30079_60.pdf

CHAPTER 60

HYDRAULIC SYSTEMS

Hugh R. Martin

University of Waterloo

Waterloo, Ontario, Canada

60.1 HYDRAULIC FLUIDS

1831

60.8

SYSTEM CLASSIFICATIONS

1847

60.2 CONTAMINATIONCONTROL

1832

60.9

PUMP SETS AND

ACCUMULATORS

1847

60.3 POSITIVEASPECTSOF

CONTAMINATION

1833

60.10

HYDROSTATIC

TRANSMISSIONS

1851

60.4 DESIGN EQUATIONS-

ORIFICES AND VALVES

1834

60.11

CONCEPT OF FEEDBACK

CONTROL IN HYDRAULICS

1852

60.5 DESIGN EQUATIONS—PIPES

AND FITTINGS

1835

60.12

IMPROVEDMODEL

1854

60.6 HYDROSTATICPUMPSAND

MOTORS

60.13

ELECTROHYDRAULIC

SYSTEMS— ANALOG

1838

1856

60.14

ELECTROHYDRAULIC

SYSTEMS— DIGITAL

60.7 STIFFNESS IN HYDRAULIC

SYSTEMS

1843

1860

60.1 HYDRAULIC FLUIDS

One of the results of the study of fluid mechanics has been the development of the use of hydraulic

oil, a so-called incompressible fluid, for performing useful work. Fluids have been used to transmit

power for many centuries, the most available fluid being water. While water is cheap and usually

readily available, it does have the distinct disadvantages of promoting rusting, of freezing to a solid,

and of having relatively poor lubrication properties.

Mineral oils have provided superior properties. Much of the success of modern hydraulic oils is

due to the relative ease with which their properties can be altered by the use of additives, such as

rust and foam inhibitors, without significantly changing fluid characteristics.

Although hydraulic oil is used mainly to transmit fluid power, it must also 1) provide lubrication

for moving parts, such as spool valves, 2) absorb and transfer heat generated within the system, and

3) remain stable, both in storage and in use, over a wide range of possible physical and chemical

changes.

It is estimated that 75% of all hydraulic equipment problems are directly related to the improper

use of oil in the system. Contamination control in the system is a very important aspect of circuit

design.

In certain industries, such as mining and nuclear power, it is critically important to control the

potential for fire hazards. Hence, fire-resistant fluids have been playing an ever-increasing role in

these types of industry. The higher pressure levels in modern fluid power circuits have made fire

hazards more serious when petroleum oil is used, since a fractured component or line will result in

a fine mist of oil that can travel as far as 40 ft and is readily ignited. The term fire-resistant fluid

Reprinted with permission from J. A. Schetz and A. E. Fuhs (eds.), Handbook of Fluid Dynamics

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

(FRF) generally relates to those liquids that fall into two broad classes: a) those where water provides

the fire resistance, and b) those where a fire retardant is inherent in the chemical structure. 1 " 4 Fluids

in the first group are water/glycol mixtures, water in oil emulsions (40-50% water), and oil in water

emulsions (5-15% water). The second group are synthetic materials, in particular chlorinated hydro-

carbons and phosphate esters.

A disadvantage with water-based fluids is that they are limited to approximately 50-6O 0 C oper-

ating temperature because of evaporation. The high vapor pressure indicates this group is more prone

to cavitation than mineral oils. Synthetic fluids such as the phosphate esters do not have this problem

and also have far superior lubrication properties. Some typical characteristics of these various types

of fluids are shown in Table 60.1.

Of all the physical properties that can be listed for hydraulic fluids, the essential characteristics

of immediate interest to a designer are 1) bulk modulus, to assess system rigidity and natural fre-

quency, 2) viscosity, to assess pipe work and component pressure losses, 3) density, to measure flow

and pressure drop calculations, and 4) lubricity, to determine threshold and control accuracy assess-

ments. The first three items are discussed in separate sections, as they relate directly to circuit design.

Lubricity, the final item, is difficult to define, as it is very much a qualitative judgment. Lubricity

affects the performance of a system, since it is a major factor in determining the level of damping

in the system, that is, viscous or velocity-dependent damping. It also affects the accuracy of operation

of a system because of its influence on the other type of friction, coulomb friction, which is velocity-

independent.

