Nesting springs

[cjccmc]

Hello,

To satisfy failsafe requirements, I need to provide two springs in case one should fail. Application is compression spring for nominal .25 dia pin. Need to have a 0.4 lb min load (per spring) at 1.25 inch ht. The spring will be compressed down to .83 inch ht where I would like the load there to be as close to the .4 lb as possible (i.e. low rate)

I originally had a single compression spring .240 dia, .018 wire, 2.0 free ht in a .255 hole. Can I simply add a second spring inside of it as long as there is radial clearance between the two on installation? Is this normal practice with no significant chance of interference and malfunction? Since the second spring is completely redundant and only there for backup there would be no sense in this approach for me if nesting two springs causes an overall higher chance of malfunction.

 

[MikeHalloran]

ASC used to publish guidelines to predict whether springs would nest with their brothers in bulk handling, but this is different.

I think you can nest springs concentrically as described, provided that there is some radial clearance and that they are of opposite hand.

However, once the first spring fails, you are not likely to then get full life from the second spring because the first spring's detritus will foul it. .... unless you have room to completely separate them, e.g. in separate chambers.

It might be time to get out the _thick_ spring design book and try to do a good fatigue life prediction for both cases.



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

A 2 inch long spring with 0.24 inch diameter has length to diameter ratio ~8. This spring will buckle for sure under compression load. Therefore, it has to be guided either on the OD on a hole or on the ID over a pin. However, it will rub on the guide resulting in friction. You can not nest them unless each one is separately guided. the inner spring will have to be guided on a pin and the outer on the hole. But, beware of the friction due to the buckle of each spring.

 

[cjccmc]

Mike: these springs are very low cycles, no fatigue issue, just a "what if" requirement that would have disatrous consequences if the spring did not function, therefore the 100% backup with second spring is needed.

israelkk: So even if both are guided you think friction may be substantial? Main concern here is reliability. Do you think having two nested springs, both guided, provides a much higher reliability than one single spring or does the arrangement pose a new possibility of malfuntion by one jamming the other or similar issue?

Springs are on an emergency door for personel escape, hand operated, low speed and cycles.

 

[MikeHalloran]

I defer to israelkk; friction is going to be a problem.
I'll go further; corrosion is likely to lock them up, so I don't see the double springs as adding anything but expense.
Speaking of expense; the given dimensions suggest that you're already painted into a corner, but a shorter spring of larger diameter should not need guides, so the issues of friction and corrosion with guides go away.


Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

If you need 0.4 lb I do not understand how two spring will do the work? If both gives ~0.4 lb then each gives ~0.2 lb therefore, if one fails the second spring will give only ~0.2 lb. Will the system still function under 0.2 lb? You mentioned that you want the 0.4 lb as accurate as possible therefore, the friction as the result of the buckled coils rubbing on the guides will affect the spring/s force greatly.

 

[cjccmc]

Thanks for your replies, I appreciate the input as springs are not an area of expertise for me.

I'll try to explain a bit better:

Need at least .4 lb to keep the door locked, and needing 100% redundancy, each spring will have to supply .4 lb in case the other fails, so when both springs are working properly there will be .8 lb of axial preload against the locking pin of this door. In an emergency the locking pin has to be manually pushed back .42 inch with one finger (thumb) so now we would like the force required to do this as low as possible, limited to 3 lb max per stated requirements. Besides the axial preload of the springs there will be friction from a radial load on the lock pin against the side of the hole that guides it. Naturally as the pin moves further back in the .42 travel to release the door the spring loads go up linearly. That is why I wanted a relatively longer free state spring, to keep the spring rate as low as I could get it in order to stay withing the 3 lb max when the pin gets to the end of travel. To allow for other friction, I want the combined force of the two springs to be 2 lb max when the pin is pushed back .42 from the starting position.

On a per spring basis this works out to: .4 lb at L=1.25, 1 lb max at L=1.25-.42=.83

I think a have room to increase the OD of the pin/spring hole to .43 if necessary. If the two springs each have their own guide, one has the walls of the hole, the smaller will be provided with a pin concentric to the hole, then each should operate as well as if the other pin were not present. True?

 

[cjccmc]

Mike: I forgot to mention, springs, pins and related hardware will be Inconel to meet high temp requirements, so I think corrosion will not be an issue for these materials.