Establishing a quality torque programme

Edited by the Engineeringtalk editorial team Nov 6, 2008

In the manufacturing and assembly world, tightening, controlling, or measuring torque fasteners is imperative for production efficiency.

An inadequately torqued fastener can vibrate or work loose; conversely, if the tension is too high, the fastener can snap or strip its threads.

Faced with these problems, manufacturers are realising that precise torque control can spell the difference between a safe, reliable, and economical product and complete disaster.

Anybody who has to tighten a threaded fastener and needs to control, monitor, or measure torque needs sophisticated torque tools.

If manufacturers want to save money, make their workplace safer, enhance product quality or reduce their exposure to liability, only specialised high-quality torque tools will get the job done properly.

A wide variety of tools are available to control or measure the torque applied to fasteners, from electric screwdrivers to large industrial wrenches, analysers, sensors, and multipliers.

These tools utilise calibrated torque setting mechanisms that may be factory pre-set or user-definable.

When the specified setting is reached, the tool gives a visual, audible, or tactile signal.

The anticipated production output, the type of materials being joined, the amount of torque required, and the specified fasteners determine the selection of tools for a given application.

Lighter materials such as wood or plastic may require only lightweight tools; likewise, heavy materials such as steel may require stronger or larger tools.

Tools should also have connection ports for an RS-232 PC cable if torque data must be gathered.

'Cam-over' wrenches utilise a ball and lobe design that allows the tool to slip free when torque is reached.

Even if the application of force is repeated, the preset torque value won't be exceeded, eliminating the possibility of over-torque.

These tools are perfect for maintenance and production applications where over-torque conditions are not tolerated.

The use of cam-over wrenches takes the operator influence out of the torque equation and offers more accurate and repeatable results than a standard 'click' type wrench.

Calibration is used to fine tune the torque control process in a production environment.

Calibration should be checked periodically to determine whether torque tools are operating at their proper settings.

Many tools don't have a locking device, and users may easily change their torque settings.

When this happens, the tool falls out of adjustment.

A regularly scheduled calibration programme enables quality control personnel to correct divergence from proper settings, whether it's because of normal slippage over time or because of adjustments to the tool.

Begin by setting a calibration interval initially based on severity of the application and the tool manufacturer's recommendations.

If the applied torque values are out of range, cut the calibration interval in half and re-test the tools.

To maintain consistent accuracy, torque tools must be checked periodically for wear or defective parts.

A properly structured preventive maintenance programme optimises tool performance and reduces unexpected downtime, thereby saving time and money in the long run.

Monitoring the number of cycles per day and total hours that a tool is used is the most accurate way to establish proper maintenance intervals.

It is recommended that tools be serviced after 100,000 cycles, or if an inspection reveals old or dry grease, parts that show signs of excessive wear, or loose screws and bolts.

Effective use of a torque analyser is a fast and reliable method of calibrating torque tools to their proper settings.

Analysers can also be used for quick tests on the line or in the lab to determine whether torque tools are holding a given setting.

They also allow quality control inspectors to calibrate torque sensors and verify torque on fasteners.

A quality torque analyser should have enough memory to record several hundred readings, and it should store calibration data for multiple torque transducers.