Quality Magazine

Testing Torque

April 25, 2008
Accurately and reliably characterizing the physical attributes of the rotating component is critical in guaranteeing consistent production quality. From automotive controls and medical devices to packaging and consumer electronics, torque testing is an essential ingredient of product design and manufacturing quality control.

Source: Mecmesin


As consumers, we typically twist, turn, wind and unscrew dozens of items in any given day. From door handles and car steering wheels, to drink bottles and stereo volume knobs, there are literally thousands of mass-produced products requiring the application of a low-level torque to operate. For manufacturers of such products, a method to accurately and reliably characterize the physical attributes of the rotating component is critical in guaranteeing consistent production quality. From automotive controls and medical devices to packaging and consumer electronics, torque testing is an essential ingredient of product design and manufacturing quality control.

A car’s windshield wiper control stalk is tested to quantify the ease of rotation. Source: Mecmesin

Why Test Torque?

First and foremost, torque testing may be used at the point of production to perform in-line quality checks. Manufacturers of bottled drinks, for example, test screw closures to ensure their bottle capping machinery consistently applies the correct torque level to achieve a good seal. This allows the manufacturer to quickly identify and amend production errors that could prove costly to their brand’s reputation.

As part of a quality management system, torque testing facilitates conformance with relevant national and international standards, as well as in-house testing specifications.

Secondly, torque testing enables designers to perfect their product’s usability. A child-resistant closure on a medicine bottle, for instance, must be too difficult for a child to compress and twist, but remain sufficiently easy for the frail and elderly to open.

A push and twist child-resistant pill bottle is tested for openability on a semi-automated torque system. Source: Mecmesin

Choose the System

With the broad variety of low-level torque testing equipment on the market, it can be difficult to know where to start. A general rule of thumb is to consider the complexity of the application, and match it with a system that will provide sufficient capacity, accuracy, repeatability, and advanced test and data functions.

Budget constraints also will play a major factor, but it will prove a false economy to buy a simple, inexpensive system for a complex application requiring sophisticated programming capabilities.

Similarly, a state-of-the-art, computer- controlled torque system will probably not be necessary for a simple in-line check on a bottle closure, and money could be wasted on advanced features that will never be used.

There are four general types of low-level torque tester: mechanical gages, manually operated digital testers, semi-automatic systems and computer-controlled torque systems.

1. Mechanical Gages

Mechanical spring-type gages have been around for a long time, and although these simple, inexpensive instruments remain on the market, recent years have seen a popularity shift toward digital torque testers.

There are three broad categories of digital system: manual, semi-automatic and computer-controlled, all of which are built around a digital torque transducer that accurately detects and converts torque loading into an electronic signal.

Digital testers generally offer far greater levels of accuracy and testing repeatability over mechanical gages. They also provide a broad range of data collection, storage and reporting facilities that can save the quality professional valuable time, as well as greatly reduce the margin for error.

2. Manually Operated Digital Testers

These tough and compact systems are affordable and simple to use, lending them well to low-tech applications such as release torque assessment of packaging screw closures on the production line; indeed, they are sometimes also known as closure testers.

To operate, the sample is loaded onto a mounting table, which typically features four rubber-coated gripping pegs, adjustable to hold a variety of sample shapes. This sits atop an enclosed digital torque transducer, which detects any rotation in the mounting table or sample. Torque is then simply applied by hand to the sample, and the resultant load is shown on an integrated digital display. The entire system is normally enclosed in a plastic, splash-proof casing to cope with the messy testing conditions of a bottling plant.

Certain advanced manually operated digital testers have the facility to detect the two peak torques associated with tamper-evident closures, and there also are systems on the market that can simultaneously detect compressive force and torque to characterize the push and twist of child-resistant closures.

Although offering digital accuracy, consideration must be given to repeatability-an inherent area of weakness in manually operated testers, owing to the variations in speed that will occur in the application of torque by hand, thereby introducing the opportunity for human error.

3. Semi-Automatic Systems

To remove human error from the equation, buyers may take a step up to semi-automated systems, on which torque is applied by a motor at a constant and predetermined speed, normally controlled by a potentiometer. This greatly increases the repeatability of tests, eliminating the inconsistencies of manual torque application.

As before, the sample base is mounted onto a lower mounting table in adjustable gripping pegs; however, this time a secondary, smaller upper mounting table grips the top of the sample. This upper table is mounted directly onto the torque transducer, which is itself mounted to an adjustable crosshead set over twin vertical posts. The motor drives the lower mounting table, and is controlled by pressing clockwise and counter-clockwise direction buttons.

