String Winding

[John Delacour]

At 22:27 01/10/01 -0400, Bill Ballard wrote:


>Thank you for that wonderful guided tour. I've always been curious about 
>that core wires tension during the winding. Is that an easy matter to set 
>quickly and reliably during the string winding? I have no idea how any 
>string winding could know what the tension of a given string on his the, 
>once this string is back in the piano and up to pitch. So there's probably 
>not reason to expect that any wire the winder might fasten to the lathe 
>would have its tension relative to eventual tensions in the finished 
>piano. I know why a wire in the lathe needs some tension. But how critical 
>are small amounts of tension when loading the core wire into the lathe.

The funniest thing I have read on this topic comes from a certain German 
Meisterpianobuilder, who states that "...it is therefore necessary that the 
strings be subjected to the same tension and elongation on the string 
winding machine as when they are installed in the instrument."  Thankfully 
he admits in the next paragraph that "I do not claim that the statements in 
this article are complete or even perfect".  In fact the basis on which he 
founds this assertion is wholly illusory and illogical.

The tension or "pull" applied to the copper covering depends, of course, on 
the gauge of the copper.  When you are winding 0.15 mm copper, it is so 
slight and critical that the greatest concentration is required in order 
for the copper not to snap, whereas when you are spinning 2.05 mm. copper 
onto a No. 22 core, the pull is such as requires a good breakfast and a 
brave outlook (one of my apprentices called it 'hero copper').  In this 
case it is necessary to apply a very firm and even pull and not to 
accelerate the machine to fast for fear of twisting the swagings on the 
steel.  In the first case, the steel wire will bow out only slightly 
towards the operative; in the second the angle will be considerable and it 
is to cater for the case of heavy copper that the machine tension needs to 
be set.

On commercial German machines I have seen, the tension is achieved with a 
coil spring and applied by guesswork.  In England some makers use pneumatic 
pressure.  For the past 15 years I have