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MCEN 3025


Spring 2009



ME ID _________________ Gear Trains

What is the most likely reason for designing a simple gear train with three spur gears (a driver, an idler, and a driven)? The most common reason for a simple gear train with three gears is increase torque or velocity while preserving the velocity direction of the drive gear. The idler does nothing more than to maintain velocity direction. In an automobile transmission, it is desirable for the output shaft to be concentric with the input shaft. Which type of gear train could be used for the transmission? C or D are the only choices below that can maintain input/output shaft concentricity. a. Simple b. Nonreverted compound c. Reverted compound d. Epicyclic (planetary) Suppose you have a compound gear train made of two identical gearsets (four total gears in the gearset--two driving, and two driven). What deduction can you make about the input and output shaft? a. The output shaft velocity ratio will be > 2 b. The output shaft velocity ratio will be < 2 c. The input and output shafts will be concentric d. The input and output shafts will not be concentric C. This describes an reverted compound gear (see below) with N5=N3 and N2=N4. We cannot deduce anything about the velocity ratios without knowing the number of teeth on the gears.




What are the advantages of epicyclic (planetary) gear trains? Epicyclic gear trains are able to attain higher rations in smaller packages. If reversion is desirable, a epicyclic gears offer this by default. Finally, concentric, bidirectional output is available from a single unidirectional input. All these features make epicyclic gears popular as automatic transmissions in automobiles. Examine the following figure of a planetary gear train. Note that the main difference between this gear train and the one shown in figure 11-16 of the text is the number of planet gears (4 versus 1).


MCEN 3025


Spring 2009

What do the extra planet gears do to the velocity ratio of the gear train? What else do they do for the gear train? The extra planet gears do not change the velocity ratio, i.e. adding one, two, or three planet gears do not alter the velocity ratio. The purpose of additional planet gears is to share the torque and loading and also dynamically balance the gear train.



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