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Testing of Elements and AssembliesMMM4035L

Laboratory Handout

Dynamics of Hydraulics Machines and Devices

by

Micha Stosiak, PhD, MSc. (Mech. Eng.)

Wrocaw 2009

All rights reserved


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1. IntroductionReciprocating plunger pumps are robust, contamination tolerant and capable of efficiently

pumping many types of fluids at high delivery pressures. As a consequence, they are widely

used in a diverse range of industrial applications, including mining (for powered roof

supports), chemical plants, reverse osmosis systems and food processing systems. The mostcommon pump construction consists of a small number of cylinders, usually mounted in-line,

each with a reciprocating piston driven by a rotating crank and connecting rod mechanism.

During the suction stroke, flow is drawn from the inlet manifold into a cylinder through a self-

acting non-return valve; various valve designs are employed although spring-load poppet or

disc valves are most frequently adopted. Fluid delivery also takes place through a self-acting

non-return valve. It is well known that the pipeline pressure pulsations produced by these

pumps are a source of noise and vibration and may have a significant influence on the

reliability of a given installation. Consequently, it is highly desirable to be able to predict

pressure pulsations at the design stage of an installation so that appropriate steps may be taken

to minimize their levels and their influence.

Considerable research effort has been devoted to the study of pressure pulsation behavior in

delivery lines of fluid power systems employing typically gear, vane or axial piston pumps

and to a lesser extent to the suction lines of these systems. However, fluid power pumps

typically employ a large number of pumping elements (nine cylinders are commonly used in

axial piston machines, for example). As a consequence they create flow pulsations and

pressure pulsations with a relatively high frequency content. Basic harmonics is described by

the formula:

][60

1 Hzzn

f

= (1)

where: n rotational pump speed [rpm], z number of pistons.

Other spectrum harmonics are described by the formula:

][60

Hzkzn

fk

= (2)

where k number of harmonics.

Moreover, some dynamic situations (e.g. sudden load increase, sudden system start or stop)

can produce dynamic pressure ripples. These pressure ripples lead to system components

damage, vibrations and noisiness.

However hydraulic systems are the most preferred source of power transmission in mostindustrial and mobile equipment due to their power density, compactness, flexibility, fast

response and efficiency. The area of hydraulics and pneumatics is also known as 'Fluid Power

Technology'. Fluid power systems have a wide range of applications which include industrial,

off-road vehicles, automotive systems and aircraft. But, one of the main problems with

hydraulic systems is the noise generated by them. The health and safety issues relating to

noise have been recognized for many years and legislation is now placing clear demands on

manufacturers to reduce noise levels. Hence, noise reduction in hydraulic systems demands a

lot of attention from industrial as well as academic researchers.

The main source of noise in hydraulic systems is the pump which supplies the flow. Most of

the pumps used are positive displacement pumps. Among the positive displacement pumps,axial piston swash plate type is mostly preferred due to its controllability and efficiency.


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The noise generation in an axial piston pump can be classified under two categories: fluid-

borne noise and structure-borne noise

Fluid-borne noise (FBN).

Among the positive displacement pumps, the highest levels of FBN are generated by axialpiston pumps and the lowest levels by screw pumps. External gear pump and vane pump lie

somewhere in between. An axial piston pump has a fixed number of displacement chambers

arranged in a circular pattern separated from each other by an angular pitch equal to

z/360= where z is the number of displacement chambers. As each chamber discharges a

specific volume of fluid, the discharge at the pump outlet is the sum of all the discharge from

the individual chambers. The discontinuity in flow between adjacent chambers results in a

kinematic flow ripple. The amplitude of the kinematic ripple can be theoretically determined

given the size of the pump and the number of displacement chambers. The kinematic ripple is

the main cause of fluid-borne noise. The kinematic ripple is a theoretical value. These ripples

(also referred to as flow pulsations) generated at the pump are transmitted through the pipe or

flexible hose connected to the pump and travel to all parts of the hydraulic circuit.

