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Gears in transmission

  1. Jan 10, 2010 #1
    Hi PFians and friends


    Have a look at this basic transmission.

    I was wondering if the gears on the layshaft somehow remain of same radius as the first gear on layshaft (I know then the mechanism has to be different), then won't be our transmission better. e.g consider the case of 1 gear( smallest gear on shaft red or layshaft and largest blue gear on yellow shaft) , here for one revolution of layshaft the blue gear will rotate very less (which will depend on gear ratio), but if the red gear here were as big as the first gear on red shaft(which takes power from engine), then the blue gear would turn more.
    So shouldn't transmissions be designed such that the gears on layshaft remain of same maximum radius possible.
  2. jcsd
  3. Jan 10, 2010 #2


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    Gold Member

    That's the whole point of having a transmission in the first place. The mechanical advantage of the blue gear turning less is what allows the engine to remain in its power band at lower vehicle speeds.
  4. Jan 10, 2010 #3
    but if the red gear is made large, then we will get a powerful first gear which will provide same torque as in above case and more acceleration in first gear.because torque to drive shaft is dependent upon blue gears and the speed of rotation on red gears
    Last edited: Jan 10, 2010
  5. Jan 10, 2010 #4
    compare it to a bicycle having gears at front and rear sprockets. In this case at front ,options for a small sprocket (first gear front) are present in combination with a large sprocket at rear (similar case as of above transmission) , but this is because having a smaller sprocket at the front will reduce driver effort. That's the advantage in case of steep hills.
    But in the above transmission, all the small and big gears on layshaft are not driven individually(as in bicycle) but are on a single shaft. So, the engine has to bear the load of rotating the big gears (no matter which gear is engaged), because engine will rotate whole red shaft. So, why not all gears be of same "maximum" possible size. Because doing this although lead in same torque(because torque to drive shaft is dependent upon blue gears and the speed of rotation on red gears) but better acceleration.
  6. Jan 11, 2010 #5
    Your notion of the bicycle is wrong. Turn a bicycle with multiple gears upside down and turn the pedal with your hand. Every single gear moves, whether it is engaged or not.
  7. Jan 12, 2010 #6
    yeah, i was wrong in case of bicycle. I apolozise if it seemed to have someone in confusion.

    But what about this:
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