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Equipment for Engineering Education

Experiment Instructions

PT 500.10 Elastic Shaft Kit

G.U.N.T. Gertebau GmbH

Fahrenberg 14

D-22885 Barsbttel

Germany

Phone: ++49 (40) 670854.0

Fax: ++49 (40) 670854.42

E-mail: [email protected]

Web: http://www.gunt.de

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i

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G.U.N

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GertebauGmbH,

Barsbttel,Germany11/2004

Experiment Instructions

Please read and follow the safety regulations before the first installation!

Publication-no.: 915.000 10 C 500 02 (A) DTP_3

PT 500.10 ELASTIC SHAFT KIT

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Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Equipment layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 Mounting the distance sensors on the safety bearing . . . . . . . . . . . . 3

2.3 Balancing the distance sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.4 Maintenance/Care. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1 Health hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Hazards for equipment and function . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1 Recording of bearing vibration on run-up and run-down . . . . . . . . . . 6

4.1.1 Required accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1.2 Preparation and setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1.3 Experimental method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1.4 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2 Recording of the shaft vibrations during run-up andrun-down of the elastic shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2.1 Required accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2.2 Preparation and setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2.3 Experimental method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2.4 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

ii


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4.3 Recording of path curves (orbits) with distance sensors . . . . . . . . . 15




4.3.4 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.4 Comparison of bearing vibrations on an unbalanced elastic shaftand on a balanced elastic shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20




4.4.4 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5.1 Technical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5.2 Suggested setups / Photographs . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.4 Items supplied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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1 Introduction

With the accessory set the response of an elasticrotor under unbalanced mass vibration can be

studied.

The subcritical, supercritical and resonance

running states can be demonstrated. Of particular

interest is a comparison of the orbits in the

subcritical and supercritical range. A further

subject area is the operational balancing of elastic

rotors.

The device allows the following experiments to be

performed:

Familiarisation with the terms critical rotationspeed and resonance

Study of the subcritical and supercriticalorbits

Influence of unbalanced mass vibration

Balancing of an elastic rotor

Influence of balancing

Influence of alignment errors

Note:

The experimental method described involves use

of the PT500.02 Instrumentation Set fromBrel &

Kjaer Vibro. However, vibration measuring equip-

ment made by other manufacturers can be used

as an alternative. The measured results still logi-

cally reflect the characteristics but depend qualita-

tively on the individual experimental setup.

1 Introduction 1


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2 Description

The accessory set comprises a 10 mm thick shaft,which is reinforced at the bearing points and the

mount of the mass disc to a thickness of 20 mm.

The nominal length between the bearings is

450 mm. In order to provide the elastic shaft with

full movement, pendulum ball bearings were used

in the bearing blocks. The safety bearing limits the

shaft movement in the near-resonance range to

non-hazardous excursions. Also, by screwing dis-

tance sensors into it, orbits can be recorded. Themass disc from the base unit serves as the rotor.

2.1 Equipment layout

2 Description 2


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Fig. 2.1 Elastic shaft, safety bearing and bearing block with pendulum ball bearing

1 2 3 4 5 2 6 7

1 Elastic shaft with 14 mm shaft end

2 safety bearing/bearing block fixing screw

3 Bearing point with 20 mm

4 safety bearing

5 Holes for distance sensors

6 Bearing block with pendulum ball bearing

7 Holes for acceleration sensors

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2.2 Mounting the distance sensors on the safety bearing

There are two threaded holes on the safety bear-ing for the distance sensors. The distance sensors

should only be screwed in far enough to prevent

contact with the shaft. To find the right reach of the

distance sensors use the extra centerin pin.

Mounting the distance sensors

Put the extra centering pin into the safety

bearing Screw the distance sensors till they contact

the centering pin

To fix the position, the distance sensorsshould be secured by the nuts. The nuts

should be tightened only lightly.

2 Description 3


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Fig. 2.2 safety bearing(A) Plastic bush

A

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2.3 Balancing the distance sensors

To balance the distance sensors, from theVibroport menu choose Task, Overall, Axial

Position, Display and Measure. The voltage

value relating to the distance can be read off here.

The voltage value of the zero distance should be

entered here inSetup.Thezero voltage distance is

approximately - 4 V. (Withno disturbing aluminium

of the safety bearing the zero value should be

around -10 V.)

Setup for measurement of axial position, numeric(Vibroport 41)

Setup 1

Input Active

Sensor type Distance

Sensitivity 8 mV/m

Unit mm

Zero display -4 V

Pos.sign for : Increasing distance voltage

2.4 Maintenance/Care

The pendulum bearings should be cleaned if they

become dirty, and lubricated with grease.

