Hydrostatic Charge Pump pressure relief and energy efficiency

[aaronschellenberg]

I am university professor working with some students developing a very small skid-steer vehicle. We are powering it with a 13 HP engine. We have purchased a hydrostatic pump system from Hydro-gear with two variable displacement piston pumps as well as an auxiliary gear pump which we will use as a charge pump for the hydrostatic closed loops as well as a source for operating some simple auxiliary functions like raising and lowering a plow blade. The catalog recommends that the charge relief should be around 600 psi. Because we are powering this with such a small engine we don't want to waste any power. The auxiliary pump produces about 2-3 gpm. If the charge pressure has to be set to 600 psi, the auxiliary pump will be wasting precious power if we use a simple relief valve to maintain the charge pressure. What would be the benefits and drawbacks of using an unloading valve to keep the charge pressure at 600 psi while stilling allowing the auxiliary pump to "unload" to tank without wasting power?

I appreciate any insights anyone has on this issue.

Aaron Schellenberg
Mechanical Engineering Professor
Brigham Young University - Idaho

 

[HPost]

Hello Aaron

There could be an opportunity to reduce the 600 PSI, but it depends on the power required to drive the auxiliary functions and the minimum pressure required to overcome the internal friction in the pump.

A relief and unloading valve won't reduce the energy consumption, it will just stop energy wastage above the level actually required to carry out the functions.

The main limitation is the internal friction in the pump. With a resistive force, a load sensing valve could be used. There is no resistive load as such in the pump. If the pressure in the piston is not sufficient to keep the head of the piston against the swash plate, the pump will run into problems. For this reason the pump designer has stated the minimum pressure required to overcome the internal friction. On a pump to pump basis, that pressure will be lower. Although the internal friction will vary with temperature and speed, so what works on one day will perhaps not work on another day.

Unfortunately you don't have much power to play with, so your options are limited. It will all be very marginal.


I can't see that you have many options at this stage.

HPOST CEng MIMechE