Oil film strength is often referred to as the anti-wear value of a lubricant, which is the ability of

the fluid to maintain a film between moving parts and thus prevent metal-to-metal contact. These

characteristics are important for the moving parts in valves, cylinders, and pumps. 5

60.2 CONTAMINATION CONTROL

There is little doubt that component failure or damage due to fluid contamination is an area of major

concern to both the designer and user of fluid power equipment. Sources of contamination in fluid

power equipment are many. Although oil is refined and blended under relatively clean conditions, it

does accumulate small particles of debris during storage and transportation. It is not unusual for

hydraulic oil circulating in a well maintained hydraulic circuit to be cleaner than that from a newly

purchased drum. New components and equipment invariably have a certain amount of debris left

from the manufacturing process, in spite of rigorous post-production flushing of the unit.

The contaminant level in a system can be increased internally due to local burning (oxidation) of

oil to create sludges. This can be a result of running the oil temperature too high (normally 40-6O 0 C

is recommended) or due to local cavitation in the fluid.

The trend towards the use of higher system pressures in hydraulics generally results in narrower

clearances between mating components. Under such design conditions, quite small particles in the

range of 2-20 microns can block moving surfaces.

Extensive work on contamination classification has been carried out by Fitch and his co-workers. 6

To take a specific example, consider the piston pump shown in Fig. 60.1. Component parts of

the pump are loaded towards each other by forces generated by the pressure, and this same pressure

always tends to force oil through the adjacent clearance. The life of the pump is related to the rate

at which a relatively small amount of material is being worn away from a few critical surfaces. It is

logical to assume, therefore, if the fluid in a clearance is contaminated with particles, rapid degra-

dation and eventual failure can occur.

Although the geometric clearances are fixed, the actual clearances vary with eccentricity due to

load and viscosity variations. Some typical clearances between moving parts are shown in Table 60.2.

Contamination control is the job of filtration. System reliability and life are related not only to

the contamination level but also to contaminant size ranges. To maintain contaminant levels at a

magnitude compatible with component reliability requires both the correct filter specification and

suitable placement in the circuit. Filters can be placed in the suction line, pressure line, return line,

Table 60.1 Comparison of Some Hydraulic Fluids

Property

Density (38 C)

Viscosity (38 C)

(99C)

Bulk modulus

(38 C and 34.5 MPa)

Vapor pressure

Mineral Oil

858.2

4.0 x 10~ 5

5.8 X IQ- 6

1.38 x 10 9

FRF (Ester)

1136.0

4.6 x 10~ 6

4.9 x 10~ 6

2.25 x 10 9

Water in Oil

980.0

0.15 x IQ- 5

Units

kg m~ 3

In 2 S- 1

Nm~ 2

2.18 X 10 9

6 x 10~ 5

6 x 10- 5

kPa (abs)

1.0

Fig. 60.1 Piston pump clearances.

or in a partial flow mode. To use a broad approach of just inserting a filter with a very low rating is

unsatisfactory from the aspects of both cost and high pressure loss. The optimization of choice can

be approached using simple computer modeling, as described by Foord. 7

Dirt in hydraulic systems consists of many different types of material, ranging in size from less

than 1 micron to greater than 100 microns. Since most general industrial hydraulics operating below

14 MPa are able to tolerate particles up to 25 microns, a 25-micron-rated filter is satisfactory. Equip-

ment operating at pressures in the 14-21 MPa range should have 10-15-micron-rated filters, while

high pressure pumps and precision servo valves need 5 micron-rated filtration. A good practical

reference for filter selection has been written by Spencer. 8

The size distribution of particles is of course random, and, generally speaking, the smaller the

size range the greater the number of particles per 100 ml of fluid. Filters are not capable of removing

all the contaminants, but for example, a 10-micron filter is one capable of removing about 98% of

all particles exceeding 10 microns of a standard contaminant in a given concentration of prepared

solution.

60.3 POSITIVE ASPECTS OF CONTAMINATION

Contamination buildup in a system can be used as a diagnostic tool. Regular sampling of the oil and

examination of the particles can often give a clue to potential failure of components. In other words,

this is a preventive maintenance tool. Many methods can be used for this type of examination, such

as spectrochemical 9 or Ferrographic 1 0 methods. Sampling of the oil can be taken at any time and

does not interfere with the operation of the equipment.

Table 60.3 shows the normally expected contaminant levels in parts per million (ppm); levels

rising above these values and particularly rates of change of levels are indicative of potential failures.

