Crack Detection Kit

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Experiment Instructions

PT 500.11 Crack Detection in

Rotating Shaft Kit



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A l l R i g h t s R e s e r v e d

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G e r ä t e b a u G m b H ,

B a r s b ü t t e l , G e r m a n y 0 1 / 2 0 1 0

Experiment Instructions

PT 500.11 CRACK DETECTION IN ROTATING SHAFT KIT

This manual must be kept by the unit.Before operating the unit:

- Read this manual.

- All participants must be instructed on

handling of the unit and, where appropriate,

on the necessary safety precautions.

Version: 1.1 Subject to technical alternations

Authors: Dr.-Ing. Detlef Abraham

Dipl.-Ing. Jack Boxhammer

Dipl.-Ing. Peter Mittasch



Table of Contents

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

1.1 Intended use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Health hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Hazards to the unit and its function . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Unit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Assembly of the flange connection for simulation of a crack. . . . . . . 9

3.3 Securing the flange connection on the shaft with the clamping set . 10

3.4 Maintenance / care . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.1 Simulation of “crack in shaft” with protruding shaft end. . . . . . . . . . 12

4.1.1 Purpose of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.2 Required accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.3 Preparation and setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1.4 Performing the experiment . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.1.5 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2 Simulation of “crack in shaft” with elastic rotor . . . . . . . . . . . . . . . . 23

4.2.1 Purpose of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.2 Required accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.3 Preparation and setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.2.4 Performing the experiment . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2.5 Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

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5 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.1 Technical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.2 Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.3 Setup suggestions / photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.4 Items supplied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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1 Introduction

The PT 500.11 Crack Detection in RotatingShaft Kit allows a shaft with a crack to be simu-

lated. Cracks caused by material fatigue are very

dangerousfor rotatingmachinesas they often lead

to the dreaded fatigue fracture, with fatal conse-

quences. As a result, early detection is vital.

A crack in the shaft influences the shaft’s vibration

behaviour by changing its rigidity. These changes

can be identified by measuring the vibrations on

theshaft andusingappropriateanalysissoftware.

Learning content/Exercises

– Change in characteristic vibration behaviour

(natural frequency, resonance speed, ampli-

tudeand phase ofvibrations) due toa crack

– Cracks identification from the change in

vibration spectrum

– Detection of cracks in rotating shafts at the a

protruding shaft end

– Understanding and interpreting frequency

spectra

– Use of a computerised vibration analyser.

– Crack in a shaft with an elastic rotor (with

retain bearing from PT 500.10)

Notice:

Performance of the experiment is described using

the PT 500.04 “Computerised Vibration Analyser”.

However, vibration measuring instruments from

other manufacturers can also be used. The quality

of the measured results depends on the individual

experimental setup but reproduces the basic

characteristics.

1 Introduction 1


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1.1 Intended use

The PT 500.11 unit is to be used only for teachingpurposes.

1 Introduction 2


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2 Safety

The experiment instructions, in particular the safety instructions, must be read

thoroughly prior to starting up the unit. Prior to starting the experiments, the

participants are to be briefed on the safety aspects and the correct handling of

the unit. The signal words DANGER, WARNING or CAUTION indicate the

probability and potential severity of injury. An additional symbol indicates the

nature of the hazard.

Signal word Explanation

DANGER Indicates a situation which, if not avoided, will

result in death or serious injury.

WARNING Indicates a situation which, if not avoided, may

result in death or serious injury.

CAUTION Indicates a situation which, if not avoided, mayresult in minor or moderately serious injury.

NOTICE Indicates a situation which may result in damageto equipment, or provides instructions on opera-

tion of the equipment.

2 Safety 3


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Symbol Explanation

Rotating shafts

General hazard location

Notice

2 Safety 4


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2.1 Health hazards

WARNING

Rotating shafts

Risk of injuries.

• Make sure that long hair, long beards,

chains, ties and loose clothing does not

come into contact with the rotating parts.

