Sun Launches Soft Start Hydraulic Relief Valve

Diesel Progress North American Edition, July, 2001 by Mike Osenga

Sun Hydraulics Corp. has introduced the model RPGT soft start hydraulic relief valve. The RPGT is a patented two-stage, pilot-operated, seated relief valve that is designed to limit maximum system pressure and also limit the rate of pressure rise. The new valve was designed by Sun Hydraulik GmbH, Erkelenz, Germany, and is manufactured at Sun's Sarasota, Fla., factory.

The new RPGT valve has a capacity of 50 gpm and a maximum operating pressure of 5000 psi. The RPGT fits into a standard Sun T-3A cavity and can be used in many existing line mount and sandwich bodies, said Sun's Rich Arter.

This is Sun's first entry into the soft start valve portion of the hydraulic relief valve market. Arter said the primary benefit of the valve is that the slow rise characteristic eliminates pressure transients or shock. The new valve has been sold into hydrostatic transmission and ground drive applications.

The RPGT valve provides protection for hydrostatic drives by reducing the "jerk" caused by sudden reversals. The valve can provide protection for hydraulic pumps and motors from pressure transients due to sudden load changes, especially with variable displacement piston pumps, as the displacement mechanism is sometimes too slow to catch pressure transients, Sun said. The valve is also suitable for cross-port application. Because of the continuous pilot flow, Sun said the valve is not suitable for load holding uses.

The RPGT valve functions as a standard adjustable relief valve except that the initial pressure setting is about 2000 psi lower than the maximum relief setting. Once system pressure reaches its initial value, the valve ramps open at a constant speed and takes from 200 to 300 m/sec to shift from its minimum to its maximum setting. This, said Sun, results in a reduction in dynamic stress for system components.

Also, by eliminating pressure transients, Sun said it may be possible to increase the system pressure without having to strengthen any components, plus the stress on those components, caused by sudden load changes, is also reduced.

Arter said the RPGT valve functions basically the same as a pilot-operated relief valve, the special feature being that the setting of the valve changes as soon as oil flows into the pilot chamber. The setting changes because oil flows from the pilot chamber across a metering orifice into the chamber between the adjustment screw and the pilot sleeve itself. This causes the pilot sleeve to move forward and compress the pilot stage spring, thus increasing the relief setting of the main stage. An adjust screw determines the maximum relief setting and minimum threshold setting. Shift time is not adjustable.

The pilot chamber has an additional small relief valve that limits the pressure in the pilot chamber to less than 500 psi. This creates a constant pressure drop across the metering orifice. The pilot sleeve then moves at a constant speed and ramps up to the effective setting of the valve at a rate independent of settings and flows.