Hammer let-off & stroke Bechstein IV (& regulation thereof)

[robinh]

I’d be very grateful if anyone knows the usual hammer checking distance etc on a Bechstein model IV. Additionally, some questions on regulating these...

I’ve started replacing hammer butt springs. It has been ages since I considered doing this on this thread http://www.piano-tuners.org/piano-forum ... f=3&t=9608 ...but before ploughing through all the hammers I thought it advisable to gather any info I can on whether I ought to be attending to anything else relating to letoff at the same time (butt leathers?) …so I don’t have to remove the hammers again!

Hammer stroke = approximately 50mm, but both the Reblitz & Wicks books I have* suggest it is always 45mm on an upright.
Hammer let-off is approximately 15mm. This seems to be more of a concern – isn’t it supposed to be 3mm? I suppose it explains it being generally loud.

Background info: key dip is expected 10mm; there isn’t any lost motion in the action; hammers seem to rest nicely on hammer rail; on a test key, I’ve tried setting let-off regulating screw up to get a smaller let-off, the full 10mm travel of the key still only raises the hammer to 7mm from strings – and to be honest that would be useless anyhow as the let off button barely touches the jack… so isn’t even slipping out from the hammer butt. At 15mm the let off works fine.

The above 50mm/15mm are true of all keys, so it isn’t just a few odd ones.

Questions.... Is it possible it was intended to be like this? Do the hammers look like they’ve been shaped enough to have taken 10mm off – seems unlikely? I wondered if a worn hammer butt leather would be likely to cause a lack of travel and hence distant let-off. I’ve photographed that – it doesn’t look horribly worn?

Butt leather picture

parts generally

15mm & adjusted 7mm let off

I'll hopefully have a look at it with John who posts on this forum, when he next has time to tune the piano; he's been ever so helpful with advising on repairs. He's rather in demand and fairly booked up at the moment though so as I'd quite like to get on with butt-spring repairs on my holidays I'd be very grateful if any of you folk have any advice on the above adjustments.

* books suggested a couple of years ago by another helpful forum member, Colin - very helpful.

[Withindale]

I imagine Bechstein is similar to other pianos, i.e. 3 mm, less in treble but others will advise.

What happens with a trial key when you reduce blow distance to 45 mm and then adjust the capstan and the let off button? Are there any spacers on the nose bolts or the action frame that push the hammer lines away from the strings? Is the key dip exactly 10 mm?

[robinh]

What happens with a trial key when you reduce blow distance to 45 mm and then adjust the capstan and the let off button?

Great, will try... Am I correct in assuming to pad the hammer rest rail by the test hammer so its blow distance is 45mm, then adjust capstan/let off accordingly to remove lost motion?

Are there any spacers on the nose bolts or the action frame that push the hammer lines away from the strings? Is the key dip exactly 10 mm?

The piano looks fairly untouched over its life - really doesn't look like action frame etc has been altered - I'll have a closer look later in the week. From the keys I looked at, key dip would be less than half a mm out from the 10mm mark. again, I'll check that.

[Withindale]

Yes, pad the hammer rail. 0.5 mm less key dip reduces hammer travel by about 2 mm.

[vernon]

Is the set-off rail loose?
Is it correctly positioned under the jacks?
It could be that it needs moving forward or back a bit

[robinh]

With a bit more investigation, I think it might be a set of issues, rather than one direct cause

Is the set-off rail loose?
Is it correctly positioned under the jacks?
It could be that it needs moving forward or back a bit

thanks. just had a good look at that: letoff/set-off rail (assume they're used interchangeably?) seems fine - buttons aligning directly above knuckle on jack

Yes, pad the hammer rail. 0.5 mm less key dip reduces hammer travel by about 2 mm.

Hammer rail padded on a test key (I know a damper is missing, I'm doing a repair on it in the workshop)

Adjusted capstan & let off accordingly. Certainly an improvement. Took out a punching (there are generally two older ones, and two newer ones (newer=only a few decades?) under the green baize to get a 'generous' 10mm. Again, marginal improvement. Most keys are 10mm already. If I were to do that on all keys, and set the backcheck further away too, it would be an overall improvement. I'm just sceptical they've been adjusted forward ever, so I guess I'd be setting them back to make up for wear in other parts. And blimy, even with proper setter I bought, they're damn stiff wires. I'm inclined to leave alone until I investigate more.

Add to the above adjustments 2 or 3 mm of hammer removal/reshaping... I'm erring to the side of the present regulation being caused by cumulative wear.

I suppose the are three questions in mind at this point

1. Hammer rest rail - If the hammer rest sits happily at 50mm, and given no other adjustments to relative positions of action/strings seem to have been made in the past, should I assume someone has removed an amount of felt/baize from the hammer rest rail? Related, does anyone have a book detailing blow distance, key dip etc on this model or know it to be any different to average.

2. Physics fun - key travel : hammer blow ratio Really good mental workout this - I'm very much enjoying it... 10mm key dip, 45mm hammer travel. A brief glance, the lever ratio of the keys about balance rail gives a smaller raise to capstan (let's say 7mm crudely)... so how is the 7mm translated into 45mm blow distance? looks like the majority of the distance gain is from the short distance between butt-fulcrum and jack>butt contact point compared to the large distance between butt-fulcrum and hammer.
Is it therefore reasonable to conclude (or have you seen in practice) that wear to hammer butt leather has a huge mediating impact on the distance the hammer is thrown? What else would wear manifest its self as? e.g. sharper or more gentle set-off trip?

3. If your butt springs were broken... or your client's were, and you wanted a quick fix to stop the hammers falling forward to rest on strings... what would you do? Decrease key travel, trip jacks early? increase blow distance with thinner baize? Only that might give a clue as to whether anything might have been done in addition to wear.

Oh, the felts under the keys if it is of any relevance to any comment, most likely not

Thanks again for ideas so far chaps.

[vernon]

viewing the keyframe it seems the backtouch is the original . likewise the one brown balance washer. Perhaps the front baizes aren't original.
The hammer-rest felt looks fresh so possibly it's different thickness from the original and has not been compensated for.(see previous posts)
If you want to alter the overall depth of touch don't for goodness' sake start removing paper washes as their function is to level up each key depth individually. You have to add or remove cards from under the frame itself.

[Withindale]

I suggest you look at the Abel piano parts catalogue and compare the actual bore length of your heads with new ones. The difference will be an estimate of the amount of filing on your heads.

[pianotechman]

Just as a thought,[ as every tech guy on here has probably already suggested 45mm hammer blow with a 10mm touch depth] what is the condition of your key bushings on the key balance? A badly worn felt bushing even on one side, can be responsible for a loss of accurate key travel even though you think you may have it set correctly. That would certainly account for it working beter with a 15mm depth!