Dr R Tiwari, Associate Professor, Dept. of Mechanical Engg., IIT Guwahati, ([email protected]) CHAPTER 6 BEARING AND SEAL SYSTEMS All rotating machineries are supported by bearings and fitted with seals. The bearings clearly constitute a vital component in any turbo-machine and a good understanding of their dynamic behaviour is a pre-requisite to the prediction of the machine’s properties. The influence of bearings on the performance of rotor-bearing systems has been recognised for many years. One of the earliest attempts to model a journal bearing was reported by Stodola (1925) and Hummel (1926). They represented the fluid-film as a simple spring support, but their model was incapable of accounting for the observed finite amplitude of oscillation of a shaft operating at a critical speed. Concurrently, Newkirk (1924) and Newkirk and Taylor (1925) described the phenomenon of bearing induced instability, which he called oil whip, and it soon occurred to several investigators that the problem of rotor stability could be related to the properties of the rotor dynamic parameters i.e. RDPs (the rotor dynamic (or dynamic) parameters are also known as: bearings/seals force (or moment) coefficients; added-mass, damping and stiffness coefficients; linearized rotor dynamic parameters; dynamic impedances). Seals are mainly used to reduce the leakage of working and lubricating fluids through the interface between machine parts. Some leakage is inevitable, and it results in axial fluid velocities though the seal in the direction of the pressure drop. The present day requirement of critical sealing applications have a diverse range of operating condition requirements such as (i) cryogenic temperature, (ii) hard vacuum, (iii) ultra-clean systems, (iv) leakage control to 10 -12 cc/sec, (v) pressures over 100 bar, (vi) temperatures exceeding 800°C, (vii) hard-to-handle liquids and gases, (viii) high pressure pulsations and (ix) rotor speeds as high as 10 5 rpm. These extreme conditions of seals are challenging tasks on the space age aviation and aerospace industries. The importance of calculations of RDPs of seals arose in the late 1970s in regard to vibration problems related to high-pressure oxygen turbopump of the space shuttle main engine. Compressors used in many industries also had the instability problems within the operating speed range. Seals in high-speed operations of turbo-machines leads to instability. The main factor, which governs the instability, is RDPs of seals. RDPs of seals are greatly dependent on many physical and mechanical parameters such as lubricant and working fluid temperatures, pressure drop, seal clearances, surface roughness and patterns, rotor speeds, eccentricity and misalignments. Designers should know the types of bearing and seal that could be used, and performance characteristics associated with each of them. The main aim will be towards the operation of bearings
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Dr R Tiwari, Associate Professor, Dept. of Mechanical Engg., IIT Guwahati, ([email protected])
CHAPTER 6
BEARING AND SEAL SYSTEMS
All rotating machineries are supported by bearings and fitted with seals. The bearings clearly
constitute a vital component in any turbo-machine and a good understanding of their dynamic
behaviour is a pre-requisite to the prediction of the machine’s properties. The influence of bearings on
the performance of rotor-bearing systems has been recognised for many years. One of the earliest
attempts to model a journal bearing was reported by Stodola (1925) and Hummel (1926). They
represented the fluid-film as a simple spring support, but their model was incapable of accounting for
the observed finite amplitude of oscillation of a shaft operating at a critical speed. Concurrently,
Newkirk (1924) and Newkirk and Taylor (1925) described the phenomenon of bearing induced
instability, which he called oil whip, and it soon occurred to several investigators that the problem of
rotor stability could be related to the properties of the rotor dynamic parameters i.e. RDPs (the rotor
dynamic (or dynamic) parameters are also known as: bearings/seals force (or moment) coefficients;
added-mass, damping and stiffness coefficients; linearized rotor dynamic parameters; dynamic
impedances).
Seals are mainly used to reduce the leakage of working and lubricating fluids through the interface
between machine parts. Some leakage is inevitable, and it results in axial fluid velocities though the
seal in the direction of the pressure drop. The present day requirement of critical sealing applications
have a diverse range of operating condition requirements such as (i) cryogenic temperature, (ii) hard
vacuum, (iii) ultra-clean systems, (iv) leakage control to 10-12 cc/sec, (v) pressures over 100 bar, (vi)
temperatures exceeding 800°C, (vii) hard-to-handle liquids and gases, (viii) high pressure pulsations
and (ix) rotor speeds as high as 105 rpm. These extreme conditions of seals are challenging tasks on
the space age aviation and aerospace industries. The importance of calculations of RDPs of seals
arose in the late 1970s in regard to vibration problems related to high-pressure oxygen turbopump of
the space shuttle main engine. Compressors used in many industries also had the instability problems
within the operating speed range. Seals in high-speed operations of turbo-machines leads to
instability. The main factor, which governs the instability, is RDPs of seals. RDPs of seals are greatly
dependent on many physical and mechanical parameters such as lubricant and working fluid
temperatures, pressure drop, seal clearances, surface roughness and patterns, rotor speeds, eccentricity
and misalignments.
Designers should know the types of bearing and seal that could be used, and performance
characteristics associated with each of them. The main aim will be towards the operation of bearings
Dr R Tiwari, Associate Professor, Dept. of Mechanical Engg., IIT Guwahati, ([email protected])
272
and seals under the action of dynamic loads, in particular the RDPs of different bearings and seals
types are discussed. These characteristics have a major influence on the overall system dynamics.
Bearing types are (i) rolling element bearings (ii) oil film hydrodynamic bearings (iii) gas bearings
(iv) squeeze film bearings and (v) hydrostatic bearings. Dynamic seals can be classified as (i) plain
seals (ii) roughened seals (iii) contact seals and (iv) brush seals. Table 6.1 one compares the criterion
of selection between the rolling and journal bearings.
Table 6.1 Criteria for selection of the bearing type
S.N. Parameter Rolling bearing Journal bearing
1. Starting friction Low High
2. Space Less axial space Less radial space
3. Load type Both radial and axial Radial or axial
4. Failure time Gradual Sudden
5. Clearance Less More
6. Deflection of shaft Less More
7. Assembly Simple Tedious
8. Maintenance Less More
9. Replacement Cheaper Costly
10. Load carrying capacity Moderate High
11. Electrical insulation No Yes
12. Noise High Low
13. High overload For short periods For moderate periods
6.1 Rolling Element Bearings
Rolling element bearings or simply rolling bearings are the most common type of bearings, it requires
less boundary dimensions and can transmit heavy and variable loads of various forms and can easily
installed and serviced. Figure 6.1 shows a cut section of the ball and roller bearings. Figures 6.2 and
6.3 show, respectively, the nomenclature and various basic geometries of a most simple ball deep
groove bearing. Various types of rolling element bearings (refer Figure 6.4) are: deep groove ball