The pump is considered to be an ideal flow source. The pressure in the system will be

determined by resistance to the flow or otherwise known as system load. The flow pulsations

result in pressure pulsations. The pressure pulsations are superimposed on the mean system

pressure. Both the flow and pressure pulsations easily travel to all part of the circuit and affect

the performance of the components like control valve and actuators in the system and make

the component vibrate, sometimes even resonate. This vibration of system components adds

to the noise generated by the flow pulsations.

Structure-borne noise (SBN).

In swash plate type pumps, the main sources of structure-borne noise are the fluctuating

forces and moments of the swash plate. These fluctuating forces arise as a result of the

varying pressure inside the displacement chamber. As the displacing elements move from

suction stroke to discharge stroke, the pressure varies from a few bars to few hundred bars

respectively. This pressure changes are reflected on the displacement elements (in this case,

pistons) as forces and these forces are exerted on the swash plate causing the swash plate to

vibrate. This vibration of the swash plate is the main cause of structure-borne noise.

Vibrations and noise reduction

The reduction of the vibrations and noise radiated from the hydraulic system can be

approached in two ways.

1. Reduction at Source - which is the reduction of noise at the pump. Reduction in FBN and

SBN at the source has a large influence on the Airborne Noise (ABN) that is radiated. Even

though, a lot of progress had been made in reducing the FBN and SBN separately, the

problem of noise in hydraulic systems is not fully solved and a lot still needs to be done. The

amplitude of fluid pulsations can be reduced, at the source, with the use of an hydraulic

damper.

2. Reduction at Component level - which focuses on the reduction of noise generated atindividual components like hoses, control valves, pump mounts and fixtures. This can be


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accomplished by a suitable design modification of the component so that it radiates the least

amount of noise. Optimization using computer based models can be one of the ways.

The reduction in source flow ripple in hydraulic systems is the most effective method of

reducing pump-generated pressure ripple and system noise.

Some methods of pressure pulsation and ripples reduction are considered below.

CAPACITANCE

The hydraulic capacitance (also called the compliance) tells us how hard it is to change the

pressure of a fluid stored by adding more of the fluid to the tank. It is the factor relating the

rate of change of volume to the rate of change of capacitive pressure (difference). It is equal

to the inverse of the elastance.

Symbols: CV

Relations:

dt

dp

Cdt

dV CV=

(3)

C

Vdp

dVC = (4)

Units: m3/Pa, m

5/N

Many theoretical considerations and experimental tests show that capacitance increase leads

to pressure ripple reduction in hydraulic systems. However, capacitance increase leads to

system natural frequency decrease. Capacitance increase can be produced in several ways: by

bulk modulus decrease (change of pipe material), by the application of an hydraulic

accumulator (fig. 1, 2), by increasing the pipes geometrical dimensions.

Fig. 1. Hydraulic accumulator in circuit.


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Fig. 2. Pressure ripple reduction due to hydraulic accumulator application.

Another effective method to reduce pressure ripples is the application of hydraulic valves

controlled in proportional technique. By replacing a typical on/off directional control valve

with a proportional directional control valve it is possible to reduce pressure ripples inhydraulic systems.

Figure 3 presents the control signal flow in proportional control technique. Figures 4 and 5

present typical valve characteristics and system response respectively.

Figures 6 and 7 present typical pressure run in stiff (fig. 6) and flexible (fig. 7) pipe in

hydraulic system. In this case pressure ripples are caused by suddenly valve resteering.

Fig. 3. Block diagram of control signal flow in proportional control technique.

Fig. 4. A typical proportional control valve flow output versus current input signal curve.


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Fig. 5. Valve operation with two forward and two reverse speeds.

Fig. 6. Pressure run for stiff pipe.

Fig. 7. Pressure run for flexible pipe.


