QMT Features: July 2007
Talk the Torque
Torque measurement is a crucial ingredient in delivering well-designed and consistently manufactured industrial and consumer rotary devices. David Mercer of Mecmesin looks into the key considerations in quantifying rotational forces.


Mecmesin's Orbis torque tester manually controlled, system


Torque is applied to an automotive stalk control as a quality control measure in production

Those with an interest in engineering are probably familiar with the concept of torque measurement, perhaps to determine the power of an engine, or maybe in relation to tightening and releasing bolts. But what about the smaller low-level torques that we, as consumers, regularly encounter? Consider a typical day; wake up, head to the bathroom, turn the lock on the door and twist the shower tap. Go downstairs, open the fridge, unscrew the milk and juice containers for breakfast. Leave the house, turn a key in the car’s ignition, and, if it’s raining, twist the windscreen wiper control stalk. At work, take calls on a flip-open mobile phone. Return home, sit back in a favourite swivel chair, crank up the stereo volume knob, dim the light switch and twist the lid off an ice-cold beer.

Sound familiar? We encounter dozens of objects every day that require the application of a torque to operate; from automotive and aerospace controls, consumer electronics and packaging to medical devices, furniture and architectural hardware. It is not a convenient accident that each of these devices performs precisely as expected, time after time. It is down to the meticulous work of design and production quality professionals, and their professional approach to torque testing.



Why test torque?

As a manufacturer of any of the afore-mentioned devices, there are three key benefits that can be derived from having a professional, systematic approach to torque assessment.

Firstly, torque testing enables designers to perfect the usability and fitness-for-purpose of a product. For instance, automotive controls designers use torque testers to ensure stalk switches are easy enough for any driver to twist, but provide sufficient resistance to give a positive ‘click’ on engagement. As another example, child resistant closures on medicine bottles must be sufficiently difficult to compress and twist to stop infants from removing the cap, whilst being sufficiently easy for frail and elderly users to open. Secondly, torque testing may be used at the point of production to guarantee superior quality manufacturing. Processors of bottled carbonated drinks, for example, perform in-line checks to ensure the capping heads on bottling machinery apply sufficient torque to achieve an hermetic seal in the lid, but not too much as to damage the bottle. Finally, torque testing can make up a vital component of a manufacturer’s quality management system, enabling conformance with relevant national and international standards, as well as in-house specifications.



Choosing the equipment

Mechanical spring-type torque gauges are the traditional weapons of choice for performing torque assessment of small rotary components industrially. Recent years have seen a shift in popularity, however, towards digital systems, owing to the far greater accuracy and reliability they can provide. Within the digital realm of torque testing there are three broad types of system; manual, semi-automatic and computer-controlled. Selection will be dependant upon the nature of the application, as well as budget constraints.

Manual testers

Manual torque testers have a compact, fully enclosed design. A mounting table sits atop the built-in electronic torque transducer, and grips the base of the sample presenting it for application of torque by hand.



Semi-automatic testers

As exemplified by the Mecmesin Vortex, semi-automatic testers have the transducer mounted onto an adjustable crosshead over twin vertical posts. The sample is gripped by both upper and lower mounting tables and the torque applied by a motorised drive system. This eliminates inconsistency in results arising from manual application of the torque. Not only are these testers more accurate than manual systems, they are also more versatile, offering a modular design to allow transducers of different capacities to be swapped over. A top-loading facility on the transducer carriage may also enable static loads to be pre-applied vertically to the sample, as commonly used to test child-resistant ‘push and twist’ closures.



Computer-controlled testers

These systems are similar to the semi-automatic testers, but are linked to a PC for fully automatic control of the drive system. This provides the best guarantee of testing repeatability, as all test variables are electronically managed. It also allows for cycle testing to be performed for unmanned collection of a significant quantity of data, or for fatigue testing. The Mecmesin Vortex-i is a typical example of a computer-controlled tester.

Once the system has been selected, the capacity of the transducer needs to be considered.  Transducers detect the applied rotational force and convert it into an electronic signal. They come in a variety of maximum torque capacities, and the selection should realistically reflect the torque range requirements of the application. If it is too low, the transducer risks being overloaded and irreparably damaged. If it is too high, the transducer may not be sufficiently sensitive to accurately detect small peak loads. Torque is most commonly measured in N.m (newton metres), although other units include N.cm, kgf.cm and lbf.ft and lbf.in. Transducers normally range from 0.3N.m for delicate sample assessments, up to 10N.m for more robust applications. Finally thought should be given to the gripping requirements of the application. Most should be met by the universal mounting plates supplied with the tester, but some applications will require custom-engineered grips to be made for greater accuracy and ease of operation.



The test

Before starting the test it’s important to consider the key parameters to be determined, and set up the equipment accordingly. Considerations include; 

i) The critical measurement(s) under scrutiny. This may sound obvious, but it is worth giving careful thought to what, precisely the test is looking to ascertain. For example, to determine the release torque of a pickle jar lid, the initial peak load will be of most interest. If testing a cola bottle with a tamper-evident closure, however, both the initial release torque and the torque required to break the bridges on the plastic ring may need to be identified.

ii) The total time period or distance (by angle) over which the test should run.

iii) The speed of torque application.

iv) The number of repeat tests required to gain statistically significant results.

v) Further unique requirements of the application. For example, an electrical dial can be connected to an ‘event’ port on computer-controlled systems, enabling the torque at which the switch’s circuit is completed to be determined.

Initial trials should be made to establish an acceptable torque range of results. These upper and lower tolerance limits may then be inputted to the system to activate pass/fail alerting during testing.

Once the measurements have been taken, the results can be stored, exported or manipulated for analysis with differing levels of sophistication depending on the chosen system type. Computer-controlled systems offer the greatest array of options, displaying results graphically with overlaid tolerance bands. Test replay and multilevel zooming functions may also be offered to allow closer interrogation of particularly interesting portions of the torque curve. l

www.mecmesin.com

  
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