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A torque wrench is a tool used to apply precisely a specific torque to a fastener such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms. It was invented by Conrad Bahr in 1918 while working for the New York City Water Department. It was designed to prevent overtightening bolts on water main and steam pipe repairs underground.
A torque wrench is used where the tightness of screws and bolts is crucial. It allows the operator to measure the torque applied to the fastener so it can be matched to the specifications for a particular application. This permits proper tension and loading of all parts. A torque wrench measures torque as a proxy for bolt tension. The technique suffers from inaccuracy due to inconsistent or uncalibrated friction between the fastener and its mating hole. Measuring bolt tension (indirectly via bolt stretch) is actually what is desired, but often torque is the only practical measurement which can be made.
Torque screwdrivers and torque wrenches have similar purposes and mechanisms.
- 1 Types
- 2 Using torque wrenches
- 3 See also
- 4 References
- 5 External links
The most basic form of torque wrench consists of two beams. The first is a lever used to apply the torque to the fastener being tightened and serves also as the handle of the tool. When force is applied to the handle it will deflect predictably and proportionally with said force in accordance with Hooke's law. The second beam is only attached at one end to the wrench head and free on its other, this serves as the indicator beam. Both of these beams run parallel to each other when the tool is at rest, with the indicator beam usually on top. The indicator beam's free end is free to travel over a calibrated scale attached to the lever or handle, marked in units of torque. When the wrench is used to apply torque, the lever bends and the indicating beam stays straight. Thus the end of the indicating beam points to the magnitude of the torque that is currently being applied. This type of wrench is simple, inherently accurate, and inexpensive.
The beam type torque wrench was developed in the late 1920s/early 1930s by Walter Percy Chrysler for the Chrysler Corporation and a company known as Micromatic Hone. Paul Allen Sturtevant—a sales representative for the Cedar Rapids Engineering Company at that time—was licensed by Chrysler to manufacture his invention. Sturtevant patented the torque wrench in 1938 and became the first individual to sell torque wrenches.
A more sophisticated variation of the beam type torque wrench has a dial gauge indicator on its body that can be configured to give a visual indication, or electrical indication, or both when a preset torque is reached. 
The dual-signal deflecting beam torque wrench was patented by the Australian Warren and Brown company in 1948.  It employs the principle of applying torque to a deflecting beam rather than a coil spring. This helps prolong the accuracy of the wrench throughout its working life, with a greater safety margin on maximum loading and provides more consistent and accurate readings throughout the range of each wrench. The operator can both hear the signal click and see a visible indicator when the desired torque is reached. 
A slipper type torque wrench consists of a roller and cam (or similar) mechanism. The cam is attached to the driving head, the roller pushes against the cam locking it in place with a specific force which is provided by a spring (which is in many cases adjustable). If a torque is demanded which is able to defeat the holding force of the roller and spring, the wrench will slip and no torque will be applied to the bolt. A slipper torque wrench will not over tighten the fastener by continuing to apply torque beyond a predetermined limit.
A more sophisticated method of presetting torque is with a calibrated clutch mechanism. One common form uses a ball detent and spring, with the spring preloaded by an adjustable screw thread, calibrated in torque units. The ball detent transmits force until the preset torque is reached, at which point the force exerted by the spring is overcome and the ball "clicks" out of its socket. The advantage of this design is greater precision and both tangible and audible feedback on reaching the desired torque. The wrench will not start slipping once the desired torque is reached, it will only click and bend slightly at the head, the operator can continue to apply torque to the wrench without any additional action or warnings from the wrench.   For this reason, it is important to stop applying torque as soon as the wrench gives the click sound. Typical accuracy level would be +/- 4% > 10 Nm and +/- 6% < 10 Nm.
A number of variations of this design exist for different applications and different torque ranges. A modification of this design is used in some drills to prevent gouging the heads of screws while tightening them. (The drill will start slipping once the desired torque is reached).
These are specialised torque wrenches used by plumbers to tighten the clamping bands on hubless soil pipe couplings. They are usually T-handled wrenches with a one-way combination ratchet and clutch. They are preset to a fixed torque designed to adequately secure the coupling but insufficient to damage it. 
Electronic torque wrenches
With electronic (indicating) torque wrenches, measurement is by means of a strain gauge attached to the torsion rod. The signal generated by the transducer is converted to the required unit of torque (e.g. N·m or lbf·ft) and shown on the digital display. A number of different joints (measurement details or limit values) can be stored. These programmed limit values are then permanently displayed during the tightening process by means of LEDs or the display. At the same time, this generation of torque wrenches can store all the measurements made in an internal readings memory. This readings memory can then be easily transferred to a PC via the interface (RS232) or printed straight to a printer. A popular application of this kind of torque wrench is for in-process documentation or quality assurance purposes. Typical accuracy level would be +/- 0.5% to 4%.
