1 GEARBOX A gearbox, also known as a gear case or gear head, is
a gear or a hydraulic system responsible for transmitting
mechanical power from a prime mover (an engine or electric motor)
into some form of useful output. A gearbox is a set of gears for
transmitting power from one rotating shaft to another. They are
used in a wide range of industrial, automotive and home machinery
application. Gear heads are available in different sizes,
capacities and speed ratios. Their main function is to convert the
input provided by an electric motor into an output of lower RPM and
higher torque. Functions of a Gearbox A gearbox is precisely bored
to control gear and shaft alignment. It is used as a
housing/container for gear oil. It is a metal casing for protecting
gears and lubricant from water, dust and other contaminants.
Gearbox Specifications There are a number of performance
specifications which must be considered while choosing a gearbox
for different industrial applications. Some of the important
specifications are : Gear ratio : The ratio may be specified as x :
1, where x is an integer. Output torque Maximum input power Maximum
input speed Gearing arrangement Reducer output Shaft Alignment
2
The Gearbox (Transmission) Types of gearing:Various types of
gearing are used on a motor vehicle. The gearboxes employ one or
more of the following: 1- Spur, teeth parallel to axis, used on
sliding mesh. 2- Helical, teeth inclined to axis to form helix. 3-
Double helical, two sets of opposing helical teeth. 4- Epicyclic or
planetary, spur or helical gears rotating about centers which are
not stationary.
Gear ratio (single gear train): The gear ratio, or velocity
ratio, between a pair of gear wheels is in inverse ratio to the
number of teeth on each. Thus:
NB/NA = DA/DB= nA/nB
NB = NA (nA/nB) Where: NA= rev per min of gear A, nA = number of
teeth on A NB = rev per min of gear B, nB = number of teeth on B DA
= Diameter of gear A DB = Diameter of gear B
3 Power, Speed and Torque: The power transmitted by a shaft is
directly proportional to the speed of revolution and the torque
acting on it
Power [kW] = 2 N T / (60 x 1000) [N.m/s] Then TA NA = TB NB For
a given power, therefore, the torque is inversely proportional to
the speed of revolution and if the re min is reduced the torque
will be increased in the same ratio (assuming 100% gear
efficiency). TB/TA = nB/nA Where: TA = torque transmitted by A TB =
torque transmitted by B Velocity or gear ratio (ig) = number of
teeth on driven gear/number of teeth on driver gear. TB = TA
(nB/nA) = TA/ ig Compound gear train: If the number of teeth on
each wheel is known, the relationship between the speed of wheels A
and D can be determined as follows For wheels A and B: NB/NA =
nA/nB, i.e. NB= NA (nA/nB) Wheel B and C are fixed on the same
shaft, so NC=NB For wheels C and D: ND/NC = nC/nD, i.e. ND = NC
(nC/nD) Substituting NC = NB = NA (nA/nB) from above, we get ND =
NA (nA/nB) (nC/nD) 4 Or ND/NA = By inspection of the layout of the
figure, it will be observed that wheels A and C are driver gears
while B and D are driven gears. Hence, from the above equation
Velocity or gear ratio (ig) = product of teeth on driven gears/
product of teeth on driver gears
ND = NA (nA/nB) (nC/nD) = NA (nA nC / nB nD) =NA/ig Types of
Drives and gearboxes There are many types of the car drives,
usually classified accordance with number of driving axles (4x2,
4x4, 4WD, AWD) and each type has a different gearing arrangement.
Also, gearbox (transmission) has different types (sliding-mesh,
constant-mesh, synchro-mesh) some of them are old-fashion and had
been replaced, and some are in use in modern cars. SLIDING-MESH
GEARBOX: The sliding gearbox was popular on cars up to about 1930,
but it is rarely used. The basic layout of a 4-speed and reverse
gearbox is shown in the figure. The various spur-type gears are
mounted on three shafts. o Primary shaft (alternative names clutch
or first motion shaft) o Layshaft (countershaft) o Mainshaft (third
motion shaft).
5 1.main drive gear 2.counter shaft 3.main shaft 4.I gear 5.II
gear 6.III gear 7.top speed engaging dogs 6 7 Primary shaft This
shaft transmits the drive from the clutch to the gearbox. At the
end, the shaft is supported by a spigot bearing positioned close to
the splines on to which the clutch driven plate is connected. The
main load on this shaft is taken by a bearing; normally a sealed
radial ball type, positioned close to an input gear called a
constant mesh pinion. The gear is so named because it is always in
mesh with a larger gear, a c constant mesh wheel, that I part of
the layshaft gear cluster. Note that a small driving gear is called
a pinion and a large gear a wheel.
Lay shaft This shaft, which is normally fixed to the gearbox
casing, supports the various-sized driving pinions of the layshaft
gear cluster.
Main shaft This splined output shaft carries spur gearwheels
that slide along the shaft to engage with the appropriate lay shaft
gears. At the front end, the main shaft is supported by a spigot
bearing situated in the centre of the constant mesh pinion. A heavy
duty radial ball bearing is fitted at the other end to take the
force of the gears as the attempt to move apart.
