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Old 09-19-2017, 03:51 PM   #1
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Default Do Die Springs loose strength over time?

I built the die spring landing gear for my Acro Sport II back in the mid 1980s and the airplane now has over 750 hours and several thousand of landings. I used the die springs available from Aircraft Spruce. The springs were 6 inches long when new. Now with the weight of the aircraft (1030 empty), the springs are 5 1/4 inches long. See photo for details.

Has anyone else changed out their die springs over time? The plane flys and lands just fine it just appears the springs may have lost some of their strength over the last 30 years.

Thoughts?


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Old 09-19-2017, 04:34 PM   #2
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As a general rule, springs get tired and lose length (or gain it, if they're tension springs). They're a wear item, though usually a pretty long-lived wear item. I'm not surprised you're seeing this result.

I would guess that, in this system, the weaker springs give you less leeway for hard landings -- you're closer to completely compressing the spring and transferring the load into the frame. New springs would give you a safer system, though not necessarily a better one, if you never fully compress the springs as you fly now. Keep in mind that it may not be possible to tell by feel if you're completely compressing the springs, and because of the way they're built, you can't use the old motorcycle trick of putting a ziptie on the tube to see how far up the fork legs compress. There's probably an old airplane trick that has the same effect, though.


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Old 09-19-2017, 05:38 PM   #3
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Steve,
If i remember right you had one of the lighter Acro 2's.
Your washers at the ends of the springs look weird like someone tried to adjust the tension by welding spacers.
When we went from 150hp to 180hp full inverted the springs moved quite a bit. i will measure them next time i fly.
Here is a Danly die spring catalog where these springs come from with a wide variety of spring rates, lengths, and diameters.
Wag Aero springs were green but i am not sure who mfd them, i will check the Wag Aero drawing.
File Type: pdf DanlyIEM-DieMaxDieSprings.pdf (1.14 MB, 50 views)
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Old 09-19-2017, 06:40 PM   #4
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Thanks, Dennis

You are correct in that I added some spacers to tighten up the gear. I did this when I first started flying the Acro Sport II back in 1986. When I first built the plane I weighed it with bathroom scales and the original weight was not correct. I redid the weight several years ago and my plane weighs 1030 lbs with oil, seat cushions, headsets, etc. So rather than being light, my Acro is about average in weight.

I did note that the current Aircraft Spruce springs are yellow while the Wag Aero Springs are green. I will need to do some more research to determine which ones might be best if I decide to replace them.
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Old 09-19-2017, 07:31 PM   #5
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Steve,
My records show 2"OD x 1" ID x 6" LG spring with 1400 to 1500 LBS per inch rate.
preloaded on strut by 1/4" so 5 3/4" length with no load.
The color of the die springs looks to be different for each mfg. so better to use the specs to order them.
1.78" solid, 1.5" max rated, 1.2" ave life, 1.02" long life.
Let us know how it works.
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Old 09-19-2017, 08:13 PM   #6
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Just a FYI on die springs. I bought mine from Wag about 8 years ago. A few months ago I started seeing some surface rust between the springs. After trying to sand between the springs while on the plane and realizing I was going to be there a long time, I hung the plane and took the 2 struts off. It was then I realized the 2 tubes had corroded solid and had no spring action at all. They had to be hammered apart. I had been landing stiff legged for God know how long. Once apart and after sanding the corrosion off, I found that the 2 tubes on each spring were almost a net tolerance between the tubes where even the slightest bit of corrosion would freeze them up. I called Wag and asked what they lubricated the tubes with when they fabricated them. The answer was "Nothing. We put them together dry."

Just thought I pass that on.

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Old 09-19-2017, 09:47 PM   #7
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Aerokroil it's the best I've found to lube things like this that are still together.
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Old 09-19-2017, 11:18 PM   #8
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Holy XXXX, Never put them together dry!! Wag strikes (out) again!
Corrosion as you found will be severe, and wear will be rapid.
These are industrial die springs, designed to run thousands of cycles each day for years, I doubt you can wear them out in an airplane.
Are you absolutely sure things have changed? It's normal for springs to compress a bit under static load, otherwise they would be too hard. Regarding welding in washers to take up some space, a better method is to add rubber washers, you can cut them from car tire side walls. That way if you have a hard landing and fully compress the spring, you still have a bit of shock absorption, instead of the shock load.
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Old 09-20-2017, 01:35 AM   #9
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I suspect Wag Aero ships the spring struts dry so builders can weld and paint them. I took mine apart and lubricated them with white lithium grease (the stuff in a spray can).

