Does washer diameter impact friction in a bolted joint? 3

[CB77]

Consider a clevis with a bolt going through it, with an elastomeric washer on both sides to prevent rotation.

In terms of maximizing the friction, I don't think the washer's diameter matters because friction doesn't care about surface area (assuming a constant pre-load from the bolt and that the washer is always completely covered by the bolt/nut heads for even clamping).

The counter-argument is that with a larger diameter washer, the resistance to rotation is greater since it's a greater moment (the friction is acting further away from the axis of rotation). But surely that's countered by the frictional force being lower throughout making this a moot point?

Who's right here?

 

[TugboatEng]

If the elastomeric washer is to use friction to prevent rotation that means it would be storing rotation in shear in the rubber like a spring. Wouldn't this make the bolt impossible to tighten in the first place as the rubber washers would recoil and loosen the bolt?

 

[CB77]

The "bolt" is really just a hand-tightened screw so its not a tremendous amount of force we're talking about here. The design is in production so it works more or less. I'm just curious about the impact of washer diameters.

 

[3DDave]

I'm inspired to suggest adding a new forum - bolts, screws, nuts, and washers. Collect these questions in one spot.

Anyway - classic friction says that the normal force and the coefficient of friction are the only controls. As Tugboat mentioned, elastomers are not compatible with classic friction theory. They tend to conform to a different class of surface defects that metal washers won't. Compare dragging a metal item over asphalt/gravel street vs a soft rubber item of similar weight or noting that, for the same contact area, doubling the normal force on the soft rubber will more than double the friction.

That said - the area of interest is the area contacted by the nut or the underhead of the bolt. Unless the bolted material is softer than the washer, I don't see much effect of changing the diameter.

The more interesting outcome from using an elastomeric washer is that if the item being bolted has faces that are not perfectly perpendicular, the deformation allowed by the washer will more uniformly load the face of the nut or the bolt underside. What happens if the force is not high enough to bend the bolt head is that some point can be the point of contact, forming a lever so that when external loads are applied, the lever causes the bolt to turn, reducing the contact friction and allowing the bolt to unthread itself. It's not usually a big effect as most screws/bolts get a large tensile stress, but in open joints, such as a channel or clevis, it's why there are cross-drill bolts and cotter pins.

If you want to prevent rotation, there are fittings that have tabs to engage both the base material and the fasteners, usually one bends the tab to match the fastener, but there are a lot of alternatives - aggressive lock nuts, castellated nuts with drilled bolts and cotter or roll pins.

Hence why I would like to see a separate thread forum for bolts, screws, nuts, washers.

My preference is for conical washers so that the first contact is with the rim against the base material, maximizing the resistance to turning.

 

[dvd]

I second the conical washer suggestion.

 

[CB77]

I should probably clarify that this is a non-critical, lightly-loaded plastic joint in a consumer goods product, so while I appreciate the discussion for robust solutions, it's a bit overkill for this.

 

[MintJulep]

Once you get larger than the "diameter" of the nut or bolt head it makes no difference, because outside of the head there is no compression of the washer.

Smaller than that, the imaginary "effective radius" will make a bit of difference.

 

[dvd]

CB77 - tension holds the nut and prevents rotation. It looks like the plastic components would be subject to differential thermal growth (even small ones)and/or deformation due to loading, and, thus loss of tension on the fastener, which would result in nut rotation. Why not forego the elastomeric washers and put a drop of loctite on the threads?

 

[CB77]

Good idea, but the joint needs to be somewhat frequently adjusted by the user (and at times completely removed) during use making loctite a no-go.

 

[rb1957]

"making loctite a no-go" ... I don't think so ... I think you can disassemble a joint which has loctite in the threads, just takes more force then would typically be seen from inservice vibrations.

another day in paradise, or is paradise one day closer ?

 

[dik]

I'd use regular washers with nylock nuts or loctite... Loctite Red discourages removal, other Loctites are 'removable'. I forgot to answer the question. The washer size makes no difference.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[3DDave]

Show of hands - how many people love getting the actual problem statement dribbled out a smidge at a time?

 

[CB77]

How many people like getting their actual question ignored and irrelevant recommendations provided instead?

Seemed fairly straightforward to me: does washer diameter impact the rotational friction?

 

[3DDave]

I covered that in the first response. It was not ignored.

 

[CB77]

Yup I saw it and appreciated the answer. I just need to mathematically convince people of this, though my integral calculus is a bit rusty.

 

[rb1957]

I think diameter has very little impact. It won't change the COF. It also doesn't change the normal force. It does change the pressure (between the washer and the parent body) which would change the local friction force, but not the sum (over all the washer). Now there may be some room to argue if the bolt is only "finger tight".

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

Ouch, calculus! For a thick steel washer most of the load is spread out in a cone-like fashion from under the head of the bolt to the part. I believe most "cone makers" use a 60 deg. included angle for the cone. I'd guess for rubber the angle would be much smaller.

If you really want to get into it, the head of the bolt will deform under load so that the pressure under the bolt head is not uniform. I think that effect is very, very small and always ignored.

 

[CB77]

Intuitively I don't think washer diameter should matter because as we've said friction only cares about normal force and COF.

But does that also apply to a radial situation like a washer where the friction is generating a moment to counteract rotation? And then the washer diameter would matter?

 

[drawoh]

I'm inspired to suggest adding a new forum - bolts, screws, nuts, and washers. Collect these questions in one spot.

forum725

--
JHG

 

[drawoh]

CB77,

My first thought about your joint is that your clevis is flexible. If you put any force whatsoever on it, it will bend. This is especially true if it is plastic. I assume your washer will move with the clevis, and the bolt and screw will make a point contact somewhere. This renders your diameter irrelevant. This also renders your joint loose and flexible, making it easy for your screw to work its way loose.

Loctite dissolves certain types of plastic. I have a bad history with microswitches and PCB components. Be careful.

Search Carr Lane for spherical washers. The spherical faces are rather roughly machined so they are not as slidey as I would like.

--
JHG