Washer Theory 1

[SimonJ]

Does anyone know where to find info on washer theory?
Are there any theorecial models/formulas that you can use to determine when a split washer is required if you know the vibration, preload etc?

Does anyone know of a good text book about washers?

Cheers
Simon

 

[nashjp]

Does anyone know where to find info on washer theory?
Simon
I recommend Bickfords books - they cover almost the full range of bolted joint problems.

On split washers, I recommend not to use them. Studies show that joints can come loose faster with split washers than when regular hard washers are used. The split ring allows movement - they bolt preload tries to squirt the washer out from under the bolt head. As the split washers move, the joint sets (preload is lost) in the joint and your parts come loose. The split ring can also act as a ratcheting mechanism to unscrew the bolt.

The most important thing to prevent vibrational loosening is to maintain preload. If you loose preload your joint will come loose. The best thing to do is have a long stretch length for your bolt.

The second thing is to minimize setting. Avoid paint, rough surfaces, abutments with low yield points, and minimize the number of joint interfaces.

Washers add more joint interfaces, but they are hard and help to spread out the concentrated load so that it does not yield the abutment. Use thicker washers not larger diameter washers. Thicker washers spread the load better and provide more stretch length for your bolt. (Size the outside diameter of the washer to allow a 60deg load cone to pass through it from the bolt or nut.)

Some kinds of abutment will have movement between surfaces (bolts holding dissimilar metals together), good joint design is the best way to make these work. Locktite will work most of the time (99% however I always find that 1%) however your abutment will still be moving about.

For small fastener sizes, toothed washers do work. Unfortunately they tear things up. For higher strength bolts they might not bite in like you would need.

Good luck holding it together-:-V

 

[SimonJ]

Thanks nashjp

I have to convince my boss that split washers are no good. Do you know where I can get copies of the studeis that show that split washers are no good.

cheers

 

[nashjp]

Simon,

Here are two quick source from the web.

(

http://www.boltscience.com/pages/vibloose.htm

...conventional spring lock washers are no longer specified, because it has been shown that they actually aid self loosening rather than prevent it.)

(

http://www.engineeredracingproducts.com/articles/art1.htm

A variety of split-locking washers are available, but are rarely, if ever, used on motorcycles. The reasons for this are that these locking washers work by "digging in" to the fastener and mating parts, causing damage...)

The comment on the most important thing to prevent vibrational loosening should have also said - Have a preload that prevents gapping (opening) of the joint due to loading. If the joint opens, things move and you lose.

Hope this helps:-V

 

[boo1]

Mechanical Engineering Design had an article that agreed with nashjp. Threadlocker compounds were the most resistant to vibrational loosening. We never use lock washer for our designs.