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.