• Only operate theunit with theprotective hood

closed.

• Switch off the motor before any modifica-

tions.

2 Safety 5


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2.2 Hazards to the unit and its function

NOTICE

At least two of the six hexagon headscrewsdistrib-

uted around the circumference must be securely

tightened with no clearance with spacer sleeves.

The remaining screws must be fitted as loose con-

nections with clearance. For safety, all screws

must always be fitted, see section 3, Fig. 3.3.

NOTICE

Do not exceed the maximum permitted values

(see technical data).

NOTICE

For the “Shaft with crack with protruding shaft end”

experiment, the maximum belt tension is 70 N.

NOTICE

Continuous operation at a critical bending speedshould be avoided. The critical bending speed

depends on the individual experimental setup.

2 Safety 6


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3 Unit description

This unit simulates the characteristic behaviour ofa shaft with a crack using an asymmetrical flange

connection.

The flange connection is provided by six screws

distributed around the circumference. Tightening

the flange connection with spacer sleeves gives a

connection that is either loose or secure depend-

ing on the installation direction of the spacer

sleeves.

When rotated with a bending load, this flange con-

nection results in intermittentseparation of thebutt

joint. This very closely resembles the behaviour of

a crack in the shaft.

To create this behaviour, it is necessary to load the

flange connection with a bending torque (e.g.

using the PT 500.14 belt drive or an imbalance

from a weight).

NOTICE

At least two screws in the flange connection must

be securely tightened to give a secure connection

with no clearance.

For safety and to prevent imbalances, all screws

must always be fitted.

3 Unit description 7


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Fig. 3.1 Shaft with no crack -

Flange connection withsix supporting screwsA = Loose connectionB = Secure connection

Fig. 3.2 Shaft with small crack -Flange connection withfive supporting screws

Fig. 3.3 Shaft with maximumcrack - Flange connecti-on with two supportingscrews

B

B

A

A

B



3.1 Layout



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Fig. 3.4 Overall view of PT 500.11 - “Crack Detection in Rotating Shaft Kit”

1 Driving shaft (PT 500)

2 Clamping set

3 Centring pin

4 Pick up disc

5 Hexagon head screws

6 Spacer sleeve

7 Flange with long shaft (output for weight)

8 Flange with short shaft (output for belt drive)

8 7 6 5 4 3 2 1



3.2 Assembly of the flange connection for simulation of a crack

– Align the flange with the shaft (Fig. 3.4, 7 or 8)and pick up disc (4) using the centring pin (3)

and securewiththe hexagon headscrews(5).

NOTICE

The discs cannot be centred using the screws

alone.

– Dependingon the function, insertandtighten

the spacer sleeves (6).

• The use of a spacer sleeve for a looseconnection can be seen in Fig. 3.5. In

this type of connection, the flange and

pick up disc are held together by the

screws with clearance. When using the

spacer sleeve for a secure connection,

turn the spacer sleeve.

• The use of a spacer sleeve for a secure

connection can be seen in Fig. 3.6. In

this type of connection, the flange and

pick up disc are held together by the

screws force with no clearance.

– Insert the shaft clamping set (2) in the pickup

disc (4).

– Fit the “shaft with crack” in the experimental

setup on one end of the short shaft using the

shaft clamping set.

NOTICE

At least two of the six hexagon headscrewsdistrib-

uted around the circumference must be securely

tightened with no clearance with spacer sleeves.

For safety and to minimise imbalance, all screws

must always be fitted.



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Fig. 3.5 Loose connectionwith spacer sleeve

Clearance

Fig. 3.6 Secure connectionwith spacer sleeve



3.3 Securing the flange connection on the shaft with the clamping set

To secure the pickup disc on a shaft with theclamping set, switch off the drive and carry out the

following steps:

– Slide the pickup disc with loosened clamping

set onto the shaft.

– Check that the clamping set is inserted flush

in the pickup disc.