Gear positions
Neutral All main shaft gearwheels are positioned so that they do
not touch the layshaft gears. A drive is taken to the layshaft, but
the mainshaft will not be turned in neutral position.
First gear The firs-speed gearwheel A on the mainshaft is lid
backwards to engage with pinion B on the layshaft; all other gears
are positioned in neutral. In this gear, the reduction in speed
that occurs as the drive passes through the constant-mesh gears, E
and F, is reduced further by the firs-speed gears, A and B. The
gear ratio (also called the movement ratio or velocity ratio) is
given by 8 Ratio = (Driven/driver) x (driven/driver) Second gear
The second-speed gearwheel C is slid forward to engage with the
layshaft gear D; all the other gear are set in the non-driving
position.
Ig2 = (F/E) x (C/D) Third gear In this gear position, gearwheel
G is slid in to mesh with gear H.
Ig3 = (F/E) x (H/G)
Top gear In this layout, fourth gear is a direct drive; namely a
gear that gives a ratio 1:1. It is obtained by
slidinggearGtoengageitsdogteethwiththecorrespondingteethformedontheendofthe
constantmeshpinionE.Engagementofthedogclutchlockstheprimarytothemainshaftand
this gives a straight-through drive. Reverse gear Sliding a reverse
gear between any two gears on the layshaft and main shaft is the
method used to change the direction of rotation of the output
shaft. The simplest arrangement usesa single reverse gear, which is
mountedon a short shaft. This shaft is positioned so that the
reverse can slide and mesh with the two first-speed gears as shown
in the figure. The gear ratio is igr = (Driven/Driver)
x(Driven/Driver) x(Driven/Driver) = (F/E) x (J/B) x (A/J) = (F/E) x
(A/B) 9 Thisisthesameratio
asforfirstgear,andirrespectivelyofthesizeofgearJ,itwillbe seen
thattheratioalwaysremainsthesame.Forthisreasonitiscalledan idler
itchangesthe direction, but does not alter the ratio.
With the idler arrangement, some drivers persistently slip the
clutch to maintain a low reversing speed. Excessive clutch wear
resulting from this practice is minimized when the reverse ratio is
setlowerthanfirstgear.Thisachievedbyusingareversegeararrangementasshowninthe
figure.Insteadofsingleidler,thecompoundreversegearhastwogearpinionsjoinedtogether.
The reverse shaft is positioned so that the reverse pinions are
able to mesh simultaneously with the appropriate layshaft and
mainshaft gears. Gear Changing When one gear is moved to engage
with another gear noise will result if the peripheral (outside)
speedsarenotthesametoavoidthis,thedriverofthevehiclehavingasliding-meshgearbox
performs an operation called double declutching. I nterlock
mechanism-
Preventstwogearsengagingsimultaneously;ifthisoccursthegearboxwilllockupandshaft
rotation will be impossible. Although the interlock device takes a
number of different forms, the arrangement shown in the figure is
one of the most common. 10 Power take-off arrangement In addition
to the mechanism use for driving a vehicle along a road, a power
supply is often required for operating external items of auxiliary
equipment. A light truck having a tipping mechanism is one example,
but the most varied application of power take-off units is
associated with specialized off-road vehicles. The figure shows a
typical power take-off arrangement that is driven from the gearbox
layshaft. Disadvantages of the sliding mesh Although the mechanical
efficiency of the sliding mesh gearbox was high, it suffered from
two great disadvantages: 1- Gear noise due to the type of gear. 2-
The difficulty of obtaining a smooth, quit and quick change of gear
without the great skill and judgment.
11 CONSTANT-MESH GEARBOX
12 1.I speed gear 2.II speed gear 3.main shaft 4.III speed gear
5.top and III speed engaging dogs 6.top gear 7.primary shaft or
main drive gear 8.counter shaft/cluster gear The main feature is
the use of the stronger helical of double helical gears which lead
to quieter operation. In this design, the mainshaft pinions
revolves freely on bushes or needle-roller bearings and are all in
constant engagement with the corresponding layshaft wheels.The gear
operation is obtained by locking the respective gear to the main
shaft by means of a dog clutch. With this arrangement the
quieter-running helical gears can be employed, and during gear
changing the noise and wear are reduced by the simultaneous
engagement of all the dogs instead of only a pair of gear teeth as
on the sliding-mesh gearbox. With single helical pinions (double
helical is economically impractical), the driving loads on the
teeth cause an axial thrust which must be resisted by thrust
washers, or shoulders, on the mainshaft.
SYNCHRO-MESH 13
14 The figure shows unite main details of. Fundamentally the box
is laid out in same manner as a constant-mesh, with the exception
that a cone clutch is fitted between the dog and gear members. The
initial movement of the selector a sleeve carries the hub towards
the gear and allows the cones adjusts the speed of the gearwheel to
suit the hub and mainshaft. Extra pressure on the lever will allow
the sleeve to override the spring-loaded balls, and positively
engage with the dogs on the gear.
