Small powerful springs

Question first:

Can anyone suggest a spring supplier who could supply 7mm springs with a rate of over 50 N/mm and ideally with a minimum order value below £18+vat & p&p?

Explanation:

I am looking for some very small, strong springs. I have some that are 6mm diameter from 16-gauge wire and an inch long. At least they were an inch long, they are now about 0.7 inches long, after being compressed with a load of about 50 kilos. This was nowhere near enough to close up the coils, but obviously overstrained them. The springs that failed had a measured rate of about 0.29mm per 2 kilos, which is about 60 N/mm (an online calculator suggested it should be 56 N/mm).

To my mind a larger diameter spring with a rate around 50-60 N/mm should be able to take these loads and recover fully.

I can substitute springs with an external diameter of up to 8mm, but I'd prefer 7mm. The highest rates I found 'off the shelf' are 12.4 n/mm for 7mm and 16 n/mm for 8mm.

These springs are for my driving car, and the other options I have are doubling up the springs, or my original idea of copying Alec Issigonis and having a lump of rubber under each suspension arm.

Neil, what amount of movement do you need, a shorter spring of same wire/OD will have a higher loading than a long one. eg 7.55 OD x 12mm long 24.18 N/mm same spring at 25mm long only gives 9.80 N/mm. Both from HPC International ( not the usual HPC Gears site)

Diet is anothe option

Edited By JasonB on 20/05/2014 20:28:36

Should add that they both need 111N to fully compress them fully

Neil

Ever since I started making things, I always managed to find a suitable spring from my scrapbox. Recently I needed a small quantity that had to be stainless steel. After much internet searching I eventually got them from Lee Springs (from their standard/stock size range

They were a bit smaller than you need but cost £1.61 each. Five of them with VAT etc cost me £9.66

I've no connection with leespring.com but was pleasantly surprised by the fast and cheap service.

Ian P

Sorry, I left out the critical bit... they need to be roughly 25mm long as the travel needs to be about 6-8mm. I have got screw adjusters to balance the springs (I will probably need to balance fore and aft once I have got my seating position sorted!)

At the moment the 6mm springs are in 1/4" holes with M5 screws poking into the ends for adjustment. I can bore out to 8.5mm (would rather limit it to 7.5) for a bigger spring, and add an end cap to the adjusters. The idea is to adjust the amount of spring projecting to be about 10mm, or 5mm under a static load of about 250 N per spring. This would give 5mm more travel before the arm bottoms out, hopefully stopping the spring from completely closing up.

<PAuse for experiment>

I've just done some experiments with 25x10x10mm lumps of rubber cut from a lorry mudflap. Sandwiched between the spring arms and the chassis they work really well. I can move them towards and away from the fulcrum to get different rates.

Better still, I've discovered that,by attaching a length of rubber in line with the suspension arm, as the rubber compresses the size of the contact patch increases, so I get an increasing spring rate with load, ideal for suspension purposes.

I thjnk I've made my decision!

At the very least, I should provide some entertainment for onlookers as I bounce down the track.

Neil

Have you considered using a pile of belleville washers, alternate ways up?

Try: www.leespring.co.uk - their catalogue has pages of sizes, thicknesses, etc, with spring rates.

They are in Wokingham, Berks UK

Cheers, Tim

Thanks Ian,

I have just had a go at the lee Springs search. The best I can get is about 14 N/mm, it seems the springs I have at 1.45mm diameter wire are unusual for such a small external diameter. I suspect they are meant for something like detent or ratchet springs that need to have a lot of force but little movement.

Neil

Hi Neil

Try Springmasters (Redditch) http://www.springmasters.com/

We use them a lot for work related items and they are very good

regards

Lee

I suggested Lee Springs as a source of 'Bellevile Spring Washers' - have another look.

Quite different from coil springs, they comprise conical washers like miniature diaphragm springs (as used in clutches in real Minis etc). I have their catalogue - pages 138 - 139. You can use them alternately, in a stack, or in pairs and alternately, etc to give lots of combinations of height, rate, etc.

Example - their 250-042-562, 0.042" thick, gives a load of 2300 newtons at flat - 520 lb. Much 'stronger' than a coil, I reckon.

Cheers, Tim

Edited By Tim Stevens on 20/05/2014 21:48:19

Springmasters are good but as Tim says, Belleville washers would be ideal in this application. I use them to regulate very high pressure gas.

graham.

Unless you have a big stack of belleville washers you won't get the travel Neil wants.

My CNC mill drawbar requires about 3/4" of movement to do a tool change, it gets this from a stack of Bellevilles about 5" long and compressed by a hydraulic cylinder.

.

Neil,

Alex Moulton, and Indespension Trailers are two names that should be high on your reading list.

Moulton used rubber springs in direct compression, and Indespension use them rather cleverly.

... Both very successfully.

MichaelG.

So did Issigonis on the original mini.

.

True ... He and Moulton worked together on it.

MichaelG.

I have used Entex Stock Springs in the past and had very good service from them. They have nothing anywhere near the spec required though. May be a useful source for others.

As an alternative to rubber how about polyurethane springs?

Yes, it was the original mini I meant - and the clutch with its belleville washer was designed by Automotive Products (ie Borg & Beck).

A simple way to proceed is to find a washer of the diameters you can manage, and quite a bit more than the max load you need. Then use as many as it takes to give the max travel you need, alternately up and down.

Example: Lee 138-015-281, od = 7.14mm, free height = 0.58mm, load fully compressed = 271N, thickness = 0.38mm.

A stack of 40 of these has a free height of 23.2mm, compresses down to 15.2mm, and gives a load from 0 to 271N as you go through that distance*. More than adequate (but possibly too expensive?).

*NB it is not accurate to think of the 'rate' of a belleville washer, as they do not obey Hooke's Law (which is why they are brilliant for vehicle clutches).

The hysteresis (self damping) of rubber is much greater, especially with some synthetics - but this might be an advantage. But it is not so resistant to the effects of high temperature. Ditto Polyurethane but more so.

And Stainless is completely sun- and weatherproof, and can be polished, of course ...

Cheers - Tim

Edited By Tim Stevens on 21/05/2014 09:15:53

Not wishing to be a smartass, but in my original post I did put "my original idea of copying Alec Issigonis and having a lump of rubber under each suspension arm" ;'

I didn't realise you could get Bellville washers that small, but somehow I think managing a stack of 40 is going to be lot more critical than a spring poking out of a hole...

> Stainless is completely sun- and weatherproof, and can be polished, of course

In that case, I'd better use rubber as it fits better with the RABA-WIWIT approach.

Neil

What's a rabbit got to do with it unless you are pulling it out of the hat. ?

Kato Entex in Nottingham will supply virtually any spring you can think of.

http://www.advanex.co.uk/

Dave