Maintenance and Use of a Torque Wrench

Use of a torque wrench prevents stripped threads and under-tightening.

A technician applies clockwise force with a wrench to tighten a standard nut or bolt. This twisting moment is known as "torque." A torque wrench is designed to tighten a fastener to a predetermined setting. This is usually measured in foot pounds, but the scale could be calibrated to inch pounds or Newton meters. There are two types of torque wrench available: the flat beam and the mechanical "clicker" type.

Flat Beam Torque Wrench

The flat beam wrench consists of a precision ground alloy steel beam with a square socket head on one end and a pivoting handle on the other. The needle on a vernier scale situated near the handle measures the deflection imposed on the beam when under load. The geometric setup of the pivoting handle focuses the force on a specific point on the beam, regardless of the various tightening techniques used by different technicians. This ensures repeatable torque accuracy, whether tightening or loosening a fastener.

Using a Beam Torque Wrench

When torquing a fastener, the technician places the correct size socket on the head of the nut or bolt and pulls the handle of the wrench toward his body. This allows him to impose a steady force without causing the needle on the vernier scale to fluctuate. It also draws the scale closer, making it easy to read. When the needle reaches the predetermined setting, the technician relaxes pressure on the handle.

Maintenance and Calibration

Since the flat beam torque wrench has no moving parts, it is virtually maintenance-free. The rugged design gives this tool an indefinite service life. If the needle on the vernier scale points to zero when the wrench is at rest, the tool is accurate to within 2 percent. However, if used daily in an operation such as an assembly line, the tool should be sent to a calibration specialist once a year for testing and calibrating if necessary, and to renew its certification of accuracy.

Mechanical Torque Wrench

The "clicker" type mechanical torque wrench incorporates a spring-loaded pawl and ratchet assembly. This causes the socket to slip with an audible "click" when the predetermined torque load is applied.

Using a Mechanical Torque Wrench

The technician sets the required torque by turning a knurled knob on the end of the handle and reading the setting on a micrometer scale on the barrel of the tool. The correct socket is used and the technician places the socket on the hexagonal head of the fastener. She then pulls the handle smoothly toward her body until the tool "breaks" with an audible click.

Care, Maintenance and Calibration

A mechanical torque wrench must be handled with care to avoid damage. The tool should never be dropped, slid across the floor, or tossed carelessly into a toolbox. A torque wrench should be kept separate from the rest of the tools and stored in its own case. To avoid stressing the spring controlling the pawl and ratchet, the technician should always restore the torque setting to zero after using the tool. After an extended idle period, the technician should adjust the wrench to half the required setting and work the mechanism five or six times before using the tool. A wrench used regularly should be sent to a firm specializing in torque wrench calibration at least twice a year for adjustment and certification.

References

• Web Bike World: Torque Wrench
• Sturtevant Richmont: M Series Memory Flat Beam Torque Wrenches
• Torque Wrench Repair - Torque Wrench Calibration - Torque Wrench Certification