I'm thinking about material like metal, plastic or rubber; size for example disks many times larger than the bolt head; and shapes like smooth, split or crimped.From my research I have read that washers are there to protect the surface - but split or crimped washers actively damage the surface. Also that they're there to provide a smooth contact, but split or crimped washers do the opposite. I have read that rubber washers help make water-resistant seals.From my experience, sometimes multiple washers are used packed together. I wonder if plastic is used purely because it's cheaper, or whether it also allows some compression. Very large washers I often see on bolts through wood and rarely elsewhere - and I can imagine that that is necessary to spread the pressure over a larger area.And related bonus question - when is it preferred or acceptable to not use a washer? via /r/DIY https://ift.tt/2Ja62cJ
