cSt, which is comparable to honey.

Factors affecting lubrication film thickness1) Operating speed

The operating speed has an important effect on the lubrication film thickness. As the speed increases a condition

known as hydrodynamic lubrication develops, which significantly increases the oil film thickness. The effect is similar to aquaplaning (water dynamically supporting a

moving body).At slower speeds higher viscosity lubricants are required to

maintain an adequate oil film. In very slow and heavily loaded applications additives such as molybdenum disulphide are

used to improve the lubrication.2) Operating temperature

Temperature within the bearing must be also considered. The oil viscosity will reduce with increase in temperature resulting

in a corresponding reduction in the oil film thickness. Lubricants with a high viscosity index are less sensitive to the

effect of temperature changes within the bearing.3) Loads

The operating load has to be considered as it has direct effect on the contact pressure and area contact between the

rolling elements and raceways. These changes must be reflected in the selection of the lubricant.

Bearing grease lubrication Grease is by far the most popular method for bearing

lubrication. It is easy to apply and retain within the bearing housing and can also function as a seal to protect the

bearing. Grease is a combination of approximately 85% oil (mineral or synthetic) and 15% soap or thickener. All soaps

function in the same way; they retain the oil in a similar manner to a sponge retaining water. The oil is allowed to

bleed out, as the bearing requires it when its internal temperature increases.

The bearing should be filled with grease with free space in the housing up to 50 % to allow room for the excess grease

to be ejected from the bearing during start up. Filling the bearing with grease should be one of the last operations to complete when mounting a replacement bearing to ensure cleanliness and minimum contamination. A complete fill is

desirable with low speed operating conditions where overheating is not a concern.

How long will the grease last ?The life expectancy of a grease depends on a number of factors, the main ones are the operating temperature, the type of grease, the operating speed and the environmental and sealing arrangement.Small lightly, loaded ball bearings may not require any re-lubrication during the machine's life time and therefore the sealed or shielded "sealed-for-life" bearings can be used.

Re-lubrication intervalsDetermining an application's re-lubrication intervals

depends on many factors. Most important are operating conditions, such as temperature, speed and load, and

bearings arrangements and types.Environmental considerations can also affect the re-

lubrication frequency. In heavily contaminated conditions such as can be found in quarries and foundries more

frequent or even continuous lubrication may be required.In some applications continuous lubrication is preferred.

This can be achieved by using a centralised system or an automatic lubricator. Ensure that the lubrication fitting is

clean, that the right type of lubricant is used, and that the correct quantity of lubricant supply is set.

For open bearings the best way to determine the re-lubrication interval is to use the SKF DialSet re-lubrication

interval calculation program

Glossary of lubrication termsGrease consistency/penetrationGreases are classified by their stiffness or consistency according to the U.S. National Lubricating Grease Institute (NLGI) and are graded from NLGI Class 000(very soft) to 6 (very stiff). This is based on the degree of penetration achieved by allowing a standard cone to sink into the grease at a temperature of 25 °C for a period of five seconds. The depth of penetration is measured on a scale of 10-1 mm. The softer greases allow the cone to penetrate further into the grease, hence the higher penetration number. This test method is in accordance with DIN ISO 2137.For normal use in bearings the range of the grease consistency between NLGI Class 1 and 3, with NLGI Class 2 being the most used.

Thickener or soapThickener or soap is the system, which holds the oil and/or additives together to enable the lubricating grease to function. The thickener system is formed from either soaps or non-soaps. The type of thickener gives the grease its typical characteristics. Soaps are based on lithium, calcium, barium or aluminum. Non-soaps are based on organic or non-organic solids, bentonite clay, polyurea and silica gel.

AdditivesAdditives are used to provide additional characteristics such

as wear and corrosion protection, friction reducing effects and preventing damage under boundary and mixed

lubrication conditions.Base oil viscosity

The base oil is the oil inside the grease, which provides the lubrication under the operating conditions. Viscosity is a measure of a fluids flow characteristics and is usually expressed in terms of the time required for a standard

quantity of the fluid, at a given temperature, to flow through a standard orifice. Since viscosity decreases with increasing

temperature, the temperature at which it is measured is always stated. The viscosity of base oils is always indicated

as a kinematics viscosity abbreviated to cSt at 40 °C and often also at 100 °C.

