Bolt torque and bolted joints: preload, where the torque goes, an indicative torque table, tightening methods, and why joints come loose
Bolt torque and bolted joints on plant and machinery: how preload clamps a joint, where tightening torque goes, an indicative torque table for property class 8.8 and 10.9 bolts calculated from T = K × F × d, torque, part-turn, tension indicator, hydraulic tensioning and ultrasonic methods, how joints fail from self-loosening to fatigue, and the practices that keep them tight. With a drawing of preload, the torque split and a tightening pattern.
Key takeaways
- A bolted joint works by preload: the bolt is stretched like a spring and clamps the parts together. Torque is only the usual way of getting there, and an indirect one.
- Most of the torque goes into friction, commonly about 90 per cent under the nut and in the threads, so small changes in friction make large changes in preload.
- Torque charts assume a friction condition. A torque set for a dry bolt will over-tighten a lubricated one, so the lubricant is part of the specification.
- Bolts mostly fail by loosening and fatigue when the preload is too low. Correct preload is the cure for both.
- For critical joints, use the manufacturer's or designer's values, tighten in passes and a pattern, and use part-turn, tension indicators, hydraulic tensioning or ultrasonic checks where torque scatter is too much.
A gearbox on a conveyor drive keeps loosening on its base. Each time, the fitters tighten the hold down bolts to the torque on the drawing, and a few months later they are loose again. The torque on the drawing was for dry bolts. The bolts go in with anti-seize, which reaches the same torque at a very different preload, and a bolt that has been stretched past its yield on one tightening never holds properly again.
This guide covers bolt torque and bolted joints on plant and machinery: how preload works, where the torque goes, an indicative torque table and its limits, the tightening methods, how joints fail, and the practices that keep them tight. Structural connections and pressure flanges have their own standards and procedures, named where they apply. The values here are indicative, and the equipment manufacturer's or joint designer's values take precedence.
Preload, and where the torque goes
A bolt is a spring. Tightening it stretches it slightly, and the tension in the bolt, the preload, clamps the parts together. While the clamp force holds, external loads mostly change the pressure between the parts rather than the tension in the bolt. That is why a well preloaded bolt survives vibration and load cycles, and a loose one fails by fatigue in weeks.
Torque is the usual way of producing preload, and an indirect one. A commonly cited split is that about half the torque is lost to friction under the nut face, about 40 per cent to friction in the threads, and only about 10 per cent stretches the bolt. A small change in friction, from a dry thread to a lubricated one or from a clean nut to a rusty one, moves the preload a long way for the same torque.
Specify the preload. The torque is only a way to get there, and only under the friction it was set for.
An indicative torque table
The table below is calculated, not measured. It uses T = K × F × d, with the preload F at 75 per cent of the proof load from ISO 898-1 for property classes 8.8 and 10.9, and a nut factor K of 0.20 for plain dry bolts and 0.14 for lubricated ones. Preloads are in kN and torques in N·m. Real values depend on the finish, plating, lubricant, washers and the joint, so use the equipment manufacturer's or joint designer's values where they exist.
| Size | 8.8 preload, kN | 8.8 dry, N·m | 8.8 lubricated, N·m | 10.9 preload, kN | 10.9 dry, N·m | 10.9 lubricated, N·m |
|---|---|---|---|---|---|---|
| M10 | 25 | 50 | 35 | 36 | 72 | 51 |
| M12 | 37 | 88 | 62 | 53 | 126 | 88 |
| M16 | 68 | 219 | 153 | 98 | 313 | 219 |
| M20 | 110 | 441 | 309 | 153 | 610 | 427 |
| M24 | 159 | 762 | 534 | 220 | 1,055 | 738 |
| M30 | 252 | 1,515 | 1,060 | 349 | 2,095 | 1,467 |
Tightening methods
Torque control is quick and suits most joints. Where the preload has to be closer, methods that measure stretch or rotation rather than torque do better. In Australia, structural steel connections designed as tensioned joints are tensioned by the part-turn method or with direct tension indicators under AS 4100 and AS/NZS 5131, not by torque alone.
