Why use washers in a bolted joint?

In the article titled Effect of lubrication on the transmission of forces when tightening a bolted joint we described how friction in the thread and under the nut affects the calculation of the tightening torque and the resulting tightness of the flange connection.

Inherent in this topic is the use of washers under the bolt head and nut. Washers are an integral part of the flange connection just like bolts and nuts. In the points below we explain why this is so.

• Reduce friction under the nut and bolt head
• thanks to washers, the exact coefficient of friction between the washer and the nut or bolt head can be determined
• define the contact area where friction occurs during tightening
• prevent damage to the flange sheets due to tightening
• thanks to the washers it is possible to bridge grooves or unevenness that may be present on the flanges
• the washers ensure an even distribution of pressure between the flange and the nut or bolt head

There are areas between the seating surface of the bolt head (nut) and the flange that affect the resulting coefficient of friction. These are surfaces that may be worn, corroded or otherwise damaged. Thus, when using washers, the surface is clearly defined in terms of hardness and roughness.

It is generally assumed that one third of the total friction in a bolted joint takes place under the nut and the remaining two thirds on the thread.

When calculating the tightening torque, the variance of friction values in the thread (µth), and under the bolt head or nut (µp) must also be taken into account. Increasing variance of these values can result in uneven tightening of the bolted joint.

Care must be taken to ensure that the nut faces and washers are treated with lubricant when assembling the bolted joint.

It is recommended to use lubricants whose friction values (including dispersion) have been measured according to EN 16047. These should be lubricants that minimise friction in the thread and thus reduce the torsional stress on the bolt. These requirements are well met by dry lubricants such as baked-on slip varnish POWERtorque LF kote 450 (See chart with friction ratios in the bolt joint).

Stainless steel fasteners after tightening without lubricant