Measuring coil spring rates

[Tim Falce]

I have four nine inch coil springs and only one is marked clearly for rate, this is 600lb, two others are marked at either 550 or 650 but it's not clear. The thickness of the coil is very similar on all four as to be so fine that it could be paint thickness making the difference. Two springs weigh 2.2kg and two weigh 2.4kg so quite a difference there. The 600lb and the unmarked one are 2.2 the poorly marked springs are 2.4kg.

I don't have a press but can I accurately measure the rates of these using only bathroom scales or would the weights be an accurate way of comparing and would the heavier ones be higher or lower rate?

[phoenix]

Originally Posted by Tim Falce

I have four nine inch coil springs and only one is marked clearly for rate, this is 600lb, two others are marked at either 550 or 650 but it's not clear. The thickness of the coil is very similar on all four as to be so fine that it could be paint thickness making the difference. Two springs weigh 2.2kg and two weigh 2.4kg so quite a difference there. The 600lb and the unmarked one are 2.2 the poorly marked springs are 2.4kg.

I don't have a press but can I accurately measure the rates of these using only bathroom scales or would the weights be an accurate way of comparing and would the heavier ones be higher or lower rate?

You might think that the heavier springs would be a higher rate - but it's not that simple! If the heavier springs have thicker wire, they could be of a higher rate. If the heavier springs have more coils than the lighter springs, they could be softer! The answer depends on three factors: wire diameter, coil ID/OD and the number of active coils.

I can get you in the ballpark if you can let me have these numbers....

First, measure the 'wire' diameter as accurately as you can, as this is a critical measurement and paint thickness can make a difference.

Then measure the ID and/or OD of the coils - again as accurately as you can. These are not such critical measurements as the wire diameter, but if you let me have both they will provide a cross check for the wire diameter measurement given above.

Finally, count the number of 'active' coils. If the springs have ground ends, then normally about 3/4 of a coil at each end of the spring is actually touching the coil above when unloaded, but at some point the coils separate (i.e. have an air space between them) If the springs don't have ground ends, and sit in 'spiral' shaped seats, then count all the open coils.

Note that the open length of the springs (9 inches in your case) is not relevent to the calculations.

[Tim Falce]

All coils are about the same wire thickness at 13.05mm or 513thou. I/S and O/S diameter are roughly the same for all at 84.76mm outside and inside at 2.490in for a 2.25 spring.
The 600lb and the one I believe to be the same have 9 coils and the other two have 10 coils.
Sorry about the mix and match with metric and imperial but i was brought up using both so I tend to go with whichever is easiest on the eye

So I assume the two 9 coils springs are the same but what would the other two be?

[phoenix]

Originally Posted by Tim Falce

All coils are about the same wire thickness at 13.05mm or 513thou. I/S and O/S diameter are roughly the same for all at 84.76mm outside and inside at 2.490in for a 2.25 spring.
The 600lb and the one I believe to be the same have 9 coils and the other two have 10 coils.
Sorry about the mix and match with metric and imperial but i was brought up using both so I tend to go with whichever is easiest on the eye

So I assume the two 9 coils springs are the same but what would the other two be?

Could you check the ID measurement? With an OD of 84.76mm and a wire diameter of 13.05mm, the ID should be 58.66mm

Thanks

[phoenix]

Back to basics, though:-

If your springs with 9 coils have a rate of 600lb, then the 10 coil ones will have a rate of 600 x 9 / 10 = 540lbs

If the wire size and ID/OD are identical.

[Tim Falce]

Allowing for rust and paint they all have an id of 58.50mm. So going on your figures that sounds about right. They were bought in pairs and 600 and 550 (being nearest to 540 you state) seems to be logical.
How much of a drop in wire diameter would it take to make a difference of 50lbs?

[phoenix]

I'm not convinced that the rates of your springs are as marked. The 9 coil ones calculate to about 900lb (907lb) and the 10 coils ones to about 800lb (816lb). I will check with my spring man during the week and come back to you on that...

