GEAR TRAINGEAR TRAINA gear train is a combination of A gear train is a combination of gears used to transmit motion gears used to transmit motion from one shaft to other shaft. It from one shaft to other shaft. It becomes necessary when it is becomes necessary when it is required to obtain a large required to obtain a large reduction in speed within a small reduction in speed within a small space. The following are the main space. The following are the main types of gear trains.types of gear trains.

GEAR TRAINGEAR TRAIN 1. SIMPLE GEAR TRAIN1. SIMPLE GEAR TRAIN 2. COMPOUND GEAR TRAIN2. COMPOUND GEAR TRAIN 3. REVERTED GEAR TRAIN3. REVERTED GEAR TRAIN 4. EPICYCLIC OR PLANETARY GEAR 4. EPICYCLIC OR PLANETARY GEAR TRAINTRAIN 5. DIFFERENTIAL 5. DIFFERENTIAL

SIMPLE GEAR TRAINSIMPLE GEAR TRAINA series of gears, capable of receiving A series of gears, capable of receiving

and transmitting motion from one and transmitting motion from one gear to another is called a Simple gear to another is called a Simple Gear Train. In this, all the gears axes Gear Train. In this, all the gears axes remain fixed relative to the frame remain fixed relative to the frame and each gear is on a separate shaft. and each gear is on a separate shaft. In simple gear train it is seen that In simple gear train it is seen that the intermediate gears have no the intermediate gears have no effect on the speed ratio and effect on the speed ratio and therefore they are known as idler therefore they are known as idler gears.gears.

SIMPLE GEAR TRAINSIMPLE GEAR TRAIN Consider a Simple Gear Train as Consider a Simple Gear Train as

shown in the figure (1)shown in the figure (1) Let N1 = Speed of driver gear in Let N1 = Speed of driver gear in

rpm.rpm. N2, N3, N4 = Speeds of the N2, N3, N4 = Speeds of the

intermediate gears in rpm.intermediate gears in rpm. N5 = Speed of driven gear in rpm.N5 = Speed of driven gear in rpm.

SIMPLE GEAR TRAINSIMPLE GEAR TRAIN

SIMPLE GEAR TRAINSIMPLE GEAR TRAIN T1 = Number of teeth on driver gearT1 = Number of teeth on driver gear T2,T3, T4 = Number of teeth on T2,T3, T4 = Number of teeth on

intermediate gearsintermediate gears T5 = Number of teeth on driven gear T5 = Number of teeth on driven gear We have N1/ N2 = T2/T1 ; N2/ N3 = We have N1/ N2 = T2/T1 ; N2/ N3 =

T3/ T2T3/ T2And N3/ N4 = T4/ T3 ; N4/ N5 = And N3/ N4 = T4/ T3 ; N4/ N5 =

T5/ T4T5/ T4Speed Ratio of the Gear Train is Speed Ratio of the Gear Train is

obtained by multiplyingobtained by multiplying

N1 x N2 x N3 x N4 T2 x T3 x T4 x T5N1 x N2 x N3 x N4 T2 x T3 x T4 x T5------------------------- = ------------------------- ------------------------- = ------------------------- N2 x N3 x N4 x N5 T1 x T2 x T3 x T4N2 x N3 x N4 x N5 T1 x T2 x T3 x T4Or, N1/ N5 = T5 / T1Or, N1/ N5 = T5 / T1Train Value is the inverse of Speed RatioTrain Value is the inverse of Speed Ratio Speed of driven gear ( N5) Speed of driven gear ( N5)

T1 T1Train Value = ------------------------------------- = Train Value = ------------------------------------- =

------ ------ Speed of driving gear ( N1) Speed of driving gear ( N1)

T5 T5

COMPOUND GEAR TRAINCOMPOUND GEAR TRAINWhen a series of gears are arranged in When a series of gears are arranged in

such away that two or more gears such away that two or more gears rotate about an axis with same rotate about an axis with same angular velocity, it is known as angular velocity, it is known as compound gear train. In this type of compound gear train. In this type of train, some of the intermediate train, some of the intermediate shafts i.e shafts other than input and shafts i.e shafts other than input and output carry more than one gear. output carry more than one gear.

COMPOUND GEAR TRAINCOMPOUND GEAR TRAIN

COMPOUND GEAR TRAINCOMPOUND GEAR TRAINUsing the same notations for speed and number of teeth for Using the same notations for speed and number of teeth for

different gears, as in case of simple gear train, different gears, as in case of simple gear train,

Here in case of compound gear train,Here in case of compound gear train, N2 = N3 and N4 = N5 N2 = N3 and N4 = N5 N1 N3 N5 N1 T2 x T4 x T6N1 N3 N5 N1 T2 x T4 x T6Speed ratio = ---- x ----- x ----- = ----- = ------- = -------------------- Speed ratio = ---- x ----- x ----- = ----- = ------- = -------------------- N2 N4 N6 N6 T1 x T3 x T5N2 N4 N6 N6 T1 x T3 x T5 Product of no.of teeth on driven gears Product of no.of teeth on driven gears i.e. Speed ratio = ----------------------------------------------------- i.e. Speed ratio = ----------------------------------------------------- Product of no.of teeth on driving gears Product of no.of teeth on driving gears

REVERTED GEAR TRAINREVERTED GEAR TRAINIf the axes of first and last wheel of If the axes of first and last wheel of

a compound gear coincide, it is a compound gear coincide, it is called a reverted gear train. Such called a reverted gear train. Such an arrangement is used in an arrangement is used in automotive transmissions, in automotive transmissions, in clocks (where the minute and hour clocks (where the minute and hour hand shafts are co-axial) and in hand shafts are co-axial) and in simple lathe where back gear is simple lathe where back gear is used to give a slow speed to the used to give a slow speed to the chuck.chuck.

