Flange Nut With Flat Washer 3

[natepiercy]

Is there any reason why a joint would contain a flat washer under the flange head of a flange nut?

Extra info:
I'm looking at a joint that holds a hardened steel disk to a hub (4 joints per disk/hub). The joint in question has a carriage bolt on the hub side of the disk (carriage head in the hub). On the other side, there's a flat washer and a flange nut. The OD of the flat washer is less than the flange diameter, but not significantly.

Thanks in advance.

 

[MikeHalloran]

I'm guessing it's either to make sure the flange nut never reaches the end of the thread,
or not quite similarly, to provide enough bolt length under tension to make the joint 'not super stiff', so you can store a useful amount of strain energy in the bolt shank to prevent loosening, without going out of your mind measuring bolt stretch or trying to measure torque with great precision to the same end.

An extreme example of providing length for bolt stretch is engine cylinder retention.
In the typical aircraft piston engine, the 'jug' is retained by short studs through flanges at the base of the cylinder. Great attention is paid to getting the torque correct so they won't fatigue, or loosen, and still a jug will fall off occasionally.
By way of contrast, VW and Corvair engines use a very long stud, severely necked in the middle, to retain the stack of jug/ head gasket/ head between the engine case and the nut. The stud stretches quite a lot, and typically survives a lot longer than the itty bitty studs used on aircraft engines.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

One reason I prefer using a flat washer under nuts is because the nut is typically the component turned/wrenched at installation. A flat washer protects the underlying part surface from abrasion at the nut face sliding contact.

 

[R4FKEN]

I remember something about the friction coefficient to take into account to calculate the required torque, depending on whether or not there is a washer in between. Can't remember the formula tho, or even the norm/standard...

So it could just be a design/calculation requirement.

 

[Tmoose]

Carriage bolt ?
Without doing some Googling that sounds like a pretty mundane low tech low stress fastener.

So there are square holes broached in the hub?

 

[natepiercy]

I think Mike hit it this one squarely on the head (strain energy). A co-worker had posed that explanation to me, but it was poorly articulated. Is it reasonable to say that "increasing the length under tension allows there to be more strain energy in the system without increasing the strain on the fastener"?

Now that I have a direction to investigate, I dug out a textbook and found some equations to back up the claim.
(1) strain = displacement / length = d/L
(2) displacement = d = F*L/(A*E)
from (1) and (2)
(3) strain = F/(A*E)
Or stated verbally, the strain on a member in tension is dependent on the tensile force applied, the cross-sectional area of the member, and the elastic modulus of the member, but not the length.
(4) strain energy = U = F^2/(2*k) = (F^2)*L/(2*A*E)
So if we want to increase strain energy, our options might be:
A. Increase F, the tensile force
B. Increase L, the length under tension
C. Decrease A, the cross-sectional area
D. Decrease E, the elastic modulus (material property)
Recall from equation (3) that options A, C, and D will increase the strain on the member. So while we may be able to change any of these properties by re-design, we will eventually reach the threshold of allowable strain on the member, and these options are no longer permissible. That leaves option B, increasing the length. And in the instance of the fastener, the simplest way to increase the length under tension is to provide more space between the nut and the bolt head by, say, adding a washer.

tbuelna,
Agreed. I was thinking the same thing, but then again, it's a hardened disk, so I didn't see the benefit from that angle. Good point, though.

R4FKEN,
This also seems like a possibility. It's hilarious, isn't it? We prescribe a torque value when what we really want is a tension - and numerous factors (such as friction) keep us from achieving our desired tension value because it interferes with the fragile relationship between the two.

Tmoose,
Square holes, yes. The disk-contacting portion of the hub is a casting; I believe the holes are formed by the mold, rather than machined in. There's not a lot of clearance on the hub side once this is in the field, so I assume that for ease of maintenance (and ease of manufacturing, for that matter) carriage bolts were used to appease customer demands.

Thanks to all who replied.

 

[R4FKEN]

From my (limited) experience, it is more guesswork than calculating, those torque values. With all the different coëfficiënts in the equation...
Secondly, applying a tension is possible, but takes special (expensive) equipment and a LOT more time than applying torque. But I've seen it being done for body flange connections of heat exchangers and reactors for oil and gas, and petrochemical industries. Not to mention nuclear reactors, that come with custom-built high-tech tensioning contraptions. But that's offtopic