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I was trying to troubleshoot bobbling hammers in a relatively new U-5 today, and could not find specifically why it was happening. All of the action parts and keys are in perfect shape.

I regulated the lost motion and let off

I experimented with blow distance, let off distance, checking; key dip was fine
I checked the strength of damper springs and hammer return springs but found nothing out of the ordinary.

Everything was adjusted correctly and functioning the way it's supposed to, except that at pianissimo if the key was not fully depressed, many of the hammers were bobbling, even if I backed the letoff out a few mm.

There is plenty of aftertouch, which I experimented with, but the problem occurs before aftertouch, as the jack is sliding across the butt. It's almost like the jack tip jumps over a ridge, causing the hammer to jump. While I didn't take touch weight measurements, the action seems fairly firm.

I explained to the customer the shortcomings of upright actions and the need to play through to the bottom of the key stroke in order to avoid this. I tune several U5s and have never noticed this before, nor has any customer that owns one complained of this problem. However, when I tune and play, I always play firmly to the bottom of the keystroke.

Has anybody noticed this on these pianos? Any solutions or suggestions?

Frustrated in San Jose...

Hi Dave,

As a lurker in the tech. forum, I can't give you any advice, but I just wanted to ask whether you could explain the term "bobbling" to me. Is the hammer striking the string(s) twice?

Regards,
Mark

try to massage the butt leather, I ran across the same thing on some U1, traced at that time to o lack of smoothness in the butt's leather curve.

initially I believe it is a bad hammer rest angle that may be the cause, as the jack then attack the butt leather on its center and the passage of the edge of the jack gives a second impulse.

But it may be also something else, for instance the position of the letoff button that could propulse the jack too fast (or too slow ?) allowing it to push the hammer a little more at some moment.

Was solved with a fine tuning of the key dip and capstan/whippen "air" space.

The real sure method to find hammer travel , hence shank rest angle, and to regulate the key dip pressure is to bend toward you all the backchecks and regulate the "drop" of the hammer so it is even and around 16-12 mm from the strings. with samples at 10 mm key dip you can then find the good travel distance for the hammer and shim or place the hammer rest felt, afterthat only the regulation may begin.

This look as the grand piano method, but actually is the original method for verticals as used in some factories.

bending the backchecks is not that bad for the wire, and if you don't it is difficult to evaluate the pressure, as the flex of the wire comes in the picture.

aftertouch is somewhat minimal on a fine regulated vertical..

Nowadays a vertical hammer may "drop" as a grand hammer, and I recall that when I find that condition, that was the first thing that the workshop head technician checked.

Dave:

The only thing that comes to mind is the letoff button rail. I seem to remember this being a problem brought up on some list. The rail was not in the correct place and was interfering with the jack. The jack was not clearing out from under the butt as it should.

Mark,

Yes, double striking is otherwise known as bobbling.

Isaac, Jeff,

Thanks for the suggestions. I'll be revisiting the piano shortly after New Year's and will check these things. I've already done some futzing with the checking and blow distance for the better jack/butt angle, and while there was some improvement, the problem was not cured.

I'll keep you posted.

I would also suggest to check the jacks for any type of burr... (reminds me of our crummy winter weather brrr) not being smooth or the angle of the jack being funny.

If the hammer butt leather is bunching up being poorly installed, or if it is faulty leather, that may help to cause it too.

On some uprights, there is not enough resistance to the hammer's forward impetus during a soft blow, particularly if the keystroke slows down near the end. The hammer will leave the jack prematurely, strike the string, bounce back, and then go through let-off, striking again. Additional spring tension will help, although that increases the touch weight.

I've find a neat tool, kind of small burnisher used for books making may be (or mechanics).