– Tighten the inner hexagon (A), if necessary

holding the outer hexagon (B) steady whiledoing so. The thread of the inner hexagon

creates a cone (C) in the socket of the outer

hexagon. The shaft then twists with the

pickup disc.

When loosening the clamping set, hold the inner

hexagon (B) steady and unscrew the outer hexa-

gon. After overcoming the initial loosening torque,

continue turning until the resistance starts to

increaseagain.Turning further presses the clamp-

ing set out of the mount and it can then be

detached by hand.

3.4 Maintenance / care

ThePT500.11accessory set is maintenancefree.



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Fig. 3.7 Layout of a clamping set(shaft clamping set)

A B C

Pick up disc

Shaft

Fig. 3.8 Clamping set



4 Experiments

Theselectionof experimentspresentedhere is not

intended to be complete. Instead, they are

intended as a stimulus for own experiments. The

results shown are intended as a guide only.

Depending on the construction of the individual

components, experimental skills and environmen-

tal conditions, deviations may occur in the experi-

ments. Nevertheless, the laws can be clearly

demonstrated.

Generally, it is important tonote that the “shaft with

crack” experiments involve very sensitive effects.

Note the following:

– The shaft should run true and not knock.

When modifying the shaft with a crack,

ensure that the centring is retained.

– The shaft shouldnot be loadedwithany addi-

tional imbalances.

– The screws on the flange should be secured

finger tight, not with force.The gap that forms

at the flange for the “shaft with crack” should

be able to “breathe”.

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4.1 Simulation of “crack in shaft” with protruding shaft end

4.1.1 Purpose of the experiment

A shaft crack at the protruding end of the shaft is to

be simulated. The simulation will be carried out

using the short shaft with flange. The constant

radial load is created using the belt drive

(PT 500.14).

– For the experiment with no shaft crack , all

six screws should be fitted with spacer

sleeves to give a secure connection with noclearance (Fig. 3.1).

– For the experiment with a shaft crack , three

consecutive screws are fitted as a loose con-

nection.

The frequencyspectra for the twoexperimentsare

to be compared.

4.1.2 Required accessories

PT 500 Machinery Diagnostics System

PT 500.04 Computerised Vibration Analyser

PT 500.14 Belt Drive Kit

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4.1.3 Preparation and setup

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Fig. 4.1 Experimental setup for “shaft with crack with protruding shaft end”

1 Belt tensioner

2 Belt

3 Small belt pulley

4 Short shaft with flange5 Pick up disc

6 Shaft clamping set

7 Acceleration sensor

8 Bearing block

9 Short shaft

10 Reference sensor

11 Setting ring

12 Bearing block

13 Elastic claw coupling14 Drive unit

15 Magnetic clamp with steel plate

16 Reflective mark

17 Large belt pulley with bearing block

14 13 11 12 11 10 9 8 7 6 5 4 3 2 1

15 16 17

B e l t r u n n i n g

d i r e c t i o n u p w a r d s



Set up based on the diagram or the following

points:

– Fit the drive unit (14) on the clamping plate.

– Connect the drive unit to the control unit.

Connect thecontrol unit to thepowersupply.

– Fit the elastic claw coupling (13) to the drive

unit (see PT 500 section 3.6).

– Set up the short shaft (9) with two bearing

blocks (12 & 8) and setting rings in such a

way that the short shaft can be connected to

the drive unit with the elastic claw coupling

(13). Also, so that it can subsequently be

secured axially to the first bearing block.

– Align and secure the drive unit and bearing

blocks. To align on the clamping plate, slide

all components forward or back at right

angles to the grooves to minimise possible

lateral misalignment.

– Axially secure the shaft with setting rings to

the first bearing block.

– Secure the fully assembled unit made up of

the pickup disc (5) and short shaft with flange

(4) (Fig, 4.2) to the end of the shaft (9) with

the clamping set (6). For the “crack in a

shaft” experiment, the flangeshouldbe fitted

as described in section 3.

