Fundementals of Torque Calibration

FUNDEMENTALS OF TORQUE CALIBRATION

1

Lesson Objectives Introduction

Definition and Examples of Torque

Conversion Formulas

Torque Calibration Traceability Standard

Types of Torque Standards

ASTM E2428 BS7882

Accuracy and Precision

Performance and Uncertainty

Potential Measurement Errors

What is involved in building a Primary Torque Standard

SECTION 2 IN CLASS DEMONSTRATION ONLY (NOT ON WEB)

Torque Wrench types

Proper Handling techniques

Torque Wrench Demonstration

2

Company History 1920s Morehouse and the U.S. Bureau of Standards started to design

and refine force calibration products (Proving Rings) for the purpose of generating an accurate force for Brinell Hardness Testing.

3

Pictured above: Morehouse Brinell Proving Ring S/N 14 Calibrated by U.S. Bureau

of Standards test # 47197 May 24, 1926 (86 years ago!)

Morehouse Proving Ring S/N 14 Calibrated in 1926 and the last calibration we have on record is July 25, 1984. In 58 years there was a shift of .012% at capacity.

4

Company History

1930s The Morehouse Proving Ring was refined and used to calibrate Material Testing Machines.

1950s Morehouse developed products for commercial industry including, Force Gauges, Morehouse Universal Calibrating Machines and Morehouse Dead Weight Primary Standards for calibration of load cells, proving rings etc..,

5

Company History

2004 Morehouse becomes the first accredited commercial calibration laboratory to offer dead weight primary standards calibrations accurate to 0.002% of applied force up to 120,000 LBF

2009 Morehouse expands force calibration range offering ASTM E74 calibrations up to 2,250,000 LBF in compression and 1,200,000 LBF in tension.

6

Company History 2010 - Morehouse finishes construction of new torque calibration

laboratory. This calibration laboratory features a primary torque calibration standard accurate to 0.002% of applied torque. This standard was acquired from the National Physical Laboratory in England which is a National Metrology Institute.

2011 Morehouse becomes A2LA Accredited for Torque Calibration

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Calibration Lab Pictures

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General Information

9


Henry Zumbrun

Morehouse Instrument Company

1742 Sixth Ave

York, PA 17403

PH: 717-843-0081 web: www.mhforce.com

Sales: [email protected]

10

Morehouse


William Lane, P.E.

Morehouse Instrument Company

1742 Sixth Ave

York, PA 17403

PH: 717-843-0081 web: www.mhforce.com

Technical: [email protected]

11

Morehouse

Contents Torque

Definition of Torque

Examples of Torque

Conversion Formulas

Torque Traceability Standard

Calibration Traceability


ASTM E2428

BS7882



Uncertainty Distributions

Potential Measurement Errors 12

Contents (continued) In Class Demonstration

Morehouse Torque Calibration Laboratory

Conceptual Design

Air Bearings

Lever and Reaction Beam

Boron Tapes

Couplings


13

The Importance of Torque Control

The object of a threaded fastener is to clamp parts together with a tension greater than the external forces tending to separate them.

When the bolt is torqued properly it remains under constant stress and is immune from fatigue.

14


If the torque is not applied properly and the tension on the bolt torque is too low, varying loads will act upon the bolt and it will fail.

15

The Importance of Torque Control Honda Recalling 50,000 Vehicles

http://www.manufacturing.net July 25,2011

TOKYO (Kyodo) -- Honda Motor Co. said Monday it will recall a combined 50,122 units in the Stream minivan, the Civic sedan and the Crossroad sport utility vehicle to repair their bolts that fix the water pump pulley to the engine, free of charge.

In a report filed with the Ministry of Land, Infrastructure, Transport and Tourism, Honda said the engines of the vehicles, made between July 2008 and July 2010, may stall due to the defect.

The automaker said it will also implement a recall for the models in overseas markets, including Latin America and Europe, where a total of 150,000 units were marketed.

The ministry said the bolts may lose tension or fracture, causing the drive belt which runs both the alternator and the water pump to circulate coolant water to come off the pulley.

16


If the tension is too high, the tightening process may cause bolt failure.

Pictured Above: Metal snap from Jeff Nihels dragster, apparently the bolts on the left exhaust manifold were over-torqued bolts then failed, manifold popped off and 4000bhp of exhaust gas launches the car in the air at over 200mph!

17


Imagine if one of the one hundred and fifty plus car engine bolts is under-torqued, it loosens overtime and eventually destroys the engine. What if the bolts are under-torqued in an airplane assembly and become loose in mid-flight?

Fastener reliability depends on controlling the tightening torque.

Other engineering factors such as fastener material, design,

pitch & surface finish may also influence the tightening torque. 18

Torque Is derived and traceable to SI units of length, mass, and time.

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

19

Torque = Force x Length

CIPM/BIPM defines 1N as the force required to accelerate one kg to one meter per second per second in a vacuum.

Most torque systems consist of an arm or disc of a known length and a set of weights.

20


Local Gravity - Determination of gravity must be done at the site performing the calibration and the elevation of each weight should be considered. (Varies by up to .2% across the U.S.)