Certain motorized semi-automatic testers have the additional facility to pre-apply static vertical loads onto the transducer carriage, to allow testing of child-resistant closures, although only torque is actually measured

Semi-automatic systems are so called because the motor control is not computer automated. As such, these systems are best suited to applications requiring greater accuracy and repeatability than can be achieved with manual systems, but not requiring a great deal of sophistication in terms of the test routine and reporting facilities. Packaging is again a common application, as are consumer electronics, toys and architectural hardware.

4. Computer-Controlled Torque Systems

At the advanced end of the low-level torque tester market is the computer- controlled system. Featuring the same physical layout of the semi-automated systems, these testers are controlled by a remote PC and specialized software. Offering the highest level of repeatability and accuracy, all independent variables are managed electronically, including speed, time and distance (angular displacement).

The associated software offers advanced data handling options, including graphical display of results, real-time test replays and automatic calculation of key parameters.

Advanced test functions meanwhile include cycle testing, automatic return to start position and pass/fail tolerance alerting.

As the most costly option, however, consideration must be given to the application, and whether it legitimately warrants the advanced capabilities. As might be expected, these advanced systems are used on more technically sophisticated products, from medical devices and electronic controls, to automotive and aerospace components.

Capacity

Whatever system is chosen, there will be a range of options for the capacity of the torque transducer it features. Consideration must be given to ensure the transducer capacity suits the application in question. Too high a capacity transducer will not accurately detect very small peak loads, whereas too low a capacity transducer risks being overloaded and irreparably damaged. Transducers normally range from 0.3 newton meter capacity for delicate sample assessments, increasing to 10 newton meter capacity for progressively more robust applications.

Custom-engineered upper and lower gripping fixtures are used to test the rotating dial on a divers watch. Source: Mecmesin

Do the Twist

So you have chosen and purchased a torque testing system, and are itching to put it to use. To achieve the best results, however, it is worth asking a few important questions before getting stuck in.

1. What is being measured?

Torque, obviously, but is that peak torque, running torque, average torque or the torque at a predetermined time or displacement? Alternatively, there may be a particular event to capture, such as the torque at which an electronic rotary switch is engaged. There are a number of possible parameters, and one must decide which are relevant and of primary concern to avoid drawing false conclusions from results.

2. How to grip the sample?

Determine how the sample will be physically oriented in the mounting table to guarantee torque is applied to the correct components. What are the desired start and finish positions, and how far in terms of angular displacement should the test run?

It may be necessary to require custom-engineered gripping fixtures if the universal mounting tables do not adequately hold particularly small or awkwardly shaped samples.

3. What speed should be used?

Like many things in life, if you go too slow you will never arrive, but if you go too fast you will miss the good bits. It is essential the test runs at the optimum speed to ensure the torque profile is accurately mapped, but in a timely fashion.

4. What is acceptable?

Initial trials on the sample can be performed to establish the range of acceptable results for quality purposes. Most digital torque testers, bar some manual testers, will enable these parameters to be entered as upper and lower tolerance limits. Audible and visual alarms can then be set to trigger when results fall outside of this acceptable region, to quickly and easily identify failures.

5. How many repeats should be performed?

From simple in-line checks to rigorous laboratory routines, the more repeat tests performed, the more statistically significant, and hence reliable, the results.

6. What to do with the results?

After the test is complete, results may be viewed, stored, exported and analyzed with varying degrees of sophistication depending on the chosen system. All good digital torque testers will provide a facility for electronic transfer of results to a peripheral device, such as a PC, datalogger or printer.

In today’s highly competitive global economy, prerequisites for a product’s success are a design process with meticulous attention to functionality, and a production process that maintains the utmost consistency in quality. If that product contains a rotary element, torque testing can help manufacturers meet these requirements, reducing production costs, minimizing rejects and improving their brand’s reputation. Q

Tech Tips

To achieve the best results with a torque testing system, it is worth asking a few important questions:
  • What am I measuring?
  • How will I grip the sample?
  • What speed should I go at?
  • What is acceptable?
  • How may repeats should I perform?
  • What will I do with the results?


Quality Online

For more information on torque testing, visit www.qualitymag.com to read these articles:
  • “The Pulse of Tightening”
  • Case Study: “Tightening Up on Torque Standards”