EngineeringMechanical EngineeringPart 13 of 15
Bolted Joint Design
A bolt is a spring that clamps. Design it as a tension member and it works; design it as a pin that happens to have a thread and it loosens, fatigues and fails.
- Stress area basis
- Preload and torque
- Shank vs thread shear
- Combined loading
Executive summary
Two ISO metric property classes cover most general engineering: 4.6, a commercial grade, and 8.8, a precision high-tensile grade. Coarse pitch is standard, with fine pitch available in selected sizes.
Bolt strength is computed on the stress area — an area based on the mean of the pitch and minor thread diameters. Testing established that the actual tensile breaking load exceeds the value computed on root area alone, and the stress area is now the accepted international basis for the strength of externally threaded parts.
Preload: what a bolt actually does
A correctly designed bolted joint carries external load through clamping friction and joint stiffness, not by shearing the bolt or stretching it further. Preload is therefore the design variable, and assembly torque is how it is delivered.
Choose the design factor
Selected on the nature of the loading — steady, repeatedly applied, or repeatedly applied with shock and possible fatigue.
Set the preload stress
Commonly taken at around 0.65 of the yield stress for the property class. This leaves headroom for the external load without approaching yield.
Find the stress area
Preload force divided by preload stress gives the required tensile stress area, which selects the bolt size from the tables.
Specify assembly torque
Recommended assembly torque for that size and class, stated on the drawing alongside the bolt specification.
Assembly torque is a proxy for preload, and a fairly rough one — most of the applied torque is consumed by friction under the head and in the threads. Lubrication, plating and surface condition all shift the relationship. Where preload genuinely matters, specify a controlled method (angle turn, bolt elongation or tension indicating) rather than relying on a torque figure alone.
The four load cases
| Case | Governing quantity | Design route |
|---|---|---|
| Tension | Tensile stress area against preload stress. | Design load = external load × design factor. Divide by preload stress to obtain required stress area. |
| Shear | Shear area against permissible shear stress. | Arrange the joint so shear is taken on the shank, not the thread. Design load divided by allowable shear stress gives the required shank area. |
| Gasketed (flexible) joint | Combined preload and external load. | Preload is a multiple of the external load per bolt; the design bolt load is the sum of both. |
| Combined tension and shear | Combined stress at the critical section. | Both stresses computed on the same section and combined, then checked against both allowable values. |
- ft
- direct tensile stress on the section
- fs
- direct shear stress on the section
- Check
- Both the maximum tensile and the maximum shear results must lie within their respective allowable values — a bolt can pass the tension check and fail the shear check on the same section.
Assumptions that make the method conservative
Friction ignored in shear
The method disregards friction between the mating faces, so that if a nut loosens the bolt still carries the design shear load. In practice friction adds a further margin.
Shear shared evenly
Where several bolts share a shear load, even distribution is assumed. Strictly this holds only for fitted precision bolts, but bolts deform under load and the distribution evens out. Combined with the friction assumption above, assuming only a few bolts carry the load would be unduly conservative.
Getting the shear plane onto the shank is a design decision expressed through bolt length, thread length and grip length. It is checked by drawing the joint stack — plate, frame, nut and washer — and confirming the unthreaded shank spans the shear interface with margin.
Worked example: combined tension and shear
A commercial grade bolt connects a mild steel plate to a machine frame 30 mm thick. The plate exerts a 10 kN tensile load and a 6 kN shear load on the bolt. A factor of safety of 3 is required.
The iteration that matters
Sizing on tension alone gives M18. Checking that bolt on the shank at design load produces a direct tensile stress of about 118 MPa and a direct shear stress of about 70.7 MPa. Combining them gives a maximum shear stress of about 92 MPa — comfortably above the allowable shear for the class. The bolt passes in tension and fails in shear.
Moving to M20 reduces the tensile stress to about 95.5 MPa and the shear stress to about 57.3 MPa. The combined maximum shear falls to about 74.5 MPa and the maximum tensile to about 122 MPa — both inside their allowable values. M20 is selected.
Closing the joint geometry
Bearing between bolt and plate sets a minimum plate thickness — here about 5.7 mm, so 6 mm nominal. But the grip stack then decides the bolt length, and the nearest standard length leaves too little shank bearing inside the plate. Increasing the plate to 12 mm restores adequate shank engagement. The joint closes at M20, 12 mm plate, 80 mm bolt length, 46 mm thread length.
Three separate constraints — combined stress, bearing on the plate, and the discrete steps in available bolt length — each pushed the design. None of them was visible from the tension calculation alone.
Gasketed joints and bracket loading
Gasketed pressure joints
Where a gasket sits in the joint, the joint is flexible and the bolt takes a share of the external load in addition to its preload.
- Calculate the total pressure loadGauge pressure multiplied by the effective sealed area.
- Apply the design factorRepeatedly applied shock loading with possible fatigue warrants a substantial factor.
- Divide by the number of boltsGiving the external load per bolt.
- Set the preload above the external loadA common design rule is a preload of roughly 1.1 times the external load per bolt, so the joint does not separate.
- Form the design bolt loadPreload plus external load.
- Size on stress areaDesign bolt load divided by allowable stress.
- Set the tightening torqueOften specified as a fraction of the standard assembly torque for gasketed work — then check the actual bolt tension that torque produces and iterate if it exceeds the assumed preload.
Brackets in bending and torsion
Bracket in bending
The bolt group resists a moment. Bolt loads vary linearly with distance from the neutral axis of the group, so the outermost bolt carries the greatest tensile load and governs the design.
Bracket in torsion
The bolt group resists an in-plane moment. Each bolt carries a shear load proportional to its radius from the group centroid, acting perpendicular to that radius, and must be combined vectorially with the direct shear.
Design checklist
- Property class chosen deliberately and stated with the bolt size.
- Design factor selected on the character of the loading, not by default.
- Sizing based on tensile stress area, not root area.
- Preload stress set as a defined fraction of yield.
- Shear plane arranged on the shank, and confirmed by the grip stack.
- Combined tension and shear checked against both allowable values.
- Bearing stress between bolt and plate checked, setting minimum plate thickness.
- Bolt length taken from the standard range, and shank engagement re-verified afterwards.
- Minimum thread length satisfied.
- Gasketed joints designed with preload exceeding the external load per bolt.
- Assembly torque specified on the drawing, with a controlled method where preload is critical.
- Bolt group geometry drawn out for brackets in bending or torsion.