Material Density - Knowing the density of the material from which the weights were manufactured.

Buoyancy the Archimedes principle applies, ie. air pressure under the weights causes an upwards force. This reduces the effective force generated by the weights and therefore the mass must be increased to allow for this. (When used in air the correction is approximately 0.015%)

21


Torque, also called moment or moment of force is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.

Torque (BS 7882 definition)- product of tangential force and length applied about a known center of rotation

22


Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

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Examples of Torque

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Examples of Torque (cont)

25


Example 1: Distance = 1m. Force = 2000 N, Torque = 2000 N-m

Example 2: Distance = 2m. Force = 1000 N, Torque = 2000 N-m

Example 3: Distance = 1 ft. Force = 1000 LBF, Torque = 1000 FT-lbf.

Example 3: Distance = 5 ft. Force = 200 LBF, Torque = 1000 FT-lbf.

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Force Conversion Factors newton (N) to LBF = MULTIPLY BY .2248089431 newton (N) to KGF = MULTIPLY BY .1019716212978

newton (N) to ozf = MULTIPLY BY 3.59694309

lbf to newton (N) = MULTIPLY BY 4.4482216152605

lbf to KGF = MULTIPLY BY .45359237

lbf to ozf = MULTIPLY BY .0625

kgf to lbf = MULTIPLY BY 2.20462262

kgf to newton (N) = MULTIPLY BY 9.80665

Kgf to ozf = MULTIPLY BY 35.27396

ozf to newton (N) = MULTIPLY BY 0.2780139

ozf to lbf = MULTIPLY BY 16

Ozf to kgf = MULTIPLY BY 0.0283495231

27

Length Conversion Factors

METER (m) to FEET (ft) = MULTIPLY BY 3.2808399

METER (m) to INCHES (in) = MULTIPLY BY 39.3700787

FEET (ft) to METERS (m) = MULTIPLY BY 0.3048

FEET (ft) to INCHES (in) = MULTIPLY BY 0.0833333333

INCHES (in) to FEET (ft)= MULTIPLY BY 12

INCHES (in) to METERS (m) = MULTIPLY BY 0.0254

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Torque Conversion Formulas

Force 1N = .2248089431 lbf Length 1m = 3.280839895 ft

1 N-m = .2248089431 x 3.280839895 = .7375621489 FT-lbf

Force 1N = .2248089431 lbf Length 1m = 39.3700787 in

1 N-m = .2248089431 x 39.3700787= 8.50745782 in-lbf

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Torque Conversion Formulas

If I need to apply 100in-ozf and have a 100mm Calibration Disc. How much weight do I need to apply?

Convert 100mm to m

1000mm = 1 m

100mm/1000 = .1m

Convert .1m to in = .1 * 39.3700787 = 3.93700787

100in-ozf = 3.93700787 in * Force

100 / 3.93700787 = approx 25.4 ozf

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Calibration Traceability Guidlines Check the website of accrediting bodies for the accredited laboratories

individual scope of accreditation

www.a2la.org

ts.nist.gov/standards/acceditation/index.cfm

www.nist.gov/pml/nvlap/

www.l-a.b.com

www.isaonline.org

www.aclasscorp.com

www.pjlabs.com

The 1st page of any ISO 17025 is useless after it has been determined that the laboratory is accredited.

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Calibration Traceability Guidelines

It is the scope of accreditation that determines the laboratorys capability.

The cert should state the best measurement uncertainty for different ranges. On this cert the best measurement uncertainty is 0.0044% for torque calibrations from 20 2000 N-m

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Torque Traceability

The Torque Machine at Morehouse belonged to a NMI (National Physical Laboratory)

This machine was verified in international CIPM test to be accurate to 0.002 % of applied torque

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Morehouse Primary Torque Standard

In 2009 Morehouse purchased the Primary Torque Standard from the National Physical Laboratory (NPL) in England and began construction of a new Torque Calibration Laboratory. Construction of the Laboratory was completed in March 2010.

On completion of construction the Primary Torque Standard was shipped to Morehouse and NPL sent a team of engineers and scientists to commission the machine.

This Primary Torque Standard was used and registered with BIPM as the national standard for the United Kingdom.

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Morehouse Primary Torque Standard

The machine was used in a recent CIPM (International Committee for Weights & Measures) international measurement comparison and demonstrated a world leading measurement uncertainty.

It is the goal of Morehouse to use this machine to promote US Commerce by dissemination of accurate torque measurements.

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CIPM Comparison

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Lever deadweight This consists of a set of calibrated masses that act on a calibrated lever arm. The system can be directly applied to the transducer in the case of an unsupported calibration beam.

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Types of Torque Standards (cont)

In the ideal case the lever arm will be connected to a bearing to eliminate bending from the weights and masses and to minimize friction. This type of system is used for static torque calibration.

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Lever deadweight typically tend to be chain loaded

Some consist of multiple weight stacks with various weights or multiple weight stacks of the same size. 39


Lever dead weight systems are typically used with two different types of torque arms.

