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Author Topic: Gas Ram To coil Spring Considerations  (Read 476 times))

Online Ronno6

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Gas Ram To coil Spring Considerations
« on: September 22, 2022, 12:57:43 PM »
It appears that replacing the Reaxix gas rem in my Umarex Octane is not currently possible.
I need to understand the design considerations of converting to a coil spring powerplant.

The Reaxis ram is 18mm OD and the fully extended length is 330mm.
The rod is 150mm long,and has an available stroke length of 137mm.
When installed, the ran had very minimal preload from what I can tell.

The ID of the spring tube (then wall metal tube inside the piston) is a tad over 20mm.
I understand that a coil spring will increase in OD as it is compressed.

If I consider using a coil spring, how would I translate these dimensions??
I would think that a coil would require about 3" of preload in round numbers, without giving consideration to the spring rate and actual preload required.
That's gettng on up to a spring of just over 400mm in free length.
I am not finding that anywhere.....

So, I have to believe that a combination of spring length and a suitable spacer to make up the difference is required?

As there is so much that I do not (currently) know, what other considerations will factor in??
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Online Ronno6

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Re: Gas Ram To coil Spring Considerations
« Reply #1 on: September 22, 2022, 06:52:40 PM »
If I cannot find a single spring long enough, can I stack 2 shorter ones?
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Re: Gas Ram To coil Spring Considerations
« Reply #2 on: September 22, 2022, 08:26:32 PM »
An engineer at Suspa offered to make one custom for $105.00.......
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Re: Gas Ram To coil Spring Considerations
« Reply #3 on: September 22, 2022, 09:00:07 PM »
An engineer at Suspa offered to make one custom for $105.00.......

I hope he understands that your application needs the gas spring to be undamped.

How much to have a custom coil spring made?    McMaster has a wide range of parts and materials, but often at double the best price.  They can ship you a custom made steel spring, but probably at twice the price of that gas spring - assuming the gas spring meets your requirements:  https://www.mcmaster.com/coil-springs/spring-type~compression/made-to-order-compression-springs/

I have to believe that there is coil spring stock that can be cut to your required length.  This link takes you to 3/4" OD spring stock (because that seems most common for airgun steel springs, near the 18 mm gas spring body you mentioned).  You can step back and select whatever OD you want from the dropdown menu:  https://www.mcmaster.com/spring-stock/od~3-4/

You should know the installed and compressed length (and the spring rate you are trying to achieve); so there are a few ways to approach this:  https://www.mcmaster.com/spring-stock/maximum-load~range~~-5637010442429/


It may be that the ID is the most important diameter to fit (a non-existent guide?).  Or the OD needs to be a slip fit in the piston ID when the spring is compressed.  What spring OD/ID/wire diameter combinations are available may dictate your choices.  A spring manufacturer no doubt would have more options.
« Last Edit: September 22, 2022, 09:04:10 PM by subscriber »
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Re: Gas Ram To coil Spring Considerations
« Reply #4 on: September 22, 2022, 09:10:19 PM »
As a mechanical buyer for 25+ years I have purchased many made-to-print springs of many different types.
I am (well, used to be) familiar with spring design parameters.
Material is something which I have not seen mentioned.
Music wire?
Chrome vanadium?
Chrome silicon?
Valve spring wire?

I used to know people at many spring manufacturers, but, as I retired 18 years ago, they are probably gone now...

Yes, the app engineer and I discussed the need for no internal damping.....that feature would preclude the use of any of their stock gas springs.
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Re: Gas Ram To coil Spring Considerations
« Reply #6 on: September 22, 2022, 09:20:57 PM »
Perhaps duplicating the original gas spring is the simplest, because it eliminates the "missing guide" problem that a steel spring would introduce.

I assume that you contacted Umarex and Pyramyd Air for a replacement gas spring.  It certainly sounds like you know where to look.

