Do the twist

Low-level torque testing is essential in the successful development and production of small rotary products, from consumer electronics and automotive controls, to medical devices and packaging

WHAT do the lid of a drinks bottle, a shower tap, a car steering wheel and a stereo volume knob have in common? They all require the application of a controlled low-level torque to operate. We commonly associate torque with the power output of an engine, but often overlook the smaller torques that we, as consumers, may encounter on a daily basis. Design and manufacturing quality control laboratories across numerous industries utilise low-level torque testing equipment as a simple, fast and inexpensive method to perfect the ‘feel’ of rotary devices, assess conformance to international standards and guarantee quality in production. This allows manufacturers to quickly identify and amend production errors that may harm the brand’s reputation.

Perhaps nowhere is this of more significant importance than in the development and production of safety-critical medical devices. Here, we look into low-level torque assessment from a broad perspective, appraising the equipment available, and the initial considerations in setting up the test, before finally looking at some examples of low-level torque applications within the medical device sector.

There is a broad variety of equipment on the market purporting to accurately assess torque, but to ensure you select the correct system to suit your laboratory and application, consideration should be given to required capacity, accuracy, repeatability and degree of sophistication in terms of advanced functions required. Space and budget constraints may also play a part in the decision; however low-level torque testers are generally compact and inexpensive in comparison to most specialised laboratory equipment.

Traditionally, mechanical spring-type gauges were widely in use, and although these cheap and simple instruments are still available, recent years have seen a shift in popularity towards digital systems. These comprise testers with a digital torque transducer at their core, to detect the applied rotational force, and convert it into an electronic signal. These offer far greater accuracy and repeatability over spring-type gauges, as well as a whole range of advanced data collection, storage and reporting facilities. Digital torque testers normally fall into one of four categories; manual, semi-automatic or computer-controlled self-contained systems, and the fourth option of small, hand-held gauges.

• Manual torque systems, so called owing to fact that the torque is applied by hand (and also known as closure testers), are small and highly portable, and most commonly used to assess the release or closure torque of screw lid containers. As such, they are a common fixture in the quality control laboratories of packaging plants across the food and beverage, cosmetics and pharmaceutical industries. Manual torque testers normally comprise a mounting table sat atop an enclosed digital torque transducer. The mounting table may have three or four rubber-coated pegs, adjustable to grip a variety of sample containers. Once the container base is firmly positioned on the mounting table, torque is simply applied to the lid by hand, and the effort displayed on an integrated digital screen. Certain testers offer the capability to detect the secondary peak load required to snap the plastic bridges present on tamper-evident caps.

• Semi-automatic torque systems represent a step up in sophistication from manual testers, the primary difference being that torque is applied by a motor, removing inaccuracies arising from inconsistent manual application. The speed of torque application is normally potentiometer controlled, and the motor activated manually using clockwise or anticlockwise direction buttons. The transducer is mounted onto an adjustable crosshead, set over twin vertical posts. A similar lower mounting table sits on the axis of the drive motor, and rotates during testing. A smaller upper table is mounted directly onto the torque transducer. Some semi-automatic testers have a top-loading facility incorporated into the transducer carriage, allowing static vertical loads to be pre-applied, typically for testing child-resistant ‘push and twist’ closures. Semi-automatic systems are typically used in applications requiring a higher degree of testing accuracy and repeatability, but where reporting and test routine sophistication are fairly low priorities. Closure testing is again a common application, as are toy and consumer electronic device testing.

• Computer-Controlled Torque Systems, although physically similar to semi-automatic testers, are controlled by a remote PC and associated dedicated software. Computer-controlled testers provide the greatest level of testing repeatability and accuracy, as all parameters are managed electronically, including speed, run-time and angular displacement. They also enable more sophisticated data collection, and display and interrogation of results, as well as a range of advanced functions, including unmanned cycle testing, automatic return and tolerance alerting. Some advanced testers feature an ‘event’ port, enabling, for example, an electrical rotary switch to be connected to the system during testing, to assess the torque at which the switch’s circuit is completed. Computer-controlled testers are commonly used in the development and production of more intricate products, such as aerospace and automotive controls and medical devices. They are, however, the most costly option, and though portable, require a greater amount of bench space owing to their reliance on a PC.