1.I speed gear 2.II speed gear 3.main shaft 4.outer engaging
unit 5.inner engaging unit 6.top gear engaging teeth 7.main drive
gear 8.top gear synchronizing cones 9.counter shaft 15 Four- and
All-Wheel Drive:
Four-wheel-drive (4WD) and all-wheel-drive (AWD) systems can
dramatically increase vehicles traction and handling ability in
rain, snow, and off-road driving. The improved traction of 4WD and
AWD systems allows the use of tires narrower than those used on
similar 2WD vehicles. These narrow tires are less expensive. They
also tend to cut through snow and water rather than hydroplane over
it. Both 4WD and AWD systems add initial cost and weight.
16 Design of gears : All gears are subjected to fatigue.
Material selected is 30 Ni 4 Cr 1with Sut=1500Mpa, Sy=1300Mpa and
hardness 440 BHN. Vehicle load is steady while multi cylinder
engine has torque fluctuations. The service factor of 1.25 is
selected. First gear ratio G1 is to be 4:1. Pair GH--- This pair is
spur and H is input gear or pinion with speed of (1/G1) times the
engine shaft speed and input torque of G1times engine mean torque.
Let Engine torque be 85Nm @ speed of 5000rpm. Estimation with Lewis
equation: Fatigue strength of material = 500Mpa Minimum number of
teeth = 18 Factor of safety = 2 Input torque G1x 85 = 170Nm Input
Speed (1/G1)= 2500 17 Assuming module=4 Diameter of pinion = 18 x
4=0.072m Pitch line velocity = V = (dn/60) = x 0.072 x 2500/60 =
9.42 m/sec. Velocity Ratio = Kv= (5.6/(5.6+V)) =0.646 Wt = Torque /
(dp/2) =170/0.036 = 4.72kN Effective tangential force = (4.72 x
1.25)/0.646 = 9.2 kN Lewis form factor for 20 full depth 18 teeth
is 0.308 Now, F= (Wt x Ks)/(Cv x m x Y x ) =4.72 x 1000 x 1.25
/(.646 x .004 x .308 x 250 x 106) =0.03m with =250MPa F=30mm which
is near to 7 times module ,hence is accepted. Pair AB --- This pair
is helical and A is input gearAssuming module = 3 Input Torque =
Engine torque Assuming helix angle = 25 Input speed = Engine speed
= 5000rpm To calculate number of teeth on gear A Center distance
between shafts is that of spur pair = TH + GTA x module / 2 = 18+(2
x 18) x 4 / 2 = 108mm 18 For helical pair center distance is = TA +
G1TH x module / 2 cos 108= TA + 4TA x 3 / 2cos25 TA = 22 TB = 44
Exact center Distance = 108 = 22+ 88 x 3 / 2cos = 23.55 Virtual No.
of Teeth = 22 / cos3 = 28.55 = 29 Lewis form factor = 0.35 Dp
=module x No. of teeth / cos =72 mm Pitch line velocity = dn / 60 =
( x 0.072 x 5000) / 60 = 18.85 m/sec. The velocity factor Kv= 5.6 /
(5.6 x 18.85) = 0.563 Effective tangential force Wt = Te x Ks / (Kv
x D/2) = 85 x 1.25 / (.563 x 0.072/2 ) = 5.24 kN Factor of safety =
2 Design stress = fatigue strength / F.S. = 500/2 = 250 MPa Now Wt
= m x F x x Y 5.24 x 103 = 0.0003 x F x 250 x 106 x 0.35 F = 19.66
say 20mm ---- accepted Pair CD for G3 G3 =1.58 = TB x TC / (TA x
TD) but TB/TA =2 TC/TD = 1.58/2 = 0.79 The pair is to be helical
and with same helix angle and module as that of pair AB and as such
total number of teeth are same. TC + TD = 22+44 = 66 19 TD = 36.87
= 37 TC = 66 37 = 29 The gear D is input gear :d = TD x m / cos =
37 x 3 / cos23.55 = 121mm Pitch line velocity = dn / 60 = x 121 x
2500 /60 = 15.84 m/sec Kv = 5.6 / (5.6 x 15.84) = 0.584 Virtual No.
ofteeth = T / cos3 = 37 / cos323.55 = 48 Lewis form factor = 0.395
Effective tangential force = Wt = Te x KsG1 / (Kv x d/2 ) Wt = 6.01
x 103 But Wt = m x F x x Y 6.01 x 103 = 0.0003 x F x 250 x 103 x
0.395 F = 20.20mm say 21mm Pair EF for ratio G2
G2 = 2.52 = TB x TE /(TA x TF ) but TB/TA = 2 T E/ TF = 2.52 / 2
= 1.26 But TE + TF = 66 TF = 29 and TE = 37 Gear F is input gearDF
= TF x m / cos = 29 x 3 / cos23.55 20 = 0.0949 m Pitch line
velocity = dn/60= x 0.0949 x 2500 / 60 =12.42 m/sec Kv = 5.6 / (5.6
+ 12.42) = 0.61 Virtual no. of teeth = TF/cos3 = 29 / cos323.55 =
38 Lewis form factor = 0.377 Effective tangential force Wt = TE x
G1 x Ks / (Kv x DF/2) =7.34 kN Wt = m x F x x Y 7.34 x 103 = 0.003
x F x 250 x 106 x 0.377 F = 0.02595m=26mm Checking the design of
spur gear pair GH as per AGMA standards:For spur gearsK= 50 / (50 +
200V) If the gears have high precision shaved and ground teeth, and
if an appreciable load is developed then, K = 78 / (78+200V) If the
gears have high precision shaved and ground teeth and there is no
appreciable dynamic load, then Kv = 1 Geometry Factor AGMA replaces
the Lewis form factor by the geometry factor J and insists upon its
use when fatigue failure is to be considered in the design.
