Kawai let-off (poor jack design?)

[Ron Overs]

Richard Moody and list,

Richard wrote;

>I have always been interested in the additional weight needed to push the
>jack through let-off. You say it will require ADDITIONAL 150 grams.?? 
>Perhaps you mean milligrams? Centegrams? 15 grams?? 


 When measuring down weight, up weight and let-off weight, it is 
important observe the following;

1) All measurements must be taken with the damper assembly lifted, ie. 
with the sustain pedal depressed if checking in the piano, or with the 
action on the bench.

2) Ensure that the measuring instruments are accurately calibrated. I use 
a Kontaktor touch weight scale (25-250 gms for checking let-off weight) 
and gram weights from APSCO (for down and up weight). The APSCO gram 
weights were originally quite inaccurate, for example, the heaviest 
weight which was marked as 32 grams actually weighed 35. I spent 
considerable time machining and grinding the weights to an accuracy of 
better than 0.01 gram (I used my RCBS rifle powder reloading scale for 
the calibration - [15.43 grains = 1 gram]).

3) The down weight is that required to move the key slowly from its 
resting position to the point where the jack tail just contacts the 
let-off button. When the jack tail reaches the let-off button the 
downward motion of the key will stop.

4) When measuring the down weight, the gram weights should be placed 
gently on the front of the key. Dropping them from a great height is 
unlikely to give results which mean anything.

5) The additional down weight is that required to execute let-off, and is 
additional to the previously measured down weight.

A procedure for measuring let-off weight

Check the down weight of the key to be tested. With gram weights 
equivalent to the measured down weight placed on the front of the key, it 
will be depressed only to the position where the jack tail is in contact 
with the let-off button. An accurate scale is used gradually to increase 
the weight on the key until let-off is executed. For example, if a note 
has a down weight of 50 grams, and an additional 125 grams is required to 
execute the let-off, then the overall weight required will be 175 grams, 
ie. the sum of the down weight of 50 grams and the additional let-off 
weight of 125 grams.

The let-off weight will vary from higher, in the bass, to lower in the 
treble. The figures I quoted in a previous post referred to measurements 
taken around note C40 (it is important to measure a couple of adjacent 
notes to obtain averages, since there will be some note to note 
variation). While key leads and/or wippen helper springs can be used to 
control down weight with a heavier hammer, heavier hammers will increase 
the loading between the roller and the jack. This results in a higher 
let-off weight in the bass section, and vice versa for the treble section.

It is important to remember that while the let-off weight is an indicator 
of the relative after touche resistance for real world actions, we are 
measuring this parameter under static conditions. When a piano is played 
normally, the let-off resistance is barely noticed.

Ron E. Overs
Sydney, Australia

Email: sec@overspianos.com.au
Website: www.overspianos.com.au