Author Topic: Proposed Foster Fitting standard  (Read 391 times)

Alan

  • Administrator
  • Hero Member
  • *****
  • Posts: 4188
  • Set the example... Don't be one!
    • Mobile Amateur Radio
Proposed Foster Fitting standard
« on: September 26, 2021, 01:09:07 PM »
I would suspect, that everyone here knows what a Foster fitting is? And if you’ve read the other threads in the forum, you’d also know that Steve and I have fostered (pun intended) a drive to convince manufacturers to standardize the size of these fittings.

Making things a bit more complicated, there is a pseudo-standard or sorts for us PCP users. However, the airsoft folks seemingly have their own pseudo-standard, which is slightly larger (≈2 mm). Since all of these are called “Foster” fittings, there are instances where the (unknowingly) incorrect fitting(s) may be used. This creates a dire situation, wherein the fittings could fail and/or separate and/or unlatch with catastrophic consequences!

Our friends at AirGunWire (https://airgunwire.com) covered this issue earlier this year. And more recently, the wonderful folks at Hard Air Magazine (https://hardairmagazine.com) have joined the fray.

All of this is fine business, but it will take a lot of doing to get the ball rolling towards a real Foster Fitting standard. Part of the need, are measurements of all of the fittings in our collective possession. Therefore, I’m asking for help from you, our members.

If you have a means of measuring ID and OD measurements (calipers, etc.), in either millimeters or inches (to at least two significant figures past the decimal), we would like you to do so. If you know who manufactured the part, that would be an added bonus. Primarily, we need the outside diameter of the male fitting's locking collar, and the inside diameter of the female fitting. Following, are the measurements Steve and I have amassed. And remember, fittings for airsoft guns are fair game too. So if you know they’re for airsoft, please so notate.

Just answer this thread with your findings, with the hope our combined efforts will bare fruit.

It is not uncommon for the diameter of a male Foster fitting (PCP pseudo-standard) to vary between 7.82 mm (.3079 in.) to as large as 7.84 mm (.3088 in.). The inside diameter of female Foster fittings, fall between 7.87 mm (.3098 in) to 8.04 mm (.3168 in.).


  • Roswell, New Mexico
Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

lester-moderate

  • Newbie
  • *
  • Posts: 4
Re: Proposed Foster Fitting standard
« Reply #1 on: September 27, 2021, 02:56:32 PM »
Good work Alan.

I will go through all my "foster" fittings again, and get the best measurements I can, and post. Someone is going to get hurt from this lack of specifications, and standardizations.

Loren

  • Sr. Member
  • ****
  • Posts: 471
    • Diamond Gun Stocks
Re: Proposed Foster Fitting standard
« Reply #2 on: September 27, 2021, 05:24:44 PM »
The other measurement that should probably be considered is the wall thickness especially on the male fittings.
  • Baldwin City KS

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2288
  • Retired and loving it!
    • Mozey-On-Inn
Re: Proposed Foster Fitting standard
« Reply #3 on: September 27, 2021, 09:34:30 PM »
"Foster" is a brand name.... Why not ask them to provide the tolerances on those dimensions, and make that the standard?.... After all, any fitting called a Foster that is not made by them is not what it claims to be.... It might be a "Foster style" fitting, but for any fitting to be claimed to be a Foster that is not should be a forgery, or at least a trademark violation.... I would think this could be solved by making all other fittings use the term "Foster compatible" if they meet Fosters tolerances.... JMO....

