TromboneChat

My Bach 36 with F attachment has developed a problem. The horn is about 25 years old. The hole that the rotor spindle fits is worn and is now slightly oval instead of round. This can easily be demonstrated by just touching the spindle (where it joins the linkage) with my finger. If you push up (toward the main tuning crook) and try to use the trigger it sticks. If you push down is works fine. The horn will stick unannounced while playing!

How would the oval hole be corrected?

Thanks for any ideas.

Howard

I think the simplest repair is simply to replace the bearing plate.

If for some reason you absolutely need to repair the one that's worn, you could plate up the hole, redrill, lap, etc. (probably cost more than the replacement plate).

Have you removed it and cleaned it?

Hmm, do you keep it regularly oiled? How much use does the horn get? IT could just need a cleaning, and may not have an oval hole. It may also just need a cleaning. Regardless of the reason, it's time to take it to a tech.

Yes, this is a job for a tech, and yes it probably also needs cleaning, but the main point is that if you can feel non-rotational movement in any direction at the spindle then the valve is worn (and/or the bearing plate is not correctly seated).

Bruce is talking about the bearing plate under the valve cap, but the OP is talking about the spindle bearing that's part of the valve casing and obviously isn't replaceable.

Yes, both bearing points (spindle/casing and bearing plate) can be swedged i.e. squeezed back to tightness, and then relapped to allow smooth movement. If the wear is significant, this might take several rounds, or not be possible at all. Rotors never have only one thing wrong with them. If there's enough wear that the spindle bearing is visibly ovalled, everything will probably need tightening up and the bearing plate seat position will need to be adjusted.

The correction for this is to swedge the bearings. A pretty quick job for a good tech with the right tool. After the bearings are swedged, a light lap will have the valve working better than when it was new. It may be necessary to address end play issues at the same time.

I remember seeing video years ago of the Bach shop doing final fit of rotors. The tech was actually tapping (pretty firmly!!) the spindle stop arm laterally, towards the tuning slide!! Such treatment could induce the type of wear you're describing, especially after some years of playing.

I wish people who are not techs would not offer "advice" on these threads....

hornbuilder wrote: ↑Fri Jun 09, 2023 2:34 pm I remember seeing video years ago of the Bach shop doing final fit of rotors. The tech was actually tapping (pretty firmly!!) the spindle stop arm laterally, towards the tuning slide!!

I can't say I'm entirely surprised, but yikes

What's the saying??

If at first you don't succeed, use a bigger hammer!?!?

I just did a nearly-new Wessex Stearn bass that showed signs of this sort of factory muppetry. Components weren't bad, but fitting seemed to have been quick and violent. Took a few passes to get back to something approaching circles. At Chinese speeds, I'm not shocked, but I would have thought that even C-S knew better than this sort of thing.

Chances are good that, if you have this amount of uneven wear in your bearings, the rotor and casing themselves are also worn unevenly, along with the thrust bearings. A loose fit in the spindle and plate bearings will allow the rotor to tilt inside the casing when actuated. Over years and years of use this will cause uneven wear patterns.

This is definitely something you need to see a good tech about. Ask them to do a pressure/leak test on the valve. Swedging the bearings will solve the "sticking" problem but if the rotor is worn unevenly and leaky, you'll need to either have the valve rebuilt or replaced.

Thank you all for your information and suggestions! I'm definitely going to take it to a good tech and discuss it with him. Much appreciated!! By the way, my work-around right now is a rubber band stretched between the rotor stem and the tube holding the trigger. Definitely solves the sticking problem but obviously not a permanent solution.

hornbuilder wrote: ↑Fri Jun 09, 2023 2:34 pm
I wish people who are not techs would not offer "advice" on these threads....

Amen, brother! Never gonna happen, though.