REVERTED GEAR TRAINREVERTED GEAR TRAIN

PLANETARY OR EPICYCLIC PLANETARY OR EPICYCLIC GEAR TRAINGEAR TRAIN

When there exists a relative When there exists a relative motion in a gear train, it is motion in a gear train, it is called a planetary or an called a planetary or an epicyclic gear train. Thus in epicyclic gear train. Thus in an epicyclic gear train, the an epicyclic gear train, the axis of one of the gears axis of one of the gears moves relative to the frame. moves relative to the frame.

PLANETARY OR EPICYCLIC PLANETARY OR EPICYCLIC GEAR TRAINGEAR TRAIN

Consider two gear wheels S & P, the axes of Consider two gear wheels S & P, the axes of which are connected by an arm ‘a’. If the arm which are connected by an arm ‘a’. If the arm 'a' is fixed, the wheels S & P constitute a 'a' is fixed, the wheels S & P constitute a simple gear train. However, if the wheel S is simple gear train. However, if the wheel S is fixed so that the arm can rotate about axis of fixed so that the arm can rotate about axis of S, the wheel P would also move around S. S, the wheel P would also move around S. Therefore, it is epicyclic train. Usually, the Therefore, it is epicyclic train. Usually, the wheel P is known as epicyclic wheel. The term wheel P is known as epicyclic wheel. The term epicyclic emerges from the fact that the wheel epicyclic emerges from the fact that the wheel P rolls outside another wheel S and traces an P rolls outside another wheel S and traces an epicyclic path.epicyclic path.

PLANETARY OR EPICYCLIC PLANETARY OR EPICYCLIC GEAR TRAINGEAR TRAIN

PLANETARY OR EPICYCLIC PLANETARY OR EPICYCLIC GEAR TRAINGEAR TRAIN

Epicyclic gear trains are useful for Epicyclic gear trains are useful for transmitting high velocity ratio transmitting high velocity ratio with gears of moderate size in with gears of moderate size in comparatively lesser space. Such comparatively lesser space. Such gear trains are used in back gear gear trains are used in back gear of a lathe, differential gears of of a lathe, differential gears of the automobiles, hoists, pulley the automobiles, hoists, pulley blocks, etc.blocks, etc.

DIFFERENTIALDIFFERENTIAL

The fact that an epicyclic gear has two The fact that an epicyclic gear has two degrees of freedom has been utilized in degrees of freedom has been utilized in the differential gear of an automobile. It the differential gear of an automobile. It permits the two wheels of the automobile permits the two wheels of the automobile to rotate at the same speed when driving to rotate at the same speed when driving straight while allowing the wheels to straight while allowing the wheels to rotate at different speeds when taking a rotate at different speeds when taking a turn. Thus a differential gear is a device turn. Thus a differential gear is a device which adds or subtracts angular which adds or subtracts angular displacements.displacements.

DIFFERENTIALDIFFERENTIALFig. shows the arrangement of gears in a Fig. shows the arrangement of gears in a

differential. The shaft S is driven by the engine differential. The shaft S is driven by the engine through the gear box and has a bevel pinion A through the gear box and has a bevel pinion A keyed to it. Bevel pinion A meshes with a bevel keyed to it. Bevel pinion A meshes with a bevel wheel B which turns loosely on the hub of gear wheel B which turns loosely on the hub of gear C. Shafts S1 and S2 form the rear axle to which C. Shafts S1 and S2 form the rear axle to which are fixed rear wheels of the automobile. Gears C are fixed rear wheels of the automobile. Gears C and D are keyed to the shafts S1 and S2 and D are keyed to the shafts S1 and S2 respectively. C and D gear with equal bevel respectively. C and D gear with equal bevel pinions E and F which are free to rotate on their pinions E and F which are free to rotate on their respective axes. The wheel B carries two respective axes. The wheel B carries two brackets that supports the bearings for gears E brackets that supports the bearings for gears E and F.and F.

DIFFERENTIALDIFFERENTIAL

DIFFERENTIALDIFFERENTIALWhen the automobile moves in a straight path, When the automobile moves in a straight path,

the bevel pinion A drives the wheel B. The the bevel pinion A drives the wheel B. The whole differential acts as one unit and whole differential acts as one unit and rotates with the bevel wheel B so that the rotates with the bevel wheel B so that the wheel C and D rotate with the same speed wheel C and D rotate with the same speed and in the same direction as B. There is no and in the same direction as B. There is no relative motion between gears C and D, E relative motion between gears C and D, E and F. Gears E and F also do not rotate and F. Gears E and F also do not rotate about their own axes. When the automobile about their own axes. When the automobile takes a turn, E and F rotate about their own takes a turn, E and F rotate about their own axes and the system works as an epicyclic axes and the system works as an epicyclic gear giving two outputs at C and D with one gear giving two outputs at C and D with one input at Binput at B