By varying soap, oil viscosities and additives it is possible to produce greases with different characteristics to suit the

wide range of bearing applications. It is very important that the type of grease chosen has the properties suitable to the

bearing type and operating conditions.

Drop pointThe drop point is the temperature at which the grease

sample, when heated, will begin to flow through an opening. This test method is in accordance with DIN ISO 2176.

The drop point does not relate to the allowable operating service temperature of the grease.

Melting pointThe melting point is the temperature at which the grease

makes the transition from a solid to a liquid state. This point is normally at a higher temperature than the drop point.

Oil separationLubricating greases release oil when stored for long periods of time or as a

result of temperature. The degree of oil separation depends upon the thickener, base oil and manufacturing method. To measure the oil separation

degree, a cup is filled with a given quantity of grease (which is weighed before the test) and a 100-gram weight placed on top of the grease. The complete unit is put into an oven at 40°C for one week. At the end of the

week the amount of oil, which has leaked through the sieve, is weighed and reported as a percentage of weight loss. This measuring method is in

accordance with DIN 51 817.

Water resistanceThe water resistance of lubricating greases is measured in accordance with DIN 51 807 part 1. A glass strip is coated

with the candidate grease, which is placed into a water-filled test tube. The test tube is immersed in a water bath

for three hours at a specified test temperature. The change in the grease is evaluated visually and reported as a value between 0 (no change) and 3 (major change) along with

the test temperature.

Selecting grease for bearing lubricationOf the 36% of bearing failures caused by poor lubrication, 50% can be attributed to incorrect grease type. Therefore, it is of critical importance to the bearing performance that the correct type of grease is selected to provide the necessary base oil viscosity at the prevailing operating temperature. All-purpose greases are inadequate for specialized bearing needs and can cause problems rather than be beneficial. Bearing applications have wide variations of load, speed, temperature and environment, and correct lubrication calls for matching the grease precisely to the bearing application.When selecting bearing lubrication grease, besides temperature, speed and load, other operational conditions must be considered. An example of this is the requirement for bearings in assemblies subjected to heavy vibrations. If a grease with a low mechanical stability is used, the grease may be destroyed by the vibrations causing the bearing to prematurely fail. The most important factors to consider when selecting grease for bearing lubrication are:   Machine type   Bearing type and size   Operating temperature   Operational load conditions   Speed range   Operating conditions such as vibration and    the orientation of the shaft in horizontal or    vertical plane   Cooling conditions   Sealing efficiency   External environment

Machine and bearing condition

To ensure long bearing life, it is important to determine the condition of machinery and bearings while in operation. Good predictive maintenance will both reduce machine downtime and decrease overall maintenance costs. To help you achieve the maximum life from your bearings, SKF have developed a series of measuring instruments that will analyse the critical environmental conditions which have an impact on bearing and machine performance.The SKF range covers the most important parameters for measuring machine condition to achieve optimum bearing performance:   Noise   Temperature   Speed   Vibrations   Alignment   Oil condition   Bearing condition

Maintenance concepts Run to failure

Run to failure occurs when repair action is not taken until a problem results in machine failure. Run to failure problems

often cause costly secondary damage along with unplanned downtime and maintenance costs.

Preventive maintenancePreventive maintenance implies that a machine, or parts of a machine, are overhauled on a regular basis regardless of the condition of the parts. While preferable to run to failure maintenance, preventive maintenance is costly because of excessive downtime from unnecessary overhauls and the

cost of replacing good parts along with worn parts. Preventive maintenance is similar to the regular service of

a car. Often unnecessary maintenance is performed.

