Joint bearing pressure and the use of unsuitable washers
I've been recently doing some updates to our online training material on joint bearing pressure. This is the pressure acting on the joint as a result of bolts being tightened. If the contact pressure is too high, plastic collapse of the surface can occur. This in itself may not be an issue, but when such plasticity occurs after the tightening is completed, the bolt preload will reduce. As a consequence, joint integrity issues can arise such as self-loosening and fatigue.
The issue over excessive bearing pressure is likely to be encountered when softer joint materials are being used, such as aluminium, and also, when mild steel instead of hardened washers are used. The strength of aluminium is typically less than half of that of the bolting materials usually used. As a consequence without having something present to increase the contact area, such as flanged fasteners or hardened washers, the surface is likely to collapse when the bolt is fully tightened.
In mechanical engineering, historically, mild steel washers have been frequently used. Such washers indent and deform when the bolt is fully tightened, some of this deformation occurring after the tightening is completed. This has been the cause of issues including accidents resulting from the preload being reduced to a level that the bolts fully loosened.
For decades bolts have had strength grades/property classes. Relatively recently was an ISO standard introduced for property classes for washers. ISO 898 Part 3 (Mechanical properties of fasteners made of carbon steel and alloy steel - Part 3. Flat washers with specified property classes) was introduced in 2018 which defines the property classes for washers. This states that a 200HV washer is recommended for an 8.8 bolt, 300HV for a 10.9 bolt and 380HV for a 12.9 bolt. It also uses the wording, shall not be used for certain washer-bolt combinations. For example, an 100HV washer shall not be used with 8.8 or higher grade bolts. Since standards contribute to what is deemed to be 'state of the art', an application using soft washers with higher strength bolts that are fully tightened, can't be regarded as 'state of the art'.
I remember on a training course once someone countering the argument about hard washers being needed based upon their experience. The company had been using mild steel washers for a long time in an application without any issues arising. One reason why the permissible bearing pressure should not be exceeded is that the preload loss becomes unpredictable and cannot be accounted for in joint calculations. The preload losses sustained by the use of such soft washers in their application may be such as not to cause any issue. In other joints, such as those having a short grip length and those more highly loaded, the use of such soft washers may well result in large preload losses resulting in joint failure. The point being that currently, there is no way to be able to predict the magnitude of the preload loss once the permissible bearing pressure is exceeded. Accordingly, it is a wise choice to limit the bearing stress to the capabilities of the joint material.
The issue of surface collapse when a soft metal is used under a fully tightened bolt or nut raises the question of the suitability and advisability of the use of tab washers in high performance joints. A tab washer is a thin, soft metal plate that is fitted under a nut or bolt prior to tightening. Once tightening is completed, a corner of the plate is folded/hammered over onto one of the flats of the hexagon of the nut/bolt. By necessity for its function, the tab washer metal must be soft so that it can be deformed without fracturing. The warning about excessive bearing pressure will apply and it will not be possible to provide limits at the design stage, on how much preload loss will result from plastic deformation of the surface following tightening. It could however be established from tests, but the modern approach is to predict at the design stage whether the joint is fit for purpose. Accordingly, we don't recommend the use of tab washers due to the uncertainty of what preload losses are going to be experienced.
Tab washers are used to prevent rotation of a tightened bolt or nut. You would think that this would be a fool proof way of preventing rotation. Surprisingly, the soft nature of the washer can work against it. On a Junker test, the off torque that is attempting to undo the bolt is of such a magnitude that it can cause the tab to be forced away from the hexagon flat so that the bolt rotates, and the preload lost. On the tests completed by Bolt Science, double tabbing (having two tabs against different sides of the hexagon faces), such movement is prevented.
As mentioned, the update to the online training course on Bolting Technology for Designers and Engineers has a module on joint bearing pressure including calculations and the limiting bearing pressures for various materials. There is also a brochure for this training, click on the link below for details.
Social Media Links
Instagram
Linkedin
Twitter
|
