“The effects of uncontrolled misalignment when calibrating Torque Transducers at NMISA and the impact this has on tracea

1. “The effects of uncontrolled misalignment when calibrating Torque Transducers at NMISA and the impact this has on traceability”Eddie Tarnow

2. Introduction • The current NMISA Torque Rigs were designed to calibrate square drive transducers exclusively due to specific customer demand • The tolerances of the square drives, machined into the torque rig beams shafts, were very tight • Tolerances on the torque transducers received for calibration however, vary widely • Adapter plates and square drive adapters are required to mount the wide variety of torque transducers into the rigs • This contributes to potential uncontrolled misalignment

3. Introduction (2) Typical UUT mounted in Torque Rig.

4. International Best Practice • Most NMIs around the world make use of Torque Rigs capable of primarily accommodating smooth shaft transducers • “ETP” hydraulic couplings are used to couple the UUT smooth shafts to the rig hollow shafts • UUTs with square drives are then coupled to these rigs through the combined use of precision machined adapters, hydraulic couplings and misalignment compensating couplings • This significantly reduces the effects of misalignment • Unfortunately, the current rigs at NMISA cannot accommodate hydraulic couplings or misalignment compensating couplings

5. International Best Practice (2) UUT with smooth shafts mounted using hydraulicand misalignment compensating couplings(Photograph courtesy of PTB).

6. Misalignment in NMISA torque rigs • Play in sliding rear plate • A sliding rear plate, (which provides the counter-torque), is required in the torque rig to accommodate UUTs of varying lengths • Although manufacturing tolerances were tight, there is still play between the sliding rear plate and the torque rig base plate • As a result, at 1000 Nm the centre of the female square drive can move away from the principle torque axis by as much as 1,2 mm

7. Misalignment in NMISA torque rigs (2) 1000 Nm Torque Rig rear plate misalignmentwith torque applied.

8. Misalignment in NMISA torque rigs (3) • Poorly manufactured square drive adapters • Male square drive adapters can be machined in the NMISA mechanical workshop to tight tolerances • Female square drive adapters however, require special manufacturing techniques such as “spark erosion” which are not available in the NMISA mechanical workshop • To save costs and reduce lead-time, commercially available “female-to-male” square drive adapters are used by NMISA • Typically, they are not machined but are forged and consequently their quality is poor • The male and female drives are not concentric to the principle axis through which the torque is applied

9. Misalignment in NMISA torque rigs (4) Square drive adapter offsets due to poor manufacturing.

10. Misalignment in NMISA torque rigs (5) • Poorly manufactured square drive adapters cont. • Selecting square drive adapters with the smallest misalignment when purchasing them would minimise these effects • Unfortunately, these measurements are not easily performed at the supplier • The misalignment dimensions of the adapters currently in use at NMISA are:-

11. Adapter Adapter Female Male Drive Drive Vertical Horizontal Dimension Dimension Error Error (mm) (mm) (mm) (mm) 19,1 (3/4") 12,7 (1/2") 0,146 -0,034 12,7 (1/2") 19,1 (3/4") -0,047 -0,071 25,4 (1") 19,1 (3/4") -0,025 0,008 19,1 (3/4") 25,4 (1") -0,023 0,074 38,1 (1 1/2") 25,4 (1") 0,044 -0,046 38,1 (1 1/2") 25,4 (1") 0,252 0,032 Misalignment in NMISA torque rigs (6) Square drive adapter misalignment dimensions.

12. Misalignment in NMISA torque rigs (7) • Mechanical play in UUT mounting and adapter-plate bolt holes • In most cases, the bolt holes for mounting the UUT, as well as those in any adapter plates required, are drilled at “nominal” size and at “nominal” positions • This gives rise to significant play when mounting the UUT in the torque rig • It is standard practice to loosely assemble the UUT and adapter plates and then tighten them once mounted in the rig • However practically, this results in all components of the mechanical torque transferring chain being mounted at the “bottom” of their respective holes • Thus the UUT is in the “lowest” position for the 0° Ref. rotational position and the “highest” position for the 180° rotational position and therefore not concentric to the torque principle axis

13. Misalignment in NMISA torque rigs (8) • Parasitic loading as a result of uncontrolled misalignment • Uncontrolled misalignment causes what is known as “parasitic loading” • Thus the strain gauges, mounted on the UUT shaft to measure the torque, (twisting), movement of the shaft around the “Z-axis”, also “sense” some of the bending moments associated with the parasitic loading of the UUT shaft in the “X-axis” and “Y-axis”

14. Misalignment in NMISA torque rigs (9) Parasitic loading effects due to uncontrolled misalignment.

15. International torque calibration methods • Currently, international best practice is described in EURAMET/cg-14/v.01 • The procedures in use at NMISA for the calibration of torque transducers, are based on this method except for the estimation of the measurement uncertainty • The EURAMET document considers both “repeatability” as well as “reproducibility” whereas the NMISA procedures consider “reproducibility” only • The reason for this is that the author is of the opinion that considering both is “double accounting” and • Experience has also shown that in many instances, “repeatability”, when calculated as per EURAMET/cg-14/v.01, is often zero (Run 1A minus Run 1 B)

16. International calibration methods (2) • According to the definitions in the VIM:- • “Repeatability” is the closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under the same conditions. • “Reproducibility” is the closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under changing conditions.