Predictive maintenanceCondition monitoring/predictive maintenance is the process of determining the condition of machinery while in operation. This enables the repair of problem components prior to failure. Condition monitoring not only helps plant personnel reduce the possibility of catastrophic failure, but also allows them to order parts in advance, schedule manpower, and plan other repairs during the downtime. With condition monitoring, machinery analysis takes two overlapping forms; predictive and diagnostic.

Condition based maintenance means repairs are only carried out when required. The most effective alternative.

Alignment

Shaft misalignment costs time and moneyShaft misalignment is responsible for up to 50% of breakdowns in rotating machinery. Those breakdown cause increased machine downtime, which translates directly into higher costs. Additionally, incorrect alignment places a greater load on machine components, resulting in increased wear and tear.

Effects of misalignment on bearing performanceMisaligned shafts generate a moment, which creates a reaction force in the shaft bearings of the driven and drive units. Care should be taken to ensure the degree of misalignment between the shafts is within the coupling manufacture's tolerances and the machinery manufacture's recommendations. A 20 % increase in load caused by misalignment will reduce the calculated bearing life by almost 50 %. Other serious effects can be wear and tear of the seals, allowing contamination to enter the bearing and lubricant to leak. Other important effects of shaft misalignment are excessive vibration and noise as well as increased energy consumption.Misaligned shafts can cause:   Increased bearing load   Reduction in bearing life   Increased seal wear   Increased vibrations   Increased noise   Increase in energy consumptionCorrect shaft alignment will minimise these effects

What exactly is misalignment?Misalignment occurs when the centre lines of rotation of two machinery shafts are not in line with one another. There are two types of misalignment: parallel and angular. In most cases, machine misalignment is actually caused by a combination of these two types.

Parallel misalignment Angular misalignment

Advantages of proper alignment   Longer bearing life   Minimal stress on couplings, reducing the risk  of overheating and breakdowns   Minimal wear of seals, lowering risk of   contamination and lubricant leakage   Lower energy consumption   Minimal vibration and noise   Increased uptime

Shaft Alignment MethodsTraditional alignment methodsTraditional alignment methods, although very common, do not often produce the exacting degree of accuracy required by today's precision machinery. The rough alignment methods still used nowadays may be quick, but they are also completely inaccurate. Another traditional method employing dial indicators offer a higher degree of accuracy, but it requires specialist operators and it is quite time consuming. Furthermore, shaft position calculation mistakes occur more frequently

Dial indicators method Rough alignment method Rough alignment method

Laser alignment methodLaser alignment methods are a marked improvement on traditional ones. A laser-driven shaft alignment device allows one to adjust alignment with far more speed and accuracy than with traditional methods. There are many devices available on the market, with different degrees of sophistication and price ranges.Since misalignment has a direct, negative, effect on bearing life, SKF also offers a range of high precision, easy-to-use laser shaft alignment tools. The SKF shaft alignment tool TMEA 1 Series is available in three variations:   TMEA 1P, shaft alignment tool with a printer   TMEA 1PEx, intrinsically safe shaft alignment    tool with a printer   TMEA 1, basic shaft alignment tool without    a printer

Belt or Pulley Alignment As in the case of shaft alignment, belt alignment or pulley alignment is an important maintenance task. When carried out correctly, it can prevent breakdowns and save considerable costs. Belt alignment and pulley alignment are synonymous, as the process of belt alignment hinges on the correct alignment of the pulleys on which the belt runs. For the sake of clarity, however, we will speak of belt alignment. Belt alignment concerns aligning the belts in a manner that results in the least wear on the belts and lowest energy loss for the machine or driver unit. In practice this means that the grooves of the pulleys are in line with one another.

Results of belt misalignmentWhen a belt is misaligned it can result in various detrimental,

and sometimes dangerous, effects. In the first place, the efficiency of the machine is reduced. The wear on the belt

itself increases, substantially reducing the life of the belt, and possibly resulting in belt failure. Apart from the annoying and

costly downtime needed to replace the belt, should an unguarded belt wear through and snap prematurely it could

cause injury to any personnel nearby. Other consequences of belt misalignment are increased vibration and noise level,

which in turn shortens the bearing's life. Unless the alignment is corrected, the new belt will not last much longer than the old one. Continuous operation in a

misaligned state will also increase the wear on the pulleys themselves, as well as their axles, shafts and bearings.

