3387069R_PrinciplesOfHydraulics

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FOREWORD

In this program we have tried to introduce you to the

Principles of Hydraulics. The ability to assemble and

disassemble hydraulic system components is only part of

the technicians responsibility. Before a technician can begin

to do a good job maintaining and troubleshooting any

hydraulic system, he must have a thorough understanding

of the information contained in the following pages.

Copyright@ 1977

By Cummins Engine Co., Inc



1.

2. Hydraulics is one of the most versatile, simplemeans of transmitting power known today. The

application of hydraulics is both ancient and

modern. The water wheel has been in use forthousands of years. Modern day equipment utilize

numerous hydraulic systems.All hydraulic systems, no matter how complex

or simple, utilize the same basic principles. If we

understand these principles, the application of

hydraulics in any system is easy to understand.The purpose of this program is to study the theoryof hydraulics, the various types of components,

and their use in a hydraulic system.

The field of hydraulics is divided into two branches

-Hydrodynamics or Hydrostatics. Hydrodynam-ics utilizes the impact or momentum of a moving

liquid to create or transmit power. Hydroelectricgenerating plants utilize hydrodynamic turbines todrive the generators. The large volume of water

flowing past the spiral blades causes the turbinesto turn.

3.

4. A more common example of a hydrodynamic

system is the engine water pump, commonlyreferred to as a centrifugal pump. As the impeller

in the pump is turned, water trapped between theblades is forced to the outside of the housing by

centrifugal force. The pump uses only centrifugalforce to move the water and does not have apositive sealing system between the inlet and

outlet port, it is classified as a hydrodynamic-typepump. For the continuation of the program we will

be concerned with hydrostatic hydraulics only.

.



Hydrostatic hydraulics is based on the principlethat a confined fluid under pressure transmits

equal force on all areas. The cylinder for exampleis closed at one end, filled with liquid and fittedwith a piston and seal. Applying force on thepiston will create pressure in the liquid that will act

on all areas of the cylinder and piston equally.

5,

6. Since the transmission of force requires the

movement of the fluid, let's discuss one of thebasic principles of hydraulics, .'fluid flow". It is the

element that gives the actuators, cylinders andmotors, motion. Flow is created by the hydraulicpump. In this example pushing the jack handle

down forces the pump piston to displace the fluidin its area, causing flow and thus raising the jack

ram.

Flow is measured in gallons per minute (GPM) or

liters per minute (LPM). A hydraulic pumpproducing a constant flow of 55 GPM would becapable of filling a 55 gallon oil drum in one

minute.

7,

8, The speed of the hydraulic system's actuators is

directly proportional to the rate of flow. We canillustrate this by using two pumps of different

flows operating identically sized cylinders. Bothcylinders have an internal capacity of one gallonwhen extended. The pump producing one GPMwill extend its cylinder in one minute. The pumpproducing two GPM will extend its cylinder in 30

seconds or half the time of the other pump. So, as

the flow of the pump is doubled, the speed of the

actuator is also doubled.



9. The transmission of hydraulic power requires the

movement of the fluid (flow) and a force exertedby the fluid (pressure). Let's take a look at the nextprinciple of hydraulics, "pressure". Pressure canbe defined as the force with which the fluid is

moved through the system. Contrary to popular

thought, pressure is created by resistance to fluidflow, not by the pump. For example, the cyclinder

on the left is being extended but has not

encountered the load. No significant pressure is

being created. On the right, the cylinder ram is

pushing the load. The resistance is creatingpressure.

10. Pressure is generally measured in pounds per

square inch (PSI). On the left a one pound weight

placed on a one square inch area pad exerts 1 PSI.In the center, a one hundred pound weight placed

on the same area pad exerts 100 PSI. On the right,

a thousand pound weight exerts 1000 PSI.

11. One of the real advantages of hydraulics is the

ability to multiply force. This can be illustrated by

comparing the similarity of a simple mechanicallever and a simple hydraulic circuit using two

cylinders. The small cylinder has a piston area of

one square inch and the large cylinder an area of10 square inches. ..the same ratio as the lever. A

100 pound weight placed on the small cylinder will

produce 1000 pounds of force from the largecylinder or will balance a 1000 pound weight. ..again the same ratio as the lever. Adding anadditional one pound weight to the small cylinderwill produce 1010 pounds of force in the large

cylinder, raising the 1000 pound load to the end ofthe cylinder stroke the same as the-Iever.

