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7-3. Tightening Torque Variation
The tightening torque of an impact wrench may vary slightly in accordance with the factors described below.
Salespersons are requested to advise the customer to confirm that appropriate tightening torque is obtained by
measuring the torque at the beginning of the tightening operations, and as necessary during tightening
operations. In addition, the torque values shown in Para. 6-2, 6-3 and 6-4 above are useful as a handy reference,
and may be utilized as tentative standards.
(1) Voltage
The tightening torque of an impact wrench is influenced by voltage. For example, the tightening torque will be
reduced to about 90% if the Model WR 22SA is operated with 90% of the rated voltage. The line voltage may
be reduced by the use of a long extension cord depending on the work sites. Take the following
countermeasures against voltage reduction caused by the resistance of an extension cord and the load
current.
Decrease the resistance of an extension cord.
Use an extension cord which is as short as possible and has a thick core.
Compensate for reduction of the voltage by using a step up transformer.
At this time, prevent the plug voltage of the main body from exceeding the rated voltage.
Advise the customer to use an extension cord which is as thick as possible (cross sectional area of the core of
the extension cord: 1.25 mm
2
or larger) if it is unavoidable to use a long extension cord on a building site. In
addition, recommend the customer to use a step up transformer to step up the voltage by the amount of
reduction previously.
(2) Effects of low ambient temperatures
The tightening torque required may be reduced at low ambient temperatures or under the influence of grease
and different torque coefficients (dependent on manufacturing and finishing processes, and specified by bolt
manufacturers).
(3) Different bolt diameter
Required levels of tightening torque may vary according to the bolt diameters. Generally speaking, the higher
tightening torque is required for the larger diameter bolt.
(4) Different materials being tightened
When a bolt is tightened into a soft material such as aluminum, plastic, wood, etc., the tightening torque
becomes considerably lower than when the bolt is tightened into a hard material such as steel.
(5) Different tightening conditions
The tightening torque may vary in accordance with bolt torque coefficient (dependent on manufacturing
processes, and specified by bolt manufacturers), bolt grade and bolt length, even though the diameters of the
bolts are the same. Tightening torque may also vary depending on the surface finishing state of tightening
materials (steel, aluminum, etc.) and materials to be tightened. In addition, if there is a seal packing or a
clearance between tightening materials, the tightening torque is decreased.