IIRC the Wag springs were 1600#/inch with a 1/8" preload, and approximately 2200# at full compression (I can check the numbers if anybody cares, they're on my work computer).

Springs in normal use within their elastic limit (not compressed within 80% of solid height) should last nearly forever, and in an industrial environment that's how they're normally used. But die springs have such short travel compared to their length that they probably go solid or close to it in a hard landing.
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Old 09-20-2017, 01:44 AM   #10
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This is a good place to mention there's some guys up in NC making a tension type nitrogen charged dampened shock strut for this application.
Can't remember their names, looks like the cool setup for the best landing ever but pricey.
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Old 09-20-2017, 02:50 AM   #11
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Quote:
Originally Posted by crankyklingon View Post
This is a good place to mention there's some guys up in NC making a tension type nitrogen charged dampened shock strut for this application.
Can't remember their names, looks like the cool setup for the best landing ever but pricey.
Would be nice on a Super Cub landing on 18" rocks.
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Old 09-21-2017, 02:03 AM   #12
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Checked my springs today. 5 1/4" long with full fuel and parachute only.
Acro spring 2017-09-20-15-13-04_AATSC_resized.jpg  
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Old 09-21-2017, 02:28 AM   #13
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Thanks, Dennis. That is the same length as mine and yours are almost new. Perhaps I should just continue to use the ones I have.
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Old 09-21-2017, 03:55 AM   #14
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Quote:
Originally Posted by EAABipe40FF View Post
Would be nice on a Super Cub landing on 18" rocks.
They're also mode of 7075-T6 and weigh about 7 pounds less than die spring struts.
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Old 09-21-2017, 03:57 AM   #15
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I have an abundance of super high durometer urethane 1/4 inch thick that I thought would make great spacers for the die springs. I'd be happy to send ya a chunk if ya wanna try it.
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Old 09-21-2017, 04:35 AM   #16
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I have always wondered, do the springs work in compression instead of tension like bungees?
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Old 09-21-2017, 05:15 AM   #17
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Quote:
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I have always wondered, do the springs work in compression instead of tension like bungees?
Yes.
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Old 09-21-2017, 12:34 PM   #18
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Quote:
Originally Posted by crankyklingon View Post
This is a good place to mention there's some guys up in NC making a tension type nitrogen charged dampened shock strut for this application.
Can't remember their names, looks like the cool setup for the best landing ever but pricey.
Sounds very interesting so would really like to hear more about these shock struts. Unfortunately you have forgotten their names, but maybe others here know who they are.
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Old 09-21-2017, 01:59 PM   #19
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If one is working tension and one is working compression, it must be more important for the gear to just move than be controlled in extension or contraction?
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Old 09-21-2017, 02:54 PM   #20
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Acmeaerofab.com
Here ya go, looks good and is sure to take the bounce out of landings.
File Type: jpg 1506005663335.jpg (21.3 KB, 90 views)
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Old 09-21-2017, 04:38 PM   #21
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Quote:
Originally Posted by TFF1 View Post
If one is working tension and one is working compression, it must be more important for the gear to just move than be controlled in extension or contraction?
In the end it doesn't matter whether the spring itself is in tension or compression; the strut as a whole is in tension... just like the screen door compression springs with the two wire yokes some people use on their tailwheels.
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Old 09-21-2017, 05:46 PM   #22
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Quote:
Originally Posted by crankyklingon View Post
Acmeaerofab.com
Here ya go, looks good and is sure to take the bounce out of landings.
Wanna bet? I'm betting I can make it bounce.
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Old 09-25-2017, 04:46 PM   #23
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I checked the length of my Wag die springs and they are 5 3/4" after around 800 landings. I went to the Acmeaerofab web site to check out the gas springs. They are beautiful, but they are also over $3000 list price, so you're going to really have to hate your die spring/bungee landings. At least the $3000 if for the pair.

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