Pendulum bearings are open and unsealed, so

they should be protected against dirt and dust.

2 Description 4


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3 Safety

3.1 Health hazards

DANGER! Risk of injury from rotating parts. Only

operate the unit with the protective hood closed.

DANGER! Risk of injury and of destruction of the

shaft. Never operate the unit without a safety bear-

ing.

3.2 Hazards for equipment and function

ATTENTION! It must be ensured that the bearings

are adequately lubricated with oil or grease.

CAUTION! Take care when handling the shaft. Do

not deform the shaft by external force (bending or

dropping).

3 Safety 5


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4 Experiments

4.1 Recording of bearing vibration on run-up and run-down

The bearing vibrations on run-up and run-down of

an elastic shaft are to be recorded. The bearing

vibrations are recorded using the acceleration

sensors.

The critical rotation speed for the bearing

vibrations is to be determined.

4.1.1 Required accessories

PT 500 Machinery Fault Trainer

PT 500.02 Instrumentation Set from Brel & Kjaer

Vibro

4.1.2 Preparation and setup

4 Experiments 6


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Fig. 4.1 Experimental setup to measure the bearing vibrations on an elastic shaft with accelerationsensors

1 Drive unit

2 Coupling

3 Bearing block with pendulum bearing

4 Acceleration sensor 1

5 Elastic shaft

6 safety bearing

7 Hole for distance sensor

8 Mass disc

9 Clamping set

10 Reflection marking for reference sensor

11 Acceleration sensor 2

12 Magnetic holder

13 Reference sensor

1 2 3 4 5 6 7 8 9 10 3 11

12 13

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Fit drive unit (1) on base frame

Connect drive unit

Mount first bearing block loosely in front ofcoupling

Insert elastic shaft with safety bearingthrough bearing of first bearing block into

coupling and place on base frame

Push mass discs onto elastic shaft withclamping set

Support elastic shaft with second bearingblock and mount bearing block loosely on

base frame

Align and secure bearing blocks and safetybearing

Secure mass disc to elastic shaft

Affix marking for reference sensor on shaft

Align reference sensor to marking Screw acceleration sensors on to bearing

blocks

Connect sensors to vibration measuringdevice

4 Experiments 7


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4.1.3 Experimental method

Acceleration sensor: AS-020

Setup settings:

Setup for bearing vibrations (Vibroport 41)

Setup 1 Setup 2

Input Active Active

Sensor type Acceleration Acceleration

Sensitivity 100 mV/g 100 mV/g

Unit m/s - eff m/s - eff

Measuring range 20 (variable) 20 (variable)

High-pass filter 10 Hz ISO 10 Hz ISO

Low-pass filter 1 kHz ISO 1 kHz ISO

Setup for reference (P-84)

Setup 1

Ext. reference Active

Trigger level 50%

Trigger edge Positive

Speed unit rpm

Speed/reference 1/1

Close the protective hood

Switch on the actuating unit for the electric

motor

Set the direction of rotation

Switch on the electric motor

Start the measurement and slowly adjust thespeed up and down (from 0 to 3000 rpm)

4 Experiments 8


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4.1.4 Evaluation

Figure 4.2 clearly shows the resonance range

(critical range)with contact on thesafetybearing.At

approximately 1250 rpm the amplitude begins to

rise (the resonance range starts). At approximately

1800 rpm the shaft strikes the safety bearing. The

resonancerange is runthrough up toapproximately

2200 rpm with limitation by the safety bearing.

Above 2200 rpm the shaft detaches from the safety

bearing, the amplitude falls further, the resonance

range is quit (supercritical range). The speed range

below the resonance range is also termed the

subcritical range.

The critical speed (here approximately 2000 rpm)is the speed at which the resonance curve reaches

its maximum without disturbance from the safety

bearing.

If the resonance range cannot be run through by

slowly increasing the speed because the shaft will

no longerdetach from the safety bearing, the reso-

nance range should be run through by increasing

the speed rapidly.

4 Experiments 9


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Fig. 4.2 Bearing vibration on run-up of unbalanced elastic shaft. (Shaft = shaft withmass disc)

Contact on safetybearing

Detachment fromsafety bearing

Maximum resonance rangewithout safety bearing

Subcritical rangeCritical range

Supercriticalrange

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4.2 Recording of the shaft vibrations during run-up and run-down of the

elastic shaft

The movement of an elastic shaft as a function of

the rotation speed is to be recorded during run-up

andrun-down. Themeasurement is taken with dis-

tance sensors in x and y direction. The critical

rotation speed for the shaft vibration is to be

determined.