The shape allow to lightly burnish the edge of jacks in a vertical without dismounting anythin (sometime they are too sharp) . And that is when I discovered that it is also possible to massage the buckskin with the same tool (it sound like a curved leaf with some thickness (and a scraper at the other side)
WHile using it on some verticals that had a jerky uneven feel for letoff, they came out as if the action was new (after a 10 min job)

Have no idea yet if the massaging of the butt stay long, but it can help, for instance with silent pianos that had the let off too close from the strings for sometime, as in this case there is something that develop in the leather and that contrary the sliding motion.
I'll try to post pics.

Of course the burnishing of the jack remain for long...

yes the spring sometime is not strong enough, or the action is not vertical.

The letoff buttons if misplaced can make the touch very heavy also.

Sometimes the most obvious solutions to problems are the most often overlooked...and I missed on this one.

My client e-mailed me that after thinking about this for a long while (he's an engineer), he decided to adjust the pedal so that it would lift the dampers as far as the key lifted them. I haven't been out to see the piano (I'll tune it in a couple of weeks), but he says it's almost completely healed.

Because it was the CHANGE in tension mid key stroke rather than the tension applied evenly that was causing the problem, this makes complete sense.

I'm glad I have smart clients who can occasionally teach me a thing or two...

I dont get it, to me its irrelevant .. you did not say the bobbling had any relation with the dampers pedal.
But possibly the damper pedal is sigtly engaged at rest help to straighten the action (not straighten really but it plays a role in the weakness of the whippen's rail and its ability to move/vibrate)


But please explain what is your customer's theory behind that . I really dont see why changing the dampers pedal travel could change the repetition at soft play.


One thing is sure, a weak action allow for many kind of trouble at FFF and at ppp. Those aluminium rails are not the material of choice in fact (I think of some brass rod with a wooden dowel within , could be an interesting idea any one ever try that ?).

Isaac,

In re-reading my original post I see that I did forget to mention that engagement of the damper pedal made the problem worse, but that was indeed the case. Apologies to all for that omission.

When the key is depressed without the damper pedal engaged, there is evenly increasing resistance against the damper spoon>wippen>jack after it contacts with with the damper lever when the hammer is half way to the string.

When the key is depressed WITH damper pedal engaged, there is no resistance against the spoon>wippen>jack from the damper spring until most of the way through the keystroke when the hammer is just about at the string, at which point there is quite a bit of tension already on the spring. That's where the bobbling occurs.

That's what I figure, anyway...Does that make more sense?

I get it. But I think it is only part of it.

also if the repetition springs are strong they push on the hammer butt, it contraries the checking, that is why aftertouch/pressure regulation is so important.

See R2 actions on Sauter pianos, with a bronze blade spring on the jack. iT is very difficult to regulate those so one can play pianissimo.

But on Yamahas the double strike is due to a lack of even progression at the jack's butt level (to me)

Also check if the action rail contact screw (against the plate) is well regulated.

Vibes in the action rail provide a lot of problems.

May be simply the piano is not well standing on its 4 feet. that also is a common source of touch problems (up to key dip change in a region)

The bobbling was actually happening before any catcher contact with the checks.

Haven't checked out that part of a Sauter action. There is only one that I tune, and it's been awhile.

Action rail screw/plate contact could be a problem, but on these pianos, they are usually set and glued in tight. I've had them cause some nasty buzzes in other pianos when the glue joint broke, though!

The floor level, or lack thereof, is a definite possibility.

Time to try to go back to sleep. Chocolates late at night keep naughty boys awake...

Why do you go to sleep , ? sun is rising and the sky is blue !!!

Naaaaaa. No one would ever try something that crazy!

You have to go back to my post, and you will see what is happening. The added resistance of the damper spring slows the keystroke, but not the hammer, so you get a double-strike.

You have such semi good explanations for everything, DBD, thanks !

But what you talk of is the beginner pianist that does not push firmly enough on the keys.

We talk of a highly polished professional pianist (Rome grand price ) that have bobbling hammers when playing softly on a vertical even with the sustain pedal engaged.

A nightmare, in brief...