Bob
  • Coalmont, BC

Alan

  • Administrator
  • Hero Member
  • *****
  • Posts: 4188
  • Set the example... Don't be one!
    • Mobile Amateur Radio
Re: Proposed Foster Fitting standard
« Reply #4 on: September 28, 2021, 03:40:57 AM »
I'm trying to use "Foster style" or similar references. But like the term "Built like a Mac Truck" has crept in with respect to calling them Foster fittings.
  • Roswell, New Mexico
Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

cometkid

  • Newbie
  • *
  • Posts: 1
Re: Proposed Foster Fitting standard
« Reply #5 on: November 29, 2021, 05:30:44 PM »
Things to consider:
1. The different fittings you are finding may be due to different design specifications/standards.  Verifying with each manufacturer their design specifications will sort that out.  But some of it can be just no uniform industry design standard.  Normally size or configurations are used to mechanically separate the weaker designs from the stronger designs.  i.e backward compatible only for the stronger designs.
2. In that article it is mentioned that quality fittings were used on the FX series air rifles.  Can these be "unofficially" deemed the standard until a more formal standard can be agreed to?  The formal standard most probably will take some time to complete and get buy-in by the industry.
3. The separation force on the coupler and male plug can be calculated;  force = 3.14159 x Radius x Radius x air pressure
for a 1/8" dia. air hole in the fitting, the separation force would be 3.14159 x .125" x .125" x 4,500 psi = 220 pounds force
for a 1/4" dia. air hole in the fitting, the separation force would be 3.14159 x .250" x .250" x 4,500 psi = 884 pounds force
4. yes, more coupler balls will mean less stress on each ball.
5. yes, the larger the coupler ball the less contact stress will be on the mating surfaces (i.e. coupler, ball, and male fitting).
6. I did a brief quarry and found that there are existing high-pressure couplers that now exist.  They are not cheap $80 give or take, but they are out there.  There may be more economical ones, but my task was to see if they existed first.  A special design will cost money too.  The one I found was capable of handling pressures well over the 4,500 psi pressures we see now. 
7. Is there an air gun association out there?  I would propose working with them and form a working committee to study this problem.  I sit on automotive committees (i.e. SAE) to generate automotive uniformity standards & processes.  This would be a more formal path, but the advantages are that it will be a true air gun industry standard. 
Summary;  If you are lacking an engineer on your committee, I would consider working with your team.  I think studying the problem as you are is the right first step, and that is to understand the problem in some level of detail.   
Good luck
  • Englewood, Florida and Livonia, Michigan
New to PCP
Handgun Action shooting, hunting, race cars, & engineering

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2288
  • Retired and loving it!
    • Mozey-On-Inn
Re: Proposed Foster Fitting standard
« Reply #6 on: November 29, 2021, 10:11:58 PM »
Quote
3. The separation force on the coupler and male plug can be calculated;  force = 3.14159 x Radius x Radius x air pressure
for a 1/8" dia. air hole in the fitting, the separation force would be 3.14159 x .125" x .125" x 4,500 psi = 220 pounds force
for a 1/4" dia. air hole in the fitting, the separation force would be 3.14159 x .250" x .250" x 4,500 psi = 884 pounds force

No idea where that math came from.... First off, you used the diameter of the hole, not the radius, so your calculations are too large by a factor of 4.... Secondly, the separation force must be calculated using the diameter of the seal (ie the OD of the male fitting at the O-ring).... If the male fitting is 5/16" OD where it plugs into the O-ring, then the separation force is PI/4 x 0.313" x 0.313" x 4500 psi = 346 lbs.... While you might think that the hole through the fitting affects the separation force, both halves are contained in a closed system at 4500 psi.... Therefore there is a force on the "end" of the tank balancing the size of the hole in the fitting (regardless of the shape of the tank, connecting hose, etc.).... It is only the OD that matters.... I left off the retaining ring on the male, and the details of the female (eg. locking balls, etc.) for simplicity....



If you ONLY had the male fitting plugged into the female, with no tank attached, and the air venting through it, THEN the size of the hole would matter, but it would be SUBTRACTED from the 346 lbs. (55 lbs for the 1/8" hole, and 221 lbs. for the 1/4" hole).... However, the pressure differential would be nowhere near 4500 psi, only a fraction of that because of the venting that is taking place.... While the diameter of the through hole has a HUGE effect on the strength of the male fitting, through hoop strength and axial strength, it does not affect the separation force between the male and female....

Bob

« Last Edit: November 29, 2021, 10:13:58 PM by rsterne »
  • Coalmont, BC