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Fully agree, most washers used by the industry are fender variety, and they are under-rated for the application.
Hi Steve,
Good to hear from you. I bet you see a distressing number of fender washers?
I’ve been writing about that for years. I did a lot of similar tests. We agree. Bottom line: if the load needs distribution, a fender washer will do no better than a common bolting washer. Most of them end up bent. If you are using a backing plate, the plate should be (better be) stiff enough that all you need is a bolting washer.
The Bolt Depot is a good source of extra thick fender washers. You have to buy a small box, but for a few common sizes, that’s nothing. Lots cheaper than core repair.
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Want to match the stiffness of an extra thick fender washer (about twice as thick) by using several standard fender washers? Remember what John said about stiffness increasing with the cube? You will need 8 standard washers, which will be more expensive and stand about 1/4- to 1/2-inch tall. Using two will probably just result in two bent washers.
Hi Drew,
Thanks for the confirmation.
Hi Drew,
Thanks for the fill. Interesting about how many fender washers you have to add to equal one thicker washer. Gets me thinking about another tip.
Fender washers are not backing plates!
They are intended for soft or thin materials, where you need to spread the force of a fastener out over a large area to prevent the material from tearing or crushing. They’re a great choice for something like a thin piece of sheet metal serving as an exhaust heat shield, or hanging an electrical enclosure on drywall / gypsum board, or securing sheets of thermal insulation.
If you use them in a situation where the bolt is torqued to a non-negligible fraction of its yield strength, they will deform into shallow cones, concentrating the load under the bolt head and defeating the entire point of having a large washer.
Fender washers are, by definition, thin. They have to be thin because they are *supposed* to deform in a way that minimizes the tearing forces and stress concentrations in the thin material where it touches the outer edge of the washer. A thick oversized washer is not a fender washer and, if properly specified, may make a fine backing plate.
Hi Matt,
Thanks for the fill on what fender washers are actually for. Drew had some stuff on that too. Just another case where the marine industry adopts something because it’s easy without doing any basic engineering. Reminds me of saildrives: what engineer in their right mind would design a drive that requires the whole engine to be removed (in most cases) every six years to change a gasket!
On structural washers, I guess I can see a few cases where they would work but I think in most real world cases on a boat they too are a bit of cop out since a bonded backer plate will be a lot better.
Oops. Matt beat me to it, and of course with better clarity. Agree 100%.
Fender washer will protect the FRP from abrasion at the surface. FRP is not great in abrasion. If there is cyclic loading at the bolt, especially if the load is off axis, the fender can protect the surface of the frp from getting worn away.
Hi Robert,
I mentioned abrasion as a secondary use of washers in a footnote. That said, if the bolt is moving due to cyclic loading the joint was not done right or the bolt was not preloaded properly: https://www.morganscloud.com/jhhtips/under-torquing-is-dangerous-too/
One point of contention: I would argue that a bent fender washer is not a useless fender washer. When a washer is bent (yielding plastically), it is still loaded on the elastic stress-strain curve and applying near yield stress level loads to the surface underneath it. There may be another discussion about long-term creep of that washer, and it clearly is not functioning the same as a thick, stiff washer, but the thick stiff washer also may not distribute load if it is installed with a couple of high points and or a dished surface, where it will put all the load on the high points or edges. Acknowledging that the elastic load (in washer bending mode) on the outer edges of the washer are not huge or optimal, it does seem to me that the standard fender washer would act somewhat like a thick hard washer with 100 tiny springs under it, distributing load under it’s surface.
I’m open to correction here and would love to hear some discussion on this.
Link to example of elastic response of a yielded specimen: https://mechanicalc.com/static/img/Materials/Theory/Original/unloading-reloading-01.png
Oh, and backing plate is clearly the MUCH better solution, no argument at all there.
Fender washers are *supposed* to bend, at least a little bit. The pressure on the thin material being fastened and protected by the fender washer will be low at the washer’s outer edge, and higher at the bolt head. This is how they are supposed to work: they spread out the stress concentration that would otherwise tear the soft / thin material where it touches the edge of the washer.
By contrast, general-purpose washers for use on hard / thick materials are not much larger than the bolt head, and produce a roughly uniform pressure on the material being fastened.
Horses for courses.
Excellent and thought provoking article. Quick question. At what point does stacking x-number of fender washers equal a space limited backing plate?
I am re-doing the “coffee can” hull-to-deck joint on my Havsfidra 20. This joint is an odd one. It was originally screwed together from the outside to locate everything, (with metal clips on the hull lip which the screws passed through,) and then heavily fiberglassed on the inside.
I was going to use 1/4” stainless elevator bolts fed from the outside and thru-bolted. Because the elevator bolts, without a bolt-head, give a flat profile, I planned to glass over the outside of the joint, adding a glassed in 60mm Coosa board rubbing strake, to which a commercial sized PVC rub-rail will be added. All this hull work is getting re-gel coated…..(I know….I know…LOL),
Lastly, now I’m wondering; since it is already glasses together inside, is re-bolting even necessary?
You have me re-thinking my bolting method. Any thoughts on this greatly appreciated. Thanks you.
Regards,
Martin
s/v Pippi,
1973 Fisksatra Havsfidra 20
Hi Martin,
I guess the first question is why are you doing this? My guess would be leaks? If so, and given that your are glassing from the outside, I would not bother with bolts, and particularly not elevator bolts since my guess is you would not be able to torque them high enough to do much good. So my approach would probably to just glass over from outside but using epoxy resin, not polyester, to make sure I got a good bond. Do note that you can’t use normal fibreglass that’s intended for polyester with epoxy, but need to source cloth for use with that resin. If it were me I would get all supplies from West System to insure compatibility. Follow their instructions to the letter and you will get a strong and water tight covering for the existing joint.
Washers serve another important purpose in a bolted joint, as a sort of sacrificial “slip plane” between the rotating bolt head or nut and the clamped material. Even when the average contact stress under the bolt head wouldn’t yield out the base material, the local contact stresses often do, and you get galling. With galling, you increase friction, reducing your effective joint preload. This represent much of what the “preload uncertainty” factor used in your bolted joint calculator is meant to address.
Washers, especially when plated or lubricated, reduce the chance that the “torqued” side of the joint galls during final torque. Even if one side of the washer galls against the fastener or clamped material, the other side will slip instead.