Single ended

Dual ended / Radius ended

Torque is generated by the application of a known force at a known radius from the center of rotation of the torque transducer.

Radius ended beams are typically designed with a +/- 8 degree usable arc within which the calibration accuracy is unaffected.

Single ended beams do not have a constant overturning moment as the dual radius arm would have and may have additional error as compared with Radius ended beams.

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Lever deadweight with pneumatic or chain loaded weight stack(s). - This system eliminates shock loading associated with manual lifting. This type of system is used for static torque calibration and offers the lowest uncertainties.

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Reference torque transducer Torque can be applied using a motor or hydraulics with the torque controlled by means of a calibrated reference torque transducer in a feedback loop.

Other similar designs may take traceability from calibrated load cells measuring the reaction force at the end of the lever arm.

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Types of Torque Standards (cont)

This type of system can be used to measure static torque but has the additional advantage of being suitable for continuous torque calibration, whereby the applied torque is applied over a much shorter time.

The disadvantage of these systems is that the uncertainty of applied torque will be much higher because the system is dependent on the prior calibration of the reference.

43

Uncertainty of Torque Standards

Regardless of the torque facility to be used, it is important to evaluate the uncertainty of the system. This should include contributions from all influencing parameters (e.g. mass, length, alignment, and environmental factors).

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ASTM E2428 Torque Calibration

ASTM E2428 Definitions

Primary torque standardsa dead weight force applied through a lever arm or wheel, with a calibrated length or radius of a known uncertainty, that is traceable to national standards. This type of standard is needed to assign Class AA loading ranges.

Secondary torque standardan instrument or mechanism, that has been calibrated by a comparison with a primary torque standard(s). This instrument will have a Class AA loading range and may be used to calibrate other instrumentation and assign Class A loading ranges to other instrumentation.

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ASTM E2428 Selection of Calibration Torque Values

At least 10 torque values should be applied to provide an adequate and unbiased sample of the full range of deviations. These values should be selected and distributed at least one calibration torque for every 10% interval. If the transducer is to be used under one tenth of capacity an eleventh point of no less than 400 x the resolution for Class A loading range and 1667 x the resolution for Class AA should be included.

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ASTM E2428 Number of Calibration Torque Values

A total of at least 30 torque applications per mode, clockwise or counter clockwise, is required for a calibration and, of these, at least 10 must be at different torque values. Apply each torque value at least twice during the calibration in both the clockwise and counter clockwise direction, as applies.

Examples:

On a flange 3 runs of 10 or 11 points should be applied

On a square drive 4 runs of 10 or 11 points should be applied

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ASTM E2428 Number of Calibration Torque Values

48

T distribution = sample vs mean, Z = population

30 +points reduces standard measurement error

ASTM E2428 Procedural Order In Calibration

After setting up the transducer and equipment, the maximum torque to be calibrated should be applied to the instrument at least twice. Morehouse recommends applying this torque value 3-4 times to achieve stability in zero torque indication.

After the pre-loading, apply torque values in sequential steps as uniformly as possible without inducing shock or vibration. (If you are manually lifting weights of a large capacity it is not acceptable to drop the weights onto the pan) Morehouse has developed dead weight machines to reduce the error associated with handling large weights. It is recommended that no more than five incremental torque values be applied without a return to zero.

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ASTM E2428 Procedural Order In Calibration

After data has been recorded for the first calibration run it is necessary to rotate the torque cell before repeating the next series of torque values.

If you are using a torque cell with a square drive you should perform up to 4 rotations (40 to 44 total points) to fully characterize the cell. - This is not required by ASTM E2428 but each orientation on a square drive will yield different outputs. Otherwise, the torque transducer should be calibrated in three different positions in each mode it is to be used (clockwise, anti clockwise).

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ASTM E2428 Calibration Data

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BS 7882 Torque Calibration

Some BS 7882 Definitions

Calibration torque torque with traceability derived from national standards of mass, length and time, and of specified uncertainty of measurement, which can be applied to the torque measuring device.

Lower limit of calibrationlower value of torque at which a torque measuring device of a given class can be calibrated.

Reference standard equipment used to generate or to measure the reference torque applied to the torque measuring device that is being calibrated

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BS7882 tests for the following

Relative error of indication - mean deflection for a given value of increasing torque minus the corresponding value of applied torque.

Relative error of interpolation difference between the value of the mean deflection for a given value of increasing torque and the corresponding calculated value of deflection for the given torque, obtained from a mathematically fitted curve.

Relative repeatability closeness of the agreement between the results of successive measurements from the same applied torque, carried out under changed conditions of measurement

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Relative reproducibility - closeness of agreement between the results of successive measurements from the same applied torque, carried out under changed conditions of measurement

Relative residual deflection maximum residual deflection obtained from all the series of torques

Relative reversibility difference between the deflection obtained from the last given torque series applied in an increasing mode and the deflection obtained from the same given torque applied in a decreasing mode.