Shaft without crack

• Six hexagon head screws with spacer

sleeves as a secure connection (Fig.

3.6)

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Fig. 4.2 Pick up disc with clamping

set, short shaft with flangeand small belt pulley. Twosupporting screws (A)

A



Shaft with crack

• Four hexagon head screws with spacersleeves as a looseconnection (Fig.3.5)

• Two hexagon head screws with spacer

sleeves as a secure connection (Fig.

3.6)

– Secure the small belt pulley (3) to the end of

the short shaft with flange using the clamping

set.

– Assemble the belt drive with the bearing

block and belt tensioner from the PT 500.14

(belt drive), align andslightly tension thebelt.

The correct direction of rotation must be

ensured (Fig. 4.1). The belt tensioner must

be fitted on the unloadedside of the belt. The

belt tension (Fig. 4.3) can be adjusted using

the belt tensioner. For each belt drive, the

adjusting screw moves a tension roller into

the loose side of the belt from below until the

belt is slightly tensioned.

– Adjust the belt tension for the shaft with

crack.

Thebelt tension shouldbe increaseduntil the

feeler fits 0.4 mm into the gap between the

flange on the side opposite the supporting

screws. A measurement should also be car-riedout on the sideopposite the belt drive (on

the side where tensile stress is acting on the

shaft).

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Fig. 4.3 Belt tension adjusted usingfeeler, or alternatively withbelt pretension measuring

unit (tension approx. 70 N)

Belt tensionadjustment

Feeler 0.4 mm



Alternatively, thetensile stressof thebelt can

be measured using the belt pretension mea-suring unit (Fig. 4.5).

Procedure:

• Switch off the belt drive.

• Push the red lever on the belt preten-

sion measuring unit downwards so that

it indicates zero.

• With your index finger in the clip, slowly

press the measuring unit onto the beltfrom above, until you feel it click.

• Read the measured value of the red

pointer pushed upwards until it clicks.

(For details, refer to the manufacturer’s

instructions.)

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Fig. 4.4 Belt drive with tensioner

Fig. 4.5 Belt pretension measuring unit



– Screw the acceleration sensor (7) into the

horizontal tapped hole in the bearing blockclose to the shaft with crack, Fig. 4.1.

– Fit thereflective marker (16) for thereference

transducer (10) on the shaft or weight.

– Fit the referencesensorwith magneticclamp

(15) and steel plate on the clamping plate

and roughly align with the reflective mark.

– Connect the reference transducer and accel-

eration sensor to the measuring amplifier.– Connect the measuring amplifier to the PC

via the USB measurement box.

– Switch on the PC and start the pre-installed

PT 500.04 software.

– Connect the power supply for the measuring

amplifier. Switch on the measuring amplifier

on the front panel.

– Align the reference sensor with the reflectivemark.

– Check the switching behaviour of the refer-

ence sensor. The second LED directly on the

reference sensor may only trip once when

scanning the reflective mark.

– In the software, open the “Sensor” window.

Select the sensor and check the settings in

the “Calibration” menu.Sensor: Acceleration

Scale: 100 mV/g

Offset: 0.0

Factor: 1.0

– Gain on measuring amplifier: 10x.

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– Check thesignalstrength in theOscilloscope

window. To obtain optimum resolution whendigitising, select the largest possible gain

factor without exceeding the measuring

range. See also PT 500.04. section 3.3.

– In the software, open the “Frequency Spec-

trum” window and check the following set-

tings.

Channel A: Channel 1

Channel B: ReferenceScan Rate: 8 k/s

Scan Time: 4 Seconds

Of Means: 1

Mode: Velocity

Window Function: Uniform

Log Or Linear: Linear

Unit Of Magnitude: rms

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4.1.4 Performing the experiment

Recording the frequency spectrum

To show the difference in the comparison, the

“shaft without crack” is recorded first.

