Part 4 modalbump tseting 02 2004

Free-Free Impact Modal Test of Shaft & Roll ResultsModal Analysis test results of Feed Roll & Shaft... Typically this would not be considered a good test setup, however; the test shoul

Free-Free Impact Modal Test of Shaft & Roll Results

Modal Analysis test results of Feed Roll & Shaft Most bending is indicated in the shaft as

X

Y Z

1 2 3 4 5 6 7

1 2

Figure 2: Rotor model, third mode 10,862 CPM Bending occurring in the shaft but not

in the roll, which is primarily a very stiff hollow cylinder The bolted joint was modeled as solid.

Figure 3: Frequency response function (FRF) of the Driving Point, Bottom Plot

Coherence of the measurement is shown in the top plot The data becomes noisy

above 500Hz because the instrumented hammer tip was not hard enough to input

adequate energy into the rotor to excite the structure in this frequency region

The FRF of the Driving Point shows

frequency of the shaft-roll at 187 Hz ~ 11,220 cpm

The rotor model calculated to 181 Hz

~ 10,860 cpm or 96.8%

Shaft & Roll Installed Impact Modal Test.

as indicated by the red dots.

Note that the response will be affected by any

looseness in the bearing assembles This may

cause non-linearity in the response and

reduce coherence Typically this would not be

considered a good test setup, however; the

test should be useful in identifying potential

problems such as loose bolts, traverse shaft

crack, and looseness in the bearing assembly

Step Two

Installed Shaft & Roll Impact Modal Test.

the bearings.

the roll.

Installed Shaft & Roll Impact Modal Test.

the bearings.

the roll.

The rotor model predicted the shaft-roll first flexure mode with the shaft

supported by bearings (Brg Stiffness Estimated) at 10,208 cpm ~ 170 Hz.

The measured FRF showed a fairly narrow range of resonance for

the 1st flexure mode In the FRF plotted, the resonance measured

204.38 Hz ~ 12,263 CPM

!

Draw Roll-Shaft Natural Freq in Hz.

Results from one machine with 23 shaft-rolls All of the rotors “rang”

except the #2 Disassembly of the #2 roll showed loose bolts The rolls

natural frequency were in the 200 Hz range with exception of roll 21.

Roll Response Measured By Im pact Modal Test

Shaft-Rolls Lbf/Mil (Dynam ic Stiffness)

The FRF from Roll #2 did not show a natural frequency Typically, modal analysis requires a linear structure but in this case we are looking for a non- linear structure with poor resonance qualities

Inspection showed a loose bolted joint

Step Three

Bump Test of Installed Shaft-Roll Assembly To Identify Defective Rotors

Compare impact data from triggered spectrum & time waveform to modal data (FRF) tests for location of roll natural frequencies

Monitor natural frequencies during operation for shifting

(A shift lower would indicate a change in the stiffness of some components in the system such as broken bolts, shaft traverse crack, bearing assembly looseness.) Follow-up with modal testing on as

needed basis

Step Three

Bump Test of Installed Shaft-Roll Assembly To Identify Defective Rotors

Bump testing did not prove to be useful in detecting shafts with loose

or broken bolts

After several tests, it was discontinued.

+

Example of time waveform and spectrum from a roll The FFT shows 12.46 mils P-P run out The time waveform shows much less amplitude P-P due

to roll off of the accelerometer The FFT is multiplied by a curve to

compensate for the roll off and yields the correct amplitude

This roll measured 37.6 mils runout Note the time waveform clearly shows asymmetrical stiffness indicating a loose joint

Frame Roll ID WO # Status Last Read Bearings Last Read Radial Runout Last Read Stiffness Worked

11/16/01 Update: Feedroll and Drawroll Condition

Spreadsheets with color coding were used for reporting the

condition of Roll-Shaft Bearings and Bolted Joint indicated condition.

Roll run out tolerance measured at the end of the roll:

> 30 Mils Schedule Maintenance

for inspection of bolts

10 Mil to 30 Mil Monitor More

Frequently

1 Modal testing was successful in identifying shaft-roll rotors with lower

stiffness, i.e., did not ring when impacted as likely candidate of loose

bolting, failed bolting, or shaft with a developing crack.

Detection Process:

2 The roll rider data was useful in identifying rolls with loose bolting

Limits were established as follows:

1 <10 Mill acceptable

2 > 15

3 > 30 Mill PP Schedule inspection of the bolts.

3 Vibration data

A Collected using the long stinger in the end of the roll cover plate

was successful in detecting defective bearings.

4 Physical inspection of bolting, shaft and roll mating faces, condition of

tapped holes, bearing fits, etc

B Data from permanent accelerometers installed on the bearing

housings were not as useful in detecting bearing defects as data collected on the end of the rolls.

1 The roll and shaft mating surface finish, flatness, hole angularity and

depth of full thread tolerances were enforced by inspection at the

repair shops

Correction Process:

2 The bolt length was reduced ½” to 6”.

3 Thread lubricant was changed to Loc-Tite Graphite which has a very

low K factor.

4 Bearing heating procedures were modified to use a temperature

controlled heater for all bearing installations.

1 Use of Modal testing to detect loose rolls was effective But, testing had

to be scheduled when a line was down We were not aware that this test method had been used previously.

2 The roll rider block with accelerometer is a modification of a well know

test method using a wood stick also called a beaver tail (sold by IRD)

After some modification of the initial block design and data collection methods, the test proved successful in identifying rolls with excessive run out

3 Vendor shop visits were very helpful to improve understanding of

the repair processes and identify problem areas

4 Modification of the repair processes, tightening of tolerances, initiation

of inspections of repaired Shafts & Rolls with documentation were

successful in improving the quality of Shaft & Roll repairs performed by repair shops.

5 Modification of Bearing and Roll maintenance installation practices

proved successful in reducing the number of improperly installed shaft assemblies.

roll-Roll Shafts With Bolt Failures

were not identified.

No data available before

1997

Bearing Failures in 2003 reduced 79% compared to 2002

Lowest number of failures documented over 10 years.