Torque wrenches are precision hand tools that are used to apply a specific amount of torque to a fastener. These adjustable-torque tools are primarily used to prevent over or under tightening of fasteners. Both over and under tightened fasteners are hazardous in different ways that could result in the eventual failure of the fastener.

Torque wrenches should only be used for tightening fasteners, and never for loosening.  The torque required to remove a fastener may be greater than the torque used to tighten it.

Types of Torque Wrenches

The most common type of torque wrenches resembles a socket wrenches with a long handle to apply torque and special internal mechanisms.  Other torque wrench head types are also available in addition to socket style, including head types resembling open-ended spanner wrenches, ring spanner wrenches, adjustable head wrenches, and hook spanner wrenches.

Torque wrenches fall under two main categories:

• wrenches that apply a predetermined amount of torque
• wrenches that measure the amount of torque applied 

Torque wrenches are manufactured with many different internal mechanisms, the most common of which are listed below. 

Beam type torque wrenches measure the amount of torque and are manufactured with a material that bends when a torque is applied. Beam torque wrenches have a secondary, small bar that is not subjected to toque and does not move when torque is applied.  An analog indicator that displays the torque applied is mounted to the handle. Beam torque wrenches are the least expensive, simplest and most durable type of torque wrench, however they are not the most accurate. 

Deflecting beam torque wrenches, also referred to as dual-single deflecting beam, transmit torque via a bending beam, as opposed to a coil spring as found in some other torque wrenches. Dual-single deflecting beam torque wrenches were invented by Warren and Brown, an Australian company. Deflecting beam torque wrenches contain a sliding scale that can be adjusted to the desired torque level.  When the desired torque is applied, the deflecting beam contacts the sliding scale and depresses a button that makes an audible sound and releases a visible indicator. 

Click type torque wrenches make a clicking noise when the selected torque has been reached while tightening. Markings on the handle indicate the toque setting. Click type wrenches contain a ball detent and a spring that is calibrated for specified torques.  When the torque is reached, the ball "clicks" out of the detent and prevents over-torqueing.

Dial torque wrenches have an analog dial that displays the amount of torque being applied. A precision mechanism pushes a pusher rod when torque is applied, that in turn pushes a pointer on the analog dial indicator.  Dial torque wrenches are available in three main configurations.

• Plain dial torque wrenches contain a single pointer style that indicates the torque value on the dial.
• Dial with signal torque wrenches have a signal, such as a light or buzzer, which is triggered when a preset torque value is reached.
• Dial with memory torque wrenches contain a secondary pointer referred to as a memory pointer.  The memory pointer is pushed by the main pointer and indicates the highest torque level that is applied.

Electronic torque wrenches use a strain gauge to measure torque and display torque values are on a digital display.  Electronic torque wrenches are very precise tools, capable of accuracies of 1% or better.  Electronic torque wrenches have other advantages over mechanical type wrenches, including:

• the ability to recall torque measurements and display them
• count the number of times a specified torque has been applied
• connection to an external signal unit that displays a signal when the torque is reached
• output of signal readings to a PC or printer for recording or analysis
• selectable units
• wireless communication technology

Programmable electronic torque wrenches, also referred to as angle wrenches, are similar to electronic wrenches, but also measures the angle of the threshold torque using a gyroscope or angle sensor.

Mechatronic torque wrenches combine the elements of click type torque wrenches with modern electronics.  Torque readings can be indicated mechanically or digitally.

Calibration

Torque wrench calibration is critical for accurate readings.  Some wrenches, such as click type, can be calibrated using weights or fish scales, but more sophisticated wrenches should be calibrated by a certified calibration professional.  Wrenches that are dropped should have their calibration checked.  Torque wrench standards are indicated by BS EN ISO 6789 – “Assembly tools for screws and nuts – Hand Torque Tools – Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure”.

Applications

Torque wrenches are used in many applications where a nut or fastener requires tightening, including industrial and automotive. A wrench's application is largely determined by the range of torque it is designed to deliver. For example, wrenches for use in automotive or heavy industry applications must be capable of delivering high torque, while tools for use in lighter industries have lower torque capabilities.

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