Multiple Washers with High Tensile Bolts ? 2

[novus2]

I have a situation where three flatwashers were used under the nuts of 1" diameter A325 bolts. The loads are dynamic. The holes are round and 1 1/16 " in diameter. Are there any prohibitions against this or any potential adverse consequences?

 

[MintJulep]

Every mating surface is a place where embedment can occur. The more mating surfaces, the more embedment.

 

[msquared48]

Other than availability of the washers, why not use one thicker specially fabricated steel plate washer if there is not enough thread on the bolt?

Mike McCann
McCann Engineering

 

[desertfox]

Hi novus2

The more faces you have the more friction you have to overcome during tightening resulting in a lower tensile preload in the bolt and consequently a lower joint clamping
for a given bolt torque.

regards

desertfox

 

[vesselfab]

What you have is a fabricator/erector that did not consider that A-325 bolts have shorter thread length on them that regular bolts do and them made up with washers.

I think I would have someone replace with proper length bolts at no charge.

 

[dimjim]

It seems like a simple solution would be to order
thicker washers rather than using multiple washers.

 

[bridgebuster]

I agree with the others - three washers and a dynamic load -you're looking for trouble.

A number of years ago, I was Resident Engineer on a bridge rehab. One task was to install modular joints on top of the existing finger joints. The new joints were held in place with threaded studs welded to the finger joints. The contractor had to shim the new joints to meet the top of overlay.

The shims were 1/8" thick, non-tapered; multiple layers were needed. It didn't take long for the new joints to break loose.

The conclusion was that the problem was causes by the shims.

 

[bvanhiel]

The more faces you have the more friction you have to overcome during tightening resulting in a lower tensile preload in the bolt and consequently a lower joint clamping
for a given bolt torque.

I think this is false. In fact, I think it is possible that you will get less friction. As you tighten the nut the interface with the least friction is the one that will move. This is probably going to be the nut-washer interface, but might be a washer-washer interface. The weakest link (washer in this case) in the chain determines the overall friction. Adding more links won't make the chain stronger.

That said, I don't have any experience with embedment or other issues, so please don't think I'm endorsing multiple washers.

-b

 

[desertfox]

Hi bvanhiel

If you can get less friction using several washers in a bolted joint which means more torque is available to generate axial tension why don't you endorse using multiple washers?

regards

desertfox

 

[bvanhiel]

I doubt you'll get much more preload, but I don't see any reason you'd get less.

Of course, even if you did get more, more is not what your after. You want to get the RIGHT preload.

There are other issues that dictate washer selection. I'm not an expert in the design of bolted joints. I just recognize that, as far as friction is concerned, adding washers is like adding links to a chain. The weakest joint governs.

-b

 

[desertfox]

I think it is possible that you will get less friction

.
Firstly you haven't answered my question and getting the right preload or more preload isn't relevant in the context
that were discussing. What I asked is if you have less friction loss during tightening of a joint why don't you
endorse using mutiple washers in the joint.
If I have a bolted joint and I have one washer to go under the nut that joint will have friction during tightening which will have to be overcome by the wrench or spanner if I am tightening to a given torque, some of that torque is lost in overcoming the friction between nut/washer and washer/joint surface.
Now if I have the same joint and three washers to go under the nut according to you I will have less friction to overcome even though I have introduced twice as many interfaces were the washers/ nut/ joint surface can experience relative movement during tightening.
In a nutshell your saying that for a given joint configuration including a torque figure, preload, bolt elongation whatever you prefer, the loss to friction during tightening will be less if you use more than one washer.
I have yet to see a specification that says if you use more than one washer under the nut your friction losses will be less during assembly, but maybe you can produce such evidence which will be of interest.

 

[desertfox]

Follow this link:-

http://www.pirate4x4.com/tech/billavista/NutsandBolts/Nuts&Bolts_signed.pdf

scroll down till you come to the heading "Why Torque" its just below table 2

 

[bvanhiel]

desertfox,

The answer to your question is: There may indeed be a good reason not to use multiple washers. I don't know for sure that there is one, but I do know that the reason you've posited (increased friction) isn't it.