12 Horsepower is also a very important element in

hydraulics which we should understand. Horse-

power (HP) is a unit of measure, like fluid flow(GPM) and pressure (PSI). In the motorized

hydraulic system there are two measures of

horsepower -engine horsepower and hydraulichorsepower. Engine horsepower with which we arefamiliar, is a measure of the rotating force (torque)and speed (RPM) of the engine output shaft.

Hydraulic horsepower is a measure of thecombination of fluid flow (GPM) and pressure

(PSI). Hydraulic horsepower equals the product ofpressure (PSI) and flow (GPM) divided by theconstant 1714; The formula looks like this:

PSI x GPM

1714 = HP



13. Understanding hydraulic horsepower and its effect

on the total hydraulic system is very important. So,

let's look at some examples.The engine shown develops 230 HP, delivering

185 HP to the main drive line and the remaining 45HP is used for the hydraulic system. The pump is

developing 35 GPM, and the system has a

maximum operating pressure of 2000 PSI. Usingour formula, we see that 35 GPM and 2000 PSI

require a total of 40 horsepower. If the main drive

line and the hydraulic pump were requiring

maximum power simultaneously, the engine wouldhave approximately 5 horsepower to spare.

14. Now if by mistake, the pressure setting in this

circuit were adjusted to 2500 PSI, our formulashows that the power required would exceedengine output. The engine would lug or stall if

both systems required maximum power. On the

other hand, if the pressure setting were too low,the hydraulic system would not produce maximum

available horsepower.

All hydraulics systems are designed to utilize aspecified flow and pressure. Changing these

values, flow or pressure, will upset the balance

between hydraulic horsepower and engine horse-

power .

Another important item we should discuss is

"pressure drop". Looking at this typical hydrauliccircuit, we see a difference in pressure registeredby each of the gauges. This is caused byresistance to flow in the components and lines.

Note that at the pump, the pressure reading is 100PSI while at the outlet it is only 10 PSI. This drop

in pressure from one end of the circuit to the other

is known as "pressure drop"."

15

All hydraulic systems will have "pressure drop"

caused by the various components. A smallamount of pressure drop is normal. However, a

large pressure drop in the system caused by a mis-

sized component can waste horsepower.

16.



17. Let's summarize what we have learned about thetheory of hydraulics.1. A confined fluid under pressure transmits

equal force on all areas.

2. Flow controls the movement and speed of

hydraulics.3. Pressure controls the force exerted by the fluid

flow.4. Hydraulic horsepower is a measure of flow and

pressure.

18. A hydraulic pump producing a continuous flow offluid must have a reserve of fluid to draw from.

This ready reserve of fluid is provided by thehydraulic reservoir. In addition to just providing

storage for the fluid, the reservoir also de-aeratesand cools the fluid.

19. The reservoir contains several important compo-nents. The pump inlet will be located at one side of

the reservoir and may be equipped with a pumpstrainer. The strainer must be kept clean, as a

restricted strainer will cause the pump to cavitate-starve from lack of fluid.

The return line should be located on theopposite side of the reservoir from the inlet andseparated by baffles. Both the inlet and return line

outlet should always be below the fluid level.

Returning fluid above the level causes aeration.Aeration can cause damage to the pump. If thereservoir is vented, the vent must be equipped with

a filter.

20 Some reservoirs are pressurized. They can be

either self-pressurized or pressurized from apositive air source. The self-pressurization systemwill achieve a slight pressure through the expand-ing of the trapped air and oil as the system warms

up to operating temperature.Pressure in the reservoir prevents foreign

material from entering and provides the pump with

a supercharging effect, reducing the possibilities

of cavitation.

.A Confined Fluid. ..

.Flow...

.Pressure. ..

.Horsepower. ..



21. Contamination in the hydraulic fluid can destroy a

high pressure pump in a very short time. Highpressure systems cannot tolerate the slightestamount of contamination. Contaminants are

removed from the fluid by the hydraulic filter.Hydraulic filters are usually located in return lines

at the reservoir. Filters can be mounted in the

individual lines or internally in the reservoir. The

filter will usually contain a replaceable element

and a relief valve.

22 The filter element is usually made from a treated

paper which can be damaged by high pressuresurges. The relief valve protects the element by

bypassing flow when a specified pressure isreached. Cold fluid resists flowing through the

element and will cause the relief to operate.

23 Hydraulic filters are rated by flow and pressure,

but most importantly by the size of particles theywill trap. The micron is a unit of measure. A

micron is one-millionth of a meter or 39 millionthsof an inch. For comparison, 70 microns is

approximately the same size as a grain of salt. Thesmallest particle seen by the eye is about 40

microns. Hydraulic filters having a 10 micronnominal rating will trap most particles 10 microns

in sizel'or larger.

