Vibration Measurement CapabilityJ. Denis16 August 2004

IntroductionXXXXX and XXXXX engines are required to meet vibration levels that are historically unheard of. It has become apparent that current vibration measurement precision, or lack thereof, represents a significant but un-quantified proportion of the specification and production values. This means that there is a broad “gray area” surrounding the specification limit where pass/fail decisions cannot be made with any certainty. Engines in this situation will either be tested repeatedly until a pass value is obtained or subjected to corrective action. Either activity incurs significant extra cost.

Any reduction in the breadth of this “gray area” of uncertainty will reduce associated costs. The initial goal must be to precisely quantify the range of uncertainty. From such information, efforts could be justified and directed to improve or reduce that range of uncertainty.

Option “A” Goals: Quantify range of measurement uncertainty only. Provide essential information to manage risk and costs associated with engine test as well as impetus for improvement.

Objective: Use only one engine with repeated testing. Test the engine in each of 3 test cells, repeating the test scheme 5 times, with random assignment, over an appropriate span of time. The minimum number of tests is 15.

Advantages: Useful for estimating vibration measurement uncertainty limits. Minimum interruption, cost and elapsed time.

Disadvantages: Contribution of some factors must be determined separately; loss of information regarding engine-cell-operator interactions. These interactions are known to exist but should be controllable within the program.

Description: A controlled program which includes statistical analysis to determine the range of uncertainty. This program is to be structured in such a way so as to isolate and identify specific areas where improvements would be most beneficial and appropriate.

General Conditions: Testing for this program requires only enough time in the test cell to perform the vibration surveys, the first test values can be taken during production or development testing. Please refer to Execution Rules herein as well as Excel file with test flow chart.

Choose 2 specific vibration survey types to run. The tests could be chosen to provide an opportunity to examine repeatability of vibration / hysteresis effects.

Choose an engine from the production population which has clocked a number of hours of running time and is known to be relatively stable.

Option “B” Goals: Quantify range of measurement uncertainty separately and proportionally with respect to engine population variance. As well as meeting the goals of Option “A”,

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Vibration Measurement CapabilityJ. Denis16 August 2004

provide a indication of the measurement system’s value as a tool for product and process improvement.

Objective: Gauge R&R study using several (3) engines. Test each of these engines in each of 3 test cells, repeating the test scheme 5 times with random assignment over an appropriate span of time. The minimum number of tests is 45.

*NOTE: Repetition of each test 5 times vs. 3 times, gives nearly twice (1.023/0.577) the precision in the estimation of uncertainty.

Advantages: Evaluate measurement repeatability as a distinct value and as a proportion of the engine population variance. Estimate engine population variance and evaluate interactions between engine, cell and operator.

Disadvantages: Greater interruption, cost and elapsed time.

Description: A controlled program which includes statistical analysis to determine the range of uncertainty. This program is to be structured in such a way so as to isolate and identify specific areas where improvements would be most beneficial and appropriate.

General Conditions: Testing for this program requires only enough time in the test cell to perform the vibration surveys, the first test values can be taken during production or development testing. Please refer to Execution Rules herein as well as Excel file with test flow chart.

Choose 2 specific vibration survey types to run. The tests could be chosen to provide an opportunity to examine repeatability of vibration / hysteresis effects.

Choose 5 engines from the production population which have clocked a number of hours of running time and are known to be relatively stable.

General Instructions The test plan is essentially the same regardless of which option is chosen and differs only in the number of repetitions. Analysis will, of course, differ between the options.

*NOTE: Potentially, many factors which contribute to measurement uncertainty can be evaluated without involving an engine test, thus the utility of completing the list of influential factors and their values.

1. Identify program sponsor and coordinator

2. Assemble a team to brainstorm and detail the list of factors which potentially affect the repeatability of vibration measurement. Suggested team makeup: Representatives from PCE, instrumentation, test cell operations, etc.

3. Categorise the aforementioned factors into blocks and block elements

4. Where reliable, high quality data is available, evaluate the contributory variance of individual blocks and elements.

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Vibration Measurement CapabilityJ. Denis16 August 2004

5. Determine whether the factors are controllable or uncontrollable (noise) for purposes of executing these particular tests (controls which may not normally be applied in production).

6. Write a test plan based on the list which identifies or specifies:

a. The expectations and requirements applied to personnel in execution of the work instructions

b. The failure modes – where the testing program can go wrong , be subverted or have the intent of the program or quality of data compromised

c. Reaction plans in the event of particular failure modes and events

d. Which factors to control and how they are to be controlled

e. Which noise factors are to be observed and recorded (create appropriate form for recording observations throughout the program)

f. The randomisation plan or testing schedule.

7. Inform and Train personnel as appropriate and execute the written instruction.

General Rules for Execution of Test Plan Instruct operators not to deviate from the plan. Allow variations to occur as they

would naturally occur.

No attempts should be made to “improve” the results – this could introduce an unacceptable bias in spite of the best of intentions.

Changes, adjustments or repairs to items that appear on the list of factors are NOT PERMITTED to be done to any TEST CELL or ENGINE without prior notice to the coordinator and team. Changes affecting any other items MUST be communicated to the team and recorded on the observation forms.

One set of vibration transducers is to be used for either option, with transducer calibration tests interleaved between engine tests. Installation of the transducers must be consistent in every way.

Runs should NOT be repeated and averaged, nor should the “best” results be recorded in favour of others.

If, within a single engine setup, several surveys of the same type are performed consecutively, the results of each survey must be recorded individually and the fact recorded.

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Vibration Measurement CapabilityJ. Denis16 August 2004

Table 1.

Example of factors contributing to measurement precision/repeatability

Between Blocks

Block within block group

category factor variance value

Cell to Cell Cell cell-cell BB between block varianceCell hardware WB within block group varianceCell hardware C cabling differencesCell hardware N connection resistanceCell hardware N connection errorsCell hardware C signal conditioning box differencesCell hardware C analysis box & software variationCell hardware C transmission of cabling/hosesCell crew WB within block group varianceCell crew C method variationCell crew N operating precisionCell crew N human factorsCell setup WB within block group varianceCell setup N offset of engine to exhaust funnelCell setup N tightness of rigging fastenersCell setup C slack in cabling/hosesCell setup C alignment of componentsCell setup C inlet attachment & support variances

Engine to Engine Engineengine-engine BB between block variance

Engine equipment WB within block group varianceEngine equipment C transducer differencesEngine equipment C inlet changesEngine equipment C strongback differencesEngine equipment N case stiffnessEngine build WB within block group varianceEngine build C build effectsEngine inherent WB within block group varianceEngine inherent N aeroacoustic differencesEngine inherent N rotor o-o-b conditionEngine inherent N resonant frequencyEngine inherent N accessory vibe levels

Test to Test Test test-test BB between block varianceTest survey type WB within block group varianceTest survey type C fast vs slowTest survey type C accel vs decelTest survey type C HRT vs other typeTest environment WB within block group varianceTest environment N cell ambient tempTest environment N engine tempTest environment N humidityTest environment N ambient windsTest environment N passing trucks, air traffic, earthquakes

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Vibration Measurement CapabilityJ. Denis16 August 2004

Figure 1.

Example of Measurement R&R process flow chart

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Prepare list, procedure &

data collection

Observ e & record all

settings, v alues & status

of noise factors

Select Option

Select engine

lottery draw to assign test cell

& crew

setup engine in cell

Perform & record vibe surveys

remove engine & store safely

Control factors

w hich impact

v ibration

Identify results, copy and

communicate to program

coordinator

consult records to

ensure consistent

setup for nex t repeat

repe

at 9

tim

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