ENGINE AR-RR 20R0908 - Caterpillar

Lock Check Valves
Some pilot operated check valves are called lock check valves. This function will allow oil flow in the reverse direction. The difference is that the lock check valve uses the pressure created by the load being moved rather than an independent pilot source to open the lock check valve. This allows oil to be returned from the opposite side of the actuator.
Illustration 5 g01063180
Lock check valves are used on both cylinders and motors to ensure no movement in either direction.Illustration 5 shows a lock check valve. The check valves on both sides of the actuator are closed or blocked. The length of the extensions on each end of the piston are shorter than the distance between the two seated check valves. Both valves are being held on their seat by the blocked oil from the actuator.
Illustration 6 g01063192
Illustration 6 shows a lock check valve. The left side is pressurized to move the load. The flow has opened the check valve on the left side. The pressure has moved the piston and its extensions to the right, pushing the check valve on the right side open to allow reverse flow back through the check valve to the control valve.The flow for opening the left valve is from the directional control valve. The reverse flow oil from the right valve returns through the directional control valve to the tank.Note: Lock check valves are used in most circuits of G-Series motor graders.Inverse Shuttle
The inverse shuttle is similar in construction to the lock check valve. The check valve on either end traps oil from flowing back into the valve. The following items are differences between the inverse shuttle and the lock check valve:
The inverse shuttle has a single source of oil that is supplied into the center chamber.
The center rod between the two check valves is longer than the distance between the two seated check valves. Thus, one or both of the check valves are always held open.
The inverse check valve is always used with an accumulator in each valve outlet to ensure that the accumulators are equally charged. The inverse check valve is normally used in the brake system to ensure that equal pressure is available for front brakes and rear brakes.
Illustration 7 g01063217
Illustration 7 shows an inverse shuttle that has equally charged accumulators. Note that the check valves are open on both ends of the valve in order to provide equal oil flow to both accumulators and the front and rear brake systems.
Illustration 8 g01063218
Illustration 8 shows an inverse shuttle valve. The accumulator on the left end has a higher pressure than the accumulator on the right end. The pressure from the left accumulator has pushed the left check valve against the center chamber. The longer rod is holding the right check valve open. The cut-in valve is now sensing the lower accumulator pressure to actuate the cut-in function. This permits flow from the cut-in valve to flow to the right side until the pressure is equalized.The pressure in the front and