• The final option available is the small hand-held unit, typically comprising a gauge/display box connected to one of a selection of interchangeable torque sensors. Sensor options include torque screwdrivers, torque wrenches or simply a torque transducer (the kind of which you would find on one of the above systems) as a freestanding unit. These configurations have the advantage of simplicity, ease of use and portability. They may also be used in awkward orientations, for example, to test the capping heads on bottle filling machinery in situ. The lack of rigid structure and standardised test position, however, provides a large margin for inconsistency between tests.

Transducer capacity must be considered, once the system type has been chosen. Semi-automatic and computer- controlled testers may have a modular design to enable transducers to be swapped at will, however this is not normally an option for the self-enclosed manual tester. Low-level torque transducers commonly range from 0.3N.m capacity for delicate sample assessments, up to 10N.m for more robust applications. It is essential that the torque range of the selected transducer realistically reflects the requirements of the application. A transducer with too low a capacity risks being overloaded and irreparably damaged, however one with too high a capacity may not be sufficiently sensitive to accurately detect small peak loads. Finally, for applications where many repeat tests of the same sample are to take place, or where the sample is a particularly awkward shape, it may be beneficial to have custom-engineered gripping fixtures made, to more effectively hold the sample in place, and allow for easier operation.

Before beginning the test, consideration must be given to the critical measurement under scrutiny. It may sound obvious, but without having clearly identified the objectives of the test, it is easy to lose sight of its purpose and draw false conclusions from results. The peak torque, the average torque throughout the test, or the torque at a specified elapsed time or angular displacement may be among the measurements of most interest. Alternatively, a specific ‘event’ during the test may be of primary concern, as in the example of the plastic bridges breaking on a tamper-evident closure. Consideration should also be given to the orientation of the sample, its start and finish positions. If fatigue testing is to be performed, how many cycles will be sufficient to rigorously assess the sample’s durability? Once these factors have been determined, the optimum speed setting should be defined, as well as the number of repeat tests required to attain statistically significant results.

If performing a quality control assessment in production, initial trials can establish an acceptable torque range of results. Good digital testing systems will then enable these upper and lower tolerance limits to be inputted, and pass/fail alerting to be established. Finally, it may be useful to lock out changes to the settings once the system has been set up, allowing for simple, single-button operation.

Once completed, the results of the test can be displayed, analysed, stored or exported with varying levels of sophistication, depending on the chosen tester. Advanced options such as real-time test replays, graphical display of results and automated calculation of key parameters are offered only by computer-controlled testers, however most digital systems will allow export of results to a PC, or other external data-logging devices.

Medical devices that are poorly designed and manufactured seriously limit the ability of a healthcare professional to effectively and safely treat their patient. Torque testing of rotary devices in production enables manufacturers to guarantee consistent quality and performance of their products. At the development stage, it also allows designers to perfect the usability and fitness-for-purpose of their devices, and strike an optimum balance between material usage and mechanical strength, thus minimising production costs.

IV components, needles and syringes are commonly interconnected via a threaded luer-lock coupling. The force required to disassemble the two components must be sufficiently high to ensure a hermetic seal in the connection, whilst remaining sufficiently low to enable quick and simple engagement and disassembly as required. A common test to assess the integrity of the seal in the connection involves tightening the assembly to a pre-determined

Another area is over-the-counter medical devices intended for home use by the patient, such as asthma inhalers and insulin injection pens. Assessment of the effort required to twist the dosage selector on an insulin pen is a simple yet crucial measurement to ensure the user may accurately self-administer their medication. Other applications such as the cap removal torque of inhalers help to ensure ease-of-use and create an overall feel of quality.

Other rotating medical device elements include; taps, valves and stopcocks on respiratory aids and blood filtration apparatus, dials and switches on electronic devices, as well as artificial joint components, all of which must quantifiably perform to international standards.

From automobiles to fighter jets, and medical devices to architectural hardware, meticulous attention to detail in design and production is an essential prerequisite for a product’s success. If that product contains a rotary element, low-level torque testing offers a quick and cost-effective method to optimise design, reduce production costs and minimise waste.

By David Mercer. David is Technical Marketing Executive at Mecmesin, producer of force and torque test equipment.