Controlled torque-tension test
Same torque. Three different Tension results.
Read The Full Test ReportWhat the controlled test showed
One torque input. A 178 kN tension difference.
In a controlled torque-tension study, three M36 × 235 mm, class 10.9 fastener installations were each tightened to the same torque of 2,596 Nm.
The resulting tension ranged from 334 kN to 512 kN, representing a 48% difference between the minimum and maximum values.
- Applied torque
- 2,596 Nm
- Tension range
- 334–512 kN
- Max–min difference
- 178 kN
- Nut factor range
- 0.14–0.22
The study used new nuts and washers, Permatex Anti-Seize Lubricant, a calibrated Hytorc HY-115 pump, a calibrated STEALTH 4 Hydraulic Torque Wrench, and a Skidmore-Wilhelm Model K.
The fastener installation and calibrated equipment used in the controlled torque-tension study.
Why the results vary
The Nut factor.
The nut factor (K) represents the combined effect of friction and other conditions on the relationship between applied torque and resulting bolt tension. When these conditions vary, the same torque can produce different levels of tension.
Where applied torque goes
Most applied tightening torque is lost to friction under the bolt head (1), in the threads (2) and between mating surfaces (3). Only a small portion stretches the bolt and generates preload.
- 10–15%
- Contributes to preload
- 85–90%
- Overcomes friction
These are commonly used approximate ranges. The exact distribution varies with fastener geometry and joint conditions.
See How It Works
Torqued. But is it tight?
We make tension visible.
Why are you torquing the bolt? To create tension. And tension creates the clamp force that holds your joint together.
But reaching the specified torque doesn’t confirm the tension achieved. Variations in the nut factor can leave a bolt under or over tightened. If the bolt bottoms out, you can register torque without clamping the joint at all.
SmartBolts® make that unknown visible. Our Visual Indication System™ responds to bolt stretch, confirming when the required tension has been reached.
You can see that the torque you applied has created the tension you need.
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The boltstretches
Tightening stretches the fastener and creates the tension responsible for clamp force.
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The gage pinresponds
An internal gage pin moves with the bolt’s elongation and activates the calibrated indicator.
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The colorchanges
Red indicates loose. The indicator changes to black when the required tension is reached.
Could SmartBolts work for you?
Tell us about your equipment, operating conditions and bolt requirements. Our engineering team can help determine whether SmartBolts® are suitable for your application.