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Tasks to execute

1. Build an hydraulic system according to scheme presented on fig. 8.

M

MP T

A B II

L1/L

2

12

34

5

6

7

8

9

L1/L

2

a b

I

Fig. 8. Scheme of test system: 1 filter, 2 tank, 3 fixed displacement pump, 4 safety

valve, 5 4/2 directional control valve with on/off solenoids, 6 flexible pipes (length L1 =

1000 mm, length L2 = 2000 mm), 7 pressure transducer, 8 double acting, single piston rod

hydraulic cylinder, 9 external load (30 kg).

1.1. First test step - pipes length (6) is equal to L1 = 1000 mm.

An electrical circuit of solenoid is operated by a monostable button normally opened. A

spring keeps valve spool in position I. The closure of the electrical circuit of solenoid b causes

the cylinder to move up. Ports P and B are connected now. Ports A and T are connected now

too. During the system start pressure is acquired by pressure transducer 7. Now pressureripple can be observed and the coefficient of dynamic surplus can be calculated. Coefficient

of dynamic surplus is described by the formula:

st

std

p

pp =

max (5)

where: pmax pressure ripple, pst stable pressure.

Capacitance of flexible pipes 6 can be calculated:

B

VC

V = (6)

where: V- volume of oil in pipes, substitute bulk modulus (for flexible pipes 750 MPa).


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1.2. Next test step - change pipes length to L2 = 2000 mm. An electrical circuit of solenoid is

operated by a monostable button normally opened. A spring keeps valve spool in position I.

The closure of the electrical circuit of solenoid b causes the cylinder to move up. Ports P and

B are connected now. Ports A and T are connected now too. During the system start pressure

is acquired by pressure transducer 7. Now pressure ripple can be observed and the coefficient

of dynamic surplus can be calculated. Capacitance of flexible pipes 6 (length L2) can becalculated too.

2. Valve 5 is replaced by a proportional directional control valve, according to the scheme

presented in fig. 9.

M

MP T

A B IIIII

L1

L1

12

34

5

6

7

8

9


valve, 5 4/3 proportional directional control valve, 6 flexible pipes (length L1 = 1000

mm), 7 pressure transducer, 8 double acting, single piston rod hydraulic cylinder, 9

external load (30 kg).

Spool of valve 5 is operated by proportional solenoids, so spool displacement is proportional

to input current. Moreover an electronic amplifier which collaborates with proportional

solenoids is equipped with rampenzeit knob to adjust the value of time delay of electricalcontrol signal. It is an efficient tool to shape transient processes in hydraulic systems (starting

or stopping hydraulic cylinder). During system operation pressure transducer 7 measures

pressure run. Now pressure ripple can be observed and the coefficient of dynamic surplus can

be calculated.

3. Reduction of pressure ripples using hydraulic accumulator.

This test step covers building the system with an accumulator according to the scheme

presented in fig. 10.


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M

MP T

A B IIIII

L1

1

2

34

5

6

7

8

9

L1

10

11


valve, 5 4/3 directional control valve with on/off solenoids, 6 flexible pipes (length L1 =

1000 mm), 7 pressure transducer, 8 double acting, single piston rod hydraulic cylinder, 9

external load (30 kg), 10 bladder accumulator, 11 check valve.

Electrical circuits of solenoids are operated by monostable buttons normally opened. Springs

keep valve spool in neutral position. The closure of the electrical circuit of solenoid b causesthe cylinder to move up. Ports P and A are connected now. Ports B and T are connected now

too. In the first phase the accumulator is being charged. Next the hydraulic cylinder moves up

softly. Pressure change can be observed. Pressure ripple is reduced.

4. Report

Conclusions should cover dynamic states comparison of considered systems (figures 8, 9 10)

with the help of coefficient of dynamics surplus d and discussion about practical rules of

pressure pulsation and pressure ripples reduction. Results of the reduction should be well

understood.

References:

1. Norvelle F. D.: Electrohydraulic control systems. Prentice Hall. 2000.2. Design and Steady-state Analysis of Control Systems. Fluid Net Publications Cracow

2002

3. Hydraulic Trainer. Bosch Rexroth Manuals.

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