How to Determine if Spring Life has been Spent

[gatech74]

I am trying to set a go, no-go limit for the die springs we use in our molding operation. One noticeable difference between new springs and old springs is the free length. We are using a Danly Die MAX Xl heavy load spring (part # 9-3220-26), which starts out with a 5" free length. In operation the spring is deflected across a range, which may reach solid length every cycle. Is there a way to calculate the loss of spring load as it correlates to the free length loss? For example: start @ 5" with 108 lb per 1/10" deflection. End @ 4.75" with ??? lb per 1/10". It seems obvious to me that the pre load on the spring will be reduced, but I need to know how much the working load changes so that I can set a limit for a maintenance department. Please let me know if you have any information I might find useful.

 

[israelkk]

According to the MAX Xl catalog you should not load the spring to solid if you need even average life. There is a maximum allowed load which is 30% of the free height. Beyond that the spring stress is too high and the spring loose from its height. It receives a permanent set. This set doesn't change the spring rate but because the spring becomes shorter the maximum load to solid become smaller.

The catalog gives allowed deflections for long life and average life. Beyond that there is no guarantie.

 

[gatech74]

Thank you for the response Israelkk. The main piece of info I was looking for was whether the spring rate changes. What I can do is take the new length and calculate the load given a particular displacement to determine the allowable free length. Since I know I am loading the spring more than recomended, the application parameters are fixed because of costs, and I need a standard spring due to volume and cost - where can I go to find a spring that will work better for my application?

 

[MikeHalloran]

You will probably need to add a _lot_ more spring pockets, and use springs with a longer free length, a shorter solid height, and a lower rate.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi gatech74

Firstly the spring rate is linear between 15% and 85% of its compressed length, outside of that its non-linear.
For the purposes of your calculation it is reasonable to assume that the spring rate doesn't change depending how accurate you want your calculation to be.
Reason I say this is that most springs either have a tolerance on the rate or a load at a specified compression, so if you measured two new springs and plotted there load at lengths you would probably find them different.
I agree with the others here in that you really need to re-design your spring or component geometry to not over stress
the spring.

Regards

desertfox

 

[gatech74]

Thank you for the responses. I am trying to find a way to add additional spring pockets into my equipment, but I have not been able to add enough to achieve my desired load without running into similar spring issues (travel to solid). Thanks again.

 

[desertfox]

Hi gatech74

What is your required pre-load and compression?

regards

desertfox

 

[gatech74]

Hey Desertfox,

I don't have a required pre-load. The spring doesn't need to be pre-loaded at all as long as I achieve a certain amount of load at given distances from fully compressed. The spring must compress to a height of 3.35" or less (fully compressed), and must produce a minimum load of 1622 lbf at this height (assuming linear spring rate). The more critical load is when the spring height is between 3.85 and 4.05". I have a target load of 1240 lbf and a minimum of 1130 lbf. Actually that is not entirely true. My target laod is around 1400 lbf, 1240 lbf is just the max I have been able to achieve. In addition the spring must have a minimum load of 640 lbf at a length of 4.35".

Thanks,
Gatech74

 

[desertfox]

hi gatech74

Have a look at this spring ref:-EH150-500 at this site:-

http://www.petersondiesprings.com/Diespringcatalogehp.asp

regards

desertfox

 

[israelkk]

desertfox

The spring gatech74 uses is a spring to use in a 2" hole or over 1" rod guide. The spring you suggested is for a 3/4" rod or for 1.5" hole. This exactly the reason why he has difficulty to find a 5" long spring that can give the high loads. He probably has 1" rod guides for the die.

According to my calculations there is no material available that can give the forces at the "compressed lengths" mentioned in his last post (notice that he doesn't speak about compressed deflection).

In the short time I invested in this spring the shortest compressed length for 1400 lb was approximately 6" (instead of 3.85" to 4.05") but such a spring has no useful life cycle.