You could contact Hector Medina via PM.  He could advise you on spring material selection.  And on sources for complete springs, or about spring stock that can be cut to length.
https://www.gatewaytoairguns.org/GTA/index.php?action=profile;u=21540
« Last Edit: September 22, 2022, 09:23:27 PM by subscriber »
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Re: Gas Ram To coil Spring Considerations
« Reply #8 on: September 22, 2022, 09:26:57 PM »
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Re: Gas Ram To coil Spring Considerations
« Reply #10 on: September 22, 2022, 09:55:16 PM »
Using this 3/4" OD stock,
my rough calculations arrive at a total spring force of 72# when cocked.
Sounds like it is almost appropriate ??

What I don't see is the c-c coil spacing....
« Last Edit: September 22, 2022, 09:57:22 PM by Ronno6 »
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Re: Gas Ram To coil Spring Considerations
« Reply #11 on: September 22, 2022, 11:01:21 PM »
Ron,

I think 72 lb cocked is rather low.  I would expect perhaps triple that, depending on airgun power.  What is the rated power of your Octane?

If your rated power is 20 FPE, the energy in the cocked spring is typically 3 times that.  60 FPE stored over a 4" piston stroke would indicated 60 x 3 = 180 lb average force.  So, depending on the spring rate, the force at full cock could be much more than 180 lb.  For a gas spring, the force at the start and end of the stroke are much closer, than for the typical steel spring.
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Re: Gas Ram To coil Spring Considerations
« Reply #12 on: September 22, 2022, 11:27:24 PM »
Ron,

I think 72 lb cocked is rather low.  I would expect perhaps triple that, depending on airgun power.  What is the rated power of your Octane?

If your rated power is 20 FPE, the energy in the cocked spring is typically 3 times that.  60 FPE stored over a 4" piston stroke would indicated 60 x 3 = 180 lb average force.  So, depending on the spring rate, the force at full cock could be much more than 180 lb.  For a gas spring, the force at the start and end of the stroke are much closer, than for the typical steel spring.

A cursory search gave me a fpe of 18.32

The McMaster 3/4" OD x 36" coil was 3.8lb/in if I recall..
Shortening the 36" coil to 15" would bring the rate to 9.12lb/in.
With a 2.5" preload and 5.43" stroke force would be the 72lb/in.
One Vortek spring suggested has a rate of 48lb/in.....quite a difference........

More stuff to learn!

Max stroke length of the gas piston is 5.43"
I do not know how much if that is actually utilized in the rifle.
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Re: Gas Ram To coil Spring Considerations
« Reply #13 on: September 22, 2022, 11:40:59 PM »
From nondeliberate memory; round numbers for a 15 FPE springer would be 100 lb preload and 200 lb cocked. 

That means that for a 4" stroke, the spring rate would be 25 lb per inch.  Proportionally more for a shorter stroke.  A 3" stroke would suggest a 33.3 lb per inch rate.

I sent a PM to Hector and Ron.  It is up to them to weigh in, if they have time and inclination.
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Re: Gas Ram To coil Spring Considerations
« Reply #14 on: September 23, 2022, 12:01:20 AM »
Thanks for the invite Peter. This stuff is out of my wheel house. I'm not familiar with the gun and I can't say for sure but the avaliable stroke and spring length sound very long to me. I'm wondering if part of both those lengths was just space taken up by the ram body and not actually used stroke. That space perhaps could be taken up by a dummy plug and a more readily available shorter spring used.

I'm just spit balling here. Without seeing the original guts of what were working with here I'm probably of little use. I do wonder if the gun itself is worth re-engineering it. That can only be determined by the owner.
Be well
Ron
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Re: Gas Ram To coil Spring Considerations
« Reply #15 on: September 23, 2022, 12:23:41 AM »
Thanks Ron

I agree that the original gas spring travel is not the same as the actual piston stroke.

Do you have an opinion about steel spring material choices - listed above by the OP?  Quoted below:

Quote
Music wire?
Chrome vanadium?
Chrome silicon?
Valve spring wire?

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Re: Gas Ram To coil Spring Considerations
« Reply #16 on: September 23, 2022, 12:29:55 AM »
To measure actual effective stroke length, the piston position when held by the sear and piston fully forward need to be measured.