54


Residual deflection algebraic difference between the indicator readings before and after the application of a single series of torques

Resolution the smallest discernable measurement interval on the torque measuring device indicator

Unlike ASTM E2428 several European standards which include DIN 51309 (German Standard) and BS7882 (European Standard) classify torque instruments by class ranges.

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BS 7882 Torque Calibration Uncertainty of calibration torques - Values for the maximum permissible uncertainty of the calibration torques applied for the determination of different classifications of the torque measuring devices.

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BS 7882 Calibration Procedure Number of calibration orientations For classes 0.05 and .1 the torque

device shall be calibrated in 3 positions each rotated 120 degrees or in 4 positions each rotated 90 degrees about the measurement axis. For all other classes, the torque device should be calibrated at a minimum of 2 different positions.

Warm up - it is recommended to allow the system to be energized for at least 15 minutes prior to calibration

Preloading procedure - preload should be applied in succession a minimum of 3 times and should be maintained for 1 to 1.5 minutes.

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BS 7882 Calibration Procedure

Select a series of at least 5 approximately equally spaced increasing values of torque from 20% to 100% of the maximum applied torque. If the calibration is required to be made below 20% then torque steps of 10%,5% and 2% of the maximum torque may be used.

After the preloading procedure apply two series of increasing torques without changing the mounting position (readings should be taken 30 seconds after the torque is applied).

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BS 7882 Calibration Procedure

After the first 2 runs have been taken for determination of relative repeatability, remount the torque device 120 degrees or 90 degrees for square drives and repeat incremental forces. The instrument will be remounted and increasing torques will be applied 1 or 2 additional times depending on the drive.

Note: If relative reversibility is required, a single series of decreasing values shall be applied at the end of a series of increasing torques

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BS 7882 Calibration Data

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ASTM E2428 VS BS7882 Uncertainty Analysis

BS7882 uncertainty analysis includes the standard uncertainty associated with the calibration torque and tests for reproducibility, repeatability, residual deflection and reversibility.

ASTME2428 uncertainty analysis tests for reproducibility with rotation.

61


It is a common mistake to assume that an accurate device is precise or that a precise device is accurate.

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Accuracy

accuracy is not the same as uncertainty!

I Can have an accurate measurement with a large uncertainty.

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Accuracy Accuracy is a measure of bias (estimate of a systematic measurement

error)

Accuracy: Closeness of agreement between a measured quantity value and a true quantity value of a measurand.

Accuracy does not tell you about the quality of the instrument.

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Accuracy Example Example: A torque cell was calibrated against a primary torque

standard and at 500.0 N-m the recorded output on the indicator was 500.5 N-m. This cell was determined to be accurate to +.1% of full scale or + .5 N-m.

Note: from this example the resolution of the instrument may be .1 N-m or .5 N-m and although the resolution may be the same as the accuracy in this example they are different.

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Accuracy Example

66

A picture in which we hope the accuracy of the time

remaining is wrong

Resolution

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Resolution - the smallest amount of input signal change

that the instrument can detect reliably

Resolution - is the ability of the measurement system to

detect and faithfully indicate small changes in the

characteristic of the measurement result.

Precision

Precision is a measure of spread

Precision refers to the repeatability of measurement.

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Precision Example

Example: 500 N-m was applied 3 times using a primary torque standard to a torque cell and the torque cell indicators recorded output was 480.01 N-m, 479.99 N-m, 480.01 N-m.

Conclusion:

The instrument is precise to +/- .01 N-m when 500 N-m is applied.

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A torque cell that may be accurate in one orientation may not repeat when rotated.

A torque cell may repeat when a torque is applied, but may not read close to the desired engineering units value.

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Uncertainty

a measurement is useless unless there is some way of estimating the associated uncertainty

good analyses should include uncertainty estimates

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Uncertainty

Statistical Uncertainty is defined as the estimated amount or percentage by which an observed or calculated value may differ from the true value.

In Metrology, two common types of uncertainty evaluation are Type A and Type B.

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Performance & Uncertainty Type A Uncertainty -

method of evaluation of uncertainty by the statistical analysis of series of observations

Repeatability condition of measurement, Inter-mediate precision condition of measurement, Reproducibility condition of measurement

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Performance & Uncertainty

ASTM E2428 calibration tests for the reproducibility condition of measurement and is an example of Type A Uncertainty

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Performance & Uncertainty Type A Example

A series of measurements are taken to determine the uncertainty of the measurement. (Standard Deviation)

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Performance & Uncertainty Type B Uncertainty - method of evaluation of uncertainty by means

other than the statistical analysis of series of observations.

Type B uncertainty - Evaluation based on information associated with a quantity value of a certified reference material, - obtained from a calibration certificate or manufactures specifications, obtained from the accuracy class of a verified measuring instrument, obtained from limits deduced from a test or experiment.

Examples include torque cell temperature effect, drift, resolution, etc..,

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Performance & Uncertainty Type B evaluation method:

The method of evaluation of uncertainty of measurement by means other than statistical analysis of a series of observations

Examples:

Based on specification

History of parameter

Other knowledge or test of the process parameter

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Performance & Uncertainty Type B Example

The temperature effect on torque cell output is +/- .004 % per degrees Celsius

The specification of the torque arm is +/-0.00006 inches

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The next example will be dealing with two types of Uncertainty Distributions.

