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Author Topic: Pin spacing  (Read 378 times))

Offline BigBird

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Pin spacing
« on: February 16, 2021, 10:24:36 PM »
I have a 1-1/8" x .083 4130 tube/ SS valve that I need to add pins to.  I was interested getting them out of the way of the top and bottom "holes".   I want to put four 10-32 screws at the side 60 degree spots (clocked from top - 12 oclock) with heads countersunk. In addition to normal action slots for a cocking handle and trigger I have other holes planned for the top and bottom.

A 10mm or 1/2" TP at the top where the screw is currently in the tube/valve.  The white is circling a gouge spot I am cutting out by making the TP.  There will be two 0.191" 10-32 screws/holes, one directly under the TP for a locating screw and another at the top behind the TP for a breech screw.

Pin placement 1 - 60


If they are too close, an alternate configuration would be 120 with the top two previous 60 "slots" and the 6 oclock position 120 degrees from the others would be a 1/4-28 or 5/16-24.

Either configuration should be enough or more than what is needed.  Also, I believe the mill certification had 120kPSI for the tensile strength of this tube.

Pin placement 2 - 120


Upside down Screws with TP location.


I wanted to use Rob's (single pin, double shear parasagittal-dual plane pin configuration) but was thinking more pin heads might be better for an alloy with less yield strength (yield during testing scares me).
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Online Rob M

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Re: Pin spacing
« Reply #1 on: February 16, 2021, 11:10:22 PM »
well , with 4 10-32s you can really do any arrangement, the key is they share the load.. generally , the times ive done 4 cap heads, Ill do 2 in a row on one side 2 in a row on the other, only drilling and tapping one side at a time.. This way , once one side is done i can lock the cap heads into place and tighten them , flip the work , and start on the next 2 knowing for a fact those 2 pin holes cannot under any circumstance shift while im drilling and tapping the other side.. theres something to be said for the " kiss " method, since after all , we know that an uneven load on tthe pins is a worst case scenario.
other methods are fine also.. But while rotating the work id have the previous pin hole with a screw installed firmly.

my single pin method with the dowel ive only used on alauminum tubes, where the tndency for the tube to deform under pin load is very high. But on steel it would be indestructible.
« Last Edit: February 16, 2021, 11:15:00 PM by Rob M »
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Offline rsterne

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Re: Pin spacing
« Reply #2 on: February 17, 2021, 01:47:43 AM »
The critical thing is not the radial spacing, but the longitudinal positioning.... so that all screws share the load equally.... Obviously you want them spaced as evenly as possible, or at least as symmetrically as possible.... but in reality, if the valve is a decent fit in the tube, using 3, 6 and 9 o'clock (like a Disco) works fine....  ::)

Bob
« Last Edit: February 17, 2021, 01:52:08 AM by rsterne »
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Offline CableStop

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Re: Pin spacing
« Reply #3 on: February 20, 2021, 12:17:41 AM »
The forces applied to a valve like this will be axial, putting the screws in sheer tension.  The amount of space between fasteners side to side is not overly important since the load will be applied front to back.  What do you mean yield during testing was concerning?  Did you manage to get holes in 4130 tubing to elongate?
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Offline rsterne

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Re: Pin spacing
« Reply #4 on: February 20, 2021, 01:15:43 AM »
The valve screws are in shear, and shear strength is typically calculated at 60% of the tensile strength.... You should be working to at least a 3.5:1 safety margin in shear.... Quite often the pocket load on the thin tube wall would be the first thing to yield if overfilled, in fact it is a good idea to have that be the weak point.... A 2.5:1 safety margin in compression is sufficient, IMO....  This is also the reason most of the time the holes in the tube are drilled to the size of the head, to spread the load out into the tube wall, especially with an aluminum tube.... The screws are tightened against a flat that is spot milled on the valve body.... or even better into a pocket in the valve body, to put the head of the screw in shear....

Bob
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Offline BigBird

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Re: Pin spacing
« Reply #5 on: February 20, 2021, 03:54:08 AM »
Ok sorry.  I thought I replied but I must not have posted it.

I compiled a spreadsheet with calculations from Bobs previous posts on a 304 SS pin.  This is the first time using the formulas.  Can you check my math?

