ROTARY VALVE S ARM LINKS: READY? DISCUSS!
WARNING: This post contains many words. If lots of words cause you to cry, I strongly suggest you subscribe to
Simplish.
My statement for the basis of this thread:
S arms are not *better* than anything else. They also are not the junk that many here seem to think. They are arms that connect the lever to the stop arm of the rotary valve stem. They have a curved shape that allows the stop arm to rotate fully without the S arm coming into contact with the stop arm's center tube. The other end is shaped the same due to the Germans having a sense of symmetry and style, so both ends are curved and they take on the S shape. It is reversible, so it is faster to assemble.
What makes them superior or inferior is
how they are connected to the lever or stop arm.
The T hinge system is simply the best, strongest, and the quietest way to connect a link arm to a lever that has thus far been used. The other end, however, creates a strong vacuum force about robust, equine reproductive organs. (You may quote me on that.)
It is difficult to damage the T hinge system, but if you do, it can be rebuilt or even probably bent back into working condition. But if it is bent or the S arm is bent, the end at the valve will not work unless it, too, is bent. The only real work you need to do to the T hinge is to re-peen the washer ends of each rod and flood it with penetrating oil or WD-40 once in a while, then use something like 30 weight or 3inONE to keep the steel rods happy and rust-free. The valve end needs to be adjusted once in a while, too, but the screw-through-bushing system is one I am not fond of at all.
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Joe's thread about DVS links (which are made from a different plastic today than they were back in the bad, old days — but that is a different topic from this thread) contained an excellent post by
@Rick Denney that reminded me of a custom job I did for a local band director about a decade ago. I played on his horn again about four months ago and was impressed enough with how well my work has stood up to a lot of use over the last decade that I have decided to put it onto my own prized 186.
He loved his S arms, but he wanted the same ball-and-socket links I had installed on my Alex back in 1995. He had loved how they worked and how easy they were to service. But he liked the look of the S arms.
I told him the S arms used a superior hinge system at the lever, but that I was no fan of the stop arm screw pivot, which could easily bind up and frequently would make a lot of noise. (These used a steel screw with a long shoulder, and the arm had press-fit metal bushing. The metal-on-metal joint would wear and start to click and just about nothing could remove that noise once it started. You could do miracles with the other end, but that stop arm end sort of sucked. This opinion is born out by the fact that Miraphone switched the metal bushings out for these Teflon bushings, but I never really liked them, either.)
My solution for this guy was to retain the S arms but replace the T hinges and the stop arm ends of each S arm link.
I used a drill rod that is a good bit thicker than what Miraphone
et al typically use. Then I made the hinge tubes out of some nickel hinge tubing that is harder to bend than the yellow brass stuff used by "the Germans". They have never once needed adjustment. My washers were of the same nickel tubing and were about 3 mm thick. I also DID NOT PEEN the hinge rod over the washer. (This is important.) Rather, I cut the hinge rod to extend 3 mm beyond the washer after it had been installed to the lever.
I threaded the hinge rod with some very fine but sort of deep threads. I had to have this done for me at a local machine shop as I did not own the tap or die with the threads I wanted, and I could not locate them online. (I think today I could find these, though.) I had the rods threaded on both ends using the same tap and die. I had the lever arms tapped to match. Having all the threads match simplified life.
The rods were threaded into the paddle levers and cranked down very tightly with no peening, as done by the factory. The free ends were threaded so that I had 4 mm of thread before the hinge "shoulder" started. This means the 3 mm washer had a short shoulder beneath it.
I then used some nice, knurled, round finger nuts that I slotted for a medium-sized flathead screwdriver. (It is easier to get to these from the side than overhead, so a straight driver works better than a wrench.) Also, the round, knurled "knobs" looked much nicer than the hex nuts I had originally wanted to use. This was important to the owner, so the hex nuts were replaced with the "knobs", which look a lot like those used on Miraphone paddle rack hinge rods. (Doing this caused me to have to grind down the ends of the hinge rods to allow a flat head to be inserted into the slots as the rods were flush with the end of the nuts when adjusted correctly.) I installed them using blue Loctite so that they did not back off. The threads extended to just beneath the washer so as it wears you can tighten the nut to remove end play.
I did this to both hinge rods in the T joint. The rods are also shorter than those made by Miraphone and friends. A longer rod or tube is easier to bend, so shortening these up a little helps to make them more stout, I think.
So, with these made in this manner, both ends are serviceable by the user. If the washers become worn over time heat the knobs with a torch for a few seconds to loosen the Loctite, crank the knob over just a bit to snug the parts up a bit, and let it cool. Blue Loctite can be lightly heated and adjusted like this many times, in my experience. Or maybe I have been lucky?
