A torque wrench is a tool used to precisely set the force of a fastening such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms. A torque wrench is used where the tightness of screws and bolts is crucial. It allows the operator to measure the torque (rotational force) applied to the bolt so it can be matched to the specifications. This permits proper tension and loading of all parts. A torque wrench indirectly measures bolt tension. The technique suffers from inaccuracy due to inconsistent friction between the fastener and its mating hole. Measuring bolt tension (bolt stretch) is more accurate but most often torque is the only means of measurement possible.
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Drives
1/4 torque wrench
3/8 torque wrench
1/2 torque wrench
3/4 torque wrench
Brands
ACDelco torque wrench
Capri Tools torque wrench
CDI torque wrench
Craftsman torque wrench
Deko torque wrench
DeWalt torque wrench
EPAuto torque wrench
etoolab torque wrench
GearWrench torque wrench
Growneer torque wrench
Hazet torque wrench
Klein Tools torque wrench
Lexivon torque wrench
Milwaukee torque wrench
Neiko torque wrench
Precision Instruments torque wrench
Proto torque wrench
Snap-On torque wrench
Suzume torque wrench
Tekton torque wrench
Vevor torque wrench
Vortex torque wrench
Wett torque wrench
Click torque wrench
A more sophisticated method of presetting torque is with a calibrated clutch mechanism. At the point where the desired torque is reached, the clutch slips, signaling the desired torque and preventing additional tightening. The most 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 a positive action at the set point. 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.
Digital torque wrench
With electronic (indicating) torque wrenches, measurement is by means of a strain gauge attached to the torsion rod. The signal generated is converted by the transducer to the required unit of force (Nm, lbf.ft etc.) 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.
Beam Type
The simplest form of torque wrench consists of a long lever arm between the handle and the wrench head, made of a material which will bend elastically a little under the applied torque. A second smaller bar carrying an indicator is connected back from the head in parallel to the lever arm. This second arm is under no strain at all, and remains straight. A calibrated scale is fitted to the handle, and the bending of the main lever causes the scale to move under the indicator. When the desired indicated torque is reached, the operator stops applying force. This type of wrench is simple but not very precise.
A more sophisticated beam type has a dial gauge indicator on its body, which can be preset to a value so that a visible and/or electrical indication is given when the preset torque is reached.
Deflecting beam
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 wrench 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 see and hear when a dual-signal wrench reaches the selected torque, since the signal can be seen and heard.
Programmable electronic torque wrench
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 (N m) (e.g. 50 N m + 90° - here the 50 N m means the snug point/threshold and +90° indicates that an additional angle has to be applied after the threshold).
"No-hub" wrench
This is a specialized torque wrench used by plumbers to tighten the clamping bands on "hubless" soil pipe couplings. It is a T-handled wrench with a one-way combination ratchet and clutch, factory calibrated to slip at a torque sufficient to seal the coupling, but insufficient to damage it. Since the ratchet is not reversible, the shaft of the wrench incorporates a folding auxiliary handle for loosening the clamps.
Mechatronic torque wrench
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.
Differences between types
Click type torque wrenches are more precise when properly calibrated—however the more complex mechanism can result in them losing calibration far quicker than the beam type, where there is little to malfunction. Beam type torque wrenches are impossible to use in situations where the scale cannot be 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. There is also the issue of increased user error with the beam type—the torque has to be read off each and every use.
For the click type, when not in use, the force acting on the spring should be removed by setting the scale to 20% of full scale in order to maintain the spring's strength. Never set a micrometer style torque wrench to zero as the internal mechanism requires a small amount of tension in order to prevent tool failure due to unwarranted tip block rotation. 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 5 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.
How to use a torque wrench
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 cheater bars to extend from the handle end can damage the wrench.
Using a socket extension does not require adjustment of the torque setting.
Storage
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.
Foot-pound or pound-foot
Foot-pound is used as a unit of torque.
In the United States this unit is often used to specify, for example, the tightness of a bolt or the output of an engine. Although they are dimensionally equivalent, energy (a scalar), and torque (a vector) are distinct physical quantities. Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.
One foot-pound is approximately 1.355818 Nm (Newton meters.)
The name "pound-foot", intended to minimize confusion with the foot-pound as a unit of work, was apparently first proposed by British physicist Arthur Mason Worthington. However, the torque unit is usually still referred to as the foot-pound (ft·lb or ft·lbf).
Nm or Newton metre
A newton metre is a unit of torque (also called "moment") in the SI system. The symbolic form is Nm or N·m, and sometimes hyphenated newton-metre. One newton metre is equal to the torque resulting from a force of one newton applied perpendicularly to a moment arm which is one metre long.
1 newton metre = 0.7375621 foot-pound
Source and additional information: Torque Wrench