PT 500.02 Instrumentation Set from Brel & Kjaer

Vibro


4 Experiments 11


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Fig. 4.4 Experimental setup to measure the shaft vibrations on an elastic shaft with distance sen-sors

1 Drive unit2 Coupling

3 Bearing block with pendulum bearing

4 Elastic shaft

5 safety bearing

6 distance sensor 1-horizontal, 2-vertical

7 Mass disc8 Clamping set

9 Reflection marking for reference sensor

10 Magnetic holder

11 Reference sensor

1 2 3 4 5 6 7 8 9 3

10 11

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For preparations see section 4.1.2, but with dis-

tance sensors instead of acceleration sensors.Balancing as per section 2.3.


distance sensor: IN-085

Setup settings:

Setup for shaft vibrations (vibration distance sensors only) (Vibroport41)

Setup 1 Setup 2

Input Active Active

Sensor type Distance Distance

Sensitivity 8 mV/m 8 mV/g

Unit m m


High-pass filter 10 Hz 10 Hz

Low-pass filter 1kHz 1 kHz

Setup for reference (with P-84)

Setup 1


Trigger level 50%


Speed unit rpm

Speed/reference 1/1

Close the protective hood

Switchontheactuatingunit for theelectricmotor

Set the direction of rotation

Switch on the electric motor

Start the measurement and adjust the speed up

and down (from 0 to 3000 rpm)

4 Experiments 12


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4.2.4 Evaluation

Figure4.5 plots the pathasa characteristicof

the shaft vibration over the speed. The

record represents the result of the horizontal

sensor. In the speed range around 2000 rpm

a critical range of the shaft vibration is clearly

seen. At approximately 1875 rpm the shaft

vibration is limited by the safety bearing. The

safety bearing limits the shaft vibration up to

approximately 2150 rpm, then the shaft de-

taches from the safety bearing and runs in

the supercritical range. The maximum reso-

nance is at approximately 2000 rpm.

4 Experiments 13


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Fig. 4.5 Shaft vibrations on run-up of the elastic shaft (horizontal)

Supercritical range

Critical range

Subcritical range

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Figure 4.6 shows the same as Figure 4.5,

except that here the speed is not increased

from 0 rpm to 3000 rpm, but is reduced from

3000 rpm to 0 rpm. The resonance range is

passed through from the opposite direction.

4 Experiments 14


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Fig. 4.6 Shaft vibrations on run-down of the elastic shaft (horizontal)

Supercriticalrange

Subcritical range

Critical range

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4.3 Recording of path curves (orbits) with distance sensors

The orbits of an elastic shaft are to be recorded at

different speeds using distance sensors. The

measurement is taken with two distance sensors

in x and y direction.



PT 500.02 Instrumentation Set from Brel & KjaerVibro


See section 4.2.2


distance sensor: IN-085

Setup settings:

Setup for orbit (Vibroport 41)

Setup 1 Setup 2

Input Active Active

Sensor type Distance Distance

Sensitivity 8 mV/m 8 mV/g

Unit mm mm


High-pass filter 10 Hz 10 Hz

Low-pass filter 1 kHz 1 kHz

4 Experiments 15


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Setup for reference (with P-84)

Setup 1


Trigger level 50%


Speed unit Hz

Speed/reference 1/1

See section 4.2.3, except that one measurement

per speed is recorded instead of a speed range.

4.3.4 Evaluation

Path curves (orbits) of the elastic shaft with

mass disc were recorded. The sensors used

were distance sensors (1=horizontal, 2=ver-

tical). Theorbits reflect the movements of the

rotor shaft relative to the centre of the shaft.The size and shape of the kinetic shaft path

are influenced by the forces acting on the ro-

tor and by the design of the machine. In the

tests on the elastic shaft the unbalance

serves as a vibration exciter. The unbalance

generates a radially acting centrifugal force

during rotation which circulates with the ro-

tor. Under ideal bearing conditions (isotropic

bearing support) and assuming an isotropicshaft, the kinetic shaft path would describe a

circular orbit. In reality, bearings used in en-

gineering are anisotropic, meaning they

have differing elasticities on their principal

axes of rigidity. Subject to equal dynamic

load from an unbalance vibration, the kinetic

shaft path assumes an elliptical shape.