Otherwise , modified Lewis form factor may be used. Fatigue
strength and modifying factor It is always recommended to carry out
a fatigue test of the material if the economy permits. In the
absence of test values for fatigue strength one can estimate the
fatigue strength of the material as: Se=0.50 Sutfor materials
having Sut < 1400 MPa Se=700 MPa
for materials having Sut > 1400 MPa 21 It is also observed
that a material behaves in a different manner under test conditions
than when operating as a component of a machine. This difference is
accommodated by introducing following fatigue strength modifying
factors. (i)Surface factor, (ii)Size factor, (iii)Reliability
factor, (iv)Temperature factor, (v)Modifying factor for
stress-concentration (vi)Miscellaneous-effect factor. These factors
are used to correct fatigue strength of the material, which
generally gives a reduced strength for design. Load correction
factors The nature of driving and driven machinery also affect the
tooth load due to torque fluctuations. This is accounted for by an
overload correction factor and it increases the actual load on
teeth. Further,wider face width, nature of mounting the gears and
shaft clearances in the bearing permit deflection of the shaft, and
load distribution along the whole width is not uniform. This is
also accounted for by multiplying the tooth load by a Load
Distribution Factor. (1).Dynamic Load Factor Select Kv = 78 / 78
+200V = 78 / 78 + (200 x 9.42) Kv=0.642 (2).Fatigue strength of
tooth material Se is Se = 700 MPa Values for following stress
modifying factors are assumed as, Surface factorKa = 0.70 Size
factor Kb = 0.894 22 Reliability factor Kc = 0.868 Temperature
factor Kd = 1 Stress conc.modifying factor Ke = 1 Miscellaneous
factor= 1 Fatigue strength of the tooth of gear Se is, Se = 0.70 x
0.894 x 0.868 x 1 x 1 x 700 Se = 505 MPa Load Modifying Factor
Multi cylinder diesel engines with a uniformly driven load give the
value of overload correction factor as 1.25 (i.e. Ko). For a face
width of 30mm and accurate mounted gears, the load distribution
factor is 1.3 (Km). Bending stress = (Te x G1 x Km x Ko ) / (d/2) x
Kv x F x m x J For pinion teeth 22 and gear teeth 44, J = 0.365,
from AGMA tables, = (85 x 4 x 1.25 x 1.3) / (0.72 x 0.642 x 0.03 x
0.003 x 0.365) = 363.8 MPa Factor of safety= Se / = 505 / 363.8 =
1.38 The design is quite satisfactory as factor of safety is above
one and can be accepted. Check for surface fatigue strength of the
material: Experimental evaluation of surface fatigue strength has
been carried out by few scientists. However, this procedure is not
economical and quick. From these experimental results, the
suggested equation for finding out surface fatigue strength of
steels is: Sc = 2.76 HB 70 Mpa, where Sc = Surface fatigue strength
HB = Brinell hardness of the material and this strength upto only
108 cycles of repeated contact stress. If the two materials have
different hardness the lesser value is generally,though not always,
used. AGMA recommends that the contact fatigue strength be modified
in a manner quite similar to that used for bending endurance limit.
The equation is, 23 SH = (CL x CH) / (CT x CR) Where SH= Corrected
fatigue strength CL= Life factor CH= Hardness ratio factor CT=
Temperature factor CR= Reliability factor Wear Factor Load Stress
Factor: Contact stresses are concatenated in a localized area near
the contact and then distributed over the whole cross-section of
the element. Therefore, the contact stress values are much higher
than average stress values. To account for this fact, a wear factor
or load stress factor is introduced from Hertzian contact stress.
