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Disk Spring for thermal effects compensation 5

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eli28

eli28

Aerospace
Oct 20, 2019
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Hello,

I have an Ultem 1000 Flange that has to be connected to a Titanium container with a screw made of steel.
I am presenting only a portion of the bolted joint for simplification:
1_q4xj5v.jpg


First of all I have to point out that my preload is limited by the ULTEM 1000 and hence it's only preloaded to 2450N.
I was looking for a solution to 2 problems:
1. Preload decrease due to exposure to cold temperature (del_T=-25 celcius degrees).
2. Preload decrease due to the (unknown) ULTEM creep in compression.

As a result I thought that adding a "spring" element with a moderate stiffness may compensate the deflections with only quite small load decrease. So I decided (as a result of limited space) to try to use (for the first time) Disc Spring for this purpose, as described in the picture above.

I have some questions regarding this design:
1. Is there a problem of stacking 10 Disks in parallel?
I couldn't find an adequate standard Disk Spring (for a #10 bolt) that may give me such forces.
It's important to point out that I have a limitation of maximum outer diameter (of the Disk Spring) of 12 mm that isn't seen in the picture above, and that's why I can't use a massive disk spring.
2. Is there another elegant solution I didn't think of and that is preferable?
3. Just to make sure I have no mistake - is the ULTEM 1000 surface will have a reaction force that is equal to the bolt axial load (that is equal to the equivalent spring force exerted by the induced deflection)? I read somewhere that as a result of internal friction between the disks the ULTEM 1000 experiences a lower force than the exerted force (of the bolt), but when I drew a Force Body Diagram I found out that for an equilibrium the reaction in the ULTEM surface has to be equal to the bolt axial force.

I am looking forward to your good advice!

Have a nice day
 
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Hi

I know nothing about Ultem 1000 however you need to take account of the stiffness of the materials you are clamping and the bolt / disc spring stiffness.
Putting it bluntly if the disc spring stiffness is much greater than the stiffness of the Ultem 1000 then all that will happen is the disc springs will not compress but instead will just embed themselves into the Ultem 1000 and you will not achieve the preload you are looking for, however without the modulus of elasticity of the Ultem 1000 and properties of the other materials present it’s impossible to tell.
If the materials being clamped have similar properties to the bolt/ disc washers then yes the load in the bolt is what the Ultem 1000 would experience.
Another way to look at what I am saying is imagine your joint as you have shown it but instead of Ultem 1000 replace it with say putty and think about what happens when I tighten the joint.
Further more you also need the expansion coefficients of the materials to do any kind of analysis.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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Hey IRstuff
I think that if I flip every other washer (serial connection) the equivalent stiffness will decrease. I calculated that for achieving 2450 N preload I need at least 8 parallel disc springs (each one's force at 75% deflection is 300 N).
 
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Hey desertfox
I attached the Data Sheet of Ultem 1000.

The 2450 N was determined after calculating which load can be sustained for prolonged durations without yielding and it's actually around 30Mpa. On the other hand a creep can't be avoided.

I used in my calculations the stiffness and Coefficient of thermal expansions of the joint components, and found out that the expected reduction in preload due to temperature change alone is about 10%.

I never used disc springs in my design, and I think that my main question is if the application is correct. It seems to me that stacking 10 disc springs in parallel is exaggerated and may bring some problems I don't think of.
 
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There are losses in efficiency when you go to tall stacks, We used to use alternating pairs.
Yes it makes the stack taller, but it works smoother.
Just remember that this is a spring, there will be a force vs height relationship that you need to watch.
The washer on the Ultem is critical, it will need to be extra thick and strong, it cannot have any deflection.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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Hi eli28

Reading your last post if you haven’t included the disc springs in your original calculations for the joint under going thermal change then the calculation isn’t valid because the stiffness of the bolt will change because of its increased length when you add the disc springs and the springs themselves will contract under reduced temperature. Can you post your calculations in here ?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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I think that if I flip every other washer (serial connection) the equivalent stiffness will decrease.
Click to expand...

Sorry, no. A stack of Bellevilles all facing the same direction makes most of them ineffectual; only the one washer with its circumference touching the destination surface is really doing anything.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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If they are all facing the same direction each washer will add to the the force required to cause a unit of deflection (plus additional friction forces between washers due to shear). So, a very stiff spring, with a lot of damping.

If the washers alternate directions, each washer will add to deflection caused by a unit of applied force. So, the spring becomes less stiff.
 
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Hey again desertfox

It took me a while to reply, but I attached a detailed document with photos and explanations.
I am sure not everything will be understood, and you all are invited to ask questions.
After I added disk springs I got a reduction of 65 N due to temperature decrease.
The calculation without the Spring Disks gave a reduction of around 150 N.
 
 https://files.engineering.com/getfile.aspx?folder=76a6293f-b393-4dcf-8b52-c1754f5c90da&file=DISK_SPRING_CALC.pdf
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Hi eli28

Thanks for the detailed analysis, if I understand it correctly you are setting the bolt preload at 2450N which is the maximum that the Ultem 1000 material can take before yielding.Now with that preload you are saying the joint is cooled and so the bolt preload also reduces and with the disc springs fitted you have calculated that the preload loss is 67.5N but what I cannot see in the calculation is what the external load on the joint is liable to be? Now the external load is important because the relative stiffnesses of the bolt and joint materials will dictate how much of the external load each component will see, so if you preload the Ultem 1000 to its maximum it can take, then any amount of the external load imposed on the joint materials will cause the Ultem 1000 material to yield and your joint will be lost.

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

I am going to give you an additional information that was achieved prior to dealing with the thermal effects.
I started by FBD for calculating the required force (with safety of factor) the bolted joint should bear - 2450N per bolt.
I checked that the ULTEM 1000 stress isn't above 40 Mpa (its yield is around 100 Mpa) when using 2450 N preload.
The limitation of 40 Mpa is for avoiding significant creep.
I have a separate analysis (in my work) in which I can see that after adding the external load, the part of the load taken by the members (by the way, surprisingly it's around 80% of the total external load) is OK and the ULTEM 1000 is far enough from yielding (around 60 Mpa).
Now, after making sure that these criteria are met - I moved forward to thinking of failure modes like thermal effects etc.
 
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Hi eli28

Well it seems to me you have it well sorted, obviously the 80% of the external load isn’t going to separate the joint or yield the Ultem 1000 but was is the magnitude of the external load?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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You've put a lot of effort into this joint. Might I suggest one of these to prove the results all over all temperatures.
Bolt Tension Meter

They aren't cheap but they are accurate. They can be used to perfectly tighten fasteners regardless of any and all rotary friction aspects as they measure only the bolt elongation and stress. You can tighten fasteners to the exact torque regardless of compression of the bolted materials or thread lubricants. Furthermore they allow you to come back and see if it has changed which seems would answer your question about the Ultem creep. With the optional temp sensor you could monitor the joint across all expected temperatures. You can map how fast and how serious the creep might be. There are a couple of companies that sell these instruments though they seem to all be on the same price train together.. I see these as the oscilloscope of fastener users. The option exists of renting one or hiring a service to measure for you too.

Keith Cress
kcress -
 
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itsmoked thank you very much!
I didn't know about this kind of instrument.
Do you have any experience with this specific company? Which company is the leading one in this field?
 
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Hi eli28

What is the value or magnitude of the external force, you stated that when you added the external force you calculated that 80% was taken or seen by the joint materials, however I couldn’t see a value for it.

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