Spring Set Removal 8

[desertwolf]

Hello everybody!
I have a problem in spring designing. in some cases we have very huge static load in compression springs that makes the spring over stress in calculations. i was thinking about pre-setting the springs to increase the max load that can apply to. now i have several questions.
1. Is this solution ok?! and is there any better one?!
2. I confused about pre-setting process. Is this necessary to compress the spring to its solid height? I mean if applied load in solid height is much more than the max load, what should we do?
3. In spring stiffness equation, spring free length has not any effect. is this means we can choose any value for this parameter below the calculated critical spring free length?

thanks in advance!

 

[desertfox]

Hi desertwolf

Normally the spring manufacturer does it, basically they make the spring slightly longer in the free length and then compress it till the spring stress exceeds the yield stress.
The spring then returns or should return to a slightly shorter free length than before, however when you then compress the spring to the length you require it should also give you the load that was requested.

thats it in a nutshell, talk to your springmaker.

desertfox

 

[israelkk]

desertwolf

Regarding you first question, this solution is OK but makes the spring more expensive and more difficult to get normal load tolerances.

Regarding your second question, how much more (in percentage) is the calculated stress in solid height than the max load stress? There is a maximum that you can get from the spring wire.

Regarding your third question, you do not specify the free length before preset you only specify the final free length after preset (if it really matter). Normally, you define free length as reference value. You need to define the max. solid height and the spring length at the load point/s.

One more issue that you need to consider is relaxation. If the spring will see the static load for long time it will loose it force. Therefore, the designed static load for short time and long time loading are not the same.

 

[ornerynorsk]

Does your design allow a hardstop near the end of travel? This will save spring life by a huge factor.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[israelkk]

ornerynorsk

Relying on a hard stop in the working system design to avoid spring preset (set remove) to solid height is a BAD!! design approach. This is the reason why it is not allowed for example by the US naval/marine use because one can accidently press the spring to solid height before it is installed into the system with the hard stop. If the spring can not be pressed by hands (very large load) then you may argue that a hard stop to avoid preset to solid height can help use a smaller wire diameter for the spring.

 

[ornerynorsk]

Israelkk, I was referring to a hardstop OUTSIDE of the spring area, not on the spring itself. This is done all the time in tooling applications.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[israelkk]

I think we do not talk about the same thing. When you do a preset on the spring the spring is manufactured longer than the final designed free length. Then the spring is pressed to its solid height. Therefore, the spring length become shorter, to fit the designed free length. This way when you press the spring later on to any point, it will never should loose length. By this operation during the spring manufacturing process a spring can be designed to work under larger stresses than the wire yield strength and even the ultimate tensile strength. This is what desertwolf was referring to.

 

[ornerynorsk]

Israelkk, I am certain we are not speaking of the same thing.

Here is desertwolfs original question under point 2 of his/her post: "I mean if applied load in solid height is much more than the max load, what should we do?"

This is the basis of my suggestion for a hardstop. My suggestion has nothing whatever to do with the spring manufacturing process. Perhaps that is where the misunderstanding came about.

Best regards,

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[tbuelna]

israelkk,

Your point about judicious use of hard stops in mechanisms is well made. The biggest problem with hard stops is that they usually create a huge instantaneous jump in system stiffness when contacted, and the dynamic loads that result can overstress the structure.

I also agree that coil binding a helical compression spring in service should be avoided. What usually initiates failure in helical compression springs that are subject to coil binding is gouging of the first/last coil surface where the free end of the wire bears against it.

A good solution (if possible) is to add a high rate secondary spring, or even use a progressive rate primary spring, that gives a rising spring rate over the last portion of mechanism travel. This is commonly done with automotive suspension springs. A progressive spring rate can be achieved using variable coil pitch winding, tapered wire diameter, or a combination of both.

