[pianotech] Balance rail profile [was Action Ratios Re...]

[John Delacour]

At 06:38 -0500 20/1/10, William Truitt wrote:

>Hi JD:
>
>>  >"The key pivot centre is not precisely at the balance pin centre;
>>  >rather it pivots about a centre which moves towards the front edge
>>  >of the key balance felt punching as the key is depressed. This
>>  >causes a change on the ratio of the key-capstan pair.
>>
>>Not on a piano with a properly designed balance rail.  Some balance
>>rails are quite obviously properly designed, and others are better
>>designed than they appear at first glance, for example the Steinway."
>
>Could you expand on this, please?  As in how you go about determining how it
>is properly designed and avoids the movement of the pivot point towards the
>front of the balance rail punching.  Any other comments in addition would be
>appreciated also.

I will try in the next couple of days to take pictures or send 
drawings of a few good balance rails, and these will show that the 
rail has a channel routed out along the front of the pins for the 
sharps to serve the same function as the angled top face in front of 
the naturals.  On many pianos the sharps are treated as second-class 
notes and will raise on the pin.  If the angle and the channel begin 
just in front of the pins or in line with the centre of the pins then 
the balance point will not move forward as the notes are played.

The design of the rail also needs to take into account the angle of 
the key when at rest.  Some pianos (eg. the Steinway grand) have the 
key horizontal when it is fully bedded; in other words the front of 
the key is elevated 10mm from the balance point when at rest.  Others 
will have the key horizontal at half-stroke (5mm ditto), and others 
may have a special rule or no rule depending on the phase of the moon.

At 10:35 -0500 20/1/10, erwinspiano at aol.com wrote:

>The only way to tidy up the pivot point on any balance rail would be 
>to cut the front edge of each punching. Come to think of it, 
>wouldn't the accelerated action bearings be a superior idea 
>considering the above stated premise?

Half punchings are fine for tidying up an imperfect balance rail, as 
are long strips of card gummed behind the line of the pins -- 
provided there is room for them without raising the designed balance 
height and key angle, though this is a very fine detail.

JD


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