The Hydrostat Valve Is Not Able To Maintain Consistent Cylinder Flow With Varying Pressures

Posted on 03/01/2025 by Fluid Power Journal in Figure It Out

By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training

A new hydraulic employee designed a circuit using a proportional valve to control the speed of a cylinder. It was specifically constructed so the flow would be consistent regardless of the forces the cylinder was exposed to. Proportional valves are directional in nature, featuring built-in flow control. These valves need a hydrostat to make them pressure-compensated, offering constant flow regardless of the actuators’ pressure demands.

The new hire’s circuit design is shown. With a hydrostat installed under the proportional valve controls, the pressure drops across the valve spool and does not control the PSI the cylinder requires. However, the new team member’s system is installed to reduce the pressure available at the cylinder with no pressure compensation.

See the Solution

A hydrostat works by sensing the pressure on a valve’s outlet and feeding it into the hydrostat’s spring chamber. This allows for consistency as the pressure drops across the valve, resulting in a constant flow. Fluctuating pressures before or after the valve have zero influence on the flow. The circuit designer discovered that an order was placed for a check valve module with the signal checks flowing in the wrong direction.

Robert Sheaf has more than 45 years troubleshooting, training, and consulting in the fluid power field. Email rjsheaf@cfc-solar.com or visit his website at www.cfcindustrialtraining.com. Visit fluidpowerjournal.com/figure-it-out to view previous problems.