Mechanical Engineering Design Notes

Design Contents


The key feature of bolted joints is that they can be dismantled comparatively easily. However they are costly in that frequently additional parts are needed (washer, nut) compared to riveted or welded joints and they require more skill / effort to assemble. For these reasons they should only be used where there is a strong possibility that the parts will at some stage require to be separated.

There are several considerations when designing a bolted joint. Firstly the required clamping force (and hence size) of each bolt and secondly the distribution of the fasteners when more than one is needed (which will normally be the case). Some other points that are important when specifying bolts / or designing a joint are given below:

1. An important factor in making a successful joint is tightening the the bolt to the appropriate torque to give the needed clamping force. This is difficult to do for short bolts, but becomes increasingly easier the longer the bolt clamping length is made. For this reason the use of short bolts for critical applications involving high clamping forces should be avoided.

2. The torque required to develop a specific tension is very dependent upon the friction in the threads - and the coefficient of friction can vary widely according to the lubricant used. Special 'torque oils' are available for use with fasteners in critical situations which give reasonably consistent friction.

3. The thread form of many bolts gives rise to a stress concentration factor of 2+ to 3+. A similar magnitude of stress concentration may present in screws where the thread runs the full length to the head. Rolled threads have lower fatigue strength reduction factors than cut threads.

4. As a rule of thumb, where a joint is to include a gasket for sealing purposes, the maximum bolt centre distance should be no more than 6 x bolt diameter. To allow for wrench access, the minimum bolt spacing should not be less than 3 x bolt diameter.

5. Bolts must be tightened so that whatever loading is applied, they are always in tension. However normally the shear stresses caused by any shear loads should be calculated (see applet below). In the case of joints subjected to cyclic loading, using a high preload will usually give a longer fatigue life than using a lower preload. For bolts used in very highly loaded fatigue applications, eg engine cylinder heads, it is common practice to tighten them till they are plastically deformed. Obviously this means they should not normally be re-used (some car engine manufacturers allow one re-use).

6. Threads in aluminium parts that have to carry high loads should be fitted with stainless steel 'helicoils'.

7. When a bolt is screwed into a block of metal, the stiffness of the threads on the bolt will probably be very different from the stiffness of the threads in the block into which the bolt is screwed. This means that almost the entire load will probably be carried by the first one or two of turns of the engaged thread. Increasing the length of engaged thread will not change this. To mitigate this problem, the cross section of the boss into which the bolt is screwed should have a carefully calculated cross section that tapers. If this is done correctly the load will be carried over several turns.

8. Three widely available grades of metric bolts are:

Grade Min. UTS, MPa Stress at permanent set limit R 0.2% MPa Stress at proof load, MPa Min. elongation after fracture % Material Notes
8.8 800 640 580 12 med. carbon, Q & T general purpose
10.9 1040 940 830 9 may be alloy steel, med. carbon steel, Q & T, or med. C steel with additives Q & T highest strength grade that should be used in a marine environment
12.9 1220 1100 970 8 alloy steel, Q & T highest strength, also used for socket head caps.

It should be noted that some of the above values vary slightly in BS 6104, depending upon the bolt diameter. Conservative values are used in the above table and in the "Calculators" below.

In engineering, the term 'screw' is strictly speaking the name for a fastener with a full length thread, this is often undesirable as where the thread meets the fastener head there will be a high stress concentration factor due to both change in 'diameter' and small fillet radius. 'Bolts' are only threaded for part of their length, leaving the full diameter for most of the length, or even a reduced diameter with large fillets to help keep stress concentration factors low for critical applications in aerospace.

The majority of screws and bolts have their threads rolled rather than cut as production is quicker than machining and rolling gives a better grain structure which usually offers superior fatigue performance.

Metric bolts have their grade number marked on their heads.
US SAE specification bolts have no head markings for the lower strength grades, 1 (410 MPa UTS) and 2 (min. UTS 480 MPa for diameters between 1/4 and 3/4 inch, min. UTS 410 MPa for diameters between 7/8 and 1.5 inch) the higher strength grades have different arrangements of 3, 5 or 6 radial lines marked on their heads and sometimes an ASTM designation number.

The sketch on the left shows a UNC bolt headwith 3 radial lines indicating a UTS of of 825 MPa for bolts with a diameter betweb 1/4 and 1 inch which corresponds to SAE grade 5. (For diameters between 1.125 and 1.5 inch, the minimum UTS is 720 MPa).

The sketch in the middle is of a metric grade 8.8 bolt head.

On the right is a sketch of a metric socket head cap in A2-70 stainless steel (a type 304 stainless steel with a minimum UTS of 700 MPa).

Another grade of stainless steel socket head caps is A4-80 which is 316 stainless steel with a minimum UTS of 800 MPa.

Interactive plot and calculator for proof loads for bolts with ISO metric coarse and fine threads

Bolt ultimate load calculator for ISO metric fine and coarse threads

Interactive applet for determining the shear stresses in an array of bolts or rivets subject to an 'offset' load.

Further information about bolts and bolted joints can be found at the 'Boltscience' web site.

David J Grieve, Revised: 9th February 2015, 6th July 2014, 27th January 2010, Original: 26th November 2002.

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