How to use a torque wrench: what is the purpose of tightening torque?

If you're a mechanic, you've probably read in a manual more than once about procedures for using the tightening torque for a nut or bolt. You might think you don't need a torque setting to change spark plugs or retighten exhaust bolts. But statistics show that bolts are often over-tightened, leading to breakage and damaged threads.

In this tutorial, we'll try to explain in a simple way what torque means, what it's used for, and how to check it.

A screw is a “fat” spring

To understand how a screw works, you have to try to think of it as a very rigid spring, but one that can be tightened when tightened (it stretches) and maintains tension due to the friction between its body and the material it tightens.

Each screw is designed to withstand a certain tension. Below this tension, the screw will not stretch enough and will eventually loosen. If the working tension is exceeded, the screw will deform and break.

Measuring tension

The key, then, is to be able to measure the tension applied to each screw. Ideally, we would be able to directly measure the screw's stretch, but this is very complicated and impractical (although, in high-tech applications, there are screws equipped with stretch sensors).

In most applications, another type of indirect and less precise estimation is used, which consists of measuring the tightening torque or torsional force when turning the screw and from this force deducing the stretch and therefore the tension.

The torque wrench

Without going into technicalities, we can define a torque wrench as a device capable of measuring the torque with which we tighten a screw or thread (its tightening torque).

There are different types of torque wrenches, these are the most commonly used:

• The simplest is the dial, where a pointer indicates the deviation generated when tightening.

• A clock, very similar to the previous one, but with a dial and a needle that rotates inside.

• Click type, where an internal spring is tensioned when the tightening torque is selected (by moving the handle) and the wrench slides, emitting a “click” when the selected torque is reached.

• Electronic. They have an internal sensor that measures the torque and emits a sound and vibration when it is reached.

Units of measurement

It's important to understand and interpret the different units in which torque data is presented. If we're given a measurement in units other than those on our instrument, we'll need to convert them:

For conversion, we can use the tables that come with the tool, a calculator, or Google. Electronic torque meters usually perform the conversion automatically.

Some manufacturers provide a torque plus an angle (usually used for cylinder head bolts, crankshaft bolts, etc.). To do this, we'll apply the torque with a torque wrench and then use a goniometer or angle gauge.

Some digital torque wrenches have this feature built in.

Tips for use

When working with a torque wrench, it's essential to understand its use and maintenance. For this purpose, we'll refer to the manufacturer's manual (it's very important to read and understand it). Likewise, we must follow the tightening recommendations in the workshop manual for the vehicle we're working on. Sometimes, in addition to the tightening torque, they also provide a tightening sequence to avoid distorting the parts during assembly.

• They are delicate instruments, do not hit them or use them to loosen screws.
• They are generally stored with the dial set to 0, so as not to uncalibrate them (see manual).
• Do not lubricate or grease the screws to be tightened, unless indicated in the workshop manual (if we modify the friction, we modify the tightening torque).
• They must be recalibrated after a certain period of use (again, consult the tool manufacturer's manual).
• If we use extensions, they must always be at 90º so that the torque does not change.
• They're useless when stored away. It's important to get used to using them at all times, reading the torque values in the manual and applying them with the tool (the torque on our forearms is very imprecise and depends greatly on what we had for breakfast).