Piano World Home Page Forums Our Most Popular Forums Piano Tuner-Technicians Forum Blubbering upright hammers...what to do?

I just wanted to ask my fellow wizards about the common ailment of blubbering hammers. The Reblitz book suggests regulating the dip, letoff, and checking, but I think there's more afoot. It seems to me that the most obvious cause would be balance rail bushings becoming compacted over time. What say you?
Thanks,
Greg Livingston

If correcting the regulation fails to solve the problem-you should investigate the center of gravity, (CG), of the hammer assembly.

You will need to disable the hammer return spring and disconnect the bridle strap. Slowly move the hammer towards the string and observe if and where it begins to fall towards the string ever so slightly. The worst area for CG is just above the overstrung section.

Shaping the hammers so there is less weight on the string side of the CG will help but it is not easy to do unless you remove any staples from the shoulders.

Also if the hammer butt jack rest felt is too thin or compressed the jack has to travel too far during escapement.

If the hammer assembly wants to fall into the string you must not set the back-checks any closer than this point.

Hope this helps.

Greg, why do you think there's more afoot? Have you tried all the obvious things and they're still bobbling?

compressed cushion make the hammer go too fast to the strings and the backcheck is not yet in position when the hammer returns.
the jack can then hit the butt

this for wear , (as bad condition of the back-checks/leather the spring of the backcheck wire can send back the hammer)

One cannot work on out of condition actions and expect them to react normally to regulation.

Not every detail is investigated in reblitz indeed, but basis are still basis.


What s may be not explained is that the jack must clear the butt with the hammer in checking position. That is just logical but an important point.

Balance rail bushing is just lowering the keys so reduce the key dip. Your question as written means that that point is not a problem, you ask for causes.

AFtertouch can create problems with bobbling hammers indeed the jack being allowed to push on the hammer butt.

Actually, I think there is less to it.

Upright hammers bobble because the butt hits the jack instead of the backcheck on the rebound. - Period - Depending on what is going on, there are a number of different solutions.

If you just filed down the hammers, set the lost motion and let-off, but the hammers double-strike with reasonable checking, then you have insufficient after touch. The dip is probably fine, but you did not set the blow distance before you adjusted lost motion. The hammer rail cloth, and the hammer rail rest felt compact probably more than anything else.

But if this is an oldly-moldy upright where everything is extremely worn and out of whack, and the old folks home just needs it to be playable, adjust the lost motion, increase the let-off and adjust the back check until it stops bobbling. No, you won't be able to play softly with control, but when everyone is wearing hearing aids...

I have also found this problem on pianos with a combination of weak hammer return springs and a piano that doesn't sit level (i.e. the front is higher than the back).

Adding to what Ed wrote about centre of gravity, and Isaac about compressed butt cushion, I'd like to add that I've encountered two actions now (both compact upright actions) that were specifically prone to bobbling on soft playing with the right pedal disengaged, but played more or less fine with the pedal depressed. I found that at the point where the spoon engages the damper lever, touchweight increased from about 55 g to more than 90 g, even 100 g in some instances. This created a marked resistance point about two-thirds through the keystroke, slowing the softly played key down and allowing the butt to lift off the jack (a "false let-off", so to speak). By the time the jack was actually starting to trip, the hammer was already rebounding from the strings.

I found that the resistance point was attributable to
... some overly stiff damper springs (and possibly jack springs) and
... some stiff damper lever and jack centres.

After easing the tight centres and evening out the damper spring tensions by easing the stiffer springs to match their softer neighbours, the bobbling had practically disappeared, without making any other changes to the regulation.

While I'm no fellow wizard, I thought I'd mention this here, because from what I've seen, typical regulation procedures and bobbling remedies for uprights don't really mention the evening-out of damper spring tensions and easing of damper and jack centres.

Very True, Mark.

Typical regulating procedures and especially correcting bobbling hammers assume that the keystroke is being completed, and not stopped because of a feeling of resistance partway through the stroke. And if let-off is not actually achieved, what happens? The butt hits the jack instead of the backcheck on the rebound.

Sure, and the wear of the damper blade bottom cloth add to that sudden braking when playing lightly.

To correct, damper spoons should be bend twice so the edge of the spoon will not go within the cloth. This is too difficult to do and un do the day the cloth is changed.

I agree with Jeff. Rule out regulation issues by doing a good regulation and observe proper jack motion after key press all the way down. Must clear butt.

Isaac may have misunderstood your question. Balance rail bushings are in the key button, not under the key. Still, they would have no affect on double stricking if they were worn.

Except to the extent that they reduce key dip, as Isaac said. But that is rarely enough to be the sole issue.

Sorry - Every time I read the title of this thread, my immediate reaction is to hug them, console them, and then get them into therapy.

Then perhaps, Marty, that is exactly what you should do.

(With some luck, it might even free up bandwidth for techs to answer legit questions.)

Thank you all for your responses. I'm going to print them all out and study them.

How do worn balance rail bushings, the felt that goes inside the balance rail hole and limits sideways (horizontal) motion, affect key dip, which is a measure of the vertical motion of the key?

A question that I imagined many would initially wonder about !

Depth of touch can indeed be reduced when balance rail bushings get worn, because the keys drop on one side where the bushing fails to hold the key perfectly horizontal. This phenomenon is demonstrated most clearly where the keys are splayed most and wear on one side is greatest.

I realize my mistake, in the original post...I meant balance rail PUNCHINGS, not bushings. My apologies for the confusion.

Nevermind as both reduce key dip.

You never level keys ? on a vertical use some pieces of +- strong paper under the key frame if necessary.