What is the best spring material?

[molds]

I have a situation where compression springs are failing at an accelerated rate. the situation is a 3.00 inch long, 1/2 diameter spring. it has a rate of 31.17 LB/IN. preloaded .200 of and inch, with a working stroke of .75 inches. the solid height of the spring is 1.560 inches. the spring material is 302 stainless steel. the spring is retained in an aluminum block at one end that has been hard coat anodized for a lenght of 1.875 inches. the oposite end is retained by the ejector bar which has a .200 deep bore for thr spring pocket. it is in a horizontal position. the spring cycles once every 7 seconds. the aluminum plate has cooling water runing through it that ranges from aprox 33f degrees to 45f degrees. room temprture is held at 68f. can anyone suggest what may help this situation? I can not increase the diameter due to space constraints, but could increase the length slightly.

 

[TheTick]

questions, suggestions...

Perhaps try a different steel alloy that has better fatigue characteristics?

Shot peening: shot peened springs have greater resistance to fatigue.

 

[CoryPad]

What are the delivery conditions for the spring? Are you using a spring temper for the wire? You may have very low strength stainless steel wire.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[molds]

The springs im using now are a Lee stock spring made from passivated 302 stainless ASTM a313. thanks for your help.

 

[israelkk]

Can you give the Lee product number?
Without this info you need to give the wire diamter, number of active coils etc.

 

[molds]

The spring number is LC072G13S. Thanks

 

[TVP]

molds,

When you state that the working stroke is 0.75 inches, does this mean the end of stroke is 2.25 inches (3 - 0.75 - 2.25)? If not, what is the spring length at the end of stroke? If the spring is not being compressed completely to solid, then there may be an issue with surface damage due to contact with the anodized layer on the aluminum bore.

 

[molds]

TVP, the spring is compressed to a height of 2.05 inches. 3-.20 preload -.75 = 2.05 compressed height. Thanks

 

[israelkk]

First of all the spring rate is 26 lb/in (not 31.17 lb/in)and the outside diameter is 0.480" (not 0.5") to work inside 0.5" hole.

From your description it is not clear to me why the spring is supported inside the hole only to 1.875 and to all its working length. Spring that long will buckle sideways unless it is completely guided and all its coils are always inside a hole or guided by an inner shaft. No coil should be left unguided.

One more point to take into consideration is that the spring will buckle inside the hole and will rub against the hard anodize hole surface resulting in scratches and wear of the wire diameter. The result is larger stresses and lower life cycle.

You didn't give the number of cycles to premature failure.

According to my very quick calculations this spring should not be deflected the 0.2+0.7=0.95". At that point the spring wire is over stressed. The stress after the extra 0.75" deflection is larger than the maximum allowed/recommended for this wire type and size.

http://israelkk.googlepages.com/home

 

[sreid]

How is the spring breaking? Describe the break.

 

[israelkk]

Same exact size and material spring can be designed and manufactured to work in those conditions but the estimated life cycles is ~53000 cycles. But, the Lee spring the way it was manufactured will yield even after one time deflection.

I do not think that just by improving the spring you will solve your problem you probably will need to do some extra changes to avoid the wear of the spring due to the rubbing in the hole and to other sideways buckle effects.

 

[TVP]

molds,

Thanks for the clarification. My very quick calculations produce the same results as israelkk-- very high stresses upon compression and buckling is a problem. I am assuming that the problem is a combination of wear against the anodized layer and high stresses when compressed more than ~ 0.75 inches.

 

[molds]

israelkk, you are right, the sprink rate is 26lb/in for stainless steel. I incorrectly typed in the music wire rate. the pocket in the aluminum can only be 1.875 deep because the stripper bar that the other end is guided by has a pocket .20 of an inch deep. the stripper bar would crash into the aluminum if it is any taller. the same spring in a Danly says that compressing the spring .900 of an inch will yeild average life. what would you suggest? I could go to a 3.5 inch long spring, back off the preload to .10 so my total use of the spring is .85 of an inch. the Danly part number is 9-0814-11. the Danly book says that the .85 should yeild long life. what do yo thing this spring is made from chromium vanadium steel. Thanks again for all your help

 

[israelkk]

The Danly spring is a die spring with rectangular wire. The spring free length is 3.5" therefore it will not fit in your system without changes is the system. If you will install it in your system the preload will be 0.7" deflection instead of 0.2".

Selecting this spring will not solve the buckling effects (rubbing and others that I didn't want to specify without knowing the actual details of your system.

Your statement that "the stripper bar would crash into the aluminum" is a factor of the way you support the spring and your specific design. This has to be changed for any 0.5" diameter and and longer than 2".

 

[molds]

I can increase the depth of the pocket to allow a longer spring. I feel that the increase in length will reduce the stress and help reduce the side load against the pocket. how should the spring be supported? if I put a pin in the center, it will still rub on the pin, correct?

 

[israelkk]

Yes, if you put a pin in the center, it will still rub on the pin.

I am not sure about the stress and cycle capabilities because a quick look at Danly's web page doesn't give the wire section dimensions, number of coils and exact material properties.

Danly use terms like "long life" but what it means? What is the guarantied number of cycles?

You still didn't tell us how many cycles you are expected from the spring to consider it OK.

How many springs you need? Only one or you will need many springs?

http://israelkk.googlepages.com/home

 

[molds]

I would like the spring to last for over 1 millon cycles if possible. I can use a Dadco nitrogen gas spring with that stroke which they say is good for 1 millon strokes minimum. I thought that a standard spring should be able to be set up to out live the nitrogen spring. there are 3 springs per stripper bar. Thanks again.

 

[molds]

I can modify the pocket to go with a spring as long as 4.5 inches if that would help.

 

[israelkk]

If you need only one spring for this specific project then it may be that you can find a spring that will do what is expected but you will probably will spend more time and money until you will be satisfied.

On the other hand a custom designed spring can be designed to your specifications and small quantity of springs can be produced. This will cost few hundred dollars but you probably will get a spring that will do the job. I assume the cost of time and money you already spent on this spring is much more than the few hundred dollars.

If you need many of this springs then the custom designed spring may be the best approach.

In the case of a custom designed spring a whole new shorter non-buckling spring with larger diameter can be designed.

 

[MikeHalloran]

I have stacked short springs along a bar, separating them with Oilite bushings counterbored to form shallow spring seats.

In your case, you could use a smaller bar and put the spring seats on the OD of the bushings.





Mike Halloran
Pembroke Pines, FL, USA