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  #1  
Old 07-16-2009, 03:45 PM
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Flat Washers

Something that has never been explained to me is when is it really necessary to use a flat washer under the head of a bolt and/or a nut? I can see it when the material being fastened is realtively thin, but often times I see flat washers specified when the material is say .250 or heavier. And I also understand their useage when the hole is elongated or a bit larger than the the fastener being used. Most of the time the holes that I drill, punch or specify myself have a tighter clearance to the fastener than the typical flat washer does. Is a flat washer still needed?

So what guidlines do you follow, or what do those of you with an engineering background say? Your input is appreciated
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Old 07-16-2009, 04:16 PM
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Interestingly enough, washers are not explained in the typical engineering curriculum. Of course fasteners are covered in machine design and stress analysis courses; they are usually treated as an elastic load element that provides a compressive stress between the two pieces being fastened. With the system in a state of compressive stress, loading will generally increase or relieve compressive stress in the pieces being fastened. The fastener itself sees relatively little change in stress during loading because the increased cross sectional area at the interface between the pieces being fastened results in a stiffer load path. Stress and loads in general tend to follow the stiffest load path.

Of course there is a point at which you relieve all the compressive stress between the elements and at that point your load starts to dramatically increase the stress in the fastener, which will cause a failure.

I can see the washer doing a few things: it spreads the load from the fastener over a broader area to prevent local shear stresses, it provides a consistant surface for the end of the fastener to provide consistent torque feedback, and it prevents the hole from rounding out under shear load.

I'm not sure what a good rule of thumb would be for determining whether or not you need a washer, but I would think a hole diameter to thickness ratio could provide a clue. It would also depend on the material(s) being fastened.
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Last edited by TheBandit; 07-16-2009 at 04:45 PM.
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Old 07-16-2009, 04:27 PM
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Straight from Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing Handbook:

"Flat washers serve as bearing surfaces to prevent bolt heads from digging into the work surfaces. They also allow more accurate installation torque or strain measurements. Finally, they serve as shims."

By the way if you do anything with nuts, bolts, rivets, etc... you should own a copy of this book.

Personally I use washers when and where ever I can, it really can't hurt (as long as the right washer is used).

Hope that helps.
Jaysin
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Old 07-16-2009, 04:39 PM
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Thanks for your input guys. This all makes more sense now

Anyone else have anything to add?
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Old 07-16-2009, 05:40 PM
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south african flat washer
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Old 07-16-2009, 06:04 PM
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Quote:
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Anyone else have anything to add?
NOPE, I do believe Jay and Bandit covered that exceptionally well...

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Old 07-16-2009, 06:05 PM
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south african flat washer

LOL... I love them washers as well, but just a we bit thinner guage ....

cheers bro.. good one...

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Old 07-17-2009, 12:30 AM
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Quote:
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LOL... I love them washers as well, but just a we bit thinner guage ....
Thin flat washers for sure!
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Old 07-17-2009, 10:00 AM
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Another thing to note. Flat washers have a 'flat' side and a 'rounded' side. The flat side goes against the part, and the rounded side goes against the bolt head. The reason for this is one of the primary reasons for flat washers.

The rounded side is built to protect the chamfer where the bolt head is attached to the body. If the flat side is used against the bolt head it has the potential to nick the chamfer, introducing a stress riser and creating a weak spot in the assembly.

The whole point of the chamfer in a bolt head is to reduce stress risers. It is actually easier to manufacture bolts without the chamfer, but even the cheapest bolts utilize one. Stress risers dramatically affect the yield strength of any piece of hardware.

All the other reasons given are also valid and legit. I was also taught that the only washer to use is a flat washer. Split lock washers at best are compressed down to a flat washer when torqued, and at worst they split open and gouge the bolt and the assembly. Thus a properly torqued bolt with flat washers is the optimum configuration.

Just what I was taught at school and at work. It makes sense to me, so I now use flat washers on both sides of each fastener whenever possible.
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Old 07-17-2009, 10:09 AM
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Originally Posted by joescj5 View Post
Another thing to note. Flat washers have a 'flat' side and a 'rounded' side. The flat side goes against the part, and the rounded side goes against the bolt head. The reason for this is one of the primary reasons for flat washers.
This is particularly important for National Aerospace Standard (NAS) bolts. They have a generous radius under the head because the material has a min UTS of 180,000 psi. I don't get to play with these bolts because they are way out of my price range...
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Old 07-17-2009, 11:34 AM
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Rule of thumb for me... if I can fit a washer I do... If a lock washer will fit under the nut... that goes in too.

