Does an Impact Wrench Properly Tighten Your Bolted Joint? Hint: Probably Not.
Here’s a recent field test showing the installing of a 2”x 4.5 nut that highlights the issue.
Left: Nuts were initially tightened with an impact wrench set to 4500 ft lbs to achieve the required clamp load of 138,000 lbs.
Right: Next, the nuts were tightened with an electric reaction arm torque multiplier wrench also set to 4500 ft-lbs.
Results
An average of 30° of additional rotation was achieved when tightening with a reaction arm torque multiplier wrench versus an impact wrench. With a clamp distance of 20 inches on this assembly, this 30° translates to an additional 68,703 pounds of clamp force as shown in the downloadable calculation spreadsheet below.
* The impact torque wrench only tightened the nut to 50% of the desired clamp load
The result of this field test are also supported by the published laboratory tests shown in the table below.
| Tightening Method | Tension Scatter +/- % |
| Ultrasonic and strain gauge | 5-12 |
| Elongation control (calibrated fastener) | 10 |
| DC electric equipment (typical) | 10 |
| Torque wrench, calibrated | 22 |
| Impluse | 50 |
| Impact or hand tightening | 60 |
Sources: VDI 2230, SR1, NASA, EduPro
Take Away
We’ve outlined in previous posts (listed below) why it’s important to achieve proper clamp force when tightening a bolted joint. Impact torque wrenches are commonly used due to their speed and ease of use. However, assembly planners should take into consideration the wide variance in results often achieved when using an impact torque wrench.
- How much a bolt stretches
- Torque explanation
- How to calculate clamp force
- Rotation angle explanation
- Cyclical loading and why it’s important to keep a bolted joint tight
- Are locknuts necessary
- How does Security Locknut work
- others at www.securitylocknut.com/blog
Having problems with bolted joints? Give us a call. We can help.
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