Torque Wrenches

A torque wrench is a tool designed to apply a specific amount of torque to a fastener, such as a nut or bolt and removes the need for the operator to 'feel' if the fastener is tightened correctly, because a torque wrench provides a signal to the operator, such as a click, when the desired torque is reached.

Correct torque helps to achieve the right clamping force, which is the squeeze that holds parts together, and ensures that components are tightened to manufacturer's specifications, preventing over-tightening or under-tightening, which can lead to equipment failure or safety hazards.

Consider the torque range required for your task and ensure the drive size of the socket being used fits the torque wrench.

The manufacturer will likely state the torque that needs to be applied for the application, as well as the torque measurement type, (Nm, ft-lb, or in-lb). The torque wrench used should have a working range which puts the target torque in the middle of the scale, and ideally it shouldn't exceed 80% capacity of the torque wrench, which minimises strain on the torque mechanism in the tool, and will likely make it more accurate. So for example, if the job at hand needs 10Nm, a 2–20Nm wrench is better than a 20–200Nm, because accuracy is usually worse at the lower end, but the torque required isn't high enough to strain the mechanism.

• Click torque wrenches emit a 'click' sound when the set torque is reached. The torque value is usually set on a scale located at the handle and are popular because they’re fast to use, simple and don't require batteries.
• Dial (dial-indicating) torque wrenches show torque on a dial gauge, as the operator tightens a fastener. They’re often used for inspection and high-precision work because the reading is clear and can show peak torque, but are typically more expensive, as well as more delicate than click wrenches.
• Digital torque wrenches display torque on a screen, which is normally located near the handle and they provide either a beep, vibration or LED alert when the target torque value is reached. Many offer peak hold, unit conversion, angle measurement and data logging, which makes them excellent torque wrench for repeatable or documented work. Digital torque wrenches do require batteries to operate though, and can come at an extra cost when compared to click torque wrenches.

While a torque wrench can physically loosen bolts, it's not advisable because it will overload the internal mechanism, as well as affect the tool's calibration and accuracy. It's best to use a wrench or ratchet for loosening fasteners, and if a fastener is particularly stubborn then an impact wrench or slogging spanner will be the best tools to loosen it.

It is recommended to calibrate your torque wrench annually or every 5,000 cycles (this number does differ between manufacturers), whichever comes first, to maintain accuracy.

Frequent calibration is crucial in professional settings where precision is essential, and we would always recommend having your torque wrench re-calibrated if it -

• Is dropped or impacted.
• Over-torqued (exceeds its maximum torque value).
• Is used to loosen stuck fasteners.
• Has been stored incorrectly (e.g. A click wrench left set at a high torque value. Click wrenches should be stored at the minimum setting (not tight) to help maintain accuracy).

After use, set the torque wrench to its lowest setting (but not zero) to relieve tension on the spring. It should also be stored in a protective case in a dry environment, away from extreme temperatures or high humidity, to prevent corrosion and maintain accuracy.

The most likely cause is either a technique or setup issue. For example mishandling the tool, either holding it and applying pressure in the wrong position, going too fast during the tightening process or even giving it more than one click on a clicker wrench.

We would recommend that if using a click torque wrench to use a slow, steady pull and stop immediately at the first click, because the click is designed as a stop signal due to the desired torque being reached.

If the torque wrench being used sits at the bottom of the wrench’s scale for the target torque, accuracy and feel are worse. Instead choose a torque wrench where the target torque sits in the middle, or roughly 20%-80% of its range.

The most accurate way to measure the torque on a fastening would be to mark the bolt and the plate with a straight line when the bolt is tight, then back the bolt off and use a dial wrench to re-tighten the bolt to the point where the lines match up. The indicator on the dial wrench will show how much torque is being applied to the fastener.

Clicking torque wrenches have a scale that is usually on the handle to set a desired torque value, and a click is produced by the internal spring-and-cam mechanism when the preset torque value is reached. Once the operator hears the click sound they should stop applying pressure immediately, because there is a very small reaction window of around 4 degrees movement, to remove pressure from the wrench, before overtightening occurs.

To use a click wrench as accurately as possible, pull smoothly and steadily, hold the wrench at the intended grip point, keep it aligned with the fastener and stop at the first click (don’t 'double click').

Slipping torque wrenches remove the risk of overtightening fasteners, making overtightening nearly impossible. This is because when the preset torque value is reached, an internal clutch mechanism in the torque wrench 'slips' free and torque is no longer applied from the torque wrench, even if the operator continues to apply pressure.

Exceeding the torque setting can damage both the wrench and the fastener. It may also affect calibration accuracy, leading to incorrect readings in future use.

For fasteners that have exceeded their specified torque, their clamp load will increase beyond what the joint was designed for. This will lead to stripped threads (common in aluminum or small screws), stretch the fastener and reduce its long-term holding ability, and even potentially snap the fastener.

If the maximum torque is exceeded on a torque wrench, then the internal mechanism will drift out of calibration and the wrench may even fail, which will lead to innacurate torque application in both instances. 

Always ensure a torque wrench is re-calibrated everytime the torque setting is exceeded, before using it again.

Straight extension bars do not affect a torque reading because they don't actively alter the lever length of the torque wrench, but there can be neglible losses of accurate torque reading from flex or friction. However, universal joint sockets and wobble extension bars will be affect torque output, due to the angular misalignment they produce, and should only be used when necessary.

Torque wrench end fittings can be attached to the end of some torque wrenches. If a torque wrench end fitting is positioned at 90° to the torque wrench, then the effective lever length stays the same and no torque correction is required. However, if the fitting is used inline (straight) from the torque wrench, then it increases the lever arm length, and the fastener will receive more torque than what the torque wrench indicates.

There are online tools such as this one, which provides the mathematical correction required to ensure torque is applied at the desired value.