Basic Hydraulics

Question 1

What is specific gravity?

Answer 1

It is the measure of density of fluid. 1 cu. ft. of liquid compared to equal amount of water.

Question 2

What is pressure?

Answer 2

Pressure is measured in lbs per square inch in the English system.

Question 3

How do you find force in hydraulics?

Answer 3

Pressure (psi) x piston area ( square inch)= force (lbs)

Question 4

What is Pascal’s law?

Answer 4

When pressure is exerted on a fluid in a container, the pressure is exerted equally in all directions.

Question 5

What is Bernoulli’s law?

Answer 5

It states the total energy of a fluid always stays constant. When fluid flow increases, pressure must decreases.

Question 6

What is the essential function of hydraulic fluid?

Answer 6

To transmit power effectively from one point to another. In order to do this fluid must be able to flow with no restrictions and be virtually incompressible.

Question 7

What is viscosity index?

Answer 7

The extent of which the viscosity of a fluid changes with temperature.a fluid with a high VI has a small change in viscocity with a change a temp.

Question 8

What is film strenght?

Answer 8

Property of a fluid to be able to maintain a lubricating film under heavy load. Fluids must have the right viscocity to be able to penatrate into the small clearances between moving parts.

Question 9

If high fluid oxidation is a problem what should be done?

Answer 9

A fluid with an oxidation inhibuter should be used. They seek out and break air bubbles

Question 10

How do you control the contaminants in a hydraulic system?

Answer 10

Can be controlled through the use of chemicals and a good maintenance program.

Question 11

What is absolute rating?

Answer 11

It is the rating of a filter or strainer to filter 100% of particles of a particular size

Question 12

What is the maximum allowable pressure drop in a hydraulic fluid?

Answer 12

The filter drop across the filtering medium that permits the passage of particles through the medium.

Question 13

Where are pump inlet strainers found?

Answer 13

Should always be placed far enough below the surface of the fluid to prevent a suction vortex from forming.but high enough from the bottom of the resoiver to not pick up sludge and contaminants.

Question 14

When are filters used instead of strainers?

Answer 14

Filters can hold a lot more contaminants and also trap elongated pieces that might make it past the strainer.pressure drop with a filter is much less than a strainer. some filters have added chemicals to add or remove additives as needed.

Question 15

What are the disadvantages of depth filters?

Answer 15

Filters in highly contaminated systems do not have as long a life if it were in a system with less contamination. Absorbent filter bed filter remove additives and stabilizers from the fluid

Question 16

One type of filter media includes metal powders that are sintered or fused together under heat and pressure to form thick rigid and porous structures. The thick mass of material provides support to the filtering surface and allows the filter to be used at high pressures.

Answer 16

True

Question 17

What components should you have to maintain reservoir cooling in a hydraulic system

Answer 17

Sufficient fluid in the System
Sufficient reservoir surface area to give up heat to the surrounding air
A free standing reservoir that allows for some degree of air circulation on all sides
A good battle in the reservoir
Thin side walls on the reservoir with sturdier bottom and top plates

Question 18

The operating temperature of the fluid in a hydraulic system can be lowered by cooling the reservoir with compressed air.

Answer 18

True

Question 19

What is the function of accumulators in a hydraulic system? Accumulators store fluid under pressure and release it on demand in a smooth, even flow, at relatively constant pressure. They also provide auxiliary power and emergency power in case of pump failure, compensate for leakage, and help cushion or absorb hydraulic shocks.

Answer 19

True

Question 20

Hydraulic pressure is caused by the load on the system and by the resistance of the hydraulic system to fluid flow

Answer 20

True

Question 21

What is the primary rating of a pump?

Answer 21

Pumps are rated by its capacity. This is also called the delivery rate, flow rate, or volume metric output. Not psi

Question 22

What is volume metric efficiency?

Answer 22

The ratio of the actual delivery rate to the theoretical displacement.

Question 23

What are the 4 major classifications of gear pumps?

Answer 23

External gear 
Internal gear
 Axial flow or screw
Cyclonic.
Not vane!

Question 24

When are internal gear pumps used in a hydraulic system?

Answer 24

They are used when the system operating pressure is below 1000 psi, and in applications that have low speeds and smaller capacities.

Question 25

How do you calculate the fluid velocity through a pipe?