This factor is dependent on two parameters. One is the elasticity
of the material, as elastic co-efficient Cp, Cp=1 / ( ((1-2p) / E
p)+ (( 1- 2G ) / EG))) And second is geometry of the curvature of
the mating surfaces known as geometry factor. I = sin x cos x mg /
(2(mg+1)) Where = Pressure angle mg = Gear ratio All these factors
are combined together to give contact stress, = - Cp (Wt / Cv x F x
dp x I )1/2 Surface fatigue strength of the 30 Ni 4 Cr 1 steel Sc =
2.76 x 440 -70 =1145 Mpa Life factorCL = 1V Reliability factorCR =
1 Hardness factorCH = 1 Temp. factorCT = 1 24 Corrected strength SH
= SC = 1145 Mpa CP calculated for Steel = 191 I = sin20 x cos20 x 2
/ (2(2+1)) = 0.01 CV = KV = 0.642 H = 260 Mpa Factor of safety = SH
/ H = 4.4 Which is much more than desired. 25 Design of shafts Such
gearboxes normally have three shafts Input shaft (I), Output shaft
(O) and Lay Shaft or Countershaft (L). Shaft carrying gears (or
pulley) to transmit power are always subjected to reversed bending
due to the power transmitting force through mating teeth at a
pressure angle in case of gears. Shafts are subjected to bending
moment. This being a multispeed gearbox a neutral position I to be
provided, so the layout should accommodate all the gears when they
are not in engagement. In an automobile gearbox, forward and
backward motion of one lever from neutral engages two pairs, while
another motion along with the forward and backward stroke enables
engagement of the remaining two pairs. Rough estimation pf the
distance between supports X,Y can be taken as 2.5 times the sum of
the face widths of all pairs of gears. =(20+21+26+30) x 2.5 =
242.5mm say 243mm Output shaft (O) and Lay shaft (L) will be
subjected to maximum bending moment, when pair EF is in engagement
as it is farthest from supports and has more tangential force than
due to pair CD. Input shaft is a cantilever and loaded at center of
the pair AB.Reaction through pair AB = force along pressure angle
=Te x Sv = Te x Sv / (dp x cos /2) =3.14kN Reaction through pair EF
= Te x Sv x G1 / (dp x cos /2 ) =4.77kNB.M. on input shaft = 3.14 x
103 x 0.020 = 62.8 Nm Reactions at A & B, RB = 3.14 x 103 x
(150+20)/150 = 3.55kN RA = 3.55-3.14 = 0.41kN Axial force = Wt x
tan = TeSvtan/(dA/2) = 1.29kN 26 Reaction at X1 and Y1 of output
shaft, RX1 = RY1 = F/2 = 4.77/2 = 2.385 kN Axial force = TeSvG tan
/ (dA/2)) =1.95kN Reaction at X, Y of countershaft, Rx = F x 105
/243 = 2kN Ry = 4.77 2 = 2.77kN B.M. on main shaft (O) B.M. = 2.385
x .105 x 103 = 250Nm B.M. on lay shaft (L) B.M. = 2.68 x 103 x .105
= 273Nm Torque on input shaft = 85 x 1.25 = 100Nm Torque on lay
shaft = 200Nm Torque on main shaft = 400Nm Maximum torque on main
shaft will be in 1st gear that is 400Nm, but the values of bending
moment will be minimum.The Soderberg criteria can be applied : d =
[32 x F.S./ {(T/Sy)2 + (M/Se)2}1/2]1/3 d = diameter of shaft F.S. =
factor of safety Sy = yield strengthSe = fully corrected endurance
strength Selecting plain carbon steel with 0.45% carbon with Sut =
600Mpa, Sy = 450Mpa and fully corrected endurance strength be
135Mpa. Factor of safety=1.8 27 For the input shaft, d = [32 x 1.8
/ {(100/(450 x 106)2 + (62.8/(135 x 106))2 }1/2]1/3 d = 0.0211m
Gear A is mounted on shaft, which needs an axial locking on one
side by shoulder and other side by circlip. This increases diameter
Standard value for bearing diameter = 25mm Let counter bore
diameter = 20mm Serration depth = 2.5mm Bearing diameter = 25mm For
the main shaft, d= [ 32 x 1.8 / {(400/ (450 x 106))2 + (250/135 x
106)2 }1/2] 1/3 =0.033m Add 6mm serrations for sliding gears. For
Bearing shoulder and for other effects add another 4mm d = 33 + 6 +
4 = 43mm For bearing selection adopt the standard size of 45mm For
the layshaft It is to be a hollow cluster gear to be supported in a
needle on a non-rotating axle supported between X and Y. The actual
B.M. will be slightly less than for an integral shaft. do3 di3 =
[32 x 1.8/{(T/Sy)2 + (M/Se)2}1/2]1/3 For a gear material 30 Ni 4 Cr
1 , Sy = 1300Mpa and Se = 390Mpa Let di = 0.025m do = 30.5 mm The
cluster gear which is treated as a hollow shaft has a length such
that the angular twist will efeect the load distribution on the
gear teeth, hence its torsional stiffness is required to be
considered. 28 Assuming Trial values of do and di = 50mm and 25mm
respectively J = (do3 di3) / 16 J = 21.47 x10-6 m3 K = (T/Q) =
(GJ/L) x (/180) G = modulus of rigidity for steel = 80Gpa L =
length of shaft = 0.200m K = 149.88 kNm/degree which is an
acceptable value. The hollow shaft on which the cluster gears are
to be forged integral, shall have inside diameter of 25mm and
outside diameter of 50mm. 29
30 CLASSIFICATION OF MANUFACTURING PROCESSES OF GEARS 1. Milling
process Disc type cutter End mill cutter 2. Gear planning process
The Sunderland process The Maag process 3. Gear shapers Rack type
cutter generating process Pinion type cutter generating process 4.