Normal and Rectangular

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Normal Distribution

Normal distribution Normal distribution is one way to evaluate uncertainty contributors so that they can be quantified and budgeted for. Normal Distribution helps understand the magnitude of different uncertainty factors and understand what is important.

The normal distribution is used when there is a better probability of finding values closer to the mean value than further away from it, and one is comfortable in estimating the width of the variation by estimating a certain number of standard deviations.

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Rectangular Distribution

Rectangular distribution is the most conservative distribution. The manufacturer has an idea of the variation limits, but little idea as to the distribution of the uncertainty contributors between these limits.

It is often used when information is derived from calibration certificates and manufacturers specifications.

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Performance & Uncertainty for Calibration performed by Morehouse

COMPANYTORQUE CELL MANUFACTURER TORQUE CELL S/N

CAPACITY 1000 N-m

ASTM E2428 Uncertainty for K=2 0.022 N-m

THE LOWEST FORCE AT WHICH THE SECONDARY STANDARD WILL BE USED 100 N-m

PRIMARY TORQUECALIBRATION STANDARD UNCERTAINTY K=1

PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY K=1 (IF APPLICABLE)

CAL DATE

CALCULATED VALUES

TORQUE CELL UNCERTAINTY IN % FOR FULL SCALE K=2 0.00220%


TORQUE CELL UNCERTAINTY IN % FOR LOWEST FORCE APPLIED K =2 0.02200%


Uncertainty Description Uncertainty Distribution DivisorStandard

UncertaintySquared

ASTM E2428 Uncertainty % at the lowest calibration torque to be used 0.01100% normal 1 1.10E-04 1.21E-08

1.10E-04 1.21E-08


UncertaintySquared

PRIMARY TORQUE CALIBRATION STANDARD UNCERTAINTY 0.001% rectangular 1.732 5.77E-06 3.33E-11

PRIMARY ELECTRICAL CALIBRATION STANDARD UNCERTAINTY (IF APPLICABLE) 0.000% rectangular 1.732 0.00E+00 0.00E+00

STABILITY OF THE SECONDARY TORQUE STANDARD OVER TIME 0.005% rectangular 1.732 2.89E-05 8.33E-10

MISALIGNMENT ERROR 0.000% rectangular 1.732 0.00E+00 0.00E+00

DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.000% rectangular 1.732 0.00E+00 0.00E+00

TEMPERATURE ERROR +/- FROM CELL SPEC SHEET 0.0040% rectangular 1.732 2.31E-05 5.33E-10

3.74E-05 1.40E-09

FOR K= 1

UC= SQUARE ROOT OF TOTAL COMBINED TYPE A AND B 0.01162%

FOR K=2

U = K * UC (UNCERTAINTY % AT THE LOWEST FORCE TO BE APPLIED) 0.023%

0.023 N-m

UC AT CAPACITY * 2 ( (UNCERTAINTY % AT INSTRUMENT CAPACITY) 0.008%

0.078 N-m

Type A Uncertainty %

Combined Type A Uncertainty

Type B Uncertainty %

Combined Type B Uncertainty

SAMPLE

0.001%

10/27/2010

MOREHOUSE

Type A and B uncertainty analysis MOREHOUSE

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Performance & Uncertainty for calibration performed with a calibrated beam and masses

COMPANY

TORQUE CELL MANUFACTURER

TORQUE CELL S/N

CAPACITY 1000 N-m





CAL DATE

CALCULATED VALUES






UncertaintySquared


1.10E-04 1.21E-08


UncertaintySquared




MISALIGNMENT ERROR 0.020% rectangular 1.732 1.15E-04 1.33E-08

DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.020% rectangular 1.732 1.15E-04 1.33E-08


1.77E-04 3.14E-08

FOR K= 1


FOR K=2


0.042 N-m


0.355 N-m





SAMPLE

0.010%

10/27/2010

MOREHOUSE


84


The combined uncertainty of a good torque transducer should be approximately 5 times better when calibration is performed using the NPL/Morehouse Primary Torque Standard as compared with a conventional Calibration Beam and Masses.

Torque transducers on the Market today are capable of uncertainties much lower than that attainable by primary standards as defined by ASTM E2428.

85

Torque Calibration

86

2+2 Joke

So the teacher assigns to Ada, Bob, Charles and Danna to go home and

figure out what is 2 + 2.

Ada, the daughter of a mathematician, asks her dad. He responds: "Well, 1

+ 1 = 2. 2+ 1 = 3. 3 + 1= 4, but it can be rewritten as 2 + 2, so 2 + 2 = 4

Bob asks his mom, who is an engineer. She takes out her HP calculator, punches

in RPN the appropiate keys, and announces: "It is 4.000000000000

Charles asks his dad, the phycisist, and he responds: "Well, it is about pi on a

zeroth order calculation

Finally, Danna ask his dad, who is an accountant: "Dad, how much is 2 + 2?" And

he responds: "How much do you want it to be?"