Programmable electronic torque / angle wrenches
Torque measurement is conducted in the same way as with an electronic torque wrench but the tightening angle from the snug point or threshold is also measured. The angle is measured by an angle sensor or electronic gyroscope. The angle measurement process enables joints which have already been tightened to be recognised. The inbuilt readings memory enables measurements to be statistically evaluated. Tightening curves can be analysed using the software via the integrated tightening-curve system (force/path graph). This type of torque wrench can also be used to determine breakaway torque, prevail torque and the final torque of a tightening job. Thanks to a special measuring process, it is also possible to display the yield point (yield controlled tightening). This design of torque wrench is highly popular with automotive manufacturers for documenting tightening processes requiring both torque and angle control because, in these cases, a defined angle has to be applied to the fastener on top of the prescribed torque (e.g. 50 N·m or 37 lbf·ft + 90° - here the 50 N·m or 37 lbf·ft means the snug point/threshold and +90° indicates that an additional angle has to be applied after the threshold).
Saltus-Werk Max Forst GmbH applied in 1995 for an international patent for the first electronic torque wrench with angle measurement which did not require a reference arm.
Mechatronic torque wrenches
Torque measurement is achieved in the same way as with a click-type torque wrench but, at the same time, the torque is measured as a digital reading (click and final torque) as with an electronic torque wrench. This is, therefore, a combination of electronic and mechanical measurements. All the measurements are transferred and documented via wireless data transmission. Users will know they have achieved the desired torque setting when the wrench "beeps."
Using torque wrenches
Click type torque wrenches are precise when properly calibrated—however the more complex mechanism can result in loss of calibration sooner than the beam type, where there is little to no malfunction, (however the thin indicator rod can be accidentally bent out of true). Beam type torque wrenches are impossible to use in situations where the scale cannot be directly read—and these situations are common in automotive applications. The scale on a beam type wrench is prone to parallax error, as a result of the large distance between indicator arm and scale (on some older designs). There is also the issue of increased user error with the beam type—the torque has to be read at every use and the operator must use caution to apply loads only at the floating handle's pivot point. Dual-beam or "flat" beam versions reduce the tendency for the pointer to rub, as do low-friction pointers.
Using handle or socket extensions requires no adjustment of the torque setting.
using a combination of handle and crow's foot extensions requires the use of the following equation:
- is the wrench indicated torque (setting torque),
- is the desired torque,
- is the length of the torque wrench, from the handle to the centre of the head,
- is the length of the crow's foot extension, from the centre of the torque wrench head to the centre line of the bolt,
- is the length of the handle extension, from the extension end to the torque wrench handle.
These equations only apply if the extension is colinear with the length of the torque wrench. In other cases, the distance from the torque wrench's head to the bolt's head, as if it were in line, should be used. If the extension is set at 90° then no adjustment is required. These methods are not recommended except for extreme circumstances.
For click (or other micrometer) types, when not in use, the force acting on the spring should be removed by setting the scale to its minimum rated value in order to prevent permanent set in the spring. Never set a micrometer style torque wrench to zero as the internal mechanism requires a small amount of tension in order to prevent components shifting and reduction of accuracy. If a micrometer tool has been stored with the setting above 20% the tool should be set to 50% of full scale and exercised at least FIVE times before being used. In the case of the beam type, there is no strain on the component that provides the reference force except when it is in use, therefore, accuracy is inherent.
As with any precision tool, torque wrenches should be periodically re-calibrated, this should happen every 5000 operations or every year, whichever comes first. In cases where the torque wrench is in use in an organisation which has its own quality control procedures, then the calibration schedule can be arranged according to company standards. If a torque wrench is operated at loads 25% or more over the maximum torque, then it must be re-calibrated.
- US 2231240, Zimmerman Herman W, "Torque measuring wrench", published Feb 11, 1941
- US 2167720, Willard C Kress, "Torque-indicating wrench", published Aug 1, 1939
- "DIAL TORQUE WRENCH REPAIR, MAINTENANCE AND TROUBLESHOOTING MANUAL" (PDF). CDI Torque Products. 2002.
- "Warren & Brown company history". Warren & Brown.
- "Warren & Brown Precision Tools Catalogue" (PDF). Warren & Brown.
- US 1860871, Wilfred A Pouliot, "Safety wrench", published May 31, 1932
- Tegger. "How does a torque wrench work?". "The Unofficial Honda / Acura Usenet FAQ".
- US 4485703, Bosko Grabovac & Ivan Vuceta, "Torque wrench", published Dec 4, 1984
- "Raptor No-Hub Torque Wrenches" (PDF). "Raptor tools".
- "Premium and Standard Manual Torque Wrenches Pre-Set and Adjustable" (PDF). ASG Jergens, Inc.
- Boeing 737-200 maintenance manuals. Wikileaks. 20. 2007. pp. 202–203.
- "Torque Wrench Extension Calculator". Norbar Torque.
- "CROWFOOT ADAPTERS". Belknap Inc.
- "The ten things you should know about your torque wrench". Norbar Torque. 2015.
- "Proper Torque Wrench Use and maintenance (Technical reference)" (PDF). Snap-on Tools. 2008.
- ISO6789 - Assembly tools for screws and nuts. Hand torque tools. Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure. International Organization for Standardization. 2003.
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