June 06 News

Talk the Torque

There are many ways of joining materials together, but take something as advanced as the modern motor car and you’ll find it's put together with hundreds of nuts and bolts and other threaded fasteners. Why? Because these types of joints are efficient, they are extremely strong and when it comes to service or repair, they can be easily disassembled.

How fasteners are tightened is critical. Too loose and the nut may vibrate free, too tight and the bolt may break. In some circumstances this may simply be inconvenient, in others it can be a matter of life and death. Correctly tightened fasteners will on the other hand function perfectly for years, helping to keep vehicles safe and in service.

Torque is a twisting force - it is measured by multiplying the force on a lever by the distance from the point of application. We all know we can tighten a nut much more easily holding a wrench near its end. If we hold the wrench at it’s mid point we need to exert twice as much force to apply the same torque. Applied torque is a measure of the tightness of a nut and bolt but its not actually the torque that holds the parts together, in fact the thread of the fastener converts rotary motion into an axial load that stretches the bolt, thereby converting torque into tension. This in turn creates a clamping force between the face on the nut and the head of the bolt, and it’s this force that holds the joints together.

The more the nut is turned the greater the stretching of the bolt and the greater the clamping force. For every joint there will be an optimum torque value and this can be accurately and reliably achieved using the appropriate torque tool.

There are just two basic categories of Torque Tool. Those that measure the torque being applied and those that signal when a pre-set torque has been reached.

Measuring tools generally have a scale, dial or digital readout. The torque is measured continuously as it is applied. Measuring tools are ideal for use in Maintenance, Quality Control and Servicing where there is a need to check a variety of torque settings. These multifunctional tools are also used in assembly operations where an operator may be required to tighten fasteners to various different torque values.

Quality Control checks at Aston Martin using a
Dial Measuring Wrench

Preset tools have to be set to the required torque on a calibrated torque tester, or in some cases on the calibrated scale on the tool itself. When the required torque value is achieved during the tightening, the tools either 'slip', 'break' or 'click' as a signal to the operator. This significantly reduces the possibility of over-tightening. These tools are ideal for applications where the operator could be unskilled but repeatable accuracy is required.

There are three main types of pre-settable torque tool.

With ‘Click’ tools there is an audible click and the operator will feel an impulse when the pre-set torque is achieved.

Click

As force is applied to the tool the torque will increase. At the pre-set torque value the tool will 'click' and there will be approximately 3⁰ of tool movement. Resetting takes place when the hand pressure is released. However, if the force continues to be applied then the fastener will be over-tightened.

An adjustable click wrench being
used to check wheel nuts.

‘Breaking’ torque tools 'break' in a controlled manner at a point along the tool’s length at the pre-set torque.

As force is applied the torque will again increase. At the pre-set torque value the tool will break through an angle of 20⁰ to signal that the correct torque has been applied. Again the tool will automatically reset when the hand pressure is released. Continued application of force beyond the 20⁰ movement can again lead to over-tightening. However the greater tool movement makes this significantly less likely.

Break

With ‘Slipping’ tools, torque again increases with the applied force. At the pre-set torque value the mechanism in the tool causes the application of torque to cease and the tool slips free for a short time until it automatically resets. Even if the application of force is repeated the pre-set torque will not be exceeded, therefore making it impossible to over-tighten the fastener.

Slip

With these Slipping Tools, a roller is pushed by a spring against a multi-lobed cam. Until the specified torque is achieved the roller transfers the driving force to the cam. At a pre-set level however, the force of the spring is overcome and the roller travels over to the next position on the cam. As this slipping action will repeat at each subsequent cam position, excessive torque can never be applied.

IBC Vehicles use TSN 'Slipper' Wrenches for
Vauxhall Frontera engine assembly.

To maintain reliable torque control, any tool requires regular maintenance itself if it is to meet the requirements of the International Torque Tool Standard ISO 6789:2003. Measuring tools need recalibration – normally annually, and preset tools need regular checks to ensure the tool has not drifted from its original preset value or the torque setting has not been accidentally changed. A range of both mechanical and electronic torque calibration analysers are available on the market to carry out these checks.

The Personal Electronic Torque Analyser System-
being hosted by a Laptop.

Used and maintained correctly, torque wrenches and torque screwdrivers will give many years of reliable service. Selecting the wrong tool for the task or using a tool incorrectly can be costly both in rework time, the cost of failed components and potential product liability and warranty costs.

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