In a closed loop hydrostatic transmission circuit, a low side hot oil shuttle valve is used to discharge supercharge oil from the low side of the loop for cooling. This shuttle valve is piloted from the pump work ports, with the highest pressure shifting the valve so that the opposite (low pressure) side of the loop is connected to tank (typically through a hot oil relief valve, into the motor case). When the pump is commanded to neutral, both sides of the loop see equal charge pressure, and the shuttle valve centers, causing all charge oil to exit the loop through the charge pump relief.

With pump reversals, the shuttle will quickly shift from one side of the loop to the other, following the highest loop-side pressure. In some fast reversal applications, such as drill or auger drive circuits, high pressure and flow transients can be transmitted outside the hydrostatic loop and can cause severe damage to components down stream of the shuttle valve, such as the heat exchanger, filter (if installed), and the motor case. Also, sudden loop flow losses can cause cavitation damage when instantaneous losses are greater than charge pump flow.

The Sun DS*D Hot Oil Shuttle with Delayed Shift

To address this circuit problem, Sun offers the DS*D-XEN hot oil shuttle with a delayed shift feature. When work port pressures reverse, this valve is designed to center quickly, but will remain centered as long as the two work ports are at a value equal to or greater than 300 psi (20 bar) plus the hot oil relief setting (e.g. with a 200 psi [14 bar] hot oil relief setting, the shuttle valve would not shift until one side drops below 500 psi [34 bar]). This valve eliminates damaging pressure reversal transients in two ways:

An additional unique feature, standard on Sun’s DS*H low side hot oil shuttles as well as the DS*D delayed shift shuttles, is an internal hydraulic spool stop which allows a small pilot flow from the work ports to the outlet. This feature allows the hot oil relief pressure setting to be verified and adjusted while the transmission is in neutral.

Although there is a slight price premium for the delayed shift version of the low side hot oil shuttle, specifying it is the “safe way to go” for any hydrostatic transmission application. The delay in shifting time has virtually no impact on overall loop cooling.