Never lost a bolt or nut this way... although friends say I still have a few screws loose in the garage and on the trail...

Paul G.
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Old 07-17-2009, 11:48 AM
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not a fan of the split lock washer, it's either nylocks or some color of locktite for me.
back in the day when I raced motorcross we drilled & safety wired everything critical but you dont see that much anymore, drop of locktite seems to be a lot easier.
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Old 07-17-2009, 01:13 PM
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Quote:
Originally Posted by SUPERD View Post
not a fan of the split lock washer, it's either nylocks or some color of locktite for me.
back in the day when I raced motorcross we drilled & safety wired everything critical but you dont see that much anymore, drop of locktite seems to be a lot easier.
Lock washers...
Only used in non critical applications.

A bolt properly applied and torqued is self locking a tension load is generated through the axis or center line of the bolt. A lock washer in the mix reduces the loading of the assembly, it is designed to compress and in theory the sharp edges of the split are supposed to bite into the fastener. However when you look at a lock washer you will see that the edges in a properly torqued fastener are not generally doing anything thus technically the bolted joint will have to loosen thus the bolt has failed before the lock washer can get a bite. Note that on critical fasteners there is not a lock washer. Look at head bolts, factory suspension bolts etc...
That said the lock washer has applications in materials that would crush of full torque was applied.
In every case I can think of a Nylock would be a better choice. However at 3x-4x the price often in an OE application the lock washer is the economic choice.
Bandits post addresses this, and Carroll Smiths book referenced above is an excellent easy read.

I've had the same supply of lock washers since 1983 but have used many, many gross of Nylocks in that same time period.
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Old 07-18-2009, 12:44 AM
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Thanks for all the replies guys. Now I have a better understanding of flat washers and some on fasteners in general. This has brought a couple of new questions however. It seems that some of you won't use split locks, how do you feel about other locking washers such as internal or external tooth? Also nylock nuts have been mentioned. I do use nylocks some but I usually prefer a steel top lock nut or a flange lock. What are your thoughts on these nuts?
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Old 07-18-2009, 01:07 AM
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Captain~ as far as I understand (and this is first hand use as well as reading) the all steel locknuts are one time use and so are the bolts or male threads that they go on to. Nylocks on the other hand are re-usable as long as the nylon is not damaged and it still is tight enough that you can't screw it on by hand. I must say that I use a lot of nylocks and I also love loc-tite where ever I can't use a nylock. By the way they also make Nylock bolts.

Jaysin
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Old 07-18-2009, 01:29 AM
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Now I've always heard the opposite.....a nylock is a one time use locking nut. Although I have re-used them, but not on critical components. The steel top lock nuts I have better luck with on the first time and on re-use, but of course that is as long as it still retains most of it's original locking ability.
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Old 07-18-2009, 02:47 AM
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I always use washers.. It makes me feel better.. LOL.. as far as nuts go.. I only know 3 basic kinds..

1-nylocks - they are a compression fit - I will reuse them only for non structure (maybe) , I normally use SS so rust may be an issue with others.

2- Stover nuts used like on frame bolting something that accually clinches the thread.. (I don't as a rule reuse them )..

3- My nuts.. Never to be reused unless she peens them gently ...



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Old 07-18-2009, 10:51 AM
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I like stover nuts for final assembly. They like to damage the bolt on removal sometimes, but hold better than nylocks.
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Old 07-18-2009, 05:47 PM
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Quote:
Originally Posted by DriveshaftJoe View Post
I like stover nuts for final assembly. They like to damage the bolt on removal sometimes, but hold better than nylocks.
Thanx DriveshaftJoe , when I said 'Jam Nuts' I was meaning 'Stover nuts'

Damn, as we get older, memory is good, but short.. I'm going to edit that before someone quotes me mistake... .. to many nut 'n' bolts in this world..hehe

Cheer'z Bro..

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Old 07-18-2009, 06:17 PM
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I will use the internal and external star lock washers on "Screws" that are not Grade 5 equivalent, and certainly I use them for electrical connections.

In a race situation I'll only use a locking nut one time.
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