BCIT: Millwright Level 3 (Hydraulics: General Knowledge 1)

What are radial piston motors? Are they fixed or variable displacement or both?

In a radial piston motor, pressurized fluid enters the cylinder block at the center and forces the pistons outward against a reaction ring to create rotation.

These motors can be either fixed or variable displacement.

What does it mean when a directional control valve (DCV) is identified by 3/2?

A 3/2 DCV is a three-way valve with three ports and two operating positions.

What is a normally open valve?

A normally open valve allows fluid to pass through it until the valve is activated.

What is an unloading valve? Are they normally open or normally closed? Where in the system are they usually installed?

An unloading valve returns the pump output to the reservoir at low pressure once the required system pressure has been reached. An internal or remote check valve maintains pressure in the system.

These valves are normally closed and are usually installed on the pump outlet line before the accumulator.

What is an open center?

In an open center configuration, all ports are connected in the neutral position. Pump flow returns to the tank, and the actuator can move in the direction of any external forces.

Hydraulic oil compresses to what percent when the pressure is 1000 psi?

Hydraulic oil compresses approximately one-half of one percent (0.5%) at 1000 psi.

What is a pressure and temperature compensating control valve?

A pressure and temperature compensating control valve maintains consistent flow despite changes in pressure and temperature. As the hydraulic fluid temperature changes and viscosity varies, a temperature-compensating rod expands or contracts to adjust the orifice and maintain stable flow.

What is the difference between throttling and non-throttling directional control valves?

Throttling DCVs allow the spool to move gradually between positions, providing infinite positioning and controlled flow.

Non-throttling DCVs move the spool quickly from one position to another without intermediate control.

What is an absolute filter rating?

An absolute filter rating means the filter will stop all particles of a specified size or larger.

What is a proportional filter?

In proportional filtration, only part of the oil passes through the filter element while the rest flows directly to the reservoir. Through continuous circulation, all the oil eventually passes through the filter.

What is a vane motor? Are they balanced or unbalanced or both? Are they fixed or variable displacement or both?

In a vane motor, fluid enters the inlet port and pushes against the vanes and rotor to create rotation. Centrifugal force and springs push the vanes outward.

Vane motors may be balanced or unbalanced and may have fixed or variable displacement.

What is a single acting cylinder?

A single-acting cylinder is pressurized in only one direction. The return stroke occurs through an external force such as gravity, a spring, or a smaller auxiliary piston.