Table 60.2 Typical Clearances in Pumps

Clearance

Range

Component

(micron)

Spool to sleeve in valve

1-10 diametrical

Gear pump tip to casing

0.5-5

Piston to bore

5-40

Valve plate to body of pump

0.5-5

Table 60.3 Some Typical Normal Contaminant Levels

Material

Iron

Chromium

Aluminum

Copper

Source in System

Bearings, gears, or pipe rust. Pistons and valve wear.

Alloyed with bearing steel

Air cooler equipment

Bronze or brass in bearings. Connectors. Oil

temperature sensor bulb. Cooler core tubes.

Usually alloyed with copper or tin. Bearing cage metal.

Bearing cages and retainers

Cooling tube solder

Bearing steel alloy

Seals; dust and sand from poor filter or air leak

Possible coolant leak into hydraulic oil

Max Level (ppm)

Lead

Tin

Silver

Nickel

Silicon

Sodium

The Ferrographic technique allows the separation of wear debris and contaminants from the fluid

and allows arrangement as a transparent substrate for examination. When wear particles are precip-

itated magnetically, virtually all nonmagnetic debris is eliminated. The deposited particles deposit

according to size and may be individually examined. By this method it is possible to differentiate

cutting wear, rubbing wear, erosion, and scuffing by the size and geometry of the particles. However,

the Ferrographic method is expensive compared to other methods of analysis. 1 1

60.4 DESIGN EQUATIONS—ORIFICES AND VALVES

The main controlling element in any hydraulic circuit is the orifice. The fluid equivalent of the

electrical resistance, it can be fixed in size or can be variable, in the case of a spool valve. The orifice

in its various configurations is also the main source of heat generation, resulting in the need for

cooling techniques and a major source of noise.

The orifice equation is developed from Bernoulli's energy balance approach, which results in the

following relationship: 1 2

e= ~rm^^^

C C C V A 0

/2Cp 1 1 - p vc )

V A 1 1 /

where Q = volume flow rate, m 3 /s

A 0 = orifice area, m 2

A 11 = upstream area, m 2

p u = upstream static pressure, Pa

p vc = static pressure at Vena contracta, Pa

C c = contraction coefficient

C v = velocity coefficient

p — mass density of hydraulic fluid, kg/m 3

These parameters are shown in Fig. 60.2, together with the static pressure distribution on either side

of a sharp-edged orifice. Experimental measurements show that the actual flow is about 60% of that

given by Bernoulli's equation. Hence, the need for the contraction and velocity coefficients. This

results in the practical form of Eq. (60.1) for typical industrial hydraulic oil

Q = 3.12 X IQ- 2 A 0 Vp 14 -/^mV 1

(60.2)

The symbols have the same definition as those for Eq. (60.1). The adequacy of Eq. (60.2) is dem-

onstrated in Table 60.4.

In the case of a variable orifice, such as that found in a spool-type valve, the orifice area is a

variable. In fact, it can be seen from Fig. 60.3 that the exposed area available for oil flow is part of

a circle. If the orifice, in this case called a control orifice or port, is of radius r and the spool

displacement from the closed position is jc, then the uncovered area is

Vena contracta

v Arbitrary downstream

pressure tap position

Cavitation effect

Vena contracta

Fig. 60.2 Static pressure distribution.

A 0 = [O- cos (0/2)] f yj

(60.3)

6 = 2 COS^ 1 [1 - (xlr)}

(60.4)

The area displacement characteristic plotted in Fig. 60.3 shows the nonlinear nature of the curve.

One of the significant differences between the theoretical valve and the practical valve is the lap.

It is not economical to produce zero lapped valves, so that only at the center position is the flow

through the valve zero. Normally, the valve is either overlapped or underlapped, as shown in Fig.

60.4. An overlapped valve saves fluid loss when the spool is central. This is fine for directional

control valves, but it produces both accuracy and stability problems if the valve is a precision control

valve within a closed-loop configuration.

An underlapped valve gives much better control and stability, at the expense of a higher leakage

rate (power loss). Many more details of valve design can be found in Martin and McCloy. 1 2

60.5 DESIGN EQUATIONS—PIPES AND FITTINGS

While orifices serve the important function of controlling flow in the system, pipes and fittings are

necessary to transmit fluid power from the input (usually a pump) to the output (usually a ram or

motor). It is important to minimize losses through these conductors as well as through other com-

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