How Does Gear Count Affect the Efficiency and Size of Gearboxes?

[Datt]

I have a question what is a difference or advantage for using two gears with high gear ratio or using three gears with lower gear ratio.Is there any advantage.

 

[Baluncore]

Welcome to PF.

Is there any advantage.

It depends on the overall ratio required. What ratio do you need?

The lightest weight and most economic gearbox will have pairs of gears, that each give a reduction ratio of about three. That comes about by considering the size and strength of the teeth.

If the ratio is greater than about 5.2 use more than one pair.

 

[jack action]

For one, adding an extra gear between two gears does not alter the gear ratio; It only changes the direction of rotation of the final gear.

To alter the final gear ratio, you need four gears, the middle two being on the same shaft. This adds extra friction losses and also extra inertia. Compared to a two-gear gear set, the final rotation is still reversed.

But the larger the gear ratio is, the more difficult it becomes to respect the combined design constraints like pressure angle, contact ratio, or undercut.

 

[cjl]

Welcome to PF.It depends on the overall ratio required. What ratio do you need?

The lightest weight and most economic gearbox will have pairs of gears, that each give a reduction ratio of about three. That comes about by considering the size and strength of the teeth.

If the ratio is greater than about 5.2 use more than one pair.

That heavily depends on the gearbox.

For very high reduction ratios, planetary gearboxes can achieve much larger reductions than that in a single stage, which can end up more compact and lighter than a multistage conventional gearbox with a similar reduction ratio. Size and weight also depends very heavily on torque handling capacity, though of course that'll scale at least fairly similarly regardless of which gearbox design you chose.

 

[jrmichler]

I have a question what is a difference or advantage for using two gears with high gear ratio or using three gears with lower gear ratio.Is there any advantage.

There is no simple answer to your question. A complete answer depends on the ratio, power, speed, size constraints, efficiency constraints, cost constraints, and more. If you get a copy of Dudley's Handbook of Practical Gear Design, and read the entire book, you will be able to understand just how difficult it is to answer your question. The book is available from Amazon: https://www.amazon.com/dp/0367649020/?tag=pfamazon01-20. I read an earlier edition, and highly recommend it for completeness and readability.

 

[Baluncore]

For very high reduction ratios, planetary gearboxes can achieve much larger reductions than that in a single stage, which can end up more compact and lighter than a multistage conventional gearbox with a similar reduction ratio.

Planetary gears are used on coaxial shafts where side forces need to be minimised. The planetary reduction unit can still be smaller and lighter again when an axial stack of planetary gears is used, rather than only one high-ratio planetary stage.