Bosendorfer action, Touch Weight Management

Bosendorfer grand action, rebuilt in Poland, was coming to our workshop to modify touch weight and get servicing.
This action showed typical polish rebuilding quality as usual i.e. poor details even cosmetically good. It has new hammers & shanks. Major worn parts were replaced. But there were many details where not good enough for the level which the client wanted to play such as too heavy touch weight, poor regulation, limited tonal possibilities. The client wanted to have standard touch weight because of too heavy at the moment. Also expected good variety of tonal palette as well as comfortable play-ability. 


Initial observation showed below results;
- Hammers were not tapered nor tailed 
- Knuckles were lubricated (?) with too much graphite (photo below)
- Key balancing was done. it seemed OK cosmetically.
- Key bushings were all tight, balance holes are loose
- Shank & whippen flanges seemed tight  
- Strike ratio by 2g weight was 5.8.


Too much graphite was applied

The elements of Equation of Balance by Stanwood protocol were like below:
- BW: 37g to 40g, Standard
- SW: #8 (3/4 medium) ~ #9 (Top medium)
- FW: relatively lighter side in bass and tenor, similar to ceiling level
- KR: 0.44 ~ 0.47 within 6 sample notes, smaller ratio and uneven
- SR: 5.4 ~ 5.6, relatively smaller than normal action 
- F: 12 g ~ 17 g, normal to a bit bigger 

These figures show actually it is within reasonable standard, but it is felt if one played it. I supposed friction was the main cause especially key bushings. 

BW:37g~40g で標準


See first half of the SW in the smart chart below which they are plotted in blue for even numbers. The green line shows target level. Red dot at mid-C is adjusted as sample. It was not felt so heavy in top treble section, so the hammers there were not required much reduction. 


SW (even numbers) plotted on Smart chart in blue

As SR was low even from standard already, it can't be decreased because the action may need too deep key depth to get after touch due to lower action ratio. So in this case, reducing hammer weight and treating Friction would be the area needed to be done first. The FW values could be adjusted by calculation of Stanwood's equation of balance before assembling. then see how SR can be adjusted later. 


Re-centering shank flanges, blush knuckles, adjusting SW, pre-voicing and tailing while hammers are out

Weight before and after tapering


Smart chart, completed SW (red)

Completed SW adjustment shows smart chart above. Maximum 1.1 g of reduction. No hammer lead was needed.


FW graph, Blue: original, Green: target and red: FW ceiling by Stanwood

Key balancing was done with traditional way in Poland I supposed. FW graph shows really uneven weighting result. Due to variety of friction level, uneven SW (blue in smart chart above) and FW, the action should be felt very unevenly. 
With this work, SW and FW were adjusted smoothly and F was corrected to standard. Final touch weight will be much better. More variety if we find still will be caused by uneven SR which is expected by uneven KR.


After regulation, the action was played and measured to check touch weight. It was felt slightly still heavy on white keys in tenor and bass. Those areas showed actually higher KR (and SR) at initial measurement. So we decided reducing SR by cutting balance punching cloths in the area (only white keys). Cut 1/3 in tenor and 1/2 in bass by reflection of KR figure.


Gluing half cut punching cloths in bass

1/3 cut cloth doesn't need gluing to key stick or paper washer



The work completed followed by touch up regulation. The action was returned and set up to the piano.

3 days Touchweight Management seminar lead by myself will be held in Sydney, Australia in April 2018. Please contact me at yuji3804@gmail.com if you are interested in participating it.

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