Most bolt problems we see come from the same place: someone picked a size that looked about right, and nobody checked how the joint actually carries its load. A bolt that is fine in a catalogue can still loosen, fatigue or tear out of the plate it is holding. This guide walks through the sequence we use, with a worked example at the end. It is a guide to the thinking, not a substitute for a design check.
1. Start with the load, not the bolt
Before you look at a bolt chart, write down what the joint has to do:
- The size and direction of the load, including any worst-case or shock loads.
- Whether the load is steady, repeated or reversing. Repeated loads make fatigue the main concern.
- Whether the bolt is being pulled apart (tension), sliced across (shear), or both.
- The environment: corrosion, temperature, vibration, and whether the joint will ever be taken apart.
If the load is a guess, say so, and add a sensible margin. A bolt sized precisely against a wrong number is still the wrong bolt.
2. Shear versus tension: know which one you have
A bolt in tension is stretched along its axis. The stress is the load divided by the tensile stress area of the thread, which is a bit smaller than the shank. A bolt in shear has the load acting across it, where two plates try to slide past each other.
Two details catch people out:
- If the threads run through the shear plane, use the thread area. If the plain shank is in the shear plane, you have more area to work with. Say which on the drawing.
- In shear, also check bearing, which is how hard the bolt presses on the hole wall. A strong bolt in a thin or soft plate can elongate the hole or tear out through the edge long before the bolt fails.
Many joints are intended as tension joints but see some shear, or the other way round. If you are unsure, check both.
3. Bolt grade and size
Metric bolts are marked with a property class such as 8.8, 10.9 or 12.9. Roughly, the first number relates to the minimum tensile strength (8.8 is around 800 MPa) and the second to the yield-to-tensile ratio. Higher grades carry more load per bolt, but they are less forgiving. They can be more sensitive to hydrogen embrittlement, over-tightening and poor plating choices. [CHECK: confirm nominal strength figures and wording against the current ISO property class standard]
For most general machine work, class 8.8 is a sensible starting point. Move up a grade only when you have a reason, not just because the joint feels important. Often it is better to add a bolt or go up a size than to reach for a higher grade.
Also consider:
- Bolt count and spacing. Leave enough edge distance and pitch so the plate does not fail first.
- Washers and hardened surfaces under the head and nut.
- Locking method if there is vibration: a prevailing-torque nut, a thread-locking compound or a different joint design.
4. Tightening torque and preload
The torque you apply is only a means to an end. What matters is preload, the clamping force the bolt holds on the joint. Good preload keeps the joint tight, shares load properly and greatly improves fatigue life, because the bolt sees only a small fraction of the changing external load.
A common approximation is:
Torque = K × diameter × preload
K is a friction (nut factor) value. About 0.2 is often quoted for plain, dry steel threads, but oil, plating, coatings and re-used threads can change it a lot. [CHECK: confirm K ~0.2 is a reasonable general figure for plain dry steel and note any ranges you prefer] Because of that, torque is an imprecise way to control preload. If the joint is critical, use a defined tightening procedure and, where needed, a method that measures bolt stretch or tension directly.
5. Worked example
A steel bracket is bolted to a frame with four M12 class 8.8 bolts. It carries a steady 20 kN load acting across the bolts (shear). The plate is 10 mm thick. The threads are in the shear plane, and there is one shear plane per bolt.
- Load per bolt. 20 kN ÷ 4 = 5 kN, assuming the bolts share evenly. In real joints they may not, so we keep a margin.
- Bolt shear capacity. The M12 tensile stress area is about 84 mm². As a rough rule, shear strength is about 0.6 × tensile strength, so about 0.6 × 800 = 480 MPa. 480 × 84 ≈ 40 kN per bolt at ultimate. [CHECK: confirm 84 mm² stress area and the 0.6 shear rule of thumb]
- Apply a factor of safety. For illustration, use 3. That gives about 13 kN allowable per bolt. Our 5 kN is well inside this. The factor you actually use depends on the consequence of failure, and on any code that applies.
- Bearing check. Bearing area is bolt diameter × plate thickness = 12 × 10 = 120 mm². Bearing stress is 5,000 N ÷ 120 mm² ≈ 42 MPa, which is low for structural steel plate. Edge distance still needs checking.
- Tightening. The proof stress of an 8.8 bolt is a little under 600 MPa, so proof load on M12 is roughly 49 kN. Aiming for about 70% gives around 34 kN preload. With K = 0.2: 0.2 × 0.012 m × 34,000 N ≈ 82 N·m. [CHECK: confirm proof stress and the 70% target preload, and whether to quote a torque value at all]
Because the bolts are well within capacity, the joint could be done with smaller bolts. We might keep M12 anyway for stiffness, handling and spanner access. Capacity is only one of the reasons for a size.
Check your own numbers
We have two free calculators to help with the arithmetic: the bolt stress calculator for tension and thread stress, and the bolt shear and bearing calculator for the shear and bearing checks above. They are good for early sizing and sanity checks. They do not replace a full design check.
Where to start
Write down the load, decide whether the bolt is in shear, tension or both, and pick a bolt that clears the numbers with a sensible margin. Then check bearing, edge distance and how you will tighten it. If the joint is lifting-related, safety-critical, fatigue-loaded or part of a structure that needs certification, get a qualified engineer or certifier involved, and follow whatever standard or code applies to your industry.
If you would like a second pair of eyes, our mechanical engineering and drafting work often includes checking joints and putting clear bolt callouts on the workshop drawings. You can ask for a quote and send us what you have, even if it is only a sketch.
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