Terminology - Hydraulic System Components: Pumps, Valves and Cylinders - HET 190103i

Question 1

Non-positive Displacement

Answer 1

• inlet and outlet passages connect hydraulically
• centrifugal force is used to create force
• outlet is restricted, pressure builds, flow recirculates, therefore reducing flow delivered to pump outlet port

Question 2

Positive Displacement

Answer 2

• do not suffer significant flow loss under pressure
• mechanically seal the path of flow between the pump inlet and outlet ports
• flow rate changes very little

Question 3

Pump Displacement

Answer 3

volume of liquid that the pump delivers when the pump input shaft rotates one revolution.

Question 4

Flow Rate

Answer 4

• amount of liquid that the pump delivers in a given period of time
• flow delivered is determined by Pump displacement and Input shaft speed

Question 5

Imperial gallon contains how many cubic inches?

Answer 5

231 in³

Question 6

Metric litre contains how many cubic cm?

Answer 6

1000 cm³

Question 7

Mechanical Efficiency

Answer 7

amount of power required to overcome the pump’s internal friction

Question 8

Slippage

Answer 8

internal leakage oil that provides lubrication for the pump’s moving parts

Question 9

Volumetric Efficiency

Answer 9

• amount of flow lost within pump due to internal leakage
• theoretical delivery vs actual delivery
• LPM or GPM

Question 10

Pressure Rating

Answer 10

maximum pressure the pump can safely withstand without damage occurring

Question 11

Gear-type Hydraulic Pump

Answer 11

• positive displacement
• least complex of all hydraulic pumps
• simple, economical and rugged
• 2 types: external and internal

Question 12

Gear Tooth Pockets

Answer 12

spaces between the pairs of gear teeth on the drive and driven gears that carries oil from inlet to outlet side

Question 13

Partial Vacuum

Answer 13

low pressure

Question 14

Internal Gear Pump

Answer 14

• consist of two gears in mesh rotating in a pump housing
• usually less efficient than external gear pump if missing pressure plates

Question 15

External Gear Pump

Answer 15

• one drive, one driven gear
• contained in pump housing
• contains pressure plates

Question 16

Pump Inlet Design

Answer 16

• significantly larger than outlet port to ensure adequate oil volume is available

Question 17

Unidirectional Pumps

Answer 17

operate in one direction only
always determine the direction of driveshaft rotation to prevent damage when servicing

Question 18

Cavitation

Answer 18

pockets of vaporized hydraulic oil form as the oil enters the inlet side of the pump. when carried to the outlet side they are exposed to system pressures and collapse (implode) with tremendous force. will increase oxidation

Question 19

Aeration

Answer 19

• inlet line is leaking, air enters inlet side of the pump.
• bubbles compress when they reach the pressure side of the pump, generate heat and cause rattling noise.
• causes system actuators to move with an erratic or jerky motion.

Question 20

Direct-acting Pressure Relief Valve

Answer 20

• normally closed pressure control valve that senses hydraulic system pressure. - commonly located between pump and directional control valve.
• protects system by limiting the maximum pressure
• always refer to manufacturer’s recommended procedures when adjusting

Question 21

Cracking Pressure

Answer 21

pressure at the inlet port when the ball is initially moved off its seat by hydraulic system pressure

Question 22

Full Flow Pressure

Answer 22

• pressure required to fully open the relief valve
• greater than cracking pressure to compress valve spring

Question 23

Pressure Override

Answer 23

pressure difference between cracking pressure and full flow pressure

Question 24

Poppet Directional Control Valve

Answer 24

is a check valve whose function is to allow oil flow in one direction and block flow in the opposite direction

Question 25

Rotary Directional Control Valve

Answer 25

employs a rotary action to open and close passageways within the valve

Question 26

Spool Directional Control Valve

Answer 26

- the most common type of directional control valve used in heavy equipment - employs linear or straight-line, movement to open and close paths of oil flow within the valve - has circular grooves to trap oil

Question 27

Feathering

Answer 27

partial movement of the spool allows the operator to control the speed of the cylinder

Question 28

Directional Control Valve Actuation Methods (3)

Answer 28

- Mechanical (lever or pedal) - Pilot (hydraulic or air) - Electrical

Question 29

Pilot Operated Directional Control Valve

Answer 29

- low-pressure hydraulics actuate circuit - used on larger hydraulic systems to reduce the amount of operator effort require to activate the valve - supplied by low-volume, low-pressure pump, or compressor

Question 30

Electrical Operated Directional Control Valve

Answer 30

- solenoids that actuate the directional control valve. - eliminates risk of oil leakage from a pilot circuit

Question 31

Open Centre Directional Control Valve

Answer 31

neutral oil from the pump can flow through the valve and return to reservoir

Question 32

Closed Centre Directional Control Valve

Answer 32

blocks the flow of oil from the pump when the valve is in the neutral position

Question 33

Hydraulic Cylinder

Answer 33

- one of the most common actuators in heavy equipment hydraulic systems - converts hydraulic power into mechanical power - linear actuator - single or doubling acting

Question 34

Single-Acting Hydraulic Cylinder

Answer 34

- operated in only one direction - acts on the surface area of the rod - cylinder rod retracts when the cylinder port is connected to the reservoir - an external force is required to retract the cylinder - commonly referred to as rams

Question 35

Telescoping Cylinder

Answer 35

- applications that require a longer stroke - provides long stroke length while remaining relatively compact when retracted

Question 36

Stages (referring to Telescoping Cylinders)

Answer 36

- nested tubular rod segments - most force is required when extending the first stage, less with each additional stage as surface area decreases

Question 37

Double-Acting Hydraulic Cylinder

Answer 37

- hydraulically extend and retract - do not require external force - aka Differential Cylinders - force and speed are different during extension than retraction - produce more force during extension