Rattle guns are one of the most useful tools that a technician can use.
However, it is also one that will cause a lot of problems if not used correctly. This is because we have very little idea of how much torque it is applying to the fasteners.
SUBSCRIBE to the PowerTorque newsletter
Amazingly, there are pages on the internet teaching people to check the torque of a rattle gun by comparing fastener torque reached with that of a torque wrench.
\Unfortunately, counting the times that a rattle gun makes an “ugga dugga” noise is not an accurate way to measure torque.
The torque a rattle gun applies will vary depending on air pressure (which changes as the compressor cuts in and out), amount of oil in the gun and simple wear and tear. This makes it difficult to estimate the accuracy and consistency of this already vague guide to bolt torque.
One major issue with rattle guns is the risk of over-tightening fasteners past their yield point. To understand the issue here, we need to look at the stages that a fastener undergoes, from elastic to plastic, when it is over tightened.
The first stage is the elastic range, which is the blue part of the bolt yield chart. This is the range where we normally tighten fasteners with a torque wrench. In this range, the stress in a fastener is directly proportional to its strain.
Engineers calculate fastener clamp load by considering the fastener’s material, the coating or whether washers are also used.
For example, Hendrickson QUIK-ALIGN pivot bolts have a special dry lubricant coating that includes zinc and aluminium.
The torque specification for these bolts makes allowance for this coating. Before installation, it is important to ensure that there are no additional compounds on the threads, like grease or paint, that could change the function of the coating.
The second stage is plastic range, which is the orange part of the chart. A bolt will enter the plastic range after it passes its yield point. In the plastic range the bolt undergoes permanent deformation. Most fasteners on a vehicle, including suspension bolts must never be tightened past their yield point.
Engineers will normally recommend a torque that includes a good safety margin to ensure fasteners are never taken past their yield point. However, some specialised bolts, such as cylinder head bolts, are designed to be operated in this range with careful torque and angle tightening steps.
If a bolt is further tightened in the plastic range, it will stretch until it passes the ultimate tensile strength point. Once it passes this point, it will start getting easier to turn until it breaks.
To operate reliably, standard automotive fasteners should never be tightened past their yield point. When using a rattle gun, it can be easy to tighten a bolt past the yield point without even realising it, which may well fail in service.
On the other hand, tightening critical fasteners with a torque wrench will ensure that the truck or trailer remains reliable and in service with the lowest overall cost.