| Method | How | Notes |
|---|---|---|
| Torque | A calibrated torque wrench or tool to a set torque | Simple and quick, but friction scatter means preload can vary widely bolt to bolt |
| Torque then angle (part-turn) | Snug tight, then a set rotation of the nut | Less sensitive to friction, the basis of structural bolt tensioning |
| Direct tension indicators | Washers with bumps that flatten at the required tension | Shows the tension has been reached, checked with a feeler gauge |
| Hydraulic tensioning | A tool stretches the bolt directly, the nut is run down, then the load released | Accurate and even on large bolts and critical joints, needs thread length and access |
| Ultrasonic measurement | Bolt elongation measured by the time of flight of sound along it | The most accurate check of preload, used on critical joints and to verify other methods |
How bolted joints fail
Most bolt failures trace back to preload. Too little and the joint slips, loosens and fatigues the bolt. Too much and the bolt yields, and a yielded bolt loses its spring. Loose and broken bolts on vibrating equipment are a common entry in root cause analysis, and soft foot and loose hold downs on rotating machines are a common cause of lost shaft alignment.
| Failure | Mechanism | Prevention |
|---|---|---|
| Self-loosening | Transverse slip under vibration lets the nut turn back | Enough preload to stop slip, and locking where needed |
| Fatigue | Low preload lets the bolt see the full load cycle | Correct preload, so the joint carries the cycling rather than the bolt |
| Relaxation | Surfaces, coatings and gaskets bed in and the preload drops | Hardened washers, fewer interfaces, a check tighten where the procedure calls for it |
| Yield and overload | Over-tightening or a load beyond the design stretches or breaks the bolt | Calibrated tools, the right torque for the lubricant used |
| Corrosion and galling | Rust, or thread surfaces welding together, change the friction and seize the joint | The right coating and lubricant, stainless fasteners lubricated |
Practices that keep joints tight
Most of good bolting is consistency: the same lubricant, calibrated tools, the same sequence and a record. The values and sequence for critical joints belong in the work instruction for the job, so they are not looked up from a generic chart on the day.
| Practice | Why |
|---|---|
| Use the specified torque and method | From the equipment manufacturer or the joint designer, not a generic chart |
| Keep the lubricant consistent | A torque set for a dry bolt over-tightens a lubricated one, and the reverse |
| Calibrate the tools | Torque wrenches and hydraulic tools checked on a set interval |
| Tighten in passes and a pattern | On multi-bolt joints, snug, then about a third, two thirds and full, in a cross pattern, then a check pass |
| Use hardened washers | They spread the load and steady the friction under the nut |
| Know what can be reused | Bolts tightened beyond yield, and tensioned structural bolts, are not reused |
| Mark and record | Paint or torque marks show a nut has not moved, and critical joints are recorded against the asset |
Common mistakes
- Using a generic torque chart in place of the manufacturer's value.
- Applying a dry torque value to lubricated or anti-seize coated bolts.
- Torque wrenches never calibrated.
- Tightening multi-bolt joints in one pass, one bolt at a time around the circle.
- Reusing bolts that have been tightened beyond yield.
- No washers, or soft washers, under the nut on a high-strength bolt.
- Treating repeat loosening as a tightening problem rather than a preload or design problem.
Common questions
How do you calculate bolt torque?
The usual approximation is T = K × F × d, where T is the torque, F the preload wanted, d the nominal bolt diameter, and K a nut factor that captures friction, commonly about 0.2 for plain dry steel bolts and lower for lubricated ones. The preload is often set at about 75 per cent of the bolt's proof load. Because K varies with the surface and lubricant, the result is an estimate, and the equipment manufacturer's or joint designer's values come first.
What is bolt preload?
Preload is the tension in a bolt after it is tightened, which clamps the joint together. A properly preloaded joint carries external loads mostly as a change in clamp force between the parts, so the bolt itself sees only a small part of the load cycle. That is what protects it from fatigue and stops the joint slipping and loosening.
Why do bolts come loose?
Mostly because the preload was too low or dropped. Under vibration, a joint that slips sideways lets the nut rotate back a little with each cycle, which is called self-loosening. Preload is also lost when surfaces, coatings or gaskets bed in. Enough preload to stop the joint slipping is the main cure, with locking devices where vibration is severe.
Does lubrication change bolt torque?
Yes, a great deal. Most of the tightening torque is spent on friction, so lubricating the threads and nut face means the same torque produces much more preload. A torque value set for dry bolts can overload and yield a lubricated bolt. The lubricant, or its absence, has to match the condition the torque value was set for.
What is the correct bolt tightening sequence?
For a joint with several bolts, such as a flange or a cover, the bolts are snugged first, then tightened in passes of increasing torque, commonly about a third, two thirds and full, following a cross or star pattern so the joint closes evenly, then given a final check pass in rotation. Pressure flanges follow their own procedure, such as ASME PCC-1.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
Related reading
Running plant like this?
Enco helps the teams that run and maintain it, with maintenance strategy, asset data and planning, condition monitoring and reporting software, and engineering studies. See the work we have delivered, or tell us what you are dealing with.