Interesting topic of discussion.
Terry

 

[desertwolf]

Thanks a lot every body!!
first of all, i cant ask the spring maker because he cant understand what im saying!! Right now im working on a hydraulic spring test machine which can apply up to 20 tons to an object and i want to do this by myself.
to israelkk.
1- can you explain little more about difficulty of getting normal load tolerances?
2- i think i understood what you said about free length. first we specify the target free length. with equation F=kl (l= free length - solid length) determine Fmax and then we can calculate max stress. in normal condition it must below than max torsional stress / 1.2. but if it is greater, we can set remove the spring. for this, tmax (max stress) must not be greater than 1.3*torsional yield strength. (how to determine torsional yield strength?). then we should make the spring with new free length = b * target free length (b=?). after pre-setting, max allowable torsional stress increases and we can apply greater load to the spring. for determining the max allowable torsional stress i use this equation: t=0.5A/d^m that A , m are constants for spring material (Cr-Va) and 0.5 is percentage that relates tensile strength to torsional stress. (refers to "Shigley's Mechanical Engineering Design, Eighth Edition" P510 Tables 10-4 , 10-6) with respect to Table 10-6 after set removing the spring, we can use 65-75% instead of 50%, and max allowable stress increases.

3- can you explain more about long term load?

4- now another question. if tmax was greater than 1.3*torsional yield strength, for set remove the spring, can we compress it till tmax = a*max torsional stress instead of compressing to solid height?


using a spring with raising k factor, is a interesting idea. but in our cases, calculations based on constant spring rate.

 

[desertfox]

Hi desertwolf

Read this link:-

http://springipedia.com/compression-stress-spring.asp

If my springmaker can't understand scragging I would find another fast!

desertfox

 

[desertwolf]

If my springmaker can't understand scragging I would find another fast!

i wish i could. but my Boss is in love with that spring maker guy!!!

 

[israelkk]

desertwolf, use 65-75% instead

 

[Cockroach]

There are a few things you can try, DesertWolf. Spring stress is basically governed by shear across the cross section of coils; play with coil diameter to achieve higher strength. You can also specify an alternate spring material noting the working stress. Third, have you looked at a rectangular profile rather than round? Typically you can get a smaller solid height while delivering greater compressive loads.

But you can put a travel limit into the design to stop a piston from fully compressing the spring. I have done this in the past using the head of a socket cap screw.

I have a spring design book by Wahl from the Spring Mfg Association. I fully recommend this as a reference to the spring device in basic design applications.

Regards,
Cockroach

 

[desertwolf]

thanks guys!
Cockroach
1- I have a limit in choosing wire size.
2- I'm working on this, but the spring material often specified by customer! but still working on this!
3- actually no! but must working on this issue too!

I design springs for using in VARIABLE SPRING SUPPORTS/HANGERS & CONSTANT SPRING SUPPORTS/HANGERS. in both of them, springs feel long term static load. how this kind of loads affect the spring stress?! (according to israelkk's post)

 

[desertwolf]

I design springs for using in VARIABLE SPRING SUPPORTS/HANGERS & CONSTANT SPRING SUPPORTS/HANGERS. in both of them, springs feel long term static load. how this kind of loads affect the spring stress?! (according to israelkk's post)

any suggestion?!!!!

 

[desertwolf]

Hi again!
im completely confused about set removing! try to explain as clear as possible however my weak english!!
is it necessary to deflect the spring until solid length? or we can just compress it to the point that spring applies the load that generates stress equals 1.3 times minimum tensile yield strength. (in conditions that we should design spring length longer because of operating deflection.)
for example we have a spring with 200mm free length and 100mm solid height. (for set remove we can not make the spring with longer free length because of exceeding the stress in solid length and distortion may occur.) after preset, free length decrease 10-30% then free length becomes 140-180mm. now with 60mm operating displacement, it can not be compressed enough. now the solution. can we make it longer than 200mm and then compress it until the stress equals to required stress to permanent deflection occurs instead of compressing to solid length?

thanks again!!

 

[israelkk]

"...after preset, free length decrease 10-30% then free length becomes 140-180mm..."This is a too much variation. I think making the spring longer before preset will not help because there is a physical limit to the preset (set-remove) you can achieve from a known spring wire. The wide range of lengths hints that the stresses during the set-remove process are too high for this wire type and diameter. You may need to redesign your spring.

If you can finally define your spring design exact requirements we can try to check if such a spring is even possible even with the set-remove process.

Outside/inside diameter or a range for the dimension?
Wire diameter? (or range)
Wire material?
Spring Load at first load point?
Spring load at second load point?
Deflection between the two load points (or spring rate)?
Maximum length at first load point (lower load)?
Maximum free length (if important)?
Maximum solid length (if important)?
Number of cycled between the two load points (if this is non static spring)?
Environment (temp, corrosion, etc.)?
Is the spring guided (inside or outside) or need not buckle under maximum?