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Spring dia increase under compression?


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#1 MatsNorway

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Posted 20 January 2015 - 15:25

I thought a coil spring would expand slightly under compression. Is this correct? is it possible to design out? Is this effect whats causing buckling on longer springs?



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#2 Greg Locock

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Posted 20 January 2015 - 21:10

Yes it does, probably not, no. You could wasp waist the spring so that under compression it becomes a cylinder.



#3 MatsNorway

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Posted 20 January 2015 - 22:18

wasp waist? If you meen guides i am. Four rods at Ø18 with even spacing.

 

How does one predict the increase in dia?



#4 Greg Locock

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Posted 20 January 2015 - 23:13

Google doesn't work on your machine?

 

O.D.solid= sqare root (D^2 + (p^2-d^2/(3.142^2))) + d

Where:

O.D. - Outside diameter
D - Mean diameter
d - wire diameter
P - coil pitch

DESIGN HANDBOOK Engineering Guide To Spring Design, 1987 Edition, Page 31, Associated Spring - Barnes Group Inc.

 

although in practice they look barrel shaped, that is because the end coils are not loaded in the same way and deflect in a different fashion. I imagine the growth in OD is proportional to load.


Edited by Greg Locock, 20 January 2015 - 23:24.


#5 MatsNorway

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Posted 22 January 2015 - 07:33

I tried.. Used the wrong words i guess and assumed this was one of these questions few talks about/hard to find info on. Got a dia increase of 1.77mm on the biggest.

 

Now for instance im trying to find the force exerted on the walls (supports/guides) when/if the springs are buckling.. My first assumption was that the force would come out like this: Spring%20load.%20buckling%20spring_zpsrw


Edited by MatsNorway, 22 January 2015 - 09:01.


#6 Greg Locock

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Posted 22 January 2015 - 11:21

 So i'm guessing you want to work something out to do with snaking loads (I've just made that up) as a long slender spring wiggles around under load. Yah good luck wid dat. we empiricists test stuff like that and write very helpful tables for other luckless bastards to use.

However, obviously there is an axial frictional load =-mu* axialforce* sin(theta)

 

mu is 0.3, theta is defined by geometry, and the rest is tedious



#7 MatsNorway

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Posted 22 January 2015 - 12:44

Hmmmm. I just want to design safe guides ensuring the spring stays within.. Guide loads as displayed is at the contact point on the guide. I just scaled everything up to make the figure more clear to read. Actually.. looking at it it now the strength of the spring goes not thru the contact point and would further complicate the model. I think the force not being at contact point creates a lever that aids in making the spring "slide" at the top mount in the opposite direction.

 

I think the figure could at least give pointers to the loads involved on the guide. adding friction makes it more accurate.  Im only asking because there is a spring and there could be something that i do not know about. I have a huge folder with tons of tables from prior googling allready, id love to have more. :)

 

Edit: these springs are not long enough to buckle on multiple places i think.


Edited by MatsNorway, 22 January 2015 - 12:52.


#8 saudoso

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Posted 22 January 2015 - 22:43

You'd better make sure the load follows the spring main axis, any reason you have to contain it from the outside? Can't you just guide it like a mcpherson suspension assembly?

 

kw_was_ist_ein_gewinde_01_en.png



#9 Lee Nicolle

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Posted 23 January 2015 - 09:24

You'd better make sure the load follows the spring main axis, any reason you have to contain it from the outside? Can't you just guide it like a mcpherson suspension assembly?

 

kw_was_ist_ein_gewinde_01_en.png

Those soft springs do cause the main coil to deflect. Usually the marks on the shock body are evidence of that. The likelihood of a broken or sagged spring is more likely. Though it can improve both ride and power down markedly if set up right. And if wrong be a major hindrance! 

I just live with one spring,, KISS principle.



#10 MatsNorway

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Posted 23 January 2015 - 09:49

I have made a seat for the upper part of the spring so it does not deflect and slide out on the press. This will probably need some work over time to find the suitable solution for the different springs. The big ones are easy to handle as the press is narrow enough to go between the guides. Meaning the guides can go all the way up.



#11 Greg Locock

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Posted 23 January 2015 - 22:46

A coilover with adjustable ride height is suboptimal for spring clash issues, first and foremost because it doesn't cause much of a problem in motorsports so you don't design for it.

 

First of all, the shock itself bends.

 

secondly, the lower spring seat is a flat plate and that is not the best way to maintain spring straightness.



#12 mariner

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Posted 24 January 2015 - 11:37

Its sligthly OT and a very sad subject but when the twin towers collasped on 9/11 people asked why they didnt fall sideways - i.e buckle.

 

I remember a skyscraper structural engineer coming on TV and explaining that  even a 110 floor building to buckle out sideways without any direct side load was very, very unlikley.

 

I suppose a coil spring is analogous


Edited by mariner, 24 January 2015 - 11:38.


#13 saudoso

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Posted 24 January 2015 - 12:56

I really didn't mean anything that convoluted for real, it's just my lack of English vocabulary.

What I meant was to have the springs aroud a cylindrical guide.

#14 MatsNorway

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Posted 24 January 2015 - 20:43

Its sligthly OT and a very sad subject but when the twin towers collasped on 9/11 people asked why they didnt fall sideways - i.e buckle.

 

I remember a skyscraper structural engineer coming on TV and explaining that  even a 110 floor building to buckle out sideways without any direct side load was very, very unlikley.

 

I suppose a coil spring is analogous

Twin towers had the load carrying structure on the outsides so when the floors collapsed they fell within the structure first. You can see it if you look closely. There is plenty of videos of failed building demolitions where the building kinda lands.. on the blown out floor and then tips over.

 

 

I guess some building engineers are in a hurry too and just make it plenty strong to deliver in time.. Btw. These springs we are testing is up to 7 tonnes when fully compressed..


Edited by MatsNorway, 24 January 2015 - 20:47.


#15 Lee Nicolle

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Posted 25 January 2015 - 02:05

A coilover with adjustable ride height is suboptimal for spring clash issues, first and foremost because it doesn't cause much of a problem in motorsports so you don't design for it.

 

First of all, the shock itself bends.

 

secondly, the lower spring seat is a flat plate and that is not the best way to maintain spring straightness.

On a coilover both ends being square cause very few problems. Yes the shocks flex but not to any great degree. The springs flex though quite a lot. Stiff factory replacement springs do sometimes cause a good deal of flex of the spring, wishbones, trailing arms. Far more than a coilover.

Though factory stuff with minor spring rate increases seldom cause any grief provided they are not too low. Better for road use then. but seldom for a racetrack. though coilover are seldom better for street use. The original springs will last a LOT longer.



#16 MatsNorway

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Posted 10 February 2015 - 13:32

Tool worked fine. We also found a interesting side effect of the stabilisation once we tested without and with support. The spring gave a higher spring load when it was bulging out to one side than when supported. Very interesting and i think i understand alot more than i can easily put down with words in english. :) Gives me lots of questions about Torsjon bars vs coilsprings in valvetrains etc.



#17 gruntguru

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Posted 11 February 2015 - 02:00

 . . . The spring gave a higher spring load when it was bulging out to one side than when supported. . . . .

Surprising. Systems usually tend towards lower energy states. Does anyone have an explanation?



#18 Greg Locock

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Posted 11 February 2015 - 04:07

Gimme an F

Gimme an r

 

you can guess the rest



#19 MatsNorway

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Posted 11 February 2015 - 19:12

Grunt. I think this explanation is onto something: Your trading spring longevity for short term strength by loading it in a way it was not designed for, thus the increased resistance.


Edited by MatsNorway, 11 February 2015 - 19:39.