How to tighten a bolt

Tightening a fastener to a predetermined load is a simple operation when done with the use of one of the many and different torque tools made for this purpose. What is perhaps less known is the implications of their use in relation to the condition of the fastener. Most tightening is sometimes performed without the necessary accuracy even when the most suitable tools are available simply because the operator is not fully aware of what is required to obtain the correct result. These notes are therefore starting with the real origin of the problem. i.e. the tightening torque.

Tightening torque

How long a bolted assembly will last mainly depends on those who perform the tightening operation, and to a much lesser degree on the designers or the bolt manufacturers, although the strength of the assembly is certainly greater if the design is accurate and the fastener is of good quality. It has proven that insufficient or excessive torque can affect the quality of the assembly, with increasing effect under more severe conditions.

Controlled Tightening

There are many systems to tighten a fastener to a predetermined load, the easier and more reliable one being the use of torque wrenches or with the help of torque multipliers when torques higher that the operator’s strength are required.

The tension applied to the bolt during torque operated tightening is however affected by frictional forces which are being produced between the threads of the bolt and nut (or threaded hole), and between the bolts head and its contact points. These forces are originated by the conditions of the bolt (finish, heat treatment, etc) and by the tightening speed.

Friction Factors

Studies on the subject have ascertained that, in extreme cases, only 10% of the torque developed by the operator is applied to the bolt, whilst the remaining 90% is absorbed by friction. Slightly lubricated bolts, as commonly supplied by the manufacturers, have a friction factor of 0.14. This factor becomes 0.10 for cadmium plated bolts and can increase to over 0.20 for non-lubricated bolts. The tightening torque for the different bolt categories have been calculated by taking two friction factors, 0.10 and 0.14. Due to the lengthy and elaborated calculations involved, it is not possible to give a mathematical ratio between these torques and others originating from different friction factors.

How to tighten a bolt

By the Turn of the nut method

1. Bring the parts of the joint into close contact over their full area by assembling and tightening enough bolts and nuts to a snug-tight condition. This condition represents a tension of approximately 10% of the required bolt tension and is achieved by the full effort of a man using a standard podger spanner or by a few impacts of an impact wrench applied after initial slackness has been taken up (when an impact wrench is used, the point at which the bolt becomes snug-tight is readily noticeable; it is the point at which the wrench changes it’s tone and impacts solidly.) Assemble the remaining bolts and nuts in the joint and make them snug-tight. 2. Make corresponding permanent marks on each nut and the protruding thread of the bolt from which subsequent rotation of the nut or bolt can be measured. Then, rotate the nuts or bolts through the appropriate number of turns indicated in the table below. 3. Tighten them in rotation, starting with bolts that are remote from the edges and finishing with bolts that are next to the edges and ends.

Stainless steel hexagon head bolts, nuts and set screws

Bolt & nut pre-load & torque data

Mechanical properties of steel bolts, screws and nuts

  1. For bolts of property class 8.8 in diameters d <16mm, there is an increased risk of nut stripping in the case of inadvertent over-tightening inducing a load in excess of proofing load. Reference to ISO 898-2 is recommended
  2. For structural bolting the limit is 12mm
  3. Applies only to nominal thread diameters d <16mm
  4. Minimum tensile properties apply to products of nominal length l>2,5d. Minimum hardness applies to products of length l<2,5 d and other products which cannot be tensile-tested (e.g. due to head configuration).
  5. Surface hardness shall not be more than 30 vickers points above the measured core hardness on the product when readings of both surface and core are carried out at HV 0,3. For property class 10.9, any increase in hardness at the surface which indicates that the surface hardness exceeds 390 HV is not acceptable.
  6. In case where the lower yield stress R eL can not be determined, it is permissible to measure the proof stress R p0,2

Recommended maximum bolt loads

Comparisons of various grades of bolts, screws & studs

ISO Fastener specification differences