Torque Calibration Theory

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Torque is a measure of the force that causes a rotational motion of an object around an axis. It acts as a twisting or turning force on an object as in opening the lid of a jar or in tightening a bolt. The amount of torque applied to the object determines how tight an object will be fastened or how fast the object will rotate.

Torque is very important for mechanics and maintenance technicians in many critical industries such as in automotive or aviation. Accurate torque readings are required to ensure that the torque placed on bolts and other fasteners is within the specifications for the job being performed. Fasteners that are not tightened correctly can become loose and too much torque can damage the threads on the fasteners. The failure of a low-cost fastener that isn’t tightened properly can lead to critical and catastrophic failures.

Torque is calculated by multiplying the force applied to an object by the distance from the center of rotation or axis that the force is applied. In its simplest form, the formula for torque is T=F x L, where T is the torque, F is the force and L is the linear length from the axis. In the case of tightening a bolt, the torque is equal to the amount of force your arm is using to move the wrench times the distance from the head of the bolt to where your hand is positioned on the wrench. Moving your hand closer or farther away from the bolt will change the amount of torque being applied to the bolt.

Because torque is a product of length and force, the units will be comprised of both a force and a length reference. The SI (International Standard) system of units is referenced to Newton-Meters (Nm) and the American system is referenced to inch-pounds (in-lb). For conversion between the systems, 1 Nm is equal to 8.85 in-lb.

There is also a rotational angle component to the measurement of torque. The ‘sin’ of the angle produced by the rotation around the axis needs to be multiplied to the torque calculation for the true torque reading. In most torque applications, the force is applied perpendicular, or at 90° to the pulling axis, such as pulling on the wrench handle. Since the sin of 90° is equal to 1, a perpendicular angle has no adverse effect on the total torque applied.

Torque Equipment Calibration

There are two main types of torque equipment that are calibrated by calibration laboratories, torque measurement devices such as torque wrenches and torque watches and torque testing devices, such as torque calibrators and torque transducers.

Torque Wrench Calibration

Most calibration laboratories use highly accurate torque transducers or torque cells to read the torque measurement and a torque loader to apply the force to calibrate torque wrenches. Most systems have multiple transducers to calibrate a wide range of torque values.

The torque loader is an important piece of equipment to calibrate torque wrenches to high accuracy. The torque loader is a device that assists in the loading application on the torque wrench. Using a loader ensures that the force is always at the correct load point on the torque wrench and that the torque angle is always perpendicular to eliminate any transducer side and end load factors. They allow torque wrenches to be tested by different calibration technicians the same way every time which eliminates potential operator-induced test errors.

ASME standards for the calibration of torque wrenches require that they are checked at 20%, 60% and 100% of tool capacity. Most torque wrenches are calibrated in both the clockwise and counter-clockwise directions, however, some torque wrenches are only made to be calibrated in the clockwise (tightening) direction and although they can be used in the counter-clockwise direction, there are no accuracy specifications for the applied torque in that direction.

Torque Calibrator/Transducer Calibration

The calibration of torque calibrators and torque transducers are also based on the torque formula. Weights are used to apply the force and a torque arm or torque wheel is used to establish the length. A torque arm or torque wheel is a device with a calibrated length that the weights are hung from. For example, a 10 lb. weight hung from a torque arm with a length of 10 inches will produce a torque of 100 in-lbs.

The required weights and the appropriate torque arm length is determined by the maximum torque range of the calibrator or transducer being calibrated. Typically, torque values are calibrated at points that represent 10%, 20%, 40%, 60% 80% and 100% of the full-scale value of the unit. Both clockwise and counter-clockwise directions are calibrated.

The measurement of torque and using quality torque calibration equipment is essential for many companies to ensure their product’s quality, safety and reliability.

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