The proof load is usually considered to be the absolute maximum load which a bolt should be asked to carry. In practice, manufacturers will design their machinery so that the bolts carry a lot less than the proof load to give a good safety factor to the design. The maximum recommended torques given in the table below assume that the threads are lightly oiled. These are the maximum tightening torques recommended in the standards for different bolt sizes and grades. Fine threads are often used where a bolt is screwed into a soft metal block, such as an aluminium casting, the thread in the block may be far weaker than the bolt and a lower torque is almost always required to avoid stripping the threads.
Many factors affect the strength of threads in components including how the thread is produced, the surface finish and the accuracy of the thread shape. The strength of the threaded connection is the strength of the weakest part so, for example, if a high tensile bolt is screwed into a softer, more ductile material such as an aluminium alloy or low carbon steel part then it will be the softer part which dictates the maximum torque which can safely be applied to the bolt.
Similarly, if the thread is made by drilling and tapping a hole the size of the drilled hole may vary depending on how accurately the drill has been sharpened and how worn the drill is, the hole might not be precisely straight and the tap might be blunt or worn. All these factors will add up to produce a thread which does not fit the bolt exactly as it should and the joint may therefore be weakened. Design Engineers have to consider all these factors and may decide that a lower torque is applicable.
This is commonly the case in lifting devices where high factors of safety are required. Correct Spanner and Allen Key sizes are given in the table below for the normal range of bolt and nut sizes found in plant and machinery. To minimise the risk of a spanner slipping and marring the corners of a fastener, it is good practise to use ring spanners or the ring end of a combination spanner whenever possible.
Metric bolts are made fractionally under their stated size, for instance an M16 bolt will have a shank diameter of typically This means that a 16mm bolt will fit through a 16mm hole but to allow for misalignment between parts it is common practice to drill bolt holes slightly larger.
These are known as clearance holes. Typical clearance hole sizes are given in the table below along with the correct tapping hole size for each size of standard and fine pitch thread.
With metric threads tapping drill size is easy to work out because it is the bolt diameter minus the thread pitch so, for example, a standard pitch M16 bolt has a pitch of 2. The table below gives tapping drill sizes for threads corresponding to the thread pitches given in the table on page 3. Note We offer a range of hex head bolts in various thread sizes and lengths. Refer to the datasheet and range overview download file for details.
If A2 stainless steel is not what you are looking for, we can also offer you the fasteners in steel and brass. We also offer the hexagon head bolt in A4 stainless steel and BZP steel.
Refer to the fasteners selection guide for details. Due to their versatility, hex bolts are used in many applications where two materials need to be joined together.
Stainless steel is useful for more than just a visual purpose. The main highlight would be the superior corrosion resistance, stopping unwanted issues like rusting. High heat resistance is also a big benefit, due to the way stainless steel is created, it can withstand an incredible amount of heat as opposed to a screw or socket made from standard steel. The only possible issue may be cost, however, a reliable and durable material will not only increase the longevity of the application, but also the sustainability.
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