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Aug 19, 2018, 11:24 PM
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Build Log

MAKING springs for oleos


I have decided to start dabbling... Learning.... about springs and how to make them.
This is for OLEOS, so that you can alter them and tune them to suit any particular aircraft and its AUW.
Most of my aircraft have INCORRECT spring/suspension strengths and qualities, seeing you only get to use what any given Oleo comes with - and they rarely come with a strength/behaviour that is truly suited. This is because a strength required has to match the aircraft AUW - which varies per any owners own 'version', and of course across the multitude of aircraft types adn constructions etc.

On my Mig-29 I discovered the oleos I used were somewhere from JUST strong enough, to "too soft". They are just strong enough to hold its AUW if it is standing statically with no external influences on it, but as soon as it MOVES they will be driven to bottom out. You would think that this can't be a good situation to have.... but in reality it IS quite good. maybe not perfect, but still a very good end result.
This is because it is actually JUST short of able to hold the AUW, and even though they fully compress they are still operating as springs - any time the aircraft tries to LIFT UPWARDS they resume operation. And that occurs as it 'bumps' its way across grass, bumps etc. So they ARE doing useful things.

This outcome showed me that oleos that 'easily' hold up an aircraft are not going to achieve this aspect of useful operation. They will pass too much 'shock' to the airframe and then it will bounce or 'pop-up.... which is not good to occur at the nose (nose gear), as it is the most likely to allow a large Angle Of Attack change - increase - which can make the aircraft lift off again after touch down.
Meanwhile if the main gear compress, you will get a certain AoA and if they de-compress (on a bumps etc) you will just get LESS AoA anyway.
So in a way you want them o operate in 'reverse'.... compress EASIER, and thus the baseline is to come UP from that.
The nose gear is the opposite case.... so you want that strong enough to "just hold up the AUW and a bit more" (a bit more extra than for the main gear)

So then it was on to some Googling and reading.....

You might think that MAKING SPRINGS is going to be fraught with difficulties..... but after reading a number of articles and YouTube videos, it was laid out as o how "simple' it really can be!
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Aug 20, 2018, 12:05 AM
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The first spring


I needed some stronger springs for the Bae Hawk, as the oleos I used on the main gear for that are WAY too soft. With its 4250g AUW they instantly bottom out fully..... showing they are WAY short of holding u the AUW.

So let's see some theory on springs.....

A spring is really just a 'shape' that allows you to TWIST a 'rod' torsionally. It converts a motion in one direction to a twist lateral to that.
When a coil moves 'down' it is rotating the rod/wire in that torsional manner.
If a coil has more spacing to another, this distance is how much TWIST it can try to exert on the rod/wire. Larger spacing will 'wind up' the wire more and it gets harder and harder to wind that up as you do it more an more.

I better start calling it 'wire'..... seeing springs are made of wire. Whilst Torsion Bars, which for EG Formula One racing cars use instead of springs, are that Rod based system. Both achieve the identical results.... just via a different 'shape'.
So try and visualise how TWISTING A ROD (or wire) needs FORCE. It will 'wind up' and then release that energy (if it was a springy material - like they DO use for these items of course). And a SPRING compressing is just a way to 'twist a wire' (rod).....

So the first item that affect spring strength is the DIAMETER of the wire - but also the mechanical properties of that wire. Springs are predominantly made from Piano wire, which is also known as Music Wire, and designed/made to be springy. The maths can get quite complex, but as a general rule when we buy some piano wire it is all 'made the same' and is just a different diameter. So Properties can be 'ignored' as they are somewhat a constant.
Thus Diameter is the main factor here....

The diameter of the SPRING..... measuring the total diameter of the coils..... affects the leverages on twisting the wire, the same as the distance between coils. So that is another factor. The same wire, wound to a larger diameter, forms a 'weaker' Spring.

If you look at an existing spring, you can just change one or more of these factors to alter the Springs result.
If the spring is too soft, then do the same number of coils and spacing and spring diameter, but increase the wire DIAMETER.
Or use the same wire diameter but open out the coil spacing - so the 'leverage' that compression motion creates is less, and thus it can't twist the wire as easily.
You will probably, possibly, need to THINK about which to do... or even to do two or more changes. The SPACE that the spring must fit into can dictate limitations.

So for my Bae Hawk spring I measured up its parameters....
6mm Spring Diameter
0.81mm wire diameter
1.0mm between coils
32mm long.

The simplest change to get more strength is to use a thicker wire.
But I only had 1.2mm piano wire.
1.2mm is 50% more than 0.81mm, but the STRENGTH of a wire you are going to twist is a function of the cross sectional AREA, not just the diameter. Area of a circle (wire) is 2 x Pi x Radius SQUARED.... so the area value increases rapidly per diameter change.

0.81mm 4.13 'units'
1.2mm 9.04 'units'
That is more than DOUBLE the area, thus more than double the strength. So that seems like a good target strength to suit the needs....
So I made that new spring up.....

HOW do you make a spring?
It is surprisingly simple and easy to do!

You feed the straight wire into a rotating shaft/rod so it forms a spring! By controlling the feed 'sideways' to the rod you will form the coil spacing.

