Nesting springs
Nesting springs
(OP)
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.
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.





RE: Nesting springs
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
RE: Nesting springs
RE: Nesting springs
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.
RE: Nesting springs
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
RE: Nesting springs
RE: Nesting springs
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?
RE: Nesting springs