Gear hobbing Axial hobbing Radial hobbing Tangential hobbing
5.Bevel gear generating Straight Bevel gear generator Spiral bevel
gear Generator 31 METHODS OF FORMING GEARS Roll forming
Inrollforming,thegearsblankismountedonashaft&ispressedagainsthardenedsteelof
rolling dies. The rolls are fed inward gradually during several
revolutions which produce the gear teeth. The forming rolls are
very accurately made & roll formed gear teeth usually home both
by not and cold. In not roll forming, the not rolled gear is
usually cold rolled which compiles
thegearwithasmoothmirrorfinish.Incoldrollforming,higherpressuresareneededas
compared to not rolling many of the gears produced by this process
need no further finishing. It becomes stronger against tension
& fatigue. Spur & helical gears are made by this process.
Stamping
Largequantitiesofgearsaremadebythemethodknownasstampingblankingorfine
blanking.Thegearsaremadeinapunchpressfromsheetupto12.7mmthinksuchgearsfind
applicationin:toys,clocks4timers,watches,water&Electricmaters&somebusiness
Equipment.After stamping, the gears are shaved; they give best
finish & accuracy. The materials
whichcanbestampedare:low,medium&highcarbonsteelsstainlesssteel.Thismethodis
suitable for large volume production. Powder metallurgy
Highqualitygearscanbemadebypowdermetallurgymethod.Themetalpowderispressedin
dies to convert into tooth shape, after which the product is
sintered. After sintering, the gear may be coined to in crease
density & surface finish. This method is usually used for small
gears. Gears made by powder metallurgy method find application in
toys, instruments, small motor drivers etc. Extrusion Small sized
gear can also be made by extrusion process. There is saving in
material & machining time. This method can produce any shape of
tooth & is suitable for high volume production gears produced
by extrusion find application in watches, clocks, type writers etc.
32 GEAR GENERATING PROCESS Gear Hobbing Hobbing is the process of
generating gear teeth by means of a rotating cutter called a hob.
It is a
continuesindexingprocessinwhichboththecuttingtool&workpiecerotateinaconstant
relationshipwhilethehobisbeingfedintowork.Forinroutegears,thehobhasessentially
straightsidesatagivenpressureangle.Thehobandthegearblankareconnectedbymeansof
proper change gears. The ratio of hob & blank speed is such
that during one revolution of the hob,
theblankturnsthroughasmanyteeth.TheteethofhobcutintotheworkpieceinSuccessive
order & each in a slightly different position. Each hob tooth
cuts its own profile depending on the shape of cutter, but the
accumulation on the shape of cutter, but the accumulation of these
straight cuts produces a curved form of the gear teeth, thus the
name generating process. One rotation of the work completes the
cutting up to certain Depth. TYPE OF HOBBING Arial hobbing This
typeoffeedingmethodismainlyusedforcuttingspurorhelicalgears.Inthis
type,firstly
thegearblankisbroughttowardsthehobtogetthedesiredtoothdepth.Thetablesideisthem
clamped after that, the hob moves along the face of the blank to
complete the job. Axial hobbing
whichisusedtocutspur&helicalgearscanbeobtainedbyclimbnotingorconvential
hobbing. Radial hobbing This method of hobbing is mainly used for
cutting worm wheels. In this method the hob & gear
blankaresetwiththeironesnormaltoEachother.Thegearblankcontinuestorotateataset
speed about its vertical axes and the rotating hob is given a feed
in a radial direction. As soon as the required depth of tooth is
cut, feed motion is stopped. 33 Tangential hobbing
Thisisanothercommonmethodusedforcuttingwormwheel.Inthismethod,thewormwheel
blankisrotatedinaverticalplaneaboutahorizontalaxes.Thehobisalsohelditsaxisorthe
blank.Before starting thecut, the hob is setatfulldepthofdie
toothand thenit isrotated.The
rotatinghobisthenfedforwardaxially.Thefrontportionofthehobistapereduptoacertain
length & gives the fed in tangential to the blank face &
hence the name Tangential feeding. GEAR SHAPING PROCESS In gear
shapers, the cutters reciprocate rapidly.The teeth are cut by the
reciprocating motion of the cutter. The cutter can either be rack
type cutter or a rotary pinion type cutter. Rack type cutter
generating process
Therackcuttergeneratingprocessisalsocalledgearshapingprocess.Inthismethod,the
generatingcutterhastheformofabasicrackforageartobegeneratedThecuttingactionis
similartoashapingmachine.Thecutterreciprocatesrapidly&removesmetalonlyduringthe
cutting stroke. The blank is rotated slowly but uniformly about its
axis and between each cutting stroke of the cutter, the cutter
advances along its length at a speed Equal to the rolling speed of
the matching pitch lines. When the cutter & the blank have
rolled a distance Equal to one pitch of the blank, the motion of
the blank is arrested, the cutter is with drawn from the blank to
give relief to the cutting Edges & the cutter is returned to
its starting position. The blank is next indexed & the next cut
is started following the same procedure. Pinion type cutter
generating process
Thepinioncuttergeneratingprocessisfundamentallythesameastherackcuttergenerating
process,andinsteadofusingarackcutter,itusesapiniontogeneratethetoothprofile.The
cutting cycle is commenced after the cutter is fed radically into
the gear blank Equal to the depth of tooth required. The cutter is
then given reciprocating cutting motion parallel to its axis
similar to the rack cutter and the cutter & the blank are made
to rotate slowly about their axis at speeds
whichareEqualatthematchingpitchsurfaces.Thisrollingmovementblowtheteethonthe
blank are cut. The pinion cutter in a gear shaping m/c may be
reciprocated either in the vertical or 34 in the horizontal axis.