Yes, the piston must be able to over travel far enough back on cocking to engage the sear.  So, that extra movement must not be impeded with a new spring, steel or gas.  If there is an auto-safety, that travel needs to be considered as well.
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Re: Gas Ram To coil Spring Considerations
« Reply #17 on: September 23, 2022, 12:33:34 AM »
Thanks for the invite Peter. This stuff is out of my wheel house. I'm not familiar with the gun and I can't say for sure but the avaliable stroke and spring length sound very long to me. I'm wondering if part of both those lengths was just space taken up by the ram body and not actually used stroke. That space perhaps could be taken up by a dummy plug and a more readily available shorter spring used.

I'm just spit balling here. Without seeing the original guts of what were working with here I'm probably of little use. I do wonder if the gun itself is worth re-engineering it. That can only be determined by the owner.
Be well
Ron

Mr. Ron,
Thanks for chiming in...
I will endeavor to take some practical measurements.
The available stroke IS very long, and that is the crux of the issue.
The original gas ram is a tad over 13" in length, 7-1/4" L cylinder; 5-7/8" L rod, and 1/2" of rod exposed at full compression.
If not used, I have a hard time understanding any manufacturer procuring such a long ram, and the compression tube and linkage to accommodate it if not fully utilized..

As for the rifle, a Umarex Octane is not a particularly valuable gun; I am going thru this exercise as an opportunity to learn...hopefully something about airguns...
I have this drive to fix things that are broken, especially when improvisation is involved. Akin to mounting a washing machine engine on a Piper Cub...

Ron,too....
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Re: Gas Ram To coil Spring Considerations
« Reply #18 on: September 23, 2022, 01:30:53 AM »
Find out what the actual stroke is. Measure the end of the compression tube to the trigger latch. Then measure the piston from the face of the seal to the trigger latch receptacle. Subtract that piston length from the first measurement and that will tell you what the actual stroke is.
You can't use the ram because afaik we don't know how much dead space it takes up with the ram body and any possible preload combined.

Once you know your stroke length, you can shop for appropriate springs. You may not need a crazy long spring.

Also does the trigger latch look compatible for a spring? Piston weight or lack of may come into play as well.
  • USA,
Hw30- .177- Vortek PG2, Tech sights
Hw30 Laminate- .177- Vortek PG2, Hawke
Airmax 2-7x32 AO. 
Hw30-.177 Vortek PG3 steel, Hawke Vantage IR 2-7x32 AO
Hw50- .177- Vortek PG3, Hawke 4X non A/O
Hw95- .177- Vortek PG2, Hawke Airmax 3-9x40
R9 SE (Blue Laminate stock) - 0.020- Vortek PG4, Hawke Airmax 3-9x40 AO
Hw97SE (Green 77 Laminate stock)-.177 Vortek PG3, Hawke 4-12x40 Airmax
R1-.177 now 0.22 - Vortek Pg4, Hawke Airmax 3-9x40 AO
P1- 0.20 now .177

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Re: Gas Ram To coil Spring Considerations
« Reply #19 on: September 23, 2022, 09:15:56 AM »
Find out what the actual stroke is. Measure the end of the compression tube to the trigger latch. Then measure the piston from the face of the seal to the trigger latch receptacle. Subtract that piston length from the first measurement and that will tell you what the actual stroke is.
You can't use the ram because afaik we don't know how much dead space it takes up with the ram body and any possible preload combined.

Once you know your stroke length, you can shop for appropriate springs. You may not need a crazy long spring.

Also does the trigger latch look compatible for a spring? Piston weight or lack of may come into play as well.

That was easy enough, almost.
The stroke to the latched position of the sear is 4 3/4"
The sear has a slot for the pivot pin that will require ana dditional .100 travel to latch, probably due ti the auto safety ??
So, I would say that the actual stroke is 4.85"
Maybe the N-forcer Mini MF19-125 will fit the bill........... 19mm OD x 4.92" stroke.........
It is extremely close, however. .007" isn't much wiggle room...
I will need to get very exact with the measurements....


Piston I.D is .935" without the sleeve....
« Last Edit: September 23, 2022, 09:24:35 AM by Ronno6 »
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