Torque tightening is quick and fine for most standard bolts, but friction makes the final bolt load vary. Tensioning stretches the bolt directly, so it gives a more accurate and even load, which matters on large or critical joints.
What you are really trying to control
The goal of tightening is the right preload: the clamping force the stretched bolt puts on the joint. Torque is only an indirect way of getting there.
How torque tightening works, and its limits
With a torque wrench, you turn the nut until a set torque is reached. Most of that effort goes into overcoming friction under the nut and in the thread. A commonly quoted rule of thumb is that only around a tenth of the torque ends up stretching the bolt.
- Friction decides the result. Oil, coatings, dirt or surface finish change the friction, so the same torque can give a very different preload. A spread of around plus or minus 25% is common.
- Big bolts need huge torque. That means large tools and reaction arms, and on site it can lead to unsafe hammering or heating.
- The bolt is twisted as well as stretched, which uses up some of its strength.
For most small and medium bolts this is perfectly acceptable, which is why torque is the everyday method.
How hydraulic tensioning works
A hydraulic tensioner fits over the bolt. Hydraulic pressure pulls the bolt straight along its axis and stretches it. While it is stretched, the nut is run down by hand. When the pressure is released, the nut holds the stretch.
- Accurate: the load comes from the hydraulic pressure, not from friction.
- Even: several tensioners can be linked to tighten many bolts at the same time, which gives an even clamp on flanges.
- No twist in the bolt.
- Needs space: the bolt must stick out above the nut, and there must be room for the tool. A little load is lost when the pressure is released, and the procedure allows for it.
Boltight hydraulic tensioners are used on wind turbines, flanges, power plants and oil and gas joints.
Mechanical tensioning: a third option
A Superbolt multi-jackbolt tensioner replaces the main nut with a nut body holding a ring of small jackbolts. Tightening the small jackbolts with ordinary hand or air tools creates a high load on the main bolt. There is no hydraulic equipment, and it suits large bolts on heavy machines, presses, crushers and mills.
Which method should you use?
| Torque | Hydraulic tensioning | Mechanical tensioning | |
|---|---|---|---|
| Accuracy of preload | Depends on friction | High | Good |
| Many bolts at once | One at a time | Yes, with linked tools | One nut at a time |
| Tools | Torque wrench | Pump, hoses and tensioners | Hand or air tools |
| Best for | Small and standard bolts | Critical flanges, wind, power | Large bolts, heavy machines, retrofits |
Keeping the load once it is set
Getting the right preload is half the job. Vibration and settling can still take it away. Read why bolts come loose to see how to keep joints tight after tightening.