Hydraulic Systems 3: Valves & Accumulators

Question 1

Difference between Continuously-Operating and Digitally-Operating DCVs?

Answer 1

Continuous can take any intermediate position, digital take discrete positions.

Question 2

Poppet / Check Valves: Why used? What are some strengths and weaknesses?

Answer 2

Used to block flow in a direction. No leakage at all and long life. Large pressure drop through valve.

Question 3

Spool Valve: General Construction?

Answer 3

2 ports with ‘spool’ sitting within cylindrical housing. Spool is actuated to allow or block flow through housing, typically spring-return.

Question 4

Choice of Valve for Single-Acting Cylinder?

Answer 4

3/2-way spool valve (3 ports, 2 positions)

Question 5

Choice of Valve for Double-Acting Cylinder?

Answer 5

4/2-way spool valve (4 ports, 2 positions)

Question 6

Common 4/3-way DCV configurations?

Answer 6

Open-Centre, Closed-Centre, Tandem-Centre

Question 7

Open-Centre Spool Valve: why use over others?

Answer 7

In centre position, all ports open, so no work can be done on system. But can move actuator

Question 8

Closed-Centre Spool Valve: why use over others?

Answer 8

In centre position, valve is hydraulically locked so actuator can’t be moved. But energy waste as fluid flows over relief valve, excess wear and heat.

Question 9

Tandem-Centre Spool Valve: why use over others?

Answer 9

In centre position, valve is hydraulically-locked so actuator can’t be moved, AND flow through system maintained so no added wear of heat.

Question 10

Purpose of Hydraulic Accumulator? Applications?

Answer 10

Acts as a back-up energy source by storing pressurized fluid that can be released into the system when needed. Can be an emergency energy source or an auxiliary source, and can be used for shock-absorption.

Question 11

Accumulator as an emergency source: how?

Answer 11

Accumulator fills with fluid under normal operation and is connected to cylinder. In event of power or fluid loss, accumulator discharges and retracts cylinder. “Back Driving” cylinder.

Question 12

Accumulator as an auxiliary source: how?

Answer 12

Accumulator can fill up and be used to assist actuation when required, providing extra power. Therefore can specify a smaller pump for the same application.

Question 13

Accumulator as a shock absorber: how?

Answer 13

Accumulator positioned near both ports of actuator. Fluid must fill accumulators before flowing through rest of circuit, so protects from pressure spikes, smooths out.

Question 14

Regenerative DCV: how does flow through system work? (Double-Acting Cylinder)

Answer 14

Flow from one port + flow from pump = flow into other port; fluid in one side of cylinder ‘helps’ pump flow in actuating the other way.