spring mounting design differences

[durablack2]

I am curious if there would be any noticeable difference in feel or longevity of two different spring mounting systems.

On one end of the system we will have a moveable, range limited rod that mates up with a compression spring. The opposite end of the spring will mate to a fixed surface. The rod can move side to side and will always be in contact w/ the compression spring. The rod will have a handle protruding outwards (90 deg from axis) and will move by hand. Around 30 lbs of spring force at max compression, 10lbs at full extension. The rod will be enclosed in a cylindrical hole so it can only move back and forth in one direction.

I am wondering if either of the two types of mounting explained below will have any kind of difference. The reason I am asking is because type A will probably have more cost, but my colleague thinks it will allow for smoother operation.

Type A: Spring will be screwed onto a threaded boss of the rod, closed and ground end of fixed side

Type B: Spring will slip fit over the boss of the rod, closed and ground end on both sides

If Type A is better than B, could adding a washer between the rod and spring for type B improve smoothness when moving the rod? Other ideas?

Pic attached.

 

[rb1957]

A would be slightly stiffer than B, 'cause you've defeated the spring action for some length of the spring.

Other than that I don't see much difference between the two.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[JohnRBaker]

I agree with rb1957...

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[durablack2]

Good point - we have limited space so that's just going to hurt us it seems.

 

[GregLocock]

B is a very typical solution for automotive coil springs. Your spring manufacturer is the best person to talk to.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[desertfox]

Hi durablack2

If you reduce the number of working coils using the type A set up then are rb1957 says the spring will effectively be stiffer than set up B, also I would add that if the spring stiffness increases in set up A then so does the shear stress in the remaining spring coils, so if you use A you ought to recheck the spring stiffness and stresses.
For me set up B is better mainly because it doesn’t effect the spring stiffness, however the one possible drawback with set up B is that the spring can possibly rotate during operation of which I have seen in practice, if this is a problem you might want to clamp one of the dead coils at one end of the spring.

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

 

[btrueblood]

"The rod can move side to side"
" The rod will be enclosed in a cylindrical hole so it can only move back and forth in one direction"

Which is it? If the latter, then B is fine. If the former, then perhaps consider a pocket or other features to keep the opposite end of the spring from shifting out of position.

 

[durablack2]

@Desertfox - thank you - i do not think rotating will be a problem but we will test to make sure no issues. I am not quite sure how we would go about clamping this end though due to how it is used and installed.. Hmm.. hopefully not an issue

@btrueblood - I was not 100% clear - the rod will move up and down, compressing the spring. The spring cannot move side to side since it will be installed in a cylinder. (in reality, it will be installed horizontally direction so i was thinking side to side when writing this, but i drew it in the vertical direction). Sorry about that confusion.

 

[TugboatEng]

There is a problem with A. Where the spring threads on to the male pilot, the motion of the spring is constrained. This will lead to cracking where the spring meets the thread. The longest life comes from a spring that is coiled to a flat end and ground. Consideration must be taken for wear at the contact points of the springs. Washers of various materials (rubber, plastic, metal) are used depending load.

 

[durablack2]

Thanks Tug. We will be mating to metal in both areas, combined with relatively low cycle numbers so I'm not too worried about wear, but I am worried about a spring cracking or failing prematurely. Seems type B will be the best method for us for durability and cost.

 

[geesaman.d]

A is how carefully tuned spring power airguns are assembled. It reduces the vibration and harmonics when the spring piston is released, but that is a very sudden action. It also requires manual fitting between the spring and the snugly-fitted end. It's a fairly specialized case compared to other compression spring applications.

The other situation that might matter is if the top component needs to be retained somehow.

 

[GregLocock]

Also used in the spring in some retractable ballpoint pens.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?