However, assuming your springs are marked correctly:

If your springs are UK made, the wire used is probably 0.5000" diameter (which is 12.7mm) without rust/paint etc as that is SWG -7 and most springs are made from standard gauge wire rather than metric or special sizes. They could be made from 13mm wire, but it is not very likely.

http://www.skegsprings.co.uk/technical/size.asp

The next standard wire size down is 0.464" which is SWG -6. Metric equivalent is 11.7856mm.

To answer your question about changing wire size, I'm not sure what the 'standard' metric sizes are, but to reduce your 600lb springs to 550 lbs springs, the wire size would need to drop from 12.7mm to 12.435mm - which is only a touch more than 10 thou! To reduce the 550lb springs to 500lb spring, the wire diameter would have to drop from 12.7mm to 12.465mm - about 9 thou. It is because these small diameter changes have such a major impact on spring rate that accurate measurements are required when calculating rates.

If your springs were made with SWG -6 (0.464") - the next standard wire size down, the 9 coil springs would have a rate of 440lb and the 10 coil springs would have a rate of 396lb.

However, using this gauge of wire, 8 coils would give a 495lb spring, 7 would give a 565lb spring and 6 would make a 660lb spring.

The usual way to design softer springs is a combination of wire diameter and the number of active coils. As you can see, using 12.7mm/0.5" wire, adding a coil to the 9 coil springs gives a 60lb drop, adding another coil to the 10 col springs would give a 49 lb drop down to 491lbs. One more coil (12) would make a 450 lb spring (a drop of 40lbs) - as you can see it doesn't appear linear, but it is if you start with only one coil: One coil would give a rate of 5400lb. - therefore 10 being 540lb should make sense and you can see that 20 coils would make a 270lb spring.

[tzei]

Bathroom scale will be fine if you compress the spring evently and use some sort of a plate to spread the load more eventy on scale as high rate springs tend to seise the scale before it reads accurately - at lest these are my findings. I used known springs (rates) to veryfy the measurements.

[dtype38]

Alternatively you could bring them round and compare them to a new spring on my hydraulic press.

[phoenix]

Originally Posted by dtype38

Alternatively you could bring them round and compare them to a new spring on my hydraulic press.

Empirical testing is always the best: I did say my calcs would give Tim ballpark figures.

The many variables, which apart from dimensions includes the alloy of the steel and it's temper - make calculations only accurate if you have all the data. As the spring manufacturer would have....

[Cameron Winton]

I would just get them tested - make sure you do your rate measurements with some preload on the springs. You want to test it in the linear range. Also keep the ends square as that will impact the accuracy.
I have heard of people using corner weight gauges to test springs!

[Tim Falce]

Originally Posted by dtype38

Alternatively you could bring them round and compare them to a new spring on my hydraulic press.

I'll do that then.
Phoenix, thanks for the help, it's very informative.

[Notso Swift]

Rate (Nm/mm) = (9909 x MD Cubed) / (Active coils x (ID + MD) x (ID + MD))

ID is Internal Diameter
MD is Material Diameter
AC is Active coils


If the material is fatigued I have no Idea what to do, so I just get them tested!


(For kg/mm x 0.10192)

[JohnD]

If you have a spring of known rate, compare that with the unknown.

Stack the springs in a sash clamp, and compress. When the known spring is compressed one inch (or whatever is convenient) then you know that it is taking what the rate is, say 600lbs (or a proportion of that if compression less than one inch). The other spring is taking the same, so that it will have shortened by an amount proportional to its rate. Say two inches, rate 300lbs, or half an inch, 1200.

John

[phoenix]

Here is a great calculator:

http://www.reliablespring.co.uk/calculator.htm

You don't need the free length or the compressed length to get a result - but you do need the other factors.

[Tim Falce]

Originally Posted by phoenix

Here is a great calculator:

http://www.reliablespring.co.uk/calculator.htm

You don't need the free length or the compressed length to get a result - but you do need the other factors.

It all comes together with that calculator and the fact I think I've been counting the number of coils incorrectly. It looks like I have 8 and 9 active coils not 9 and 10.
With that calculator it works out that two of the springs are definitely 600lb (calculate at 602lb) and the other two are 550lb (545lb)

Thanks for the help and sorry for the misinformation.