Answer 25

You must know the flow rate in gallons per minute and the inside Cross-sectional area of the pipe or the inside diameter of the pipe.

Question 26

The pressure lost in a hydraulic line depends on the flow velocity of hydraulic fluid through the line.

Answer 26

True

Question 27

What type of pipe is recommended for use in hydraulic systems?

Answer 27

Only black steel pipe is recommended for hydraulic systems. Galvanised pipe is not used because the galvanising may flake off and flow through the system where it can clog smaller orfice’s or passages.

Question 28

Most hydraulic hoses are rated with a safety factor of 4. However surge pressures, which can reach twice the level of working pressures, must be considered when selecting hoses with a safety factor of 4.

Answer 28

True

Question 29

Directional control valves are used to control fluid flow in hydraulic lines, and to start, stop, or change the direction of motion of hydraulic cylinders and motors

Answer 29

To

Question 30

The most common type of automatic 2 way valve is solenoid operated.

Answer 30

True

Question 31

Most solenoids are only strong enough to open against a spring with a force of 15 lbs. Therefore, greater hydraulic pressures require smaller valve ports.

Answer 31

True

Question 32

Why are spool valves the most commonly used in hydraulic systems?

Answer 32

Because of their construction they are partially balanced. Easy to operate when used in high pressure systems.

Question 33

Three-way spool valves have 3 primary or working connections. Pilot lines are not considered primary lines

Answer 33

True

Question 34

A three-way diverter valve can be used in 2 ways to control a hydraulic motor. The simplest way is to use the valve as in on off valve that allows fluid to flow either to the motor or to the reservoi

Answer 34

True

Question 35

Most 4 way vows are manufactured with 3 operating positions.

Answer 35

They are middle or non actuated, actuated to the left, and actuated to the right. If the centre position is not used, it is called a crossover position and the valve becomes in effect a 2 position valve.

Question 36

How is a pressure relief valve rated?

Answer 36

It is rated by its flow capacity and operating differential pressure. The differential pressure for a relief valve is the difference between the set or maximum pressure point and the cracking pressure point.

Question 37

Describe sequence files and the resemblance to direct acting relief valves.

Answer 37

They are very similar in construction except the Flow from the outlet of a sequence valve is that line pressure.

Question 38

What are pressure reducing valves?

Answer 38

They are normally open 2 way valves and are actuated by downstream fluid pressure. 2 types, direct acting and pilot operated. The primary reason for using a pressure reducing valve is to limit the pressure in one part of the system to below that of the main part of the system. A relief valve cannot be used for this purpose since the pressure in the entire system would be reduced

Question 39

What is hydraulic shock?

Answer 39

Hydraulic shock occurs when a hydraulic system starts up or performs any of its operations. The pressure rises rapidly and the high pressure relief valves are unable to reduce the pressure surge quickly enough

Question 40

Flow control valve in the hydraulic system are usually positioned how?

Answer 40

Meter in - regulate fluid flow on the inlet side of the actuator
Meter out - regulate fluid flow on the outlet side of the actuator
Bleed off - bleeds off excess fluid in circuits where only a portion of the main flow is required to control the actuator

Question 41

Hydraulic cylinders convert the pressure and movement of hydraulic fluid into straight line mechanical force in motion.

Answer 41

True

Question 42

The double acting piston cylinder can exert force in either direction.

Answer 42

True

Question 43

If fluid is directed to the rod and of the piston, the retracting stroke becomes the power stroke when this occurs the cylinder is called a pull type cylinder

Answer 43

True

Question 44

How do you select a cylinder for a hydraulic system?

Answer 44

You must 1st determine if the cylinder can withstand the system pressure, You must also find how much force the cylinder must exert and how far and how fast it must travel. Then determine whether the system can deliver the required fluid quantity

Question 45

All hydraulic motors are of the positive displacement type which means that they have a mechanical seal somewhere between the inlet and outlet ports.

Answer 45

True

Question 46

How is a hydraulic motor rated ?

Answer 46

They are rated by its pressure range. This range spans the operating pressures of the motor. Motor pressure ratings vary from 100 to 5000 psi.

Question 47

What equation must be used to find overall efficiency of a hydraulic motor?

Answer 47

Overall efficiancy= output hp/ input hp