24 The pump is the most important component in the

hydraulic system. Its purpose is to convert

mechanical power into hydraulic power by causingthe fluid to flow. All other components in the

system are dependent upon this flow for their

operation.



25. The hydrostatic pump is a positive displacement-

type pump. That is, for every revolution or strokethere will be a given amount of fluid produced. The

hydrostatic pump also has a positive seal betweenthe inlet and outlet. There are two types of

hydrostatic pumps -fixed displacement andvariable displacement. Do not confuse positivedisplacement with fixed and variable displacement.As we said, all hydrostatic pumps are positive

displacement. We will discuss fixed and variablenow.

26. Fixed displacement pumps produce a specified

amount of fluid with each revolution of the pump

shaft, or at a given RPM. To produce more flow

output the shaft RPM must be increased, and forless output, the shaft RPM must be decreased.

Compare this to the old fashioned water wellpump. With each complete stroke of the handle,the pump produces a given amount of water.

Pumping the lever fast will produce more waterthan when pumping it slowly. But with each fullstroke only a specified amount of water is pro-

duced.

27. Three types of fixed displacement pumps are

commonly used. They are the gear pump, the vanepump, and the piston pump.

28. The gear pump is composed of two meshing gears

-one drive gear, one driven gear -inside a

housing. At the inlet of the housing as the gearsunmesh fluid will fill the space between the gearteeth. As the gears continue to rotate, the fluid willbe trapped between the teeth and the inside of the

housing. Continual rotation will carry this trappedfluid to the outlet port of the housing where the

teeth again mesh. As the gears mesh the spacebetween the gear teeth is displaced and the

trapped fluid is forced out the outlet port of the

housing. Flow through the gear pump is then, inthe inlet port, around the outside of the gears and

through the outlet port. It is sometimes thoughtthat the fluid flows through the center of the gears

in the direction of their rotation. This is, however,untrue.



29 The vane type pump is also used in manyhydraulic systems. A slotted rotor attached to the

drive shaft rotates inside an elliptical cam ring.Vanes fitted in the rotor slots follow the innersurface of the cam ring as the rotor turns.Pumping chambers are formed between the vanes

and the elliptical shape of the cam ring. Vacuum iscreated at the inlet as the space between the rotorand ring increase. Fluid entering here is trapped in

the pumping chambers and then forced into the

outlet as the chamber space decreases.

Another fixed displacement pump is the pistonpump. Piston pumps operate on the principle that

a piston reciprocating in a bore will draw andexpel fluid with each stroke. Most manual hydrau-

lic jacks incorporate piston type pumps. Two basictypes of fixed displacement piston pumps are

available, the radial and axial designs. The radialpump utilizes a rotor with internal pistons in a

radial configuration. The rotor is offset in astationary ring. As the rotor turns the pistons will

follow the inner surface of the ring and reciprocatein the rotor.

30

31 In the axial swash plate design pump, pistons

reciprocate in a rotating cylinder block, which isturned by the drive shaft. Each piston is connectedto the swash plate by a pivoting shoe. As the

cylinder block rotates, the pistons will follow the

swash plate. Since the swash plate is at an angle tothe cylinder block, with each revolution the pistonswill reciprocate. The stroke of the pistons depends

on the angle of the swash plate. Maximum angle isapproximately 17 degrees.

32. The cylinder block rotates against a valve plate.

Ports in the plate are arranged so the pistons pass

the inlet port as they are being pulled out and passthe outlet port as they are being forced in.

