Torque Wrench Used With an In-Line Extension
A line-maintenance task requires the four mounting bolts of a hydraulic pump to be torqued to 90 N·m. Access is restricted, so the calibrated torque wrench must be used with a rigid in-line extension. The wrench has an effective length of 300 mm measured from the centre of its square drive to the centre of the hand grip; the extension adds a further 60 mm in line with the wrench. The tool tag shows a current calibration. A second fastener on the same installation is specified in Imperial units as 800 lbf·in. The engineer intends to set the wrench directly to 90 on the metric scale for the first bolts and to 800 on the dual scale for the second, using the extension both times.
1. Calculate the indicated wrench setting needed to achieve 90 N·m at the bolt with the extension fitted, and state the principle behind the correction.
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Principle. The wrench clicks when the bending moment at its own drive reaches the set value. With an in-line extension, the hand is further from the fastener, so the same hand force produces a larger moment at the bolt than the wrench indicates. The extension therefore acts as a torque multiplier.
Correction formula (in-line extension only):
T_indicated = T_desired × L / (L + E)
where L = 300 mm (wrench length) and E = 60 mm (extension).
T_indicated = 90 × 300 / 360 = 75 N·m
Check: at a 75 N·m indication the hand force is F = 75 / 0.300 = 250 N. Applied at (0.300 + 0.060) = 0.360 m from the bolt, this gives 250 × 0.360 = 90 N·m at the fastener. Correct.
Judgement points: the correction applies only to extensions in line with the wrench handle. A crows-foot set at 90° to the handle does not change the moment arm and needs no correction. The extension must be rigid and free of slop, and the calibration must remain valid for the assembly used.
2. Convert the second fastener's specification of 800 lbf·in to SI units and give the indicated setting required with the extension fitted.
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Conversion: 1 lbf·in = 0.112985 N·m
800 × 0.112985 = 90.39 N·m ≈ 90.4 N·m
(Sanity check in the other direction: 90.4 / 0.112985 ≈ 800, and 1 N·m ≈ 8.851 lbf·in, so 90.4 × 8.851 ≈ 800. Consistent.)
Indicated setting:
T_indicated = 90.4 × 300 / 360 = 75.3 N·m (round to 75 N·m on a typical scale)
Why this matters: the two fasteners are, to within rounding, the same torque expressed in different unit systems. They should not be treated as different tasks, and the same extension correction applies to both. Converting first and correcting second avoids the common error of applying the length correction to the Imperial figure while the wrench is set on the metric scale.
3. If the wrench is set to 90 N·m with the extension in place, what torque does the bolt actually see? Explain the consequence and why the dual scale does not solve the problem.
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Actual torque at the bolt:
T_actual = T_set × (L + E) / L = 90 × 360 / 300 = 108 N·m
That is 18 N·m, or 20 %, over the specified value - a serious overtorque on a pump mount.
Consequences:
- Bolt preload rises by roughly 20 %, eating into the margin between working load and proof load; the bolt may yield or fail in service.
- Threads in an aluminium or magnesium housing can strip; the housing is often the more expensive part.
- Clamping distortion of the pump flange can misalign the drive coupling.
- The joint is now non-compliant, so it cannot be signed off as meeting the manual.
Why the dual scale is irrelevant: the dual scale only converts units on the same dial; it knows nothing about the tool's effective length. It cannot compensate for a moment-arm change. The correction must be calculated by the person doing the task and recorded, and any task that needs an extension should have the corrected figure written on the job card so the next shift does not repeat the error.