stretching a compression spring

[bullpuppy666]

I am a computer engineer not a mechanical engineer so my question is pretty basic.


What effect does strechiing a compression spring beyond it's limit has on it's rate. For example if I have a spring that is has a 2 inches free length with a rate of 8 lbs and I stretch it to so it that it's free length is 4 inches what would the rate of the spring be?

 

[desertfox]

Hi bullpuppy

The spring rate would increase if you stretched the spring so that its mean diameter suffered a permanent reduction,
however you would also have issues with bending stresses
in the wire if the pitch angle of the coils is greater than 15 degrees.
If you can tell us what you are trying to do we might be able to help you further.

regards

desertfox

 

[sreid]

In general the answer is that the spring rate stays the same. The spring rate is determined primarilly by the length and diameter of the wire, neither of which changes when the spring is streched.

 

[desertfox]

Hi sreid

The stiffness of a spring is given by the formula:-


k = G*d^4/(8*D^3*Na)

where G = modulus of rigidity

d = wire dia

D = mean diameter of spring

Na = number of active turns

If you permanently stretch the spring you will reduce the mean diameter of the spring which in the formula is raised the the cubed this will increase the spring rate.

regards

desertfox

 

[israelkk]

One additional problem is that the spring may have gone a preset (set remove, scargging) process. If you stretch it you destroy it and the spring may have a permanent set when you compress it the same deflection as it was designed to.

Sencond problem is that if the spring was correctly designed it was designed such that the stress at solid highet will be less than the yield or the ultimate torsion stress. Now that the spring can deflect more than it was designed to the spring will see much higher streses than allowed when you compress it all the way.

http://israelkk.googlepages.com/home

 

[bullpuppy666]

I am an amateur gun smith and I have a rifle that has a design flaw that I am trying to correct. The design flaw is that it is using an extension spring as a recoil spring mounted in the front of the bolt instead of a compression spring at the rear of the bolt (as most semi-automatics). The problem is that ejected shells get caught up in the spring and do not eject properly. In addition it is possible for the bolt not to close properly and a dangerous head separation occurs.

Current extension spring has the following characteristic:


Spring 3.41" = 5 lbs
5" = 11 lbs
Rate = 4lbs

I am making the modification as add on/replacement parts only with no alteration to the rifle so I have some constraints.

To add a compression spring the spring will have to ride on a rod and into a hole. The total length of the cylinder is 2” and the rod is 1.6 inches with the spring on the rod and in the cylinder and the rod overlaps the cylinder by about .2”.

The total movement of the system is only 1.59 inches. I need to add a hole (in an attached bolt weight) and I like to make the hole as small (<=.25”) as possible so to reduce amount of weight I had to add to another part of the system.


I tried to find a stock spring but cannot find one long enough and the price for a custom one is far more than I am willing to pay.


So I need a compression spring about .25” and long enough to start with an initial load of 5 pounds and will end up at 11 pounds at 1.59 inches.

 

[israelkk]

This is not a simple case. Your spring has to last millions of cycles therefore, has to be designed for fatigue or even shocks. It will not be cheap to design and manufacture.

Is there a way to contact the rifle maker?

 

[sreid]

Desert Fox,

Springs, as we well know, are well analyzed and understood. Too often, I feel, we tend give the complex answer to a simple question. You are, of course, correct; if the wire has a fixed length and you stretch the spring, clearly the diameter changes and the spring rate changes. But it is a secondary effect that I feel clouds the first order, basic understanding of how springs work.

 

[bullpuppy666]

I have talk to the rifle maker and he is not motivated to make changes to his design. I like the rifle so much I decided to make the changes myself and send him a completed example.

Generally the recoil springs used on rifles are made using standard music wire. They tend to last about 10,000 cycles. They do not fail so to speak but will weaken and cause feeding or ejection problems that call for a replacement. (The extension spring is much more dangeous if the spring breaks) Most recoil springs on semi-auto's are contained inside a cylinder or ride on a rod. In my case I am going to do both. It should be pretty straight forward since the resulting system will have the same characteristics of the original system..

 

[israelkk]

If the spring get weakened after 10000 cycles then it is badly designed or this is what acceptable in the trade. It is probably over stressed or it is on purpose to sell more springs. Assuming that it is overstressed then it maybe a bad design or the space avaialable didn't allow a better spring without making it too expensive. Can you list the extension spring dimensions, outside diameter, wire diameter, material, number of coils, end coils type, first loaded length and load, second loaded length and load, etc.

 

[MikeHalloran]

Because of the envelope, gun springs tend to be way, way too long to be "good" designs by ordinary spring design standards.

Because the extract/eject/feed/insert cycle normally happens too fast for the human eye to see, the dynamics of the springs are at least as important as the static numbers.

