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Henry A. Zumbrun  |  05/19/2011

Henry A. Zumbrun’s picture

Bio

Measuring Torque

Uniform realization of torque measurement is needed in the United States

From the tightening of nuts on automobiles and airplanes to the connections on wind generators and other structures, the accurate measurement of torque may be essential for safety and the financial future of many companies in North America. Unfortunately, unlike other countries such as Germany, Mexico, or the United Kingdom (UK), the United States has lacked a traceable torque standard against which companies can verify their torque instruments. Companies wishing to verify torque measurements must send capital and equipment to foreign countries for calibration. Companies not following this practice are making torque measurements based on an assumed length and force, which has proven to be inaccurate. These imprecise measurements may be caused by the calibrated beam deflecting and the introduction of side loads, both of which are not accounted for when the beam and masses are calibrated.

In “A Study of the National Force Measurement System” published in 1975 by NIST, author Donald E. Marlow writes, “Torque has been identified as an element of the force measurement system which is probably inadequately served. Traceability of the measurements back to the basic standards of force and length is tenuous at best. In some cases the path is not presently visible.”

Further, a recent comparison study between several torque calibration laboratories, done by the American Society for Testing and Material (ASTM) Committee E28, Calibrations Subcommittee, revealed several laboratories were operating outside of their uncertainty budgets, proving that inaccuracies are occurring throughout North America in many of its support structures such as oil-drilling machinery, transportation vehicles, and common household items. If a traceability chain is not established, incorrect measurements will continue to occur, and the potential for product failures will continue to rise.

Inaccurate torque measurements may cost a company time and capital, not to mention reputation. When a bolt is over-torqued, it may break or cause failures to occur elsewhere, and extraction of the broken bolt in the assembly piece, or the failure elsewhere, will increase production costs. One potential solution might be to decrease the applied torque on this product to lessen the chances of over-torquing the bolts. However, under-torqued bolts pose different hazards, such as loosening over time and causing failure of an assembly. For example, there are more than 150 bolts in a car engine; if one is under-torqued, it may loosen and destroy the engine. If the bolts are under-torqued in an airplane assembly and become loose in midflight, catastrophic damages and loss of life may occur.

In an attempt to eliminate this scenario, some airplane manufacturers have opted to over-design airplanes by increasing the torque specifications to account for the potential error in torque measurement. Not only does this over-design process raise production costs, it also increases development time, forcing current aircraft to fly beyond their estimated life cycle. In turn, that can cause mechanical failure, flight cancellations, job eliminations, and the use of older, less fuel-efficient planes. In the most extreme cases, such scenarios could set the stage for bankruptcy.

Accurate torque measurement may help benefit North American companies by reducing product cost, improving product quality, and most important, improve safety. Reduction of costs associated with over-design, along with accurate torque measurements, will help North America compete with Europe and other nations that are not forced to over-design, reengineer, or increase product costs due to failures from inaccurate torque measurements.

To address this issue, companies based in the United States should use an accredited calibration lab that has torque standards traceable to a national metrology institution such as Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute (NMI) of Germany; the National Physical Laboratory (NPL), which is UK’s NMI; or the Centro Nacional de Metrologia (CENAM) in Mexico. Be sure to ask your lab which NMI they use and ask to see verification of the claim.

Another option is to use an accredited U.S. calibration lab that has a verified torque standard in-house. For example, the torque standard at Morehouse Instrument Co. was acquired through NPL, and the standard has had intercomparisons done with other national metrology institutions for torque measurement.

I can’t place enough emphasis on intercomparisons. Generating an accurate torque measurement requires more than just a traceable length and force measurement. Traceable force and length measurements will not test for parasitic effects, misalignment, beam deflection, worn drives, and bending forces on an unsupported beam. The effect of a misalignment error of just 0.1 percent may triple the combined system uncertainty. Intercomparisons, either those done with an in-house standard such as that at Morehouse or with the standards at national metrology institutions, ensure that all standards are within tolerance. All labs should be able to show you the results of those tests. Without proper verification through an intercomparison, a calibration laboratory will not know its true measurement uncertainty.

So when selecting a lab to perform torque calibration, be sure the lab is accredited by A2LA or NVLAP, and ask if it has traceability to an NMI that has national standards for torque such as PTB or CENAM. Doing so will ensure that your torque measurements are exactly what you expect them to be.

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Henry A. Zumbrun’s picture

Henry A. Zumbrun

Comments

Torque Not Traceable through NIST

Is Henry saying that NIST is not providing traceable standards for torque calibrations?


 


 

NIST does not provide Torque Calibration Services

N.I.S.T traceability through length and force does not mean that the lab performing the torque calibration is doing it accurately.  

NIST cannot close the loop for verification of a torque system.  To do this you have to use another NMI such as PTB,CENAM or now that we(Morehouse) have the equipment from a NMI (NPL)  we can verify a torque transducer that a company can use to verify their torque equipment is accurate.  

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