– Prepare the “shaft without crack ” for the

experimental process, i.e. fit all six screws

with spacer sleeves as a secure connection,

see section 3, Fig. 3.1.

• Check that allpartsarefittedsecurely.

• Close the protective hood.

• Switchonthe control unit for the motor.

• Set the direction of rotation (deter-

mined by the belt drive - the belt

tensionershouldacton thelooseside).

• Set the speed to zero.

• Switch on the motor.• Set the speed, e.g. to 2400 rpm.

• Plot the frequency spectrum.


• Switch off the motor.

– Prepare the “shaft with crack ” for the exper-

imental process, i.e. fit three adjacent screws

with spacer sleeves as a secure connection

and the other three as a loose connection,

see section 3.

NOTICE

For modifications, ensure that the centring is

retained. In other words, at least two screws

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must always secure the pickup disc and the

short shaft with flange.• Process: As described above for the

“shaft without crack ”.

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4.1.5 Evaluation

Secondorder vibrations in the frequency spectrum

are characteristic for the “shaft with crack”. These

are caused by the anisotropic rigidity of the shaft.

The shaft passes through the area of lower and

higher rigidity twice per revolution. This results in

vibrationswith doublethefrequency of thespeed.

First of all, the frequency spectrum for the “shaft

without crack” (six supporting screws) is dis-

played. Here, the fundamental vibration at 40 Hz

and the associated higher harmonics (f Dx

) can be

seen. Most of the other vibrations are caused by

thebelt drive,whichhas its fundamental frequency

at 8.8 Hz (measured). The shaft for the large belt

pulley rotates at half the speed f D/2

.

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Anisotropic:

Anisotropic rigidity is a directi-on-specific rigidity.

Fig. 4.6 Frequency spectrum of protruding shaft end without crack (six supporting screws)

n = 2400 rpm

40 Hz, f D = drive, f R = belt drive

f D1

f D2 f D3

f D4

f D5 f R1

f R2

f R3

f R4

f R5

f D/2 f R6

f R7

40.0

8.8

17.6

26.5

35.2

20.0

44.0

52.8

61.6

80.0 120.0

160.0

200.0



For the “shaft with crack”, the fundamental vibra-

tion (f D1

) with the higher order harmonics (f D2-D5

)

can also be seen. The amplitude of the fundamen-

tal vibration for the “shaft without crack” is greater

than in the “shaft with crack” setup.

The “shaft with crack” also clearly shows the char-

acteristic rise in the 2nd order vibration f D2

. This is

an indicator of the crack.

Calculated belt frequency:

f n U

LR

AR

R

60

2400

60

1979

9128 7

min mm

mmHz

1 ..

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Fig. 4.7 Frequency spectrum of protruding shaft end with crack (three supporting screws)

n = 2400 rpm

40 Hz

f D1

f D2

f D3

f D4

f D5

f R1

f R2

f R3

f R4

f R5

f R6

f R7 f D/2

40.0

8.8

17.6

17.6

26.4

35.2

44.0

52.8

61.680.0

120.0

158.9

200.0



4.2 Simulation of “crack in shaft” with elastic rotor

4.2.1 Purpose of the experiment

A shaft crack with an elastic rotor is simulated

using the long shaft with flange, the short shaft

(PT 500) and a weight (PT 500). For comparison,

the curves from the 1st and 2nd order response

analysis with and without a crack in the shaft are

plotted.

The shape of the response analysis curve shows

whether and in what speed range the amplitude ofthe 2nd order vibration speed rises sharply. Typi-

cal orbit curves can only be expected if this

happens.

The displacement sensors will then be used to plot

the orbit curves.

4.2.2 Required accessories

PT 500 Machinery Diagnostics System

PT 500.04 Computerised Vibration Analyser

PT 500.10 Elastic Shaft Kit

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4.2.3 Preparation and setup

Set up based on the diagram or the following

points:

– Fit the drive unit (12) on the clamping plate

(i.e. on the far left, so that there is space for

the remainder of the setup and the hood can

be closed).

