There have been a number of articles about fasteners that have mentioned friction, and as we have discussed, this has an important effect on the relationship between tightening torque and tension. Although torque-based tensioning of fasteners is not ideal, having a large number of unpredictable variables, we often don't have much choice other than to use this method.
The torque-tension relationship relies on a number of geometric parameters that are often under our control, or are things that we can measure directly. One thing we need to ensure, where we use a washer under the head of a bolt or under the seating face of a nut, is that the washer remains static while we tighten the bolt. Otherwise, we are having to overcome friction at an interface with a different dimension from those we have used in our calculation of washer-face friction. This gives another layer of uncertainty to our calculation.
The washer dimensions are generally larger than those of the nut face, and the contribution of friction from the interface between the washer and whatever it sits on will probably be larger than you were expecting. Washer rotation should only be prominent when the friction coefficient between the washer underside and the adjacent part is lower than that between the nut face and the washer. This may happen if the casting is a little oily and the nut and top of the washer are dry. Of course, this isn't the usual situation, but it may happen in less than ideal circumstances - late night, rushed build and so on.
Another good reason for preventing any rotation of the washer, particularly in joints with high contact pressure, is to prevent the washer from damaging the casting if it rotates. There are many ways of doing this, but the most popular is to cause some 'damage' to the underside of the washer. Centre-punching the washer to produce dimples with slightly raised lips means that, on tightening, there is a local high-pressure region and the casting material will 'flow' into the dimple, thereby locking the washer in place. Doing a similar operation with a chisel mark across the washer face will have a similar effect.
Some people produce machined washers with a narrow machined groove across them for the same reason. Any feature that gives a local high pressure will have the same effect. Be careful though to ensure that these anti-rotation features are on the underside of the washer when it is installed.
Other schemes are often more complex, and involve non-round washers in specially shaped pockets to restrain rotation, or other similarly involved practices. Many of these alternatives to the centre-punched or chisel-marked washers are impractical from the point of view of packaging them within an engine. The simple methods don't require any extra space or special machining methods.
The centre- or chisel-punching methods are only really suitable for thick washers, as they cause excessive deformation of a thin washer, and this is likely to have the wrong effect and run the risk of sitting the nut face on a local high point(s), with little lubrication and therefore increased friction.
For those who feel that punching or similarly marking their washers is the work of heathens, then machining a groove or centre drilling the face of the washer is a good alternative that has less chance of distorting the washer.
Fig. 1 - Washers, if they rotate during tightening, affect the relationship between tightening torque and fastener tension
Written by Wayne Ward