AXIAL SWASH PLATE

~~~~~.h Swash

"V Plate

\

Drive

Shaftaylinder ---

Block

Angle~"



Variable displacement pumps are becoming popu-lar in industrial and construction equipment. The

variable displacement pump has the ability to varyflow output from maximum down to zero while

operating at a constant shaft RPM. Some modelsare pressure controlled, providing maximum flowat low pressure and lesser amounts of flow as

pressure increases. Other models are manually

controlled. Some manually controlled models canreverse the direction of their flow.

33.

The axial piston pump is used for variable

displacement. Operation of the variable displace-ment swash plate pump is identical to the fixed

displacement except for the function of the swashplate. In the variable pump, the swash plate is

mounted in a yoke and can be pivoted to changeits angle. The length of the piston stroke deter-

mines the output of the pump. Thus at maximum

swash plate angle, the pistons will have theirmaximum stroke and output. As the angle of plate

is decreased, so will the piston stroke and pump

output.

34,

Some variable pumps have the ability to changethe direction of their flow. The swash plate mayhave a total sweep of 34 degrees, 17 degrees in

either direction. In view "A", the pump swash platecontrol lever is positioned fully to the left. The

pump is developing maximum output through port1. In view "B", the pump swash plate control has

been moved to the center or zero degree position,and the pump is likewise developing no output. Inview "C", the pump swash plate control has beenmoved fully to the right. The pump has reversed its

direction of flow, developing maximum output

through port 2.

35

SWASH PLATE

Pressure compensating variable pumps monitor

system pressure and then vary their output flowaccording to the amount of this pressure. The

ability to match output flow to system pressure

allows the pump to utilize maximum available

horsepower in many different working conditions.This is accomplished by a spring loaded

pressure compensating cylinder which controlsthe angle of the pump swash plate. The springtension in the cylinder will hold the plate at

maximum angle. As system pressure increases it

will act on the cylinder piston, compressing the

spring and decreasing the swash plate angle.

36.



37. In the next three slides we will see the pump reactto a pressure increase, producing maximum flow

when the pressure is low, less flow with increased

pressure and minimum flow when pressure is high.Note that in each case the pump utilizes allavailable horsepower although the flows and

pressures are different in each.In this illustration the cylinder is lifting a small

load, and only a small amount of pressure is beingcreated in the circuit. The pressure is not sufficientto compress the cylinder spring and the pump is

producing maximum flow output. The cylinder willoperate at maximum speed. Calculating the

hydraulic horsepower from the flow and pressurewe see that the pump is utilizing all available

horsepower .

Now more load has been added to the cylinder,

creating more pressure in the system. Thisincrease in pressure has partially compressed the

spring, decreasing the swash plate angle, and flowoutput of the pump. With less flow output from thepump, the cylinder will operate slower but will beable to lift a larger load. Calculating the hydraulic

horsepower we see that the pump is again utilizing

all available horsepower.

38

39. A very large load has now been added to the

cylinder, creating maximum system pressure. Thepressure has completely compressed the cylinderspring and the swash plate angle has decreased to

its minimum. The cylinder will operate at itsslowest speed but now will produce maximum

lifting capability. Again we see that the pump isutilizing all available horsepower. When the system

pressure drops, the compensating cylinder will

adjust 't:he flow output to match the system

pressure.

40. Hydraulic fluid, unlike gases, cannot be com-

pressed and stored for later use. In some applica-tions, it is advantageous to have a supply of

hydraulic energy. Accumulators provide the abilityto store a small amount of hydraulic fluid under

pressure. Accumulators are sometimes incorporat-ed into a system to absorb shocks and surges in

the pressure and flow. Three basic types ofaccumulators are available, the spring type, weighttype, and charged gas type. We will discuss the

charged gas type to understand how the accumu-

lator works.



41 Gas charged accumulators are usually pre-

charged with an inert gas such as dry nitrogen.Oxygen, air or other non-inert gases must not beused as an explosion could result.

Several types of gas charged accumulators areavailable. Piston type accumulators use a piston to

separate the gas and fluid. The bladder typeaccumulator uses a synthetic rubber bladder

inside the cylinder to contain the gas.

42. Operation of the accumulator is simple. Theaccumulator is placed in the pump pressure line.

As system pressure increases during a work cycle,the accumulator will fill with fluid and the gas will

compress. The amount of fluid which the accumu-lator will accept depends on the amount of gas

charge pressure and the amount of systempressure. Usually the accumulator will be com-

pletely filled when the hydraulic system reachesnear maximum pressure. If there is a drop in

system pressure, the gas charge will force the fluid

out of the accumulator to supplement the pump

flow.

43. The effective use of hydraulic power depends

upon precise control of the fluid flow produced bythe pump. This control function is provided by thedirectional control valve. The directional controlvalve usually consists of a moveable spool in a

machined bore in a valve housing.

The valve spool has a series of machined under-

cuts which when moved in the housing will eitherinterconnect or block the housing ports. There are

numerous valve spool and housing configurationsbeing used today. Regardless of the desigrihowever, the directional control valves basic

function is to control the direction of flow through