The form nice 'ends' you just feed in the wire with no real 'walk' down the shaft/rod so that two coils touch, then begin the spaced out coils (walk). Do the last two coils touching also. Then you can grind the ends to be flat, not sloped anymore.

So get an electric drill (many other things could be used) and fit a rod of a bit smaller diameter than you need for the internal spring diameter - because once the spring is formed it will 'spring out' a bit in diameter. In my case to get a 6mm OUTER diameter, with 1.2mm wire.... 6 - 1.2 - 1.2 = 3.6mm, I used a 3.4mm rod so it would 'spring out' to that 3.6mm approx. It is not really critical to get it all perfect....

Put a 90deg bend/tang into the piano wire so you can feed it into the drill chuck - between its jaws - and this will prevent the wire from NOT staying put as you wrap it around and down the rod/shaft.
Turn the drill chuck by HAND! Keeping the wire feed taught as you control the 'feed angle' to form the starting parallel coils and then the spaced out coils...

Simple... done!!
A keen eye can produce very good spacing equality. And even if there is some error in that it is hardly going to be measurable in your working oleo.

After making that new 'double strength' spring I found there is some 'other' factor in the maths that I do not know, because the strength came out more like TEN TIMES stronger!
I can compress the original spring reasonably easily, but the new 1.2mm spring cannot be compressed at all pretty much!

So 1.2mm diameter wire is TOO large for the aims.....

It was such a nice looking/made spring too..... the very first attempt.

Oh.... when the spring is made you really want to ANNEAL it. Put it into an oven at 250degC region, for one hour at least. Then let it AIR cool.
This will restore the properties it had before you 'mal-treated it' via wrapping it around in a tight circle.
I suspect for our uses the spring would still work fine, for long term, even without this annealing.... but if you can do that, then you may as well.

....
Aug 20, 2018, 12:13 AM
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Thread OP

Spring two.....


I have bought several wire thickness now.
0.81
0.925
0.975
1.0

Once the 'first few' springs are made then the experience gained will help make future needs easier to get close within one, or a few, springs being made up.
Remembering that most Oleo spring strength cases, as they come, are way off what you really need..... so getting 'much closer' is still going to be of good benefit.
Oh, it took me about FIVE MINUTES to make the first spring, so it is not some complex and time consuming task.

I have at least four sets I need to make up for various aircraft, so that 'data' - and growing in content per every spring made - should give me a 'table' I can record for future uses.
I will actually strength test them individually - via 'direct drive' into that piston/spring - to record the springs actual strength (Gram per mm). But then also in Trailing Link Oleos, where the LEVERAGE they impart into the Piston/Spring will alter the weight it will truly carry.
Aug 20, 2018, 10:08 AM
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Thread OP

Spring Two - continued


So the next Spring to make was using the 1.0mm piano wire.
I made one that was 32mm long, as per the originals, and put it into the Oleo to test it. not heat treated first....

In compression strength terms it was perfect! It held up the Bae Hawk AUW - versus the original springs just fully compress in an instant.
The 'hand pressure' required to begin compression, and to get it to bottom out, was 'exactly' as required and aimed for. It holds up the AUW when static, and needs a low amount of extra pressure to begin to compress, and to compress fully.

I removed the Spring to have as a reference to make the second one.... and I found it was SHORTER. It was now 25mm long instead of 32mm.
I put it back in and it STILL WORKED perfectly fine. Hmmmm......

I measured the Oleo 'space' where the Spring goes from one Oleo end until the piston 'home'/extended position. That was 25mm.....
So the Spring had compressed to match that... it SEEMS.
I decided to make TWO more springs, each 28mm long instead of 32mm. Just so they would have some Preload still.
I heat treated these and then installed them to test them.
They had, as far as I could tell, equal compression strength. They were again perfect for the task. Just as the 32mm had been.

If you have a longer spring, which needs preload to fit, this means some of the coil GAPS are already being 'used up' by the coils compressing.
So the 32mm Spring would have less remaining room to compress more - BUT both the 32mm and 28mm version could fully compress, which shows the 32mm version had enough gap to spare/waste anyway.
When I removed the 28mm Springs they were also now 25mm !!!
Hmmm, heat treating them did nothing. Or at least heat treating them and letting them AIR COOL - as per what all the sites/articles said to do.
Maybe they would fair better.... or no better at all..... if they were cooled (quenched) in OIL instead.....???
At the moment, after LOTS of full compressions, they still operate as required, and when I removed one it was still 25mm.
MAYBE a spring will compress SOME amount naturally - much like it will 'uncoil outwards' after removing it from the mandrel it is wound around. The same sort of principle of metal elasticity? But none of the sires/articles had mentioned that would, or could, happen. So..... ????

As long as they KEEP working fine, that is all that matters. But will they?

The new Springs were not as 'perfectly' made as my first ones, but that seems unimportant in the % of variation it will cause. The coil spacing 'errors' will create a 'progressive spring', which is no issue really.

The end result is a BETTER SPRING, well suited to the load (AUW) requirement.... exactly as aimed for.
So 1.0mm wire diameter - rather than the original 0.81mm diameter - did that....


....


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