Advantages:- The gears produced by the method are of very high
accuracy. Both internal & external gears can be cut by this
process. Non convential types of gears can also be cut by this
method Disadvantages:- The production rate with gear shaper is
lower than Hobbing There is no cutting on the return stroke in a
gear shaper Worm & worm wheels cant be generated on a gear
shaper GEAR CUTTING BY MILLING Disc type cutter For cutting a gear
on a milling m/c, the gear lank is mounted on am arbor which is
supported b/w a dead centre & a lieu centre in the in dering
head. The cutter is mounted on the arbor of the cutter must be
aligned exactly vertically with the centre line of the indexing
head spindle. The table of m/c ismovedupward until thecutter just
touchesthe peripheryofgearblank. The verticalfeed dial is set to
zero. The table is then moved horizontally until the cutter clears
the gear b lank. The table is then moved upwards by an amount Equal
to the full depth of the gear tooth The vertical movementmaybeless
ifthegear is tobecut intwoor morepassesAfterthis, the longitudinal
feed of the table is engaged. The gear blank moves under the
rotating cutter & a tooth space is cut. After this, the
movement of the table is reversed so that the cutter again clears
the gear blank. The gear blank is then indexed to the next position
for cutting the second tooth space. This procedure
isrepeateduntilalltheteethhavebeenmilled.Thereisaflatcirculardisctypecutterandthe
plane of rotation of the cutter is radial with respect to the
blank. End Milling cutter In this method the cutter rotates about
am axis which is set racially with respect to the blank & at
thesametimethecutteristraversedparalleltotheaxesoftheblankThecuttingedgetieona
35
surfaceofrevolution,Sothatanyaxialcross-sectionofthecuttercorrespondstotheshape
required for the space b/w two adjacent teeth on the finished
wheel. The milling m/c used in this method is vertical milling m/c
The End mill cutter is mounted straight on the milling m/c spindle
through a chuck.
1)ThedisctypeofcutterisusedtocutbigspurgearofcutterisEmployedforthemanufacture
of pinion of large pitch.
2)Thismethodisveryslowsinceonlyonetoothiscutatatime.Toovercomethese
drawbacks, multiple tools shaping cutter head is used to cut all
the tooth spaces of the gear at the same time. Advantage:- Gear
milling is a simple, Economical & flexible method of gear
making. Spur, helical, bevel gears and racks can be produced by
this method The major disadvantage of this method is that a
separate cutter must be used not only for every piton but for every
no. of teeth. Bevel Gear Generating The teeth of bevel gears
constantly change in form, from the large to the small Encl There
are to common types of bevel gear generators, on cuts straight
teeth & other cuts spiral teeth. Straight Bevel gear generator
Forgeneratingstraightbevelgears,therollingmotionsoftwopitchconesareemployed
motionsoftwopitchconesareemployedinsteadofpitchcylinder.Inthismethod,two
reciprocatingtoolswhichworkontop&bottomsidesofatooth&arecarriedonthemachine
cradle. The cradle & work roll up together with the gear blank
at the top of roll, when a tooth has been completely generated, the
work is withdrawn from the tool and the m/c inclined, while the
cradle is rolled down to the starting position. The operating cycle
is repeated automatically until 36 all the teeth in the gear have
been cut. Theadvantagesofthis processarethata
previousrougheningcutis not necessary,thussaucing one handling of
the blank, longer cutter life, improved quality of gear and less
set up time Spiral bevel gear Generator
Inthismethod,arotatingcircularcuttergeneratesspiralteeththatarecurved&obliqueproper
toothprofileshapesareobtainedbyrelativemotioninthem/cb/wworkcutter.Them/chas
adjustment by which both spiral bevel gears & hypoid gears can
be generated. Spiral bevel gears have an advantage have on
advantage over straight bevel gear is that teeth are Engage with
one another gradually by eliminating any noise & shock in their
operation. Gleason Method
Inthismethod,twodiscmillingcuttersareemployed,fig.Thetoolsformtheblanksofatooth
simulatingthebasiccrownwheel.Cutterteethareintermeshingandthediscsareinclinedto
each other at the pressure angle (usually 20*). The following
motions are involved while cutting a tooth: 1.The rotating cutters
revolve about their axes to provide the cutting action 2.They
travel in planes passing through the sides of the teeth on the
imaginary crown gear to shape the teeth along their teeth.
