Bolted Joint - Setting The Torque

Setting The Torque

Engineered joints require the torque to be accurately set. Setting the torque for fasteners is commonly achieved using a torque wrench. The required torque value for a particular fastener application may be quoted in the published standard document or defined by the manufacturer.

The clamp load produced during tightening is higher than 75% of the fastener's proof load. To achieve the benefits of the preloading, the clamping force must be higher than the joint separation load. For some joints, multiple fasteners are required to secure the joint; these are all hand tightened before the final torque is applied to ensure an even joint seating.

The torque value is dependent on the friction produced by the threads and by the fastened material's contact with both the fastener head and the associated nut. Moreover, this friction can be affected by the application of a lubricant or any plating (e.g. cadmium or zinc) applied to the threads, and the fastener's standard defines whether the torque value is for dry or lubricated threading, as lubrication can reduce the torque value by 15% to 25%; lubricating a fastener designed to be torqued dry could overtighten it, which may damage threading or stretch the fastener beyond its elastic limit, thereby reducing its clamping ability.

Also, if the fastener rather than its associated nut is torqued, then the torque value should be increased to compensate for the additional friction; fasteners should only be torqued if they are fitted in clearance holes.

Torque wrenches do not give a direct measurement of the clamping force in the screw, and indeed much of the force applied is lost just to overcoming friction.

More accurate methods for setting the clamping force rely on defining or measuring the screw extension; for instance, measurement of the angular rotation of the nut can serve as the basis for defining screw extension on thread pitch. Measuring the screw extension directly allows the clamping force to be very accurately calculated. This can be achieved using a dial test indicator, reading deflection at the fastener tail, using a strain gauge, or ultrasonic length measurement.

There is no simple method to measure the tension of a fastener already in place other than to tighten it and identify at which point the fastener starts moving. This is known as re-torqueing. An electronic torque wrench can be used on the fastener in question, so that the torque applied can be constantly measured as it is slowly increased in magnitude; when the fastener starts moving (that is, becoming tightened) the required torque magnitude briefly drops sharply, and this drop-off point is considered the measure of tension.

Recent developments enable tensions to be estimated by using ultrasonic testing. Another way to ensure correct tension (mainly in steel erecting) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When the orange rubber strands appear, the tension is correct.

Large-volume users (such as auto makers) frequently use computer controlled nut drivers. With such machines, the computer in effect plots a graph of the torque exerted. Once the torque reaches a set maximum torque chosen by the designer, the machine stops. Such machines are often used to fit wheelnuts and normally tighten all the wheel nuts simultaneously.