Design flaw.
I’ve had another seal come off it’s thread (stripped it), sooner rather than later thanks to the rigours of the MG-firing mode. And thankfully so, because it allowed me to ascertain the reason.
The stem I designed has too much mass, such that when the valve snaps shut after firing all that weight pulls disasterously on the seal’s thread. It may have failed only because of the stress induced by the MG-mode, or it may have failed eventually anyway. Regardless, I’m going to reverse the method of attaching the head so that the mass of the stem is significantly reduced.
Posted for those who are interested by such stuff. I’m finding it a lot of fun. 😀
All Replies
Viewing 14 replies - 1 through 14 (of 14 total)
The threads are tearing out.
Adam, I thought he meant the threads on the delrin/nylon side were tearing out. The metal stays put. So makes sense to make up the nylon with a metal nut so there is metal to metal thread contact. If the threads on the stem (metal) are tearing out, that’s something else entirely. Need to go shorter on the thread count (less tpi), or else change the stem material to fix that.
Lang, I’d tap the delrin if I were you. In metal, the very strongest threads you can make are actually made how you describe; not by cutting with a tap, rather, by using a forming tap that basically moves the metal around and systematically work hardens it in the thread pattern you want. This is not a V form tool, but yields the very best, strongest threads. You rarely see it in production work. If you were a paddock rat once and see a lot of race machines, you might see some. But of course delrin and nylon don’t work harden, so you can’t do this. I think you are just crowding the delrin over, and although it is very flexible for a semi solid, It probably sustains microfractures after a certain amount of deflection so there might be local weakening of the substrate near the threaded area. I’d guess a normal tap would produce a stronger thread. And there is plenty of spring back to hold the thing in place.
aloha,
walt
aloha,
walt
Nothing wrong with the one I got.
Nowt wrong with my graphics card either, but I upgraded that…. 😉 😆
Greedy bastard, what’s wrong with the one you have now?
Stop flappin’ ya gums and make me one 😀
The threads are tearing out.
The weird thing is that with a 2mm thread it never happened. Pablo’s version didn’t do it, though I think the stem in his was lighter (I lose track).
Up until today’s design I was drilling a 2.5mm hole in the seal and force-screwing the rod into it without tapping it first. I figured the tighter it was the better, but now I’m thinking that maybe that method was damaging the thread as it cut it. Today I properly tapped & died both parts and the fit seemed much better. I’m not sure how good the friction-coefficient of nylon is, though Delrin is worse. The constant vibration/shock of the valve snapping shut probably loosens the grip regardless of any thread-stripping that happens too.
Yay!
Can you get those in 3mm? 😯
Walt, up until the current design I was limited by the tolerances I was giving myself on the lenth of the seal-rod and the length of thread on it, which dictated where the head of the stem was at. With the current design I’ve given myself a whole bunch of leeway on that, so your suggestion is defintely doable. Adam’s lock-nut instead of a regular one would be the cherry. 😀
Walt,
The threads are tearing out.
If leaking when using a cap maybe the air is leaking along the shaft if you are using a press on seal. Thread it as you have been and using the cap as additional backing. The threads will seal the air leak. Also the delrin will act as your lock washer, no need to double the nuts. Just cinch the delrin up against the nut to reduce moving mass.
walt
You could use a nylon lock nut to make it stay put. Red locktite on a single nut would also secure it.
I tried a piece of brass as a rear cap, but for some odd reason it stopped the nylon from sealing once it was done up tight behind it. It was a bit rough and ready though.
Two nuts locking onto each other behind the seal would probably do a good job though, with expoxy to seal any minor leakage (though I doubt any air would get past the thread in the nylon?).
Thanks chaps, the nut idea sounds like the way ahead if the current seal comes adrift again. Probably worth doing regardless, just for peace of mind. 😀
Instead of threading the delrin or whatever you are using for the seal material and relying on the threaded area to retain the seal, you may wish to consider backing up the seal with a seat behind it, to function as a cap and apply the threads to the metal cap instead, so the weak point is eliminated entirely.
walt
Could you drill and thread all the way through the seal and place a nut behind it to keep it from tearing out the threads?
Suh-sweet! 😀
I think. 😆
Power through the head is no longer regulated by the size of the aperture through it, it’s now regulated by the thickness of the striking-collar around it. Which means the bore remains the same (6mm), which means the head is no longer being blown off like a projectile. Which also means the head can remain push-fit instead of needing to be screwed on and off. No more auto-cocking and MG-mode either.
With a collar-thickness of .75mm I’m getting 8ftlb with 1g pellets, which is 4ftlb under the limit. Looks like it’ll be bang on the limit with 1mm.
With the un-restricted head I’m getting 950fps on the 21-grain Magnums. That’s with 20mm of spacer behind the spring. If I put another spacer in I’ll get a fair bit more (was getting 1050fps+ with 25mm of spacer on the last design).
The stem’s weight is also considerably reduced with the new design, so hopefully the seal will stay put now. I did a far better job of threading this time (used the lathe instead of doing it by hand). Ultimately they could be epoxied too, but it would be nice to see it work without (I believe Blod epoxies his though).
A very satisfying afternoon’s labour. 😀
Viewing 14 replies - 1 through 14 (of 14 total)
- You must be logged in to reply to this topic.
The threads are tearing out.
Adam, I thought he meant the threads on the delrin/nylon side were tearing out. The metal stays put. So makes sense to make up the nylon with a metal nut so there is metal to metal thread contact. If the threads on the stem (metal) are tearing out, that’s something else entirely. Need to go shorter on the thread count (less tpi), or else change the stem material to fix that.
Lang, I’d tap the delrin if I were you. In metal, the very strongest threads you can make are actually made how you describe; not by cutting with a tap, rather, by using a forming tap that basically moves the metal around and systematically work hardens it in the thread pattern you want. This is not a V form tool, but yields the very best, strongest threads. You rarely see it in production work. If you were a paddock rat once and see a lot of race machines, you might see some. But of course delrin and nylon don’t work harden, so you can’t do this. I think you are just crowding the delrin over, and although it is very flexible for a semi solid, It probably sustains microfractures after a certain amount of deflection so there might be local weakening of the substrate near the threaded area. I’d guess a normal tap would produce a stronger thread. And there is plenty of spring back to hold the thing in place.
aloha,
walt
aloha,
walt
It was the thread in the nylon that was stripping, not the thread on the metal rod.