I am not arguing with your conclusion, just how you got there.

You'll note that the article you quote states:

Avoid using multiple flat washers, as the relative motion between them and the nut and the joint ALTERS the friction under the nut face

"ALTERS" does not mean increase. I am suggesting, with my links in a chain analogy, that it can only mean a decrease in friction.

Great article by the way, but not exactly the most reputable source. I would feel a bit awkward defending my decision with an article from an off-roading magazine. Especially one that has "Xtreme" in the title.

The OP is using A325 bolts, so I'm assuming it's a structural application and that there are specific codes that probably apply. I'm not an expert in that field, I don't know the codes, and I don't know the answer.

-b

 

[HgTX]

If turn-of-the-nut is used, does the number of washers matter?

Hg

Eng-Tips policies: faq731-376

 

[jetmaker]

You could concievebly get less friction with the use of multiple washers during the installation. The reason I say this is that based on the combination of materials the washer/washer interface may have a lower coefficient of friction than the bolt head/washer or washer/substrate coefficient.

To give an example, very coarse bolt head and substrate... and 2 back-to-back highly polished washers with a squirt of oil between.

I think the big issue with using multiple washers during installation is that the required torque is now a bigger crapshoot than before.

jetmaker

 

[CoryPad]

HgTX,

Turn-of-the-nut could improve the performance of a joint with multiple washers, but it doesn't remove all of the problems. These problems can include imperfect fit due to crown, burrs, or surface roughness. VDI 2230 has a calculation for preload loss due to settling, and this will depend on the number of surfaces. But, the settling losses can be small, especially if the preload is high, so the problem can be managed.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Bolteng]

There are a couple of reasons why you shouldn't use multiple washers in a joint.

1. VDI 2230 indeed states setting per surface.

2. The most common washers are too soft for the most common bolts. This means, if you don't look at the hardness of the washers, the surface pressure will probably be too high, which causes embedding (3 washer create more loss of preload than 1).

3. Dynamic external load (vibration) on the bolt will cause loosening (loss of preload). The Junker test proves this. The more flat washers you use, the more possible surfaces that can cause loosening.
The use of a flat washer combined with a spring washer is even standardised, but definitely not correct in an application with dynamic loads.

In an application with dynamic load, preload is very important because of fatigue. In this case I think you should probably use a good locking product (under the head of the bolt and the nut). Examples: Tensilock, Ripp, Schnorr, Nordlock, or a chemical locking.

 

[Tmoose]

"common washers are too soft for the most common bolts. "

HOLOKrome suggests materials with hardness > Rockwell 85 B /~80,000 psi UTS won't indent under their SHCS when run washerless at full torque.

Some bolted joint models with a single washer > approximatley 0.04 inch thick indicate the stress distribution flares out quickly in 3X washer thickness steps, so the interfaces between washers 1 thru 17 have a pretty easy time of it. That squares pretty well with my experience that the head of a SHCS will burrow into a Home Depot ungraded washer, but the second washer looks fine. if the clearance hole is too big regular thickness washers will simply bend into a belleville shape.

A nice thick grade 8 ground washer is a beautiful thing.

 

[Bolteng]

Tmoose,

You are right in saying Rockwell 85 B /~80,000 psi UTS wont't indent.
What I meant with the most common products is that in machinebuilding for instance an 8.8 bolt (~116,000 psi UTS) is very common.
The most common washer in the market (in Europe) is the flat washer DIN 125-1 140 HV (Rockwell 75 B).

In that case the washer is too soft for the surface pressure.

According to VDI 2230 the stress will flare out between the nut and the bolthead. The surface pressure on the second washer will be lower than on the first. But, if the washer is too soft the stress will probably not flare out enough, and can result in deformation of the second washer.

It all comes down to the use of washers with a correct hardness.

 

[gtaw]

Since it appears that you are addressing a structural steel application and you are installing an A325 bolts and matching nut, why not refer to the AISC Steel Construction manual and the section by the research council on high strength bolts.

I would provide more explicit information, but I'm on the road and don't have access to the information I have in mind.

Best regards - Al