– Connect the drive unit to the control unit.

Connect thecontrol unit to thepowersupply.

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Fig. 4.8 Experimental setup for “shaft with crack” simulation with elastic rotor

1 Acceleration sensor horizontal

2 Bearing block

3 Long shaft with flange

4 Pick up disc

5 Vertical displacement sensors6 Horizontal displacement sensors

7 Retain bearing

8 Short shaft with marking

9 Reference sensor

10 Bearing block

11 Elastic claw coupling

12 Drive unit

13 Magnetic clamp with steel plate

14 Setting ring

15 Reflective mark16 Weight with clamping set

17 Clamping set

18 Spacer sleeve

19 Hexagon head screws

12 11 10 9 8 7 6 5 4 3 2 1

13 14 14 15 16 17 18 19



– Fit the elastic claw coupling (11) on the drive

unit.

– Loosely fit a bearing block (10) on the clamp-

ing plate flush in front of the elastic claw cou-

pling.

– Slide the short shaft (8) through the bearing

of the first bearing block (10)on the drive unit.

Slide the setting rings (3) onto the shaft in

such a way that the shaft can subsequently

be fixed in place at the first bearing block.

– Slide the weight (16) with clamping set and

retain bearing (7) onto the short shaft and

loosely attach so that the fully assembled

“shaft with / without crack” unit can subse-

quently be fitted on the free end of the short

shaft.

– Secure the fully assembled unit (shaft with

crack, Fig. 4.9), consisting of the pickup disc

(4) and long shaft with flange (3) (asdescribed in section 3.3) on the free end of

the short shaft (8) with the clampingset (17).

– Fit the second bearing block (2) onto the free

end of the shaft with flange(3) (seeFig.4.8).

– Connect the short shaft (8) to the elastic claw

coupling (11).

– Align and secure the drive unit and bearing

blocks. To align on the clamping plate, slideall components forward or back at right

angles to the grooves to minimise possible

lateral misalignment.

– Axially secure the shaft with setting rings to

the first bearing block.

– Secure the weight (16) on the shaft with the

clamping set.

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Fig. 4.9 Fully assembled unit(shaft without crack)



– Screw the displacement sensors (5 and 6)

into the horizontal and vertical tapped holesin the retain bearing (2) in sucha way that the

transducersarenot in contact with theshaft.

– Fit thereflective marker (15) for thereference

transducer (9) on the shaft or weight.

– Secure the accelerationsensor (1) in the hor-

izontal tapped hole in the bearing block (2).

– Fit the reference sensor onto the clamping

plate with the magnetic clamp (13) and steelplate.

– Connect the reference sensor and accelera-

tion sensor to the measuring amplifier.

– Connect the measuring amplifier to the PC

via the USB measurement box.

– Switch on the PC and start the pre-installed

PT 500.04 software.

– Connect the power supply for the measuringamplifier. Switch on the measuring amplifier

on the front panel.

– Align the reference sensor with the reflective

mark.

– Check the switching behaviour of the refer-

ence sensor. The second LED directly on the

reference sensor may only trip once when

scanning the reflective mark.– Connect the displacements sensor on the

rear of the measuring amplifier.

Channel 1 - Horizontal = 0-180°

Channel 2 - Vertical = 90-270°

– For assembly and adjustment of the dis-

placement sensor, refer to PT 500.10, sec-

tion 3.2 and 3.3.

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– Open the “Sensor” and “Calibration” win-

dows in turn in the PT 500.04 software.Select the sensor and check the settings in

the “Calibration” menu.

For the experiments with acceleration

sensors:

Sensor: Acceleration

Scale: 100 mV/g

Offset: 0,0Factor: 1,0

– Check the signal strength in the “Oscillo-

scope” window. To obtain optimum resolu-

tion when digitising, select the largest possi-

ble gain factor without exceeding the mea-

suring range (see also PT 500.04, section

3.3). Selected gain on measuring amplifier

here: 10x

For the experiments with displacement

sensors:

Sensor: Displacement

Scale: 1.25 V/mm

Offset: 0,0

Factor: 1,0

For assembly and calibration of the displace-

mentsensors, refer to PT500.10, section 3.2

and 3.3.

