Pneumatic torque wrenches and proper bridge bolt load

For many years the tightening and torquing of bridge bolts traditionally have been done with impact guns being checked on a Skidmore/Calibore bolt load testing device for verification.   While this has always been acceptable in the past, many engineering and inspection firms are now requiring calibrated shut off tools to be used in place of impact guns. The Skidmore/Calibore used in conjunction with an impact gun verifies that that tool has the capacity to go to the required bolt load. The issue with this method, is that it’s operator dependent on the time the tool operator continues to hold down the trigger on the impact. There is no shut off when the correct bolt load is achieved. Therefore, the tool operator could over or under apply the proper bolt load required.

The answer to this issue is to use a calibrated pneumatic torque wrench, in place of the impact gun.

Pneumatic torque wrenches are calibrated on a torque transducer to +/- 5 % accuracy, but more importantly, they have a torque repeatability of +/- 3% at the point where they shut off when achieving the set air pressure/ torque.

The following is the proper set up of a pneumatic torque wrench used on bridge bolts:

  • A common bridge bolt , 7/8” diameter, grade A325, will have target bolt load of 38 -50 Kips.
  • The typical torque range of this bolt and size and grade is 500-700 ft/lbs. (see charts for this)
  • Use a calibrated pneumatic torque wrench that has this torque range capacity
  • Test this bolt with the pneumatic torque wrench on a Skidmore/Calibore device to achieve required Kips
  • Lock in the set air pressure on the pneumatic torque wrench regulator when the tool tightens to required Kips (bolt load)
  • The pneumatic torque wrench will now stall out at that air pressure point, achieve desired kips/load with a tool repeatability of +/- 3%.
  • All the tool operator now has to do is to tighten the bolt until the tool automatically shuts off.

It should be noted also this this pneumatic torque wrench method of bridge bolt tightening is much faster than using and impact gun also. It’s proven out on the Skidmore/Calibore the pneumatic torque wrenches achieve the correct bolt load 2 time faster than impact guns.

For more information on calibrated pneumatic torque wrenches, contact Maxpro Corporation who is a master dealer for RAD Pneumatic torque wrenches and an ISO 17025 accredited calibration lab for this equipment.

 

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