Standard washer deformation under reduced bolt torque

[vbaldwin]

I currently work with polyurethane parts that we must bolt together. Because of the reduced max compressive stress limit on the PU, I perform non-standard bolt torque calculations in order to preserve the integrity of the PU. For my current project, I noticed that when the fasteners are torqued, I am seeing washer deformation. This surprised me since we are already not reaching max bolt torque. Some information:

5/8-11 UNC, finished hex head cap screw, 316
5/8 washer, ASME B18.21.1 (for soft material), 0.656 ID x 1.750 OD x 0.10 thk (in), 316
5/8-11 UNC Nut, 316
PU hole ID = 0.657 in
Currently torquing to 65 ft*lbf = about 7.3 kip preload
Max compression on PU = 5.5 ksi

I calculate my maximum compressive force as (area under washers) x (max compression on PU) = 11.4 kip

Anyone have a calc that can help me determine why my washer is deforming, or how to determine proper washer thickness?

Thanks!

 

[3DDave]

In a typical washer there is a nearly direct path from the fastener to the fastened material so that the periphery of the washer sees little load so there isn't anything but a compressive load between the two surfaces. In this case the PU does not provide sufficient support under the fastener causing the load on the one side to be unopposed by the fastener on the other side; basically you made the washer into a trampoline with a concentrated load in the middle.

Edit: To fix this you need a much thicker washer.

 

[LittleInch]

I agree. Your error in the equation above is using the whole of the washer area to spread the force. I think at best you want to consider twice the area of the face of the nut only. so if you took a 1" radius instead of your 1.75 for the washer you might see why.

As soon as a washer starts to deflect, even not visibly, any of the outside area becomes not in contact with the PU.

This gives you about max 3 kip preload. You have 7....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

I would suggest putting metal sleeves in the holes and avoid using special washers and torques. In my experience with plastic there is so much creep it really doesn't matter how you tighten it, it will still loosen eventually.

 

[vbaldwin]

Thanks for the responses. I ran a quick FEA to visibly see what is going on. As I increase the thickness of the washer, many of these issues change significantly. Bad thing is, can't get a standard off the shelf washer that is thicker but not larger in OD.

LittleInch, I will keep that theory about the face of the nut in mind.

 

[desertfox]

Hi vbaldwin

Have a look at this link it shows how the pressure under the bolt head is theoretically distributed as a cone angle.

https://roymech.org/Useful_Tables/Screws/Joint_Stiffness.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BrianE22]

I've used shaft collars for washers when I want a thick washer:

https://www.mcmaster.com/collars/set-screw-shaft-collars-8/

 

[goutam_freelance]

What kind of deformation do you see?
I think your washer area is much spread out and thin (0.1 in) and so the area of washer near bolt hole is experiencing greater bearing stress and deforming more. So washer also may deform more near the bolt holes and the outer rim may look lifted up.
I suggest you use a thicker washer such that the distribution of stress is more uniform in PU. This will also ensure that your max bearing stress of 5.5 ksi is not exceeded anywhere.

Otherwise your calculation looks OK.

 

[hydtools]

Stack multiple washers.

Ted

 

[TugboatEng]

I see in your diagram that you are using nylon insert lock nuts. How do you plan to accurately torque your fasteners to such low levels? How do you plan for end users to do the same? I'm going to say it again, sleeve the hole and then none of this is a problem. Set the length of the sleeve to accurately control the amount of compression of the part.

 

[LittleInch]

More bolts or a bigger flange?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mfgenggear]

can inserts be installed

 

[Denney Zenge]

The dishing of the 316 washers is caused by insufficient bearing area between the washer face of the capscrew and the I.D. of the washer. I calculate worst case bearing area of only 0.190 sq.in. The lock-collar nut subtracts ~ 35 in-lbs. up front. It also matters which end of the fastener you are torqueing. (Standard torque tables are for tightening the nut.) With only 745 in-lbs. applied to an all-316 stainless assembly, I calculate you are only developing 1,689 lbs. preload on the joint. The problem with the washers is they are too soft and of insufficient thickness. Adding more of the same washers won't alleviate the problem. 316 washers can't be hardened by conventional heat-treatments. Solution hardening or precipitation hardening are options, but you should consult your washer supplier on this. I'm attaching my calculation sheet for your info. I'd be happy to provide a more thorough analysis and suggested approaches if you contact me directly with more information(no charge, just doing this to share what I've learned over my 40+ years.



Denney M. Zenge
MobileTech Engineering

 

[goutam_freelance]

On further study it is found that PU is very soft, E=1 to 1.2 GPa only. With the given load, PU will deform by about 0.7 mm. So the deformation you see is mostly due to deformation of PU. Even if you increase thickness of washer, there will be substantial deformation of PU. For mitigation of this use larger and thicker (non-standard) washer and/or use more number of bolts.

Engineers, think what we have done to the environment !