sizing a reduction gear

[Mark Emaus]

Ok, another dumb question..

I am going to have to reduce the engine rpm down to run a hyd pump. The engine runs a max of 4400 rpm but the max torque rpm is at around 3600. So if I am trying to turn a Hyd. Pump at its peak efficiency. Would it be better to have the reduction of the hyd. pump match the max rpm or the engine, or match the rpm that creates the most torque? I will use a planetary gear to make the reduction.

Thanks

Mark

[baeckmo]

Question is: what are you going to use the hydraulic power for? Is the hydraulic pump driving something that will use the full engine power, then you have to gear for that rpm. If the hydro is used for something needing a fraction of engine power in addition to a mechanical output, then there will be some other balance to be found. The answer to this q will lead to the final answer!

[Mark Emaus]

the hyd pump will drive the prop shaft.

[baeckmo]

This means you are going to use the full engine power at max rpms. The question of wether the engine can be operated for long periods at 4400 rpms is not relevant for the hydraulics per se, but must of course be checked first.

Since hydraulic power is Pressure * Flow, the pump's design pressure will be the primary input factor, along with engine power, rpm and pump efficiency. With a fixed pump displacement known, you end up with the required pump rpm. The engine power at the max torque rpms will use the max pump pressure, while the max power operating point will see a reduced pessure at a higher flow. So you have to check both operating points, because they are limited by different combinations of flow * pressure.

DON'T FORGET to check pump inlet velocity for possible cavitation limits at the max rpms!!!

There are a number of threads dealing with hydraulic transmission in this forum, where you can find useful info.