the system.

44.

.



directional control valves can

valve spools. These controlin systems when more than one

by the same pump. Numerousof spools can be incorporated into

these valves.Valves can be arranged to operate only one

function or several functions simultaneously.

46. Accurate control of hydraulic pressure is essentialin hydraulics. To prevent over-pressurization and

protect the system from damage, pressure reliefvalves are incorporated into the system. The relief

valve senses pressure and opens to bypass flowwhen its pressure setting is reached. The relief

valve is usually placed in the pressure line betweenthe pump and the control valve. Two basic types of

reliefs are commonly used -the simple relief andthe compound relief. Regardless of the type, thefunction of the relief valve is the same. We will

discuss the simple relief valve to understand the

relief operation.

47. A large spring positioned between the adjustmentand the valve piston holds the piston seated.

System pressure will act on the area of the pistonexposed to the inlet. The tension of the main

spring determines the pressure setting of the valve.

In view A, pressure in the inlet of the valve is lowerthan the setting of the valve, and the pistonremains closed. When pressure on the piston is

sufficient to overcome main spring tension, the

valve Wrll open. In view B, the pressure setting of

the valve has been reached. The piston will raiseoff its seat and allow flow to bypass. The valve will

close when pressure drops below its setting.

48. Flow control valves act to limit or control fluidflow. They are sometimes used to slow or reducethe flow to or from a cylinder or motor. A simpleflow control valve is the orifice type. It is simply a

reduced size opening in the line which causes apressure drop. The smaller the orifice, the more

the flow is restricted. The adjustable needle valveperforms the same function as the orifice but

provides an adjustment.



49. Flow divider valves divert part of the main flow toanother operation or to the reservoir. A simple flow

divider valve might consist of only a spring loadedspool with an internal orifice and an additionalport. As the fluid flows through the orifice in the

spool, pressure will build up, forcing the spoolagainst the spring. As flow increases, the spool will

move farther to the right, incovering the additionalport. Main flow is now divided between the two

ports. Numerous types of flow dividers areavailable and are used for many applications.

50. A simple type of check valve provides a one way

direction flow. The valve will free flow in one

direction and block flow in the other.

51 This check valve provides free flow in one

direction and restricted flow in the other. This typeof check valve might be used to slow the action ofa cylinder in one direction.

52. Check valves can be pilot operated. This valve

functions the same as a normal check valve

providing free flow in one direction and no flow inthe other. The valve has an additional pilot control.When pressurized, the pilot piston will open the

valve for free flow in either direction.



53. The actuator is another hydraulic component.Actuators convert hydraulic energy -flow and

pressure -into mechanical energy. Hydrauliccylinders provide linear power and hydraulic

motors provide rotary power.

54. The hydraulic cylinder is a common type of

actuator. The cylinder consists of a cylinder tube,piston, and rod.

55. Cylinders can be single or double acting. Single

acting cylinders can be activated in only one

direction and must be returned mechanically or by

gravity. Double acting cylinders have two portconnections and can be hydraulically activated in

either direction.

56. Hydraulic motors operate in reverse to pumps,

converting flow and pressure back into rotary

energy. The same basic types of pumps are alsoused for hydraulic motors. With few exceptions,

motors are identical to pumps. Since motors arebi-directional and pumps are not, the sealing

system behind the pressure plates may be some-what different. The motor requires an externaldrain to vent excess lubrication and internal

leakage.

CYLINDERS

\;%%tt TUBE

"~ ~

""'/"

PISTON



57. Now that we have had a look at some of the basic

hydraulic components, let's see how they are usedin a complete system.

This system operates a double acting hydraulic

cylinder. The pump is a gear type. The valve is asingle spool type with an internal relief valve. The

reservoir has an external filter and a pump inletstrainer. This system is commonly referred to as

an open loop type system, since all flow produced

by the pump returns to the reservoir before

recycling.

When the system is in neutral with the pumprunning, flow developed by the pump will go

through the valve and back to the reservoir with no

pressure build up.

58. When the valve is activated to raise the load, the

valve spool will direct flow through port "A" to thebase of the cylinder. As the flow meets the

resistance of the load, pressure will build up. The

relief valve will protect the circuit from over-

pressurization. Return flow from the cylinder willflow through the valve to the filter and reservoir.

Centering the control valve will hold the cylinder

and load in position.

59 This manually controlled variable piston pump isdriving a fixed displacement piston motor. This iswhat is called a hydrostatic closed loop circuit,

since it does not use a control valve or return flowto a reservoir. The units are connected directly to

each other. This type of circuit is used to drivesmall tractors and off-road construction equip-

ment. Sometimes it is referred to as a hydraulic

transmission. It provides infinite speed control ineither direction, allowing the engine to be run atfull RPM to power the main equipment systems.

60 From this introduction to the principles of hydrau-lics, it is easy to see why hydraulics is so widely

used for the transmission of power. No other

power transmitting system provides the versatilityof hydraulics.



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3387069R_PrinciplesOfHydraulics