A torque wrench is a precision tool used to tighten bolts and nuts to a specified torque instead of simply making them as tight as possible. Torque matters because bolts act like springs that clamp parts together, and the correct clamping force keeps machines safe and reliable. Too little torque can let parts loosen, while too much torque can stretch bolts, strip threads, or damage components.
In workshops, torque wrenches are essential for engines, wheels, bicycles, aircraft parts, and many assemblies where exact tightening is required.
A click-type torque wrench uses a calibrated spring and internal release mechanism to signal when the set torque has been reached. The user sets the desired torque on the handle scale, attaches the correct socket, and pulls smoothly at the handle until the wrench clicks. The torque depends on both the applied force and the perpendicular distance from the bolt axis to where the force is applied.
Correct technique, calibration, and using the proper units are all necessary for accurate results.
Key Facts
- Torque is rotational effect: τ = rF sin θ.
- For a 90 degree pull on the handle, torque is τ = rF.
- Common torque units are newton meters, N m, and pound feet, lb ft.
- 1 lb ft = 1.356 N m.
- A click-type torque wrench signals the set torque but does not prevent over-tightening if you keep pulling after the click.
- Pull from the marked handle grip and keep the wrench square to the bolt for best accuracy.
Vocabulary
- Torque
- Torque is the turning effect of a force about an axis, equal to force times the perpendicular lever arm.
- Lever arm
- The lever arm is the perpendicular distance from the axis of rotation to the line of action of the force.
- Click-type torque wrench
- A click-type torque wrench is a calibrated wrench that makes a click when the preset torque is reached.
- Calibration
- Calibration is the process of checking and adjusting a measuring tool so its readings match a known standard.
- Socket
- A socket is a removable tool head that fits over a nut or bolt head and connects it to the wrench drive.
Common Mistakes to Avoid
- Pulling after the click is wrong because the click is the signal to stop, and extra motion can apply more torque than intended.
- Using the wrong units is wrong because 80 N m and 80 lb ft are not the same torque, so the fastener may be under-tightened or over-tightened.
- Holding the wrench away from the marked grip is wrong because it changes the effective lever arm and can make the torque reading inaccurate.
- Using a torque wrench to loosen stuck bolts is wrong because sudden high loads can damage the calibration or internal mechanism.
Practice Questions
- 1 A mechanic pulls perpendicular to a 0.35 m torque wrench with a force of 120 N. What torque is applied to the bolt?
- 2 A wheel lug nut must be tightened to 95 N m. If the torque wrench handle length is 0.50 m and the pull is perpendicular, what force should be applied?
- 3 A student sets a click-type torque wrench correctly but keeps pulling after it clicks. Explain what may happen to the bolt or threads and why this is not proper technique.