The Importance of Torque in the Fastening Process

Most people understand the basics of torque and how it’s applied to a fastener. As a refresher, torque is applied to a fastener in a rotating action, overcoming friction and causing the fastener to stretch. This stretch creates the tension that keeps the fastener fastened. It’s measured by applying a force against distance.

Measuring Torque

Torque can be measured in many ways, and the most common is with a torque wrench that has graduated scales, either in NM (Newton Metre), LBF/FT (Foot Pound-Force) , LBF/IN, or KGF-CM etc. These scales are a measure of applying the relevant force (at a measured distance) to overcome the friction. Other ways of measuring this torque is by utilising calibrated power tools. In a production environment, it’s too expensive to check every fastened joint with a torque wrench, so power tools are a preferred choice for this sort of task.

Torque and Technique

We use tools such as spanners, torque wrenches or powered screwdrivers to achieve this level of torque required in manufacturing, but are we also achieving the clamping force needed? For example, if a bolt becomes cross threaded during the fastening process but reaches the specified torque, it will not be fastened correctly. Therefore, it might obtain the required torque, but the desired clamp force will not have been achieved. Alternatively, if we allow an operator to do a visual check, we can’t be sure it is fastened to the correct torque at all, because there is simply no objective way to measure it. This can have significant, even critical, consequences in certain applications, and so it is important to check consistency and accuracy throughout the process.

Verification

Error proofing, or poka yoke (pronounced PO-ka yo-KAY), is a way around this and previously only available on high value electric tools. Today’s manufacturing environment benefits from improvements in electronic motor monitoring, allowing manufacturers to offer very accurate error proofing, with minimum investment required for smart assembly tools such as screwdrivers and nutrunners.

The tool is programmed to count the number of rotations after it’s reached a “snug” torque, also known as the point of first resistance. As well as snug torque, smart tools also consider the required rotations and angles simultaneously to ensure a fastened joint passes OK. The repercussions from a fail can result in something simple, such as the smart tool showing a red light, or it can set off a whole chain of IOs (inputs / outputs) that could stop a whole work cell.

In addition to the range of notifications smart tools can provide, some entry level tools are now offering the ability to store the data, such as run-downs, and have IOs to allow barcode scanning or other means of part identification. This is ideal for different run-down strategies, where an assembly can consist of different fasteners and torque requirements.

Calibration

Typically, your torque tools are calibrated against a “known” source, such as a torque tester that is calibrated by a certified laboratory, that conforms to an industry standard i.e. NATA, NADA etc.

Achieving high quality in assembly has never been easier with the new generation of smart assembly tools. High quality tools provide manufacturers with the ability to attain quality, consistency and efficiency without the need for large investment. With this in mind, all manufacturers need to ask themselves whether they have the right tools to do the job.