Click on the desired file format. Components Related To This Article. Recently Viewed Components You've not discovered any products yet. Current Projects. Log in to view your projects. Looking For Bespoke, Precision Manufacturing? Get A Quote Now. Discover more. This short guide is intended to give a very basic introduction to ISO metric nuts and bolts. It is intended for those working in plant and machinery maintenance to give enough information on to ensure that correct replacement parts are identified and used.
Few parts are as critical in the assembly of modern machinery as the nuts and bolts which hold it all together and, at first sight, it seems that all metric bolts are basically the same. In fact, of course, nothing could be further from the truth. There are no fewer than 5 different threads for different applications and 10 different standard strength grades defined for each size of bolt.
The situation on is almost as complex with metric nuts which come in a wide variety of types full, thin, nyloc, castellated….. The difference in strength between different grades is quite dramatic: the highest standard grade being capable of carrying more than three times as much load as the lowest grade. If you are responsible for repairing and maintaining machinery, particularly if that machinery is involved in lifting operation or other safety critical applications then you will be aware of the importance of fitting the right parts in the right way.
This short guide is intended to give you the information you need to be able to correctly identify and use the metric nuts and bolts most commonly found in plant and machinery. Every effort has been made to ensure that the information contained in this document is correct but no guarantee can be given nor liability accepted for errors or omissions.
We are always grateful for any comments or criticisms of our technical publications and are happy to offer advice on all aspects of plant and machinery maintenance. Although various special threads are defined by the ISO standard only two are in common use in most machinery, the others are mainly used in specialist equipment such as precision instruments, optical instruments, etc. The pitch of a thread is the distance between two adjacent threads and is measured in millimetres. The most common thread diameter and pitch combinations found in plant and machinery are listed in the table below:.
Except for the very lowest grades, metric nuts and bolts all carry identification markings which indicate their strength. On the head of each bolt are two numbers separated by a decimal point, the full list includes ten grades from 3. The bolt head should also have a code — normally two or three letters — indicating the manufacturer.
Usually the markings are on the top of the bolt head but sometimes they are on the side. The first number gives the breaking strength of the steel in the bolt. For example, a grade Similarly a grade 8. A bolt will begin to stretch before it finally breaks, the second number on the head of the bolt indicates how much of the breaking strength the bolt can stand before it starts to stretch.
Similarly a grade 9. The strength grade marking is more difficult to see on metric nuts, sometimes it is given as a number, sometimes as a pattern of a dot and a line where the position of the line indicates the grade like the hands of a clock. Where the grade is given as a number it may be stamped on the top of the nut or on one of the flat sides. Clock face grade marks are shown in the illustration right.
Put the dot at the twelve O-Clock positions and the line shows the grade. The rule is that the grade of the nut should always be the same as the bolt grade or one grade higher so: a grade 8. A grade
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