Hydraulics

Question 1

Define pressure

Answer 1

Pressure is the force on a unit of area. Also a resistance to flow.

Question 2

What is meant by gauge pressure?

Answer 2

Gauge pressure is the pressure indicated on the pressure gauge without including atmospheric pressure.

Question 3

Why is pressure transmitted almost instantly through hydraulic fluid?

Answer 3

It is almost incompressible

Question 4

Hydraulic actuators can be either____________ or _____________ actuators.

Answer 4

Linear (cylinder) or Rotary (motor)

Question 5

The speed that an actuator can rotate is directly related to what two things?

Answer 5

Flow & Area

Question 6

The fluid law that applies to a stationary fluid and is the foundation of all hydraulic theory is?

Answer 6

Pascal’s Law

Question 7

What would the reservoir capacity be in an open loop hydraulic system?

Answer 7

3 times the volume of the rated capacity of the pump.

Question 8

What would the reservoir capacity be in a closed loop hydraulic system?

Answer 8

3/4 the volume of the charge pump capacity.

Question 9

Doubling the inside diameter of a pipe will?

Answer 9

Quadruple the inside cross-sectional area

Question 10

If a single double acting cylinder were replaced with one of larger diameter, how would this affect a hydraulic circuit?

Answer 10

Greater force/ Slower speed

Question 11

A high resistance to flow creates?

Answer 11

Heat increases

Question 12

The greatest waste of power in a hydraulic system is?

Answer 12

Maximum Pressure Relief Valve

Question 13

Flow in a hydraulic system is created by?

Answer 13

The pump

Question 14

When an actuator is moving, the pressure in a hydraulic system is?

Answer 14

The load pressure.

Question 15

Laminar flow of oil is a condition where:

Answer 15

Fluid particles move parallel to the walls of the conductor

Question 16

According to Bernoulli’s principle, If the velocity of a fluid increases the pressure will?

Answer 16

Decrease

Question 17

Which proportional DCV control loops provides the most accurate flow control?

Answer 17

Closed outer loop

Question 18

What is the pressure control valve that is both internally and externally piloted?

Answer 18

Brake

Question 19

What is the pressure override of a pressure relief valve?

Answer 19

The pressure difference between full flow and cracking pressure.

Question 20

A proportional directional control valve controls the speed of an actuator by?

Answer 20

The use of metering notches

Question 21

What is the main drawback to meter-out flow control?

Answer 21

Pressure intensification

Question 22

Maximum Pressure Relief Valve

Answer 22

• controls maximum pressure in system

- pressure overcomes spring and relieves flow to tank.

Question 23

Pilot-Operated Relief Valve

Answer 23

• Pressure relief. Controls pressure in system.
• Pressure in pilot stage overcomes springs and is drained to tank. This relieves pressure on upper portion of balance piston opening valve for full flow to tank. This lowers the pressure override of valve. (More efficient.)

Question 24

Unloading Valves

Answer 24

• Set max pressure in system, while unloading the pump
• Relieves pressure in system beyond check valve. Setting max pressure. Check valve allows pump to go to tank unloaded (no pressure). Should pressure drop in system valve will close and pump flow goes to system.

Question 25

Pressure Reducing Valve

Answer 25

- Reduce pressure in a branch of a circuit. | - Allows flow into branch until pressure builds to setting which closes valve. (setting a max pressure in that branch).

Question 26

Sequence Valve

Answer 26

- Controls order of operations in circuit. - Allows multiple cylinders to function in order by restricting flow into the second cylinder until the first cylinder reaches full extension and pressure inside system builds to open valve to second cylinder.

Question 27

Counterbalance Valve

Answer 27

- Control vertical cylinders against the force of gravity - Internal- Builds up pressure in lower port which opens valve. (Negatives: 1. Varying loads require different valve settings. 2. Pressured lower port causes an opposing force upwards limiting potential lowering force.) -External - Pressure from inlet opens valve and allows lower port to drain to tank at no pressure. (Positives: Max possible downward force.)

Question 28

Brake Valve

Answer 28

-Prevent over-speed and excessive pressure build up in motor. Pilot: Internal and external to balance pressures inside motor. - Internal pilot allows valve to open if pressure inside motor builds up, and closes valve if pressure inside motor drops. -External pilot allows valve to open when pressure builds up at inlet. This is to ease startup pressure. External pilot will also close the valve and "brake" the motor in a runaway situation. On a bi-rotational motor, dual brake valves are used (Cross-port relief valves).

Question 29

Pilot-to-open Check Valve

Answer 29

- Used to lock cylinders in place without internal leakage. - External pilot line from inlet to cylinder - Ratio must be larger than cylinder ratio. - If valve senses external pressure at cylinder inlet, check valve opens allowing fluid from discharge end of cylinder to foe to tank. (Unlocks Cylinder).

Question 30

Meter-in Flow Control

Answer 30

- Controls speed of actuator. - Located on inlet line into actuator. - opening or closing restriction in line to control flow. - Controls flow into an actuator, thus controlling it's speed. Only works with opposing loads. (Will not control a runaway load).

Question 31

Meter-Out Flow Control

Answer 31

- Controls speed of actuator. - Located on discharge line of cylinder. - Opening or closing restriction in line to control flow. - Controls flow out of cylinder to maintain speed. Possible area of pressure intensification. ( When in Doubt, Meter Out).

Question 32

Bleed-Off Flow Control

Answer 32

- Controls speed of actuator. - Opening to closing restriction of flow to tank. - Bleeds off flow from system to tank to reduce flow to cylinders, controlling speed. Only works with opposing loads.

Question 33

Bypass Compensator Pressure Compensator

Answer 33

- Maintains set speed of cylinder regardless of pressure. - Only works as a meter-in, into cylinder. - Flow is set with needle valve and maintained by 20 psi spring. - Maintains cylinder speed by maintaining flow across the valve by opening or closing the bypass to bleed off flow not required to tank. Efficient because pump not working at system pressure. Used only as meter-in.

Question 34

Restrict or Pressure Compensator

Answer 34

- Maintain set speed for cylinder regardless of pressure. - Will work as meter-in or meter-out. - Flow set with needle valve and maintained by 20psi spring. - Maintains cylinder speed by maintaining flow across the valve by opening or closing balance piston (Load pressure+20psi spring). - Disadvantage is excess flow is set at max system pressure over relief valve.

Question 35

Proportional Valves

Answer 35

- Control direction of fluid as well as flow (Speed). - Valve position is adjusted by increasing/decreasing voltage to solenoids. This shifts the spool proportionally. Flow is adjusted by position of spool and by design of metering notches. - Can accurately control flow rate in/out of actuators. Allows for infinite positions, speeds and accelerations within the range of the valve.