The SPX Smiths Series SMR low-pressure power unit —from SPX Fluid Power, Rockford, Ill. — drives its gear pump with a bi-rotational 12-V DC electric motor. By reversing the polarity of the signal to the motor, flow through the pump also reverses. This combination can operate simple double-acting circuits by initiating reversing actuation without directional control valves. (A load-holding version of the power unit includes a pilot-operated check valve in each port.)

In a typical application, the power unit runs a system that positions a rigid protective cover that closes over the dump bed of a rubbish truck so nothing blows out while in transport. The cover folds out of the way along the side of the bed during loading. A reliable hydraulic system was chosen for this mobile application because it easily generates the relatively high force required to move the cover. The SPX Smiths power unit eliminates the need for directional control valves, which holds overall system cost down.

Four 3-in. bore 18-in. stroke single-acting hydraulic cylinders operate the cover. The cylinders work in tandem sets — performing like two double-acting cylinders. Each set is coupled together by a chain that drives a gear. The gear is mounted on a shaft that rotates a mechanical arm. When the power unit is actuated, the arm lifts the cover from the side of the truck bed up and over the side, unfolding as it rotates until it closes together. When the signal to the motor is reversed, the cylinders stroke in opposite directions, causing the gear to rotate back, lifting the cover and folding it back along the side of the truck bed.

The truck driver uses a simple handset controller for the reversing solenoid that selects the motor's direction of rotation. (In case of an emergency or power failure, a portable battery-operated drill can rotate a shaft extension on the rear of the motor.)

With 2000-psi operating pressure and a 1-gpm flow rate, this system generates approximately 12,000 lb of force and 5233 ft-lb of torque on the shaft. This system can generate cylinder speeds of 9 in./sec, but in this case — to minimize the kinetic energy at the end of rotation — an in-line flow control is installed to hold the open and close cycle times to about 10 seconds.

For more information, visit or call (815) 874-5556.