Torque Calibration

87

There are several ways to measure an instrument and arrive at the same

answer, however if you are not verifying your equipment against known

standards then the answer can pretty much be whatever you want it to be.

Problems With Torque Measurement In The United States

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Accurate Torque Measurements require traceability back

to a National Metrology Institute (NMI) or Morehouse

Without verification of the torque system by inter comparison with a National Metrology Institute or Morehouse some of the Type B uncertainty error in the torque calibration system will be unknown.

Problems with Torque Measurement In The United States

Generating an accurate torque is more complex than having a N.I.S.T traceable calibrated beam and N.I.S.T traceable calibrated masses.

A comparison study done by the American Society for Testing and Material ASTM Committee E28, Calibrations Sub-committee between several torque calibration laboratories, revealed several laboratories were operating outside of their uncertainty budgets, proving that inaccuracies are occurring throughout North America.

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Problems with Torque Measurement In The United States

PICTURED ABOVE A NORBAR TORQUE WRENCH CALIBRATOR. THIS SYSTEM SUPPORTS THE TORQUE WRENCH AS TO NOT INTRODUCE ADDITIONAL HUMAN ERROR.

IT IS IMPORTANT THAT THE TRANSDUCERS IN THIS SYSTEM ARE CALIBRATED BY A LABORATORY FOLLOWING THE PROPER CALIBRATION PROCEDURES TO ENSURE THAT THE TRANSFER FROM THE TRANSDUCER TO THE TORQUE WRENCH IS ACCURATE.

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Potential Measurement Errors

Using mass weights instead of force weights

Introduction of bending and parasitic effects

Misalignment

Calibrated beam deflection and temperature

Drives on the beam or transducer being worn

Bending forces on an unsupported beam are unavoidable

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Using Mass Instead of Force Weights

It is very important that the gravitational value for the Laboratory is established. The effect of not doing this could be a variation in the force produced by the weight of perhaps 0.25% or more of reading. It is therefore strongly recommended that you establish the local value of gravity (g) for your Laboratory and use weights that have been calibrated at that gravitational constant.

The ideal solution is to have the gravity measured on site by the national geological survey agency.

92

Weight Handling

The physical lifting and un lifting of weights onto a platform creates additional error. This error is due to the un controlled application of the weight onto the pan. Handling weights and placing them onto a pan may cause vibration error, tape misalignment error and introduce a pendulum or swinging effect.

To minimize this error it is recommended to use a fixed chain loaded or pneumatic weight stack in which the weights can be applied as uniformly as possible.

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Introduction of Bending and Parasitic effects

The influence of bending and parasitic effects on a transducer will be dependent on the particular design of that transducer. However the reproducibility of the device, when calibrated in different orientations, can often be a good indicator of the influences of bending.

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Calibrated beam deflection and temperature

The material the beam is machined from will determine the appropriate amount of weight that can be supported by the beam. Using more weight than the beam is designed for may cause significant deflection.

The properties of the material of the beam will expand and contract changing the length of the beam.

The temperature effect can be minimized by maintaining the room temperature of the calibration laboratory at the same temperature the beam was calibrated. Any temperature corrections can be made based on the temperature coefficient of the material the beam.

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Misalignment

Alignment - practically it is very difficult to perfectly align a transducer in a torque machine. Usually there will be some misalignment due to the mismatch of the two axes that will give a radial, angular, or axial misalignment or any combination of the three. One way to minimize this is through the use of flexible coupling elements.

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Misalignment (Cont.)

What are the actual effects of a misalignment error?

What does this do to the combined uncertainty?

97

Combined Type A and B Uncertainty Example assuming 1% misalignment error. COMPANY

TORQUE CELL MANUFACTURER

TORQUE CELL S/N

CAPACITY 1000 N-m





CAL DATE

CALCULATED VALUES






UncertaintySquared


1.10E-04 1.21E-08


UncertaintySquared




MISALIGNMENT ERROR 1.000% rectangular 1.732 5.77E-03 3.33E-05

DISSEMINATION ERROR (FOR CALIBRATION LABORATORIES) 0.020% rectangular 1.732 1.15E-04 1.33E-08


5.78E-03 3.34E-05

FOR K= 1


FOR K=2


1.155 N-m


11.550 N-m





SAMPLE

0.010%

10/27/2010

MOREHOUSE


98

Misalignment Error

A misalignment error of .1% will more than triple the combined uncertainty.

A misalignment error of 1% will raise the combined calibration uncertainty from .355 N-m at full scale to 11.55 N-m.

99

Worn drives

Transducer clamped in a mounting bench for an unsupported beam calibration. The fit of the squares and rigidity of the mounting plate are important factors. (note: the above picture is an example of a transducer properly mounted)

If these fittings are worn the proper torque will not be generated. It is not uncommon for this error to exceed 10% of the desired torque value.

