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Overview of Bolt Tightening Values

by - September 06, 2018


Screws and bolts are probably one of the most important parts of any machinery. This small piece of metal finds its use in almost every single object that we use. It is most commonly used for joining two pieces. This method is more commonly used as compared to other methods such as riveting and welding. The benefit of threaded bolts and screws are easy assembly, disassembly and most importantly cost.


The tightening of bolts
A screw is exposed to tensile load, to torsion and a torque. The amount of bolt torque tightening applied is set according to our need. If we need a tighter joint, more torque needs to be applied. Bolt tensioning is now the preferred method of tightening bolts and studs in all critical applications. Bolt tensioners are designed for operation in a wide variety of applications including pipeline flanges, heat exchangers, pressure vessels, compressor covers, boiler feed pumps, windmills and many others. Since this method of joining is widely used, a table is made for generalised values for types of bolts with a certain type of threading or its size. This table of metric bolt tightening torque values helps a lot in simplifying the overall use as these values are standardised and remain constant for almost all conditions. It should be noted that this method is used to pre-determine clamping force for regular use only. This approach has a number of advantages over the method where a direct stress, and hence preload value, is assumed in the bolt.

Different amount of torque for the different purposes
For high thread friction values, a high torsional stress results in the bolt. Less of the available strength of the bolt is being utilized in such a case to generate preload. In the extreme case when a nut has seized on the bolt thread, all the applied bolt torque tightening is sustained as torsional stress with no preload being available. In the other extreme, low thread friction results in higher preload. For more critical applications, it is recommended to calculate these values experimentally. This is because the values in the table are calculated for general cases with some percentage of error.

Avoid the use of torque table for significant applications
·         On observing the table, one can see that due to indirect variables such as lubrication and friction, deviation up to 25% can be observed.
·         This is why one should avoid using the metric bolt tightening torque values for critical applications because, at a large scale, even the smallest percentage of error can damage the system.
·         This applies to washer faced or double chamfered hex nuts or bolts if a bolt is torqued having a width across flats of approximately 1.5 times the nominal diameter and having threads free of interference.
·         This approach has a number of advantages over the method where a direct stress, and hence preload value, is assumed in the bolt. For high thread friction values, a high torsional stress results in the bolt.
·         Less of the available strength of the bolt is being utilized in such a case to generate preload. 
·         In the extreme case when a nut has seized on the bolt thread, all the applied torque is sustained as torsional stress with no preload being available.
·         In the other extreme, low thread friction results in higher preload. 


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