A brake valve performs the same function as a counterbalance valve, but it is designed to overcome a key disadvantage. An example will illustrate this point. Suppose a direct-operated counterbalance valve is used in a motor circuit. There are times when the motor load can overrun, and braking is needed for those times. This valve is set at 350 psi, and the pressure drop across the motor to supply required torque is 350 psi. The pump must develop 350 + 350 = 700 psi. The pressure drop across the counterbalance valve is converted to heat; consequently, half the hydraulic power is wasted. A brake valve overcomes this disadvantage.
As shown in the functional diagram in Fig. 3.29, a brake valve has an internal pilot passage and a remote pilot passage. Suppose the spring is set for 1000 psi. When pressure at the internal pilot reaches 1000 psi, the piston pushes the spool upward to open the valve.
The area of the piston is much less than the area of the bottom of the spool. A typical area ratio might be 10:1. The remote pilot applies pressure directly to the bottom of the spool; consequently, only 100 psi is required to compress the spring and open the valve. Pressure required to open the valve is 1000 psi at the internal pilot and 100 psi at the remote pilot.
A brake valve is used in a circuit as shown in Fig. 3.30. It requires 100 psi at the motor inlet to keep the valve open. As long as the load on the motor requires more than 100 psi, the brake valve does not affect circuit efficiency. If the load starts to overrun, and the pressure drops below 100 psi, the brake valve closes. It requires 1000 psi at the direct (or internal) pilot (Fig. 3.29) to open the valve. This 1000-psi pressure drop across the brake valve converts the mechanical energy of the overrunning load to heat energy and slows the load. When pressure at the inlet builds to 100 psi again, the brake valve opens.