3.Atthesametime,theyparticipateintherelativerollingmotionbetweenthecuttersand
blank to obtain the required tooth profile. Indexing takes place
after each tooth space has been completed and the machine is fully
automatic in its motions. When gear has been completed, the machine
stops, the cutters withdraws the work piece can be changed with
little delay. This type of machine is a high production rate
machine and very useful for dealing with large batches of identical
gears. 37 Gear finishing process:- The following processes are
generally used for finishing of gears Gear shaving Gear sharing is
the most common method for gear finishing.In this method, a very
hard gear is used to ramous fine chips from the gear tooth profile.
The sharing cutter can be: Rotary type or
Racktypeinrotaryshoring,thecutter&thegearareruninmesh.Astheyrotate,thegearis
traversedlongitudinallyacrosstheshavingcutterorvieversa.Therotarysharingcutterhasa
member of peripheral gashes or grooves to from a series of cutting
Edges. The cutter & Gear are set up in a gear shoring m/c with
crossed axes in the form of spiral gearing. The usual angles are
10* to 15*. In rock sharing, the cutter is in the form of a rack.
During the operation, the gear is rolled in mesh with the cutter.
The cutter is reciprocated & at the End of Each stroke is fed
into the year Gear grindings Grindings is the most accurate method
of gear finishing.By grinding, teeth can be finished either by
generation or forming. In forming, the work is made to roll in
contact with a fiat faced rotating grinding wheel, corresponding to
the face of the imaginary rack meshing with the gear. One side
ofthetoothisgroundatatimeAfterthegrindingwheelisgiventheshapebyspaceb/wtwo
adjacent teeth. Both flanks are finished together. The second
method tends to be rather quicker, but both give equally accurate
results and which of the methods is to be used depends upon the
availability of the type of grinding m/c. Disadvantage:-
Considerable time is consumed in the process Low production
capacity Grinding wheels are Expensive. 38 Gear lopping It is
another extensively used process of gear finishing & it is
accomplished by having the gear in
contactwithoneormorecastironlapgearoftrueshapetheworkismountedb/wcentre&is
slowly driven by rear lap. It is in term driven the front
lap&atthesametimebothlapsarerapidlyreciprocatedacrossthegearface.Eachlaphas
individualadjustment&pressurecontrol.Afineabrasiveisusedwithkeroseneorlightoilto
assist the cutting action. The largest time of gear lapping is
about 15 minutes. Prolonged lapping damages the profile. Shot
blasting
Itprovidesafinishingprocessresemblingthatproducedbylappingalthoughithasother
functions,suchasremovingslightburrs,reducingstressconcentrationintoothfillets&
sometimes providing slight tip & root relief to teeth Phosphate
coating It is a chemical process which attacks the treated ferrous
surface and leaves a deposit on it about
0.01mm.inthickness.Itpreventsfromscuffing,
particularlyinhypoidgears,bypermittingthe Engaging tooth Surface
under the prevailing boundary lubrication conditions. Gear planning
This is one of the oldest methods of gear production but is still
extensively used. It employs rack
typecuttersforgenerationofspur&helicalgears.InvolutesrackhasstraightEdges&sharp
cornerscanbe(Easily)manufacturedeasily&accuratelyTherearetwotypesofgearplanning
machines,onebasedonTheSunderlandprocess&theotheronTheMaagprocessBoththe
methods are identical in principle but differ in m/c configuration
& detail. The Sunderland process
Inthismethod,thework(gearbalance)ismountedwithaxishorizontal&thecutterslideis
carried on a saddle position that moves vertically downward as
cutting proceeds. For cutting super
gears,thecutterreciprocatesparalleltotheworkaxis(but)becauseitcanbeswiveledinthe
vertical plane to any desired angle. The m/c is also used for
cutting single helical gears. The cutter
isgraduallyfedtothedesireddepthofteethafterwhichthedepthremainsconstant.
39 Simultaneouslythegearblank isrotating&rack is
traversedatatangent,the motionofrack& blank being geared to act
on their respective pitch lines. This relative motion beings fresh
part of the blank & rack into contact & thus causes the
teeth of the cutter to generate wheel teeth of the cutter to
generate wheel teeth. The indexing really consisting slopping the
rotation of the blank & causing the rack to moue. The process
is repeated until the blank has completed one revolution. The maag
process
Inthismethod,theworkismountedonthem/ctablewithitsaxisvertical.Therackcutteris
carried in a cutter head: that is made to moue in a vertical plane
but the actual direction of motion can be set at any desired angle.
Principal of gear planning
Thecutterduringitscuttingstrokeisincontactwithseveralteethatthesametimebutwith
differentpartofeachtooth,itplanescomparativelyanarrowstriponeachtoothateachstroke
and a different part of each tooth is submitted to the action of
the cutter at the next stroke.