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4.2.4 Performing the experiment

4.2.4.1 Plotting the 1st and 2nd order response analysis

– For example, prepare the “shaft without

crack” for the experimental process, i.e. fit all

six screws with spacer sleeves as a secure

connection. Refer to section 3 for details.

• Select the “Acceleration” under “Sen-

sor” in the menu in the PT 500.04 soft-

ware (check the configuration if neces-

sary).

• Open the “Tracking Analysis” window

from the menu in the PT 500.04 soft-

ware. Make the following settings in the

window:

Channel: Channel 1

Mode: Velocity

Graph Order A: 1

Graph Order B: 1

Scale: Auto




• Set the desired direction of rotation.


• Switch on the motor.

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Notice

The required printer must be set as thedefault printer before starting the pro-

gram.

• Either plot the response analysis chart

manuallyor continuouslyandautomati-

cally (selected here: Continuous).

• Start the measurement and slowly

increase the speed (from 0 rpm to3000 rpm), while observing the curve

plotted in the program window. The

measurement can be carried out con-

tinuously using the software or manu-

ally using individual values.

• To finish continuous measurement at

the end, click on the “Continuous” but-

ton.

• Print out the results on the default

printer.

• Switch off the motor.

– Prepare the “shaft with maximum crack” for

theexperimental process, i.e. fit twoadjacent

screws with spacer sleeves as a secure con-

nection and the other four as a loose connec-

tion. Refer to section 3 for details.

• Process: As described for the “shaft

without crack” in this section.

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4.2.4.2 Plotting the orbit curves

– Prepare the “shaft without crack” for the

experimental process, i.e. fit all six screws

with spacer sleeves as a secure connection.

Refer to section 3 for details.

• Select “Displacement” under “Sensor”

in the menu in the PT 500.04 software

(check the configuration if necessary).

• Open the “Orbit Analysis” window from

the menu in the PT 500.04 software.Make the following settings in the win-

dow:

Mode: Displacement

Order ---

Scale 1-10-100-1000

For a scale greater than 1, the value

read in the diagram has tobedivided by

the selected scaling value.




• Set the desired direction of rotation.


• Switch on the motor.• Set the required speed and plot the

orbit curve. The speeds selected

should be from the ranges in which the

2nd order amplitudes are at their maxi-

mum.

• Save or print the results.

• Set another speed and plot a new orbit

curve.

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– Prepare the “shaft with crack” for the experi-

mental process, i.e. fit two adjacent screwswith spacer sleeves as a secure connection

and the other four as a loose connection.

Refer to section 3 for details.

• Process: As described for the “shaft

without crack” on the previous page.

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4.2.5 Evaluation

For the elastic shaft, the occurrence of 2nd

order vibrations, such as those that occur on

the shaft witha crack for example,dependon

the load on the shaft. This changes with the

speed due to resonant vibrations. Therefore,

a measurement ata fixed speed may not pro-

vide any conclusions.

The response analysis over a wider fre-

quencyrange reliably identifies the2ndorder

vibrations sought.

The orbit curves are a further characteristic

indicator of a shaft with a crack. These

should meet where 2nd order vibrations

occur.

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Fig. 4.10 Response analysis for elastic rotor without crack, 1st and 2nd order. The 2nd order ampli-tudes (B) are smaller than and below the 1st order amplitudes (A) over the entire speedrange. (channel: 1; mode: Velocity)



This kind of response analysis can also be

carried out with a real rotor (e.g. power sta-

tion turbine) when a stoppage is due. It is

essential that comparative data for the

undamaged rotor is available.