100

Bending Forces on an unsupported beam

In this case it is important to be aware of the influence that bending can have on the measurement result and, where possible, to be able to quantify this. The mounting of the transducer is very important. The transducer should be held as rigidly as possible on the mounting plate. The fit of the squares should be as good as possible to minimize any slack

101


A Comparison Between Supported and Unsupported Beams For Use in Static Torque Calibrations by Andy Robinson and Barry Pratt revealed that the weight of unsupported calibration beams in relation to the capacity of the transducer is critical.

102

Figure 8 shows:

Transducer A (2kN-m transducer calibrated with 1.5 kN-m unsupported beam)

Transducer B (200N-m transducer calibrated with .5 kN-m unsupported beam)

Transducer A (1kN-m transducer calibrated with 1.5 kN-m unsupported beam)


Transducer B 200 N-m transducer was then calibrated with the 1.5kN arm.

Figure 10:

500 N-m beam weighs 16.6 kg

1.5 kN-m beam weighs 23.4 kg

103


104

Conclusion:

The weight of the unsupported calibration beam in relation to the capacity of the load cell has a significant effect on the transducer output.

The worst affected region is 0 20% where the weight of the beam is most significant in relation to the torque being measured.

Under certain conditions an unsupported beam achieved results within 0.02% of the reference value from the 20 100% range. (Transducer C which was a 2kN-m shaft type)


Bearings are often used to support the beam and eliminate bending effects due to the weight of the lever beam and weights. However depending on its quality the bearing itself may introduce errors such as friction and the performance of the bearing still requires evaluation

105

Verification by inter comparisons with a National Metrology Institute or Morehouse

Verification of the torque system will allow the laboratory to diagnose and correct problems.

Verification will give confidence in torque measurement.

Verification can be done with equipment owned by Morehouse and rented to the end user or it can be done with an existing torque transducer using the Primary Torque Standard at Morehouse Instrument Company or at a NMI.

106

In Class Demonstration

Primary torque standardsa dead weight force applied through a lever arm or wheel, with a calibrated length or radius of a known uncertainty, that is traceable to national standards.

107


108

Morehouse Single Ended Beam Known Length of .7169658 meters, Weight = 23.03955N (We will tare this out for our example)

Morehouse Force Calibrated Weights = 12.54244 LBF or 55.79155754 N


109

We will be comparing the single end beam and mass calibration against a torque cell

calibrated by our primary standard

machine

The uncertainty of the torque cell is .019 N-m

At 40 N-m U = .0475% of applied At 80 N-m U = .0238% of applied At 120 N-m U= .0158% of applied

In Class Demonstration Expectations

At 40 N-m we expect the cell to read .7169658m x 55.79155754 = 40.000 N-m within +/- .019 N-m



Per ASTM E2428 our system should have an uncertainty better than .012% of applied when combining the uncertainty for force and length. In actuality the uncertainty on this system is .018%. We know length within +/- .005 in and weight within .003% (this includes gravity correction for this area) for K=2

We would expect the results to be accurate to +/- .019 N-m +/- .03% of applied for k=2. We must then reduce everything to K=1 and sum the squares

110


For us to have confidence in this system we should have agreement as follows for k=2 For K =1 Torque cell = .0095 N-m , Single Beam system = . 01% We must divide .0095 N-m by the applied torque and square it. Then we must add .01 ^ 2 and take the square root. Finally multiply by 2 and we should get the results below.

111

At 40 N-m results should be better than .052% of applied = +/- .021 N-m




Target 40 N-m within +/- .021 N-m

Actual = (this must be filled in from test)





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Conclusion: Having not known the actual results from the test we are expecting that the uncertainty of this system is much greater than .02% of applied torque.

Without verification of the system the end user would be performing torque calibrations while claiming their system uncertainty is better than .02% of applied and they would have the proper calibration certificates for length and force to back up this claim.

Verification or proficiency testing against a torque standard from a NMI or lab that has verified their own equipment with a torque cell calibrated by a NMI should be done before any torque system is put into use and prior to any accreditation process.

113

What is so special about the NPL machine located at Morehouse Instrument Company?

The next series of slides deal with the detail and work that went into our torque calibration laboratory

The slides also include a detailed analysis of the NPL machine and the great lengths that were taken to achieve a torque uncertainty of 0.002% of applied torque

114

Morehouse Torque Calibration Laboratory

115

Construction of the Torque Calibration Laboratory.

A 68 deep 12x16 pit was dug inside our current building. The pit was reinforced and a center plinth constructed.

The plinth features a heated floor to control the temperature. NPL did not have heated flooring.

Since we knew some of the problems NPL had with airflow we designed our room to keep any air from blowing directly on the machine. From preliminary testing we have been able to maintain temperature at 20 degrees Celsius +/- 0.2 Degrees Celsius.

116

Installation of NPLs Primary Torque Standard

The Machine arrived at Morehouse

Mannie Panesar and Michael Harrison from NPL arrived to start the installation.

Andy Robinson and Andy Knott (pictured bottom right) from NPL arrived to verify the machine was within 0.002% of applied torque.