4 Experiments 16


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In the following orbits the shape of the orbit

depends on the differing elasticity on theprinciple axes of rigidity and the differing

resonance points on the two planes. The

differing resonances of the two planes (x/y)

create differing levels of vibration for the

horizontal and vertical planes depending on

the speed.

4 Experiments 17


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Fig. 4.7 Orbit with subcritical speed, n=626 rpm; differing elasticity on princi-pal axes of rigidity is seen

Fig. 4.8 Orbit with subcritical speed, n=1560 rpm

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4 Experiments 18


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Fig. 4.9 Orbit with maximum excursion in the horizontal plane, n=1850 rpm.If the speed is increased further the critical range of the vertical

plane also becomes relevant. Extension in y direction (axis-2)increases.

Fig. 4.10 Orbit on entry into critical range. Because the horizontal resonanceis before the vertical resonance and the horizontal elasticity isgreater, at the speed (n=1793 rpm) a greater elasticity in thehorizontal plane follows

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4 Experiments 19


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Fig. 4.11 Orbit at 1936 rpm. Maximum amplitude on horizontal planeexceeded, amplitude on vertical plane has reached maximum.

Fig. 4.12 Orbit at 2205 rpm. Critical ranges have been exceeded. Amplitudeon horizontal plane is greater than on vertical plane.

Fig. 4.13 Orbit at 2875 rpm. Critical ranges have been exceeded. Amplitudeon horizontal plane is greater than on vertical plane.

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4.4 Comparison of bearing vibrations on an unbalanced elastic shaft and

on a balanced elastic shaft

On an unbalanced elastic shaft the bearing

vibrations are recorded in the speed range up to

3000 rpm. This measurement is repeated with the

same shaft when balanced, and the results are

compared. For single-plane balancing see experi-

ment instructions in base unit, PT 500 section 5.2.

When selecting the balancing speed it must be

ensured that the speed is not within the resonance

range. In the resonance range there is a markedphase changeaswell asa rise in amplitude.These

majorchanges in theresonancerange cause such

significant errors during balancing that there is no

point in performing it. The balancing speed should

be before or after the resonance range.


See section 4.1.1


See section 4.1.2


See section 4.1.3

4.4.4 Evaluation

Figure 4.14 clearly shows the resonance range

with its likely maximum without limitation by the

safety bearing at approximately 2000 rpm. The

resonance range begins at the start of the rise at

1550 rpm, at 1800 rpm the shaft strikes the safety

4 Experiments 20


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Fig. 4.14 Permissible speed range forbalancing (light);Impermissible range (dark)

Amplitude

Phase

Speed

Resonance range

Speed

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bearing, and at 2230 rpm it detaches from it. The

maximum amplitude limited by the safety bearingis approximately 11 mm/s on detachment from the

safety bearing.

Figure 4.15 shows much smaller amplitudes in the

range 1550 rpm to 2500 rpm, where theresonance range was. The amplitudes only rise to

7 mm/s in the range 2500 rpm to 3000 rpm. The

shaft does not strike the safety bearing.

4 Experiments 21


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Fig. 4.15 Bearing vibrations of an unbalanced elastic shaft

Fig. 4.16 Bearing vibrations of a balanced elastic shaft

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5 Appendix

5.1 Technical data

Elastic shaft

Shaft diameter: 10 mm

Bearing diameter: 20 mm

Shaft end diameter: 14 mm

Length: 530 mm

Material: St (1.4125) 56HRC

Bearings:

pendulum ball bearing

Type: 1204TV

d = 20, D = 47, B =14

Reference diameter: 33 mm

Ball diameter: 6.35 mm

Number of balls per row: 12

Number of rows: 2

A speed of n rpm gives the following rotational frequencies:

Speed of inner ring 1,0 n min-1

Fault frequency of inner ring: 7,14 n RPM

Fault frequency of outer ring: 4,86 n RPM

Fault frequency of ball: 5,00 n RPM

Cage rotation speed: 0,41 n RPM

Ball rotation speed: 2,5 n RPM

5 Appendix 22


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30

530

7050 50

245

Fig. 5.1 Elastic shaft without mass disc; figures in mm

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5.2 Suggested setups / Photographs

5 Appendix 23


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Fig. 5.2 Experimental setup to record the orbits on the elastic shaft with distance sensors

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5.3 References

Rotordynamik [Rotor Dynamics] ; R.Gasch,

R.Nordmann, H.Pftzner; Springer-Verlag Berlin -

2nd edition

5.4 Items supplied

1x PT 500.10 Elastic Shaft Kit

1x Experiment instructions, PT 500.10 Elastic

Shaft Kit


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pt50010e

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