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Fig. 4.11 Response analysis for elastic rotor with crack (three supporting screws), 1st and 2nd order.For the 2nd order (B), the increase in the amplitude can be clearly seen in the range1500 rpm to 2900 rpm.The 2nd order amplitude rises above the 1st order amplitude (A).



To indicate the increase in 2nd order vibra-

tions depending on the formation of a crack,several response analyses are superim-

posed in a single chart. Here, even small

cracks (five screws tight) show significant

variations from the shaft with no crack.

The progress of the crack formation can be

clearly discerned.

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Speed rpm

Fig. 4.12Fig. Plots for 1st order response analysis with different levels of cracks

A m p l i t u d e m m / s

Three supporting screws (1)

Four supporting screws (2)

Five supporting screws (3)

Six supporting screws (4)

1

2

4 3

1st order response analysis



Characteristics of the shaft with crack:

• Second order amplitude is greater than firstorder in certain ranges.

• The crack causes the 2nd order amplitude to

increase.

The progress of the crack formation can again be

clearly discerned here.

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Fig. 4.13 Plots for 2nd order response analysis with different levels ofcracks

A m p l i t u d e m m / s

2nd order response analysis

Speed rpm

1

23 4

Three supporting screws (1)

Four supporting screws (2)

Five supporting screws (3)

Six supporting screws (4)



Orbit curves enable the 2nd order vibration com-

ponents to be effectively identified. This method isideal in practice for rotors with floating bearings,

which are fitted with stationary orbit detection.

First, we plot the orbit curve for a “shaft with no

crack”. A more or less circular shape can be identi-

fied here.Allchartswere plotted using thesettings:

Mode: Travel

Scale: 10

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Fig. 4.14 Shaft without crack, n = 2040 rpm



If there isa crack, the 2nd order vibrations result in

loops in the orbit curve.Depending on the phasing,different shapes can be formed.

Characteristic orbit curves with distinctive 2nd

order vibration of a shaft with crack at different

speeds.

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Fig. 4.15 Shaft with crack, three supporting screws;

n = 728 rpm; hardly any 2nd order vibrations.

Fig. 4.16 Shaftwith crack, threesupporting screws;

n =1463rpm;small 2ndorder vibration component.



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n = 1785 rpm; large 2nd order vibrationcomponent.


n = 2226 rpm; large 2nd order vibrationcomponent.



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n = 2470 rpm; with 2nd order vibrationcomponent.


n = 2842 rpm; with no 2nd order vibrationcomponent.



5 Appendix

5.1 Technical data

Max. length: 250 mm

Flange diameter: 90 mm

Weight approx.: 5 kg

Flange hexagon head screws:

DIN 933-8.8 M8 x 20 mm

Maximum permitted bending torques:

Short shaft for the belt pulley

Max. permitted bending torque on the shaft:

15,9 Nm

i.e. max force vertical to the shaft with a lever

arm of l = 106 mm:

150 N

Long shaft for the weight

Max. permitted bending torque on the shaft:

3,9 Nm

With a lever arm of l = 220 mm, the maximum

force vertical to the shaft is:

15,5 N

Continuous operation at a critical bending speed

should be avoided. The critical bending speed

depends on the individual experimental setup.

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5.2 Symbols

Belt frequency (calculated):

f n U

LR

AR

R

60

2400

60

1979

9128 7

min mm

mmHz

1 ..

where

n : Drive shaft speed in rpm

LR

: Belt length (912 mm)

U AR

: Circumference of drive roller (197.9 mm)

60 : Min to s conversion factor

f R

: Belt frequency in Hz

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5.3 Setup suggestions / photos

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Fig. 5.1 “Shaft with crack” experiment with protruding shaft end

Fig. 5.2 “Shaft with crack” experiment with elastic rotor and retain bearing



5.4 Items supplied

1x PT 500.11Crack Detection in Rotating

Shaft Kit

1x PT 500.11Experiment instructions


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Documents

Crack Detection Kit