1GEAR TRAINSGear trains are combinations of two or more gears which arearranged to transmit power from the input shaft to the outputshaft. The gears in a gear train can be a combination ofdifferent types of gears. OUTPUT Bevel gears INPUT Spiral gears Spur gearsGear trains are used when there is a need to orient the inputand output shafts for specific spatial requirement and whenthere is a wide difference in speed ratio between the input andoutput shafts. In gear trains, we assume, unless otherwisespecified, that the modules of the gears are the same.
2Types of Gear Trains 1. Simple gear train In a simple gear train, there is only one gear per shaft. 2. Compound gear train In a compound gear train, there can be more than one gear per shaft. 3. Epicyclic gear train Ring gear or Internal gear Planet arm Sun gear Planet pinions In an epicyclic gear train, the gears are mounted in shafts that are moving relative to a fixed axis. In this gear train, at least one gear axis is on rotation relative to the frame.
3SIMPLE GEAR TRAINS ω3 ω2 ω1 T1 T2 T3Considering Gear 1 and 2, both rotates at opposite direction: 1 T 2 2 T1 2 T2 1 T1 (1)Considering Gear 2 and 3, both rotates at opposite direction: 2 3 2 T2 3 T3 (2)Substitute (1) to (2): 1T1 3T3
4Hence, 1 T VR 3 3 T1We can see that the velocity ratio of between the input and theoutput shaft of a simple gear train is independent of theintermediate gear 2, the intermediate gear will just change thedirection of rotation of gear 3. The positive sign of the velocityratio signifies that both the input gear 1 and output gear 2 arerotating at the same direction.Different Types of Simple Gear Trains(a) 1 3 2 1 T VR 3 3 T1
6COMPOUND GEAR TRAINS ω4 ω2 ω1 ω3 T2 T1 T3 T4Considering Gear 1 and 2, both rotates at opposite direction: 1 T 2 2 T1 2 T2 1 T1 (1)Considering Gear 2 and 3: 2 3 (2)Considering Gear 3 and 4, both rotates at opposite direction: 3 T 4 4 T3 4 T4 2 3 (3) T3Substitute (3) to (1): T 4 4 T T2 1 T1 3
7Hence, 1 T T VR 2 4 4 T1 T3 Pr oduct of teeth on driven gears VR Pr oduct of teeth on driving gearsFor compound gear train, the speed ratio is decided by all the number of teeth in the gear.MACHINE TOOL GEAR TRAIN APPLICATIONSA gear box is an enclosed metal casing of gear assembly that is used to generate severaloutput speed from one input speed, such as in automobiles and machine tools. T9d T8c T10e Upper shifting gear block Spindle T3a T4b T5c T7e T6d T1a T2b Electric Motor Lower shifting gear blockFigure shows a layout of machine tool spindle with six speeds. In a machine tool gear box,we use slide cluster to change the engagement of the gears. This is different withautomobile gear boxes because in a machine tool gear boxes, we have to stop first themachine before we can change the gear engagement.