Nature of misalignmentAs with shaft misalignment, there are various types of belt

misalignment. In practice, a combination of different misalignments is often encountered simultaneously. For this

reason it is important that an alignment instrument both diagnoses and informs you of the nature of your machine's

misalignment. The following illustrations show the three different types of belt, or pulley, misalignment.

Vertical angle misalignment Horizontal angle misalignment Parallel misalignment

Vertical angle or twisted misalignmentThis type of misalignment occurs when one of the pulleys has an angular error from the vertical angle plane. This is usually caused by incorrect positioning of the machine and can be corrected by lifting either the front or rear feet of the motor to which the pulley is attached, depending on the direction of the vertical angle error. This differs from shaft alignment in that one never moves both feet in a vertical direction to reach the desired result.

Horizontal angle misalignmentHorizontal angle misalignment occurs when the driver and the driven unit are not positioned parallel to each other. Incorrect positioning of applications, such as the motor, can cause this type of misalignment. This misalignment can generally be corrected by moving the front or rear feet forward or backward on their guides, depending on the direction of the horizontal angle error, in order to twist the motor around its centre.

Parallel misalignmentParallel misalignment is the least complex form of misalignment. It is either caused by the incorrect positioning of the motor along its shaft axis; positioned too far forward or backward compared to the other shaft, as shown in illustration A below, or by incorrect positioning of the pulleys on their respective shaft; one of the pulleys needs to be adjusted on the shaft, as shown in illustration B below.

AB

Belt Alignment Methods

Belt alignment can be carried out by one of two methods, namely the traditional method and the laser one. Traditional methods usually employ strings and/or straight edges. Laser methods utilise a laser beam, which allows for a far higher degree of accuracy.

Traditional methodsMuch of the accuracy of traditional methods depends on visual judgement with the naked eye. These methods, which are the most widely used, involve either using visual judgement alone or visual judgement in combination with a straight edge and/or length of string.

Measuring parallel and angular misalignment using a straight edge or a piece of string

The advantage of these traditional methods is the limited length of time needed for adjustment, although the use of a straight edge takes more time than visual judgement alone. The major disadvantage is the lack of accuracy. Some belt

manufacturers recommend a maximum misalignment of 0.5° or even o.25° which is difficult, if not impossible, to

accomplish by using visual judgement. These low-tech methods involve plenty of trial and error,

naked eye approximation and rough estimation. All of which leave a large margin for error, which is unacceptable for

precision pulley alignment. Laser methods

Laser methods are a marked improvement on traditional methods. A laser device allows one to adjust alignment with far more speed and accuracy than with traditional methods.

Systems available on the market can be categorised according to the way the devices can be attached to the pulley and the way they align. In general there are two

groups, one aligns the face of the pulleys and one aligns the grooves of the pulleys

Methods using the face of the pulleyMost products on the market belong to this group. These

products use the face or side of the pulley as a reference for aligning the pulleys and belts.

AdvantageThe advantage of this method is that it can also be used for belt types other than V-belts, such as timing belts. V-belts,

however, comprise the majority of belts found on the market, especially in industrial application.

DisadvantageThe main disadvantage is that only the face of the pulley is used as a reference. This means that only the faces of the pulleys are aligned with each other. This method results in

varying degrees of accuracy when the pulleys are of different thickness, brands or kinds (e.g. one single belt

pulley and one multiple belt pulley) or when the faces are not well finished.

Pulley groove methodIn contrast to the product group mentioned above, only SKF,

Fixturlaser and Hamar Laser devices align the grooves of the pulleys in which the belt runs, thereby substantially

increasing accuracy irrespective of the thickness, brand or type of pulleys.

SKF BeltAlign TMEB 1 is equipped with specially designed V-guides, which allow the tool's two components to fit easily into the grooves of the pulleys. This is also true for pulleys where the face is obscured by other equipment as the case

may be.