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exact gear ratio applications

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autoguru

autoguru

Automotive
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Are there any applications where it's benefical to have the drive ratio be as exact as possible? (Other than machining).

 
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If you mean 'as close as possible to a specific ratio', land speed record or salt flats cars. Ideally, you want the car to be drag-limited to the speed that corresponds to its peak power rpm. But on a run of finite length you probably want a slightly shorter ratio.

Norm
 
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Yes, it is vitally important that some gear ratios should be exactly one ratio rather than something close but not exact. One obvious example is where the gearbox output shaft has Hookes joints - if you have the wrong gear ratio then the engine noise output and that from the driveline will heterodyne, which sounds terrible.

Cheers

Greg Locock

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normpeterson, can you explain why salt flat cars needs an precise drive ratio?
 
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He already did

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Given that maximum power occurs at some specific rpm, and that the power curve is likely to be rather peaky for this endeavor, if you're geared such that the engine rpm required to hit the speed permitted by the available power is either higher or lower than the rpm where peak power is developed, you won't make it to that speed. Geared too tall and you become drag-limited before you get to your power peak. Geared a little too short and you pass through the peak power rpm at a lower speed than what the power would ultimately support and become drag-limited on the high rpm side of the power peak, as the power is dropping off.

Norm
 
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Thanks. I understood that. But I thought that was for land speed cars, not salt flat cars.

I think another area where it's desirable to have an very exact drive ratio is for for spreaders or other implements where it is desireable for the action of the implement to be proportional to the speed of the vehicle/tractor across the ground.

I don't think a belt is desirable in this case since slippage is highly undesirable for accurate works.
 
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Another area to look at in gearing is running an engine in its most efficient RPM/load to maximize the powertrain efficiency, assuming the vehicle speed is known or constant.
 
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The camshaft in a 4-cycle engine must run at EXACTLY 1/2 the speed of the crankshaft.
 
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Clocks

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
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