>At 11:17 AM -0700 18/6/04, Phillip Ford wrote: > > >One thing that nobody raised in this discussion was anything about the >jack / knuckle contact point and its relation to the 'magic line'. >Most action >manufacturers seem to set up their actions so that the contact point is on the >line between wippen center and hammer center at letoff. We may say >that letoff occurs >at a certain point. However, the process of letoff is really >happening over some >portion of the key stroke, from the time the jack tender first >contacts the letoff >button until the jack escapes. During this time the jack is >rotating about its own >center and the contact point is moving slightly. I wonder if, with >a conventional >action arrangement, having the shank go slightly beyond parallel is >doing something >beneficial to help the jack escape more easily or transfer a little >more power to >the hammer. Any thoughts about that? > >Phil Ford I've been reading the posts Phil but haven't felt any contribution compulsion until now. I've written about this more than once on the list. On the several occasions that we've discussed action geometry (there was a particularly active flurry of discussion immediately following the Reno convention in 2001). I've suggested that the improvement in the jack/roller contact relationship is one of the main benefits in setting the bore distance so the hammer shanks are over-horizontal when the hammer strikes the string. Many are aware of the benefits of placing the capstan/heel contact on the line of centers at half blow, while ignoring the fact that for most conventional modern actions, the jack/roller contact is around 7-8 mm below the line of centres at rest, reducing only to 1-2 mm below the line of centres at let-off. If someone were to set up a capstan/heel to contact this poorly, they'd be howled down by the myriad of techs who know better. But when it comes to the contact between jack and roller, that particular aspect of the action seems to be immune for the normal laws of good design. Indeed, some on this list and elsewhere have defended the poor relationship between the jack/roller contact, claiming that it is important for the 'feel' of the modern action. After building five grand pianos with our new action, I am now convinced that such claims are 'bollocks'. For those on the list who are not familiar with the design of our action, one of its many design improvements was to position the jack/roller relationship so that it is on the line of centres at half blow, ie. 3.5 mm below the line of centers at rest, and 3.5 above at let-off. A line drawing of the action can be found at; http://overspianos.com.au/actn.html So getting back to your post Phil, yes I do believe that setting a conventional action to over-center will and does help the efficiency of the action. When standard actions are set up so that the hammer shank is basically horizontal at strike point, they tend to have a pretty ordinary 'touch'. One high end concert piano I look after in Sydney has a generic boring of the hammers (ie. they are not bored for the individual piano), and a keybed to string distance in the bass which is a little shorter than usual. This action, which has no obvious friction issues or other vices, does not play very well. The shanks in the bass are under centre at the strike point. We've changed similar actions from on or under centre to over-centre in the past and noticed a significant improvement in the touch. However, such modifications need to be approached with caution. Some pianos have such deep pin blocks that there is a risk that over-centering the action can result in contact between the shanks and the pin block. Ron O. -- OVERS PIANOS - SYDNEY Grand Piano Manufacturers _______________________ Web http://overspianos.com.au mailto:info@overspianos.com.au _______________________
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