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Target Uncertainty

NPLs Torque machine target specification

118

Torque range 2 Nm 2 kNm

Deadweight loading

Target uncertainty 1 x 10-5

Clockwise and anti-clockwise

Incremental and decremental

Working envelope 1 m3

Conceptual design

2 kNm lever deadweight machine

Realised uncertainty 0.002 %

Vertical design pure torque generated via identical weightstacks located at either end of the lever beam

Twin beam carbon fibre lever arm mounted on a central air bearing

119

Conceptual design

Vertical torque axis necessitates a change in force direction

Vertical force generated by the weightstacks is converted to a horizontal force connecting to the lever arm

Achieved using boron fibre tapes and a pulley air bearing

120

Conceptual design

Lever beam can be rotated independently of the torque transducer

Pulley system is castored to allow rotation about its vertical axis

The same beam and weightstacks used for clockwise and anticlockwise measurements

121

Conceptual design

Ranges

Primary: 20 Nm to 2 kNm in 30 steps

Secondary: 1 Nm to 100 Nm in 28 steps

Other features

Continuous torque calibration

Ability to introduce and measure misalignments

Asymmetric torque mode

122

Air bearings

Criteria

123

Requirement to provide both radial and axial

stiffness to support the predicted forces.

Minimal friction target (0.1 to 0.5) Nm

(Friction is a combination of boundary and viscous

shear of air film plus turbine torque)

Air bearings

Central pivot air bearing

Double thrust and air bearing system

Upper bearing enables rotation of the lever beam

Lower bearing supports the transducer

Incorporates a drag cup damper to compensate for the natural frequency of the system

124

Air bearings

Boundary and viscous shear influences are time dependant and so can be ignored

Turbine torque measured with calibrated micro air jet

220 Nm

This is constant and is tared so target uncertainty is not exceeded

Castor pulley bearing system provides horizontal and vertical rotation

Designed to carry a load of 1 kN transmitted by suspension tape at supply pressure of 345 kPa (50 psi)

125

The lever and reaction beam

Designed by NPL materials team

Criteria

126

Lightweight construction to

reduce inertia

Very low deflection under load

Thermal expansion of beam

considered critical


Two parallel 2 m long highmodulus carbon fibre tubes

Stainless steel end fittings and central boss

Calculated coefficient of thermal expansion = 0.3 x 10-6 K-1

(Steel is 16 x 10-6 K-1 m/m)

Finite element analysis used to provide confidence in design

127


Beam has nominal 1.000 000 m length measured to a relative uncertainty of 3.3 x 10-6

Length is monitored using a comparator

With an uncertainty of 2.0 x 10-6

data show that length has changed by less than 3 m over a 1 m length

Overall length uncertainty contribution - 5.3 x 10-6

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Boron fibre tapes

Criteria

129

No deformation under load

Lightweight

Flexible

Boron fibre tapes

Epoxy pre-impregnated tape of 150 m boron fibre tapes

Chemical vapour deposition of a boron fibre gas on to fine tungsten wire

Sandwiched and bonded between a titanium end connector

5 mm tape used for torques up to 100 Nm

20 mm tape used for torques up to 2 kNm

130

Boron fibre tapes

Tape is required to change orientation

20 mm tape has 90 joint made from carbon and glass fibre

5 mm tape has a pre-formed twist

131

Control Software

Graphic user interface Visual Basic 6.0

Several working modes

Displays real-time machine outputs

Complex motion profiles weightstack platforms slow as each weight is added or removed

The lever beam is re-datumed after each weight increment

Built In safety features

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Couplings

Adaptability - capable of incorporating most shaft, flange, and square drive transducers

Based around a hydraulic friction joint

Incorporates Rexnord flexible couplings eliminating radial, axial, and angular misalignments

133


Relative expanded uncertainty 2 x 10-5

Traceable to base SI units of mass, length, and time

Major contributors

Force & alignment of force

Length

Alignment of boron tapes

Validated by NPL through intercomparison work with other labs

Validated by NPL at Morehouse through intercomparison with work done by NPL before the machine was dismantled and then after the machine was reassembled at Morehouse. 134

Installation of NPLs Primary Torque Standard

Comparison graph showing agreement within 0.003 % of applied torque with data taken before decommissioning the machine at NPL and after installation at Morehouse

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0.000

0.005

0.010

0.015

0.020

0.000 671 48

0.000 671 50

0.000 671 52

0.000 671 54

0.000 671 56

0.000 671 58

0.000 671 60

0.000 671 62

0.000 671 64

0.000 671 66

0.000 671 68

0.000 671 70

0 400 800 1,200 1,600 2,000

Incremental Difference

/ % Cal Factor / mV/V per N.m

Torque / N.m

Raute 2kNm NPL

MIC

Difference

Conclusion

Commissioning completed in December 2005 at NPL

Commissioned May 2010 at Morehouse Instrument Company

Achieved target of realizing uncertainties at world leading levels via an novel and innovative design

Dissemination of the unit of torque expected to have a significant impact in US industry

Flexibility of the machine will provide a powerful research tool

136

This concludes this section on

torque calibration

Any Questions?

137

Fundementals of Torque Calibration

Documents

Transcript of Fundementals of Torque Calibration