How to Calibrate a Torque Wrench

What You'll Need
Torque Wrench
20 pound Weight
Marker or Pencil

A torque wrench is a tool that is used to apply a certain torque to a fastener like a nut or bolt. It looks like a socket wrench with special mechanisms in it. It prevents bolts from being too tight on underground water main and steam pipe repairs. Torque wrenches are used when screws and bolts must be tightened as much as they can be. Torque wrench calibration is important to the safety and effectiveness of repairs for the car and home, and for the torque wrench itself to function properly. You can use a torque wrench safely to suit your needs by keeping it calibrated. Below is a step-by-step guide to calibrate a torque wrench.

Step 1 – Marking Points

Use a pencil or marker to mark the center point of the wrench head on the back of the torque wrench.

Step 2 – Taking the Measurements

Next, take the measurements. Measure the center point to the point where you apply the most pressure when you use the wrench. Write this measurement down as 'distance one'. If the wrench measures in inch pounds, write down this measurement in inches as 'distance one'. If the wrench measures in feet pounds, write down the distance measurement in feet. 

Step 3 – Using the Weights

Use the vise to horizontally clamp the wrench bit. Hang a twenty pound weight from the wrench handle using the string.

Step 4 – Total Measurements

Move the weight along the wrench handle until it measures at 40 foot pounds or 480-inch pounds. Measure the distance from the center point on the wrench head to the string and write this measurement down as "distance 2."

Step 5 – Calibration Ratio

Using a calculator, divide distance 2 by distance 1 and this will give you the calibration ratio. The ratio is the difference between the wrench settings and the force needed to get a “click” at that setting.

Step 6 – Setting Torque Wrench

Set your torque wrench for a specific application. You can do this by taking the torque of the bolt and multiplying the required torque of the bolt by the calibration ratio. 

Torque wrenches should be calibrated annually. Expect this to cost about $25 to $35 if you take your torque wrench to a decent shop to be calibrated properly. You can purchase a digital adapter for torque which lets you calibrate the wrench yourself. The digital torque costs around $50. Adjustment and repair of the torque wrench usually runs around $15 per quarter hour.

Step 7 - Storage

Keep the torque wrench lubricated and clean. After each use, always turn the scale back to zero to prevent the spring inside the wrench from setting and causing the calibration to drift. The torque wrench is the only practical way to measure bolt tension. Proper maintenance ensures a longer life for the torque wrench.