Valve force for a .959 valve:
Area: 0.7223157684
x
PSI: 3000
=
Force at valve: 2,167   

Screw strength:
the YFS 10-32 screw tensile strength is 145,000  * .60 = 87,000 *  0.076 (area) = 6651 /3 (margin)= 2217 foot pounds each screw
x 4
= 8,868

Yes, that seems like a lot if my math is right.  FWIW I couldnt find the yield for these screws but this is good information.
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Offline rsterne

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Re: Pin spacing
« Reply #6 on: February 20, 2021, 05:05:46 PM »
Not sure where you got the area in shear of a 10-32 screw.... but it looks like you took the diameter of the head, and ignored the hex hole for the Allen key.... I use an area of 0.047 sq.in. for the head of a 10-32 screw.... Otherwise your math looks fine.... High tensile SHCSs are usually rated at 170 Ksi for tensile, which would be 102 Ksi in shear.... So, I would use (170,000 x 0.60) = 102,000 x 0.047 = 4794 lbf. / 3 = 1598 lbf. x 4 screws = 6392 lbf.... Since your required load is 2167 lbf. and you already allowed for the 3:1 safety margin, you should be more than OK.... PROVIDING that the pockets in the valve are deep enough that the shear line is actually going through the head of the screw....

If, on the other hand, the shear line is through the threads, the area is only 0.020 sq.in.... This would give 102,000 x 0.020 = 2040 lbf. per screw.... For 4 screws, that is 8160 lbf., divided by the load of 2167 lbf. gives a safety margin of 3.7:1, which should still be OK.... If the screws you are using are only 145 Ksi tensile strength, then you would be a bit less (but still just over 3:1) with the threads in shear....

Bob
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Offline BigBird

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Re: Pin spacing
« Reply #7 on: February 22, 2021, 01:21:33 AM »
Not sure where you got the area in shear of a 10-32 screw.... but it looks like you took the diameter of the head, and ignored the hex hole for the Allen key.... I use an area of 0.047 sq.in. for the head of a 10-32 screw.... Otherwise your math looks fine.... High tensile SHCSs are usually rated at 170 Ksi for tensile, which would be 102 Ksi in shear.... So, I would use (170,000 x 0.60) = 102,000 x 0.047 = 4794 lbf. / 3 = 1598 lbf. x 4 screws = 6392 lbf.... Since your required load is 2167 lbf. and you already allowed for the 3:1 safety margin, you should be more than OK.... PROVIDING that the pockets in the valve are deep enough that the shear line is actually going through the head of the screw....

If, on the other hand, the shear line is through the threads, the area is only 0.020 sq.in.... This would give 102,000 x 0.020 = 2040 lbf. per screw.... For 4 screws, that is 8160 lbf., divided by the load of 2167 lbf. gives a safety margin of 3.7:1, which should still be OK.... If the screws you are using are only 145 Ksi tensile strength, then you would be a bit less (but still just over 3:1) with the threads in shear....

Bob

Thanks for the check and correction.  I'll have to incorporate the allen key space into my calculations somehow.  145kpsi was listed as nominal tensile strength by the manufacturer in the brightonbest distributor.

https://www.brightonbest.com/catalog/NZ/YFS_technical.pdf

Now I can see where the strength of Rob's 1 dual plane pin has more strength as it is solid without the allen head. A few weeks ago I started pinning a 1"  tube valve and started pinning with this method. I haven't done any more with it yet.  Are there any additional calculations for the one pin?  It is obviously on a different plane.

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Offline BigBird

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Re: Pin spacing
« Reply #8 on: February 22, 2021, 02:03:56 AM »
The forces applied to a valve like this will be axial, putting the screws in sheer tension.  The amount of space between fasteners side to side is not overly important since the load will be applied front to back.  What do you mean yield during testing was concerning?  Did you manage to get holes in 4130 tubing to elongate?