One trick I was taught in making levers or hinges that is
very important if you want them to have no play but also be free-moving and quiet:
Never cut the hinge tube end to be perfectly flat or square with whatever it has to ride against. The tube end needs to be just slightly crooked so as to allow a minimum of surface contact. This is what the big boys do, and it is part of the reason these parts wear down and start to click. This is a perfect example of designing in a fail point that you can control or adjust so that your other more expensive parts do not fail on you.
EXAMPLE: In a 4-wheel drive Jeep or truck, you design your front axle so that the component that will fail first is the one that is easiest to get to or the least expensive. If you truss up the axle tube to take more excessive loads, beef up your axle shafts from mild steel to a high-tech alloy, replace your differential ring and pinion with the same alloy, upgrade all your bearings, etc. it becomes so bombproof that if you *do* manage to mess it up it will sort of grenade itself and cost a LOT of money and time to fix. So you upgrade your U joints in the steering knuckles to be much stronger, but not as strong as all the spendy stuff. That way, if you break your axle it will almost certainly be a U joint, which is easy to get to and cheap to replace.
So, reducing this back down to tiny tuba parts…
If you bevel the end of your hinge tubes so that there is no space between the high point and the bracket the tube lives in, you can just barely file down the point of that bevel until it fits perfectly, and then it is only barely contacting the bracket at that tiny point on the end of the hinge tube. The bevel only needs to be off by about a quarter of a millimeter to work well, but that quarter of a millimeter is important as it removes about 80% of the contact patch between the two metal parts. If you make the ends of the tube perfectly flat it will bind until you file off enough for it to move freely, and then it will have end play and could make noise. A slight angle to the end of the tube is an elegant solution to a tight fit that makes no noise and moves very freely. I will try to take a photo of this using the valve levers on my old 186. As a beginning tech, I thought this was just sloppy work, but I have since come to realize it was years of experience that led the builder to cut those hinge tube ends a bit crooked.
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After a decade of hard use, the owner has never had to take them to anyone for adjustment. They are as quiet as when they left my house in 2011.
On the stop arm ends of his S arms, I installed a ball and socket. I used the same parts that I used on my Alexander in 1995. These have never had to be replaced. They are essentially bombproof. However, they have become scarce over the years. The maker has changed hands several times, and while they are still available, finding them is difficult today. They do not advertise in RC aviation magazines or online. To get them you have to "know a guy…" as they say. The last time I had to search for these I ordered like a hundred last time in case they go out of production.) They are a black oxide-coated 4-40 threaded ball and a black nylon socket with an unthreaded shank. You heat it and then thread your rod in. The socket shank is hex-faced, so you hold it in some parallel pliers and twist it onto the threaded rod. The socket hardens with the new threads impressed into the shank. The socket snaps onto and off of the ball. It can be difficult to get these on and off the ball, but once snapped on fully they work very well. They do not need to be greased or oiled, though I have tried both. If you grease them, regular, old axel grease works best. Lithium does not seem to play well with these. Oil does is no improvement over dry. The grease seems to slow them a little but removes the very soft click they make.
I machined (fancy word for Dremeled) the ends of his S arms away to a post that could be rounded and then threaded to 4/40. I heated the threaded ends and screwed the sockets on and quickly adjusted them to fit, then let them cool and harden. I had to drill out and tap the stop arms. I also had to Dremel off the upper tab for the old shoulder screw, then machine around the stop arm until it had clearance for the ball and socket, and then so that it looked nice.
In the end, this tuba had new, stronger, and adjustable T hinges between the levers and the S arms, and a nice ball and socket on the stop arm, with the original Miraphone S arms still in place. In the ten years of fairly hard use this tuba gets, the owner has told me he has heated and tightened the knobs on the T hinges to take out slop/click only once, and that was just last year, so they held adjustment for NINE YEARS, only needing to occasionally have 3inONE oil dripped onto the rods annually, or whenever he washed the horn and got water in them.
So I think I may do this to my beloved 186 so it can once again have those sexy S arms. I will use the nice Minibal setup at the stop arm, though. I like that a lot. But I
really like how my custom T hinges worked out over the long haul, and they keep the arms in perfect alignment with the stop arm, so the little silencer disc would not be needed. Maybe. I'm not sure about that, but who really cares whether or not the nickel silver disc with the nifty neoprene bumper material is needed or not?
So a big
THANKS to Rick for making me remember this very successful modification I did years ago, as today it becomes relevant to my old 186, which had them removed prior to my buying it. I wish I had them. I have some others, but they are off of an abused tuba and will require a lot of tap-tapping to get them flat and nice looking again.
Whatever.
Time to go outside and start hacking on my Kurath F tuba again…
)
