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A torque wrench is a
device used to precisely set the torque 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 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.
Beam Type of torque wrench
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.
Click Type of torque wrench
A more sophisticated method of presetting torque is using a calibrated
clutch mechanism. At the point where the desired torque is reached, the
clutch slips, preventing over tightening. The commonest 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.
Differences between the two types of torque wrenches
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 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.
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