17. International calibration methods (3) • Since NMISA reports the “Mean Indicated Torque” for a specific “Applied Torque”, the uncertainty contribution for “variability” would normally be the Experimental Standard Deviation of the Mean (ESDM). • However, measurements are performed in four rotational positions, viz, 0°; 90°; 180° and 270° which constitute changing conditions • Therefore the uncertainty contribution for “variability” is obtained from the Experimental Standard Deviation (ESD, normally designated Sx), rather than the ESDM for runs 0°A, 90°, 180° and 270° • This uncertainty contributor includes variability due to both “repeatability” and “reproducibility”

18. Impact of misalignment on measurement uncertainty Inter-laboratory comparison results.

19. Impact of misalignment on measurement uncertainty (2) • In this example, the larger measurement uncertainty for NMISA, when compared to the other laboratory, was solely due to the “reproducibility” uncertainty contributor • This can be directly ascribed to the effects of the uncontrolled misalignment in the NMISA rig as both laboratories performed the measurements according to the same procedure • However, the other laboratory used an “un-supported beam” whereas the NMISA beam was a “supported beam” design

20. Impact of misalignment on measurement uncertainty (3) Measurement results for a typical 1000 Nm transducer calibration.

21. Impact of misalignment on measurement uncertainty (4) • Experience has shown that uncontrolled misalignment affects different torque transducers in different ways • Two same nominal range torque transducers, calibrated in the same rig, using the same method, can result in vastly different expanded uncertainties, solely due to the different “reproducibility” uncertainty contributions

22. Impact of misalignment on measurement uncertainty (5) Measurement results for the calibration of 1000 Nmtorque transducer “A”.

23. Impact of misalignment on measurement uncertainty (6) Measurement results for the calibration of 1000 Nmtorque transducer “B”.

24. Effect of large measurement uncertainty on disseminated measurement traceability • Most of the SANAS accredited Torque Laboratories are required to support accuracies on torque wrenches of at least ± 4 % (ISO 6789) • However modern direct reading torque wrenches with digital readouts have accuracies of ± 1 % • This necessitates importing measurement traceability, by means of Reference Standard Torque Transducers at measurement uncertainties less than this • In the two examples above, the Reference Standard Torque Transducers can only achieve ± 1 % and ± 2,1 % respectively, at the 100 Nm point, after applying corrections for the error • Thus, calibration of these torque transducers at NMISA may no longer be able to provide adequate traceability

25. Proposed new NMISA 5 kNm Torque Rig design • The current 1 kNm torque rig at NMISA has the following deficiencies as a result of its outdated design:- • The beam bends under loading resulting in corrections being required • Misalignment compensating couplings cannot be accommodated resulting in large measurement uncertainties • The range is limited to 1 kNm which can no longer support the requirement from industry which is now beyond 5 kNm • The weights have to be loaded by hand - which for a 1000 Nm transducer is equivalent to loading 4250 kg bags of cement

26. Proposed new NMISA 5 kNm Torque Rig design (2) • The current 1 kNm torque rig at NMISA has the following deficiencies as a result of its outdated design cont.:- • Hysteresis cannot be controlled due to manual loading • The rig has inadequate distance between centres and an insufficient “swing over” radius, which limit the size of transducer which can be calibrated • For these reasons, NMISA is considering the design and manufacture of a new 5 kNm torque rig

27. Proposed new NMISA 5 kNm Torque Rig design (3) • The most important design considerations of the new rig are as follows:- • Extension of the range to 5 kNm • Increased distance between centres • Increased swing-over radius • Stronger beam to reduce bending • Accommodation of smooth shaft transducers, flange type transducers and square drive transducers • Mechanically controlled loading of the weight stack • Holding of the beam stationary during loading of the weights to reduce hysteresis • A “counter-rotating” gearbox • Misalignment compensating devices • Possible future automation in mind

28. Proposed new NMISA 5 kNm Torque Rig design (4) • Several design options have been considered, including mounting the torque shaft vertically, using a jockey weight to allow infinitely variable torque values, using a mechanised/automated deadweight stack system to load a misalignment compensated, hydraulically coupled, smooth shaft accommodating system. Etc. • Currently, a mechanised/automated deadweight stack system appears to be the most feasible since it will share many of the design aspects already finalised for the design and manufacture of a 5 kN deadweight force machine.

29. Proposed new NMISA 5 kNm Torque Rig design (5) Torque Beam with interchangeable deadweight stack.

30. Proposed new NMISA 5 kNm Torque Rig design (6) Deadweight stack detail.

31. Proposed new NMISA 5 kNm Torque Rig design (7) Torque transducer mounting details – smooth shaft UUT transducer shown.

32. Proposed new NMISA 5 kNm Torque Rig design (8) Hydraulic Coupling with round-shaft-to-square-drive adapter.

33. Conclusions • The current 1000 Nm torque rig at NMISA is reaching the point where it will no longer be able to provide adequate measurement traceability to SA industry • This is largely as a result of it only being able to calibrate square drive torque transducers up to 1000 Nm at measurement uncertainties between ± 0,3 Nm and± 1 Nm which arise as a result of uncontrolled misalignment • A new rig design is proposed which will address these problems

34. Thank-you for listening • Contact details:- • Eddie TarnowE-mail: eptarnow@nmisa.org • Tel: +27 12 841-3138