Multiple Spring Deflection Question

[TokerM]

I need a little help in figuring out spring rates for a system I'm working on. I'll try to describe it as best as possible:

I want to lower a growing stack of material (tubes) from an elevation to the floor. As more tubes are stacked, more weight is added to the system and the load lowers to the floor. Easily done with a strap attached to a spring.

For clearance issues, I need to put a spring on either side of the strap (both fixed ends.) And for design stability/durability, there will be 2 springs on each end. 4 springs total on each strap. (There are multiple straps in the line to hold a rack of tubes.)

Rough web sketch:

/\/\/\/\------strap------/\/\/\/\
\/\/\/\/------strap------\/\/\/\/

It's just one strap, and the system is designed for compression springs.

There will be a total compression of 24" in the system, and I would like to have full extension of the strap (compression of the spring,) at 140lbs.

This is where I start to get lost. I'm assuming this is a total rate of 5.8#/in, is that also the spring rate? How does Hooke's law apply to the multiple springs in the system? Using a double spring on each end would halve the rate to 2.9#/in, but how does each individual spring change based on the parallel springs?

I'm hoping someone can check my math/correct & explain what these numbers have to do with spring constants. Final mass numbers are just a guess, I'd like to err on the side of lighter loads when it comes to actual spring selection. My gut tells me that if I go with 4 springs that have a rate of 2-2.5 lbs/in I should be good?

 

[Bob S]

At each end, the combined spring constant is 11.7 pounds per inch. If this is shared between two springs, the constant for each spring is 5.8 pounds per inch.

 

[TokerM]

Thank you Bob, I'll use a general rate of 5#/in.

I'm getting confused on actually selecting a spring to purchase. I'm looking at buying a 36" long spring from McMaster-Carr, and they only seem to list spring constants and coils/in. I don't see how those numbers translate to the rate.

On one side of the system, the free length (not including pre-compression,) of the spring would be 20.5" and compressed max length is 6.5" (14" compression on this side.) On the other side the free length will be 16.5" with compressed max of 6.5" also (10" compression, giving my final 24" required.)

Both spring sides can have the same rate (short side bottoms-out before the long side,) allowing me to spec one long spring, cut-to-length as req'd. But I'm not sure how to find the size of a spring that will result in a given rate (wire dia, coils/in, etc...) I.D. of the spring should be ≥ 0.56" (0.5" guide rod.)

I'll try to attach a pdf sketch of the section assembly to help explain.