Hello. Nedzigon's back!
Let's jump right in: what do you know about the role of washers?
The most recognized washers are flat washers and spring washers, and most people are familiar with their use. Many, however, may have been using them out of habit without ever thinking about it.
Today, let's get serious about the role of washers.
Flat washers prevent sinking of bearing surfaces
Flat washers come in various sizes for one screw diameter, from tiny to extra-large.
＞＞List of sizes for flat washers regulated by JIS
The main role of flat washers is to increase the size of a screw’s bearing surface area, and reduce the surface pressure applied on the fastened object. Looseness can result when the bearing surface sinks under the surface pressure, so using a flat washer to reduce the surface pressure is very effective. This is especially crucial if the contact area between a screw’s bearing surface and the object being fastened is small when fastening to a slot, for instance, or if the object being fastened is made of soft material such as aluminum or resin.
Incidentally, since flat washers are basically manufactured by pressing, one side has rounded corners and one side has burrs, as shown in the figure below.
There is no fixed rule about which side goes up and which goes down, but given the issue of bearing surface pressure mentioned above, it's more effective to put the burred side facing down. That said, since the burrs could mark the surface of the fastened object during tightening, it would be better to put the rounded side down if you want to avoid plating peeling.
Do spring washers actually reduce loosening?
Spring washers result from cutting out part of a flat washer so that the washer can form a twisted shape. By this means, as the name implies, spring action (elastic force) operates; these washers are said to be effective against looseness through increased frictional force and the way the cut part bites into the bearing surface.
＞＞List of sizes for spring washers regulated by JIS
As far as Nedzigon is concerned, though, spring washers are hardly effective in loosening prevention.
The elastic force of the spring washer is extremely small when compared to the axial force of screw tightening with the stipulated torque. When the elastic force of a spring washer just starts to take effect, the screw is already in a loosened state.
For example, in contrast to the maximum allowable axial force of 23,000 N for an M8 steel hexagonal socket head cap bolt with a 10.9 strength ranking, the approximate 500 N of force required to compress an M8 spring washer is not much more than a tenth of the size (Nedzigon test result).
＞＞Maximum allowable axial bolt force found here
Undoubtedly, loosened screws are prevented by friction due to elastic force.
The locking effect due to ‘biting’ is ultimately limited to cases in which the material of the fastened object is softer than the metal washer. Since ‘biting’ means that the bearing surface is scratched, it’s not good for repeated installation and removal. There are examples of use as a paired set with flat washers in order to avoid damaging the bearing surface, but, because all the flat washers have been ‘bitten’ they end up rotating, providing almost no locking effect.
The graph below shows the results from a Junker vibration test of a screw using both a spring washer and a flat washer. You can see that it is loosened and the axial force drops over time.
And since looseness occurs with an NAS vibration test, too, it would seem that spring washers have no effect against looseness caused by vibration.
＞＞NAS vibration test video
This has all gotten a little lengthy, but there are various causes for looseness, some of which may be effectively handled with spring washers. However, I think that tightening with the proper torque is more important in preventing looseness.
I would certainly like to discuss the "various causes of looseness" in greater length on another occasion.
But for today, please forgive me for finishing here.