Over-Torquing: A Technical Guide to Its Dangers and Prevention

Torque, in physics and mechanics, is the rotational analogue of linear force, which is sometimes referred to as the moment of force, or moment for short.  When driving a screw, torque is applied by the screwdriver rotating around its axis.  It all sounds very simple and straightforward, doesn’t it?  If screws are chosen correctly and withstand a pre-determined drive-in torque, what’s left is controlling the force applied to the rotating axis and everything will be nice and dandy.

Then why is it that over-torquing seems to be so prevalent among both DIY users and professionals alike, which often lead to various issues such as damaging the fastener, stripping threads, or even causing structural damage to the component being fastened?

Below are the cause of over-torquing which we’ve observed over the years in the field:

Using impact drivers (in basically every situation)

1. Using impact drivers (in basically every situation)

Although an impact driver looks like a drill, it’s not designed to drill holes.  It works by using rotational force + bursts of impact + high torque levels to drive-in fasteners. The rotation normally works in a two-steps-forward, one-step-back pattern, with the bursts of impact up to 50 times per second when encountering resistance. This function makes an impact driver significantly more effective at driving in fasteners than drills. They are normally designed for fixing to timber substrates, as wooden fibers are a real force of nature and can sometimes be very tough to penetrate and clear through, which is why its burst of impact pattern is so effective.

2. Failing to set the torque limiter correctly on the drillers:

For self-drilling screws, setting the correct torque is critical, especially when fixing sheets to metal substrates.  There is an optimal rpm (rounds per minute) to create the most effective drill and drive process – too slow, and the screw will fail to penetrate, too fast, and the screw will burnout prematurely during the process.

3. Not knowing when to stop:

Not knowing when to stop: Knowing when to stop is probably the most difficult yet most important skill of all when it comes to fixing any kind of fasteners.  When fixing screws to different substrates, there are always signs, albeit often quite subtle, of when the substrates are joined properly.  It normally requires basic knowledge and oftentimes a bit of finesse, experience, and practice to get it right.

A very simple point of reference when fastening screws with EPDM washers, is to first slow down when the EPDM washer comes in contact with the substrate below, make a full stop, and then gently tap the trigger a couple of times until the EPDM is compressed to the point that it almost flush / in-line with the screw flange.  Go any further and you’re in over-torquing territory and run a huge risk of the EPDM washer being squashed and completely destroyed.

How do we prevent over-torquing, one may ask?

Over-Torquing Prevention
  1. Proper training: Ensure that personnel using the tools are adequately trained in the correct techniques. For example, the techniques for drilling a No. 5 point screw will differ from drilling a No. 3 point screw, or reduced point screws.
  2. Pay attention to the material that the screws are drilling through, and affixed to. It is very common for personnel to focus on the material that the screws are drilling through, but unaware or even ignore the substrates that the screws are affixed to.  For example, when fixing a 0.7mm metal roofing sheet to a 12mm metal substrate, a No. 3 point self-drilling screw is defenitely not cut for the job.
  3. Check, and check again – the importance of routine inspection. This habit of constantly checking works in all kinds of situations.  As a screw manufactuter, our screw machine operaters not only have to do routine checks on the machinery during production, a very important part of their job is to do routing lot and batch checks of the screws itself.  It is never to late to check again to make sure that dimensions are within tolerance ranges.

By follong the basic principles listed above, not only can over-torquing be prevented, but it can also ensure that you as the user will not be the cause of the problem.