308C CR Excavator Hydraulic System – Check Valve (Load)

Illustration 1 g00875350

Boom I control valve (load check valve OPEN and boom RAISE)

(1) Load check valve

(2) Parallel feeder passage

Load check valve (1) performs two jobs. First, load check valve (1) prevents oil loss from a high pressure circuit to a lower pressure circuit. For example, the bucket cylinder is moved under a light load, and the boom cylinders are raised at the same time. The high pressure oil of the boom cylinders wants to flow toward the low pressure side of the bucket cylinder. Load check valve (1) prevents the boom from lowering.

Load check valve (1) also prevents the boom from moving down when the boom is first activated at a slow speed. When the boom starts moving up at a slow speed, parallel feeder passage (2) of the boom control valve has partial flow to the hydraulic tank. Without load check valve (1), the oil in the boom cylinders would flow through parallel feeder passage (2) to the hydraulic tank. This causes the boom to lower. Load check valve (1) prevents flow of pressure oil from the head end of the cylinders to the hydraulic tank.