Fitting Teflon Bushings

[John Musselwhite]

At 02:02 PM 3/10/99 -0600, Ron wrote:

(John)
>>Excessive friction in these centres is a major problem, IMO. In addition to
>>the loss of power and heavy touch, if I recall my thermodynamics correctly
>>friction translates into heat and I have some concerns about the effect of
>>even slightly heated metal rods running through a precision-reamed Teflon
>>bushing, especially as the assembly is exposed to the effects of gravity as
>>it cools. 

(Ron)
>Wouldn't it be nice if it worked this way? Bushings would be self sizing
>because the tight ones would heat the pins enough to ease the bushings, and
>the properly fitted ones wouldn't generate the required friction. Think of
>all those expensive little reamers with nowhere to go. 

What I was getting at was a tight pin heating up, getting slightly larger
and tighter still causing more heat as the note is repeatedly played. Then
as the pin cools it has a greater opportunity for the Teflon to "flow"
around the pin, deforming it slightly. 

I once watched a fairly well-known S&S technician in an "emergency" use
heat to size teflon bushings. He pushed one side of the pin  part-way out,
held a lighter to it a couple of seconds, pushed it in and did the other
side, all the while telling me NEVER to do it this way. His explanation was
that the bearing surface of the Teflon should be cut into it so you have a
fresh clean surface. Heating the pin just squeezes the Teflon slightly and
aneals the compressed surface which alters the density around the hole,
offering a greater chance for the bushing surface to deform or perform
incorrectly. This is what has led me to think about the effects of even a
minor amount of heat from friction on the bushing. 

			John

John Musselwhite, RPT -  Calgary, Alberta Canada
Registered Piano Technician   http://www.musselwhite.com
email: john@musselwhite.com