In the case of recoil- powered mechanisms, as I think this one is, the kinetics of the entire system are important to its function, e.g., you may have to change the recoil springs to deal with a slightly different cartridge.

The springs are subject to shock, and to surge, and to abrasion from the guiding tubes or rods and the particles that are generated by powder combustion. When the springs' dynamics change even slightly, the gun fails to cycle properly, which limits its utility and greatly annoys the owner. No one would intentionally underdesign a product sold to an irritated gun owner. Not twice, anyway.





Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi sreid

Bullpuppy's question was what happens to the rate of the spring if he stretches it and not how the spring works.
Secondly now we know what the application is regarding a gun recoil mechanism I am glad he didn't just stretch the spring on your advice as it might well have let him down with serious consequences.

Regards

desertfox

 

[bullpuppy]

Yes MikeHalloran I agree with your accessment. This application is a simple blow back design (it is not so simple) and, most if not all, small caliber semi-automatics tend to have long springs. There are three dynamics involved in the simple blow back. The most critical is the amount of force on the front of the bolt face at the moment of ignition.

a) the force of the hammer on the bolt when fired
b) the weight of the bolt
c) the recoil spring force

These all have to be correct to prevent case rupture and to allow proper cycling. Too much force will prevent the rifle from cycling. Too little force will cause case failure. Of the three the force of the hammer spring and the weight of the bolt contribute the most with the recoil spring mostly controling the cycle time.

From what I am getting from this is that a shorter spring is better. In this system the spring fully compressed must not be more than 1.8 inches. So I should try to design find a spring on minimum length to satify the requirements.

 

[desertfox]

Hi Bullpuppy

I agree with israelkk on this your spring needs to be designed for fatigue and just buying one of the shelf and or
modifying an existing spring would not be suitable.

regards

desertfox

 

[sbozy25]

Yes, I agree with them as well. Contact a spring manufacturer and send them the specs you are looking to meet. Let the professionals design a part for you that will work. This can be expensive, but if you find a good one, they may give ou some prototypes to try in hopes that you will convince the gun manufacturer it is the way to go and they would get the job.

 

[israelkk]

Not all spring manufacturers can design a spring (from my experience most of them unqualified for design). They mainly know how to manufacture and to verify that the spring will give the force and deflection and it is not overstressed. But they luck the broad perspective and knowledge how to correctly design the spring to meet the system requirements. Spring is seldom designed for itself it is always a part of a system. Usually the spring manufacturer has no ability or qualification to analyse the complete system and how it affects the requirements from the spring. Most manufacturers use a software for spring verification/design ((usually from the spring manufacturer association) but the common software will not account for relaxtion, stresses due to surges, etc.

It is best to use a qualified designer that has a broad understanding of the system requirements in combination with a respected manufacturer to make sure that the designed spring can be manufactured.

 

[sbozy25]

Well I suppose my company is an exception. We hire degreed engineers and send them through all kinds of training. We have a very talented staff that understands pretty much everything there is to know about springs. Now I am young and still learning, but we have a few guys that have been doing this for 30+ years. In fact many of the design manuals you may use, they wrote. Our CEO is even the president of SMI...

But yes, I do agree with you. There are many companies out there that do not understand the design side of things. But if you stick to the big players in the spring industry, they tend to have a good design staff.

 

[bullpuppy666]

Thanks for all the feed back and it all has been most helpful. My budget does not allow a custom spring to be made but I did find a stock spring that looks promising.

The characteristics
Lenght = 11"
OD = .240
WD = .028
Rate = .77 lbs
Load = 4.56
Active Coils = 121




So I read somewhere that if you reduce the number of active coils by 50% then you double the rate. Is this true?

If that is the case then I need to determine at what length would the spring have a rate of 4 lbs and see if that will work within my constraints. What would the formula be to determine that?










Outer Wire Free Part Total E-mail sales@springsfast.com
Dia. Dia. Length No. Rate Load Coils Ends Material Finish Web

http://www.springsfast.com

(in.) (in.) (in.) (lbs / in) (lbs)

0.240 0.028 11.000 661 0.77 4.56 121.0 O M N

 

[desertfox]

Hi bullpuppy

Yes if you half the number of active coils you double the rate, however you will also probably overstress the spring so that it takes a permanent set the first time you compress
it to the length you want and more importantly the force you think you have at an intial compressed length will actually be less due to the spring taking a permanent set
(spring wire exceeded its yield stress).
My advice don't modify a spring, in addition for your application you need the spring ends squared and ground so that the spring seats squarely between the two metal faces
this again is unlikely to be achieved by modifying one.

regards

desertfox

 

[sreid]

Have you looked through the Century Spring Catalog to see if they have something that will work for you.

http://www.centuryspring.com/