The 4130 tubing has a lower yield. Looking back on Lloyds testing of crosman tubing yielding plus the online calculator for 4130 makes me think that I probably shouldn't test at more than 2x the operating pressure.  I havent used 4130 tubing before but it is some really strong tensile strength so should be "safer".
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Online Rob M

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Re: Pin spacing
« Reply #9 on: February 22, 2021, 02:23:15 PM »
maybe Bob can do  the math on the single Pin dowel method.. I know its strong enough to deform most tubes before failure, So ive never had any trepidation using it ( i only do it with 1/4 inch dowels )  This assumes the pin is 4140 or 4130
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Offline BigBird

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Re: Pin spacing
« Reply #10 on: February 22, 2021, 10:04:54 PM »
maybe Bob can do  the math on the single Pin dowel method.. I know its strong enough to deform most tubes before failure, So ive never had any trepidation using it ( i only do it with 1/4 inch dowels )  This assumes the pin is 4140 or 4130
Ok.  Wait?  Are you using 7/8 OD tubing?  I started mine on 1" x .083 and I can't go to .25" or it will hit the stem.  I don't think I could have drilled it lower as the pin would show at the apex of the valve.  Maybe that is ok because the valve won't be squeezed around your pin but it also leaves less tube between the pin "heads".  Maybe you could made your throat and valve stem off center.  Id have to make a jig for that if I ever wanted to go back to work on it again.

As far as pin strength, how about HSS rod or O1 tool steel?  I have worked myself into an odd size of 15/64" (.234) drill  but I bet I could step up to 6mm.

I will attempt to do the math but defer to Bob.
I plugged the numbers based on a solid pin at both ends

Valve force calc:
0.83 diameter, 0.415 radius, area 0.5410607948, at 3600 PSI fill = 1947.818861 foot pounds at valve

For a .234" dowl, each pin head has a shear area of 0.04301 sq. inches         
 using 60% of 145000 psi for an unspecified high strength dowel (4130 or HSS ?),         
that gives a shear strength of (0.04301 x 87000) = 3741.458 per pin for a total of 7482.9156 foot pounds      
and ratio of 1: 3.8
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Online Rob M

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Re: Pin spacing
« Reply #11 on: February 22, 2021, 10:31:21 PM »
my last one was 1.25 inch tube , 1 inch valve , so it was easier to clear the valve stem.. As you step down in  tube diameter obviously a smaller pini s doing the same trick since the force on the valve pushing rearward is less.. I dont think id use anything thoroughly hardened like HSS. ( basically to avoid it being brittle.. prefer bend , not snap ) thaats just me though  , it would proably be fine.
« Last Edit: February 22, 2021, 10:36:59 PM by Rob M »
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Online Nvreloader

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Re: Pin spacing
« Reply #12 on: February 23, 2021, 12:27:16 AM »
IIRR,
The Sentry air guns uses a round steel rod thru the valve/psi tube + 4 screws also,
I'll have to mike/measure everything, follow up is this info, 22/25 cal are the same.......

Steel rod 1.040" long x .235" dia and 4 screws into valve thru psi tube walls, O/S tube dia =1.190"
Valve O/S dia = 1.015".
HTH's
Don
« Last Edit: February 23, 2021, 12:30:10 AM by Nvreloader »
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Offline rsterne

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Re: Pin spacing
« Reply #13 on: February 23, 2021, 01:05:06 AM »
Pins are fine, but with most tube walls, the yield on the tube wall will usually be at a lower pressure than when the pin shears (assuming a 3:1 or greater safety margin on the pin).... Yes, there are two shear planes on a through pin....

Bob
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Offline BigBird

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Re: Pin spacing
« Reply #14 on: February 24, 2021, 09:42:35 PM »
Thanks.  I'm going to turn down a 1/4 longer shoulder 12.9 SHCS to fit my single pin set up.  Hopefully that will not change the mechanical properties? It will be stronger than 1020 DOM steel.  On the other end of the spectrum (too hard), I agree HSS might not be the best pin due to the temper (straw).  I could stick it in the toaster oven till it is spring (blue) but I don't want to chance that method.

I'm usually working on a few guns at the same time.  When I get bored with one piece of the gun I move on to another or another gun.  So thanks for all the different pin info.  I started on the one pin and wanted to think about the single pin arrangement a while for safety's sake.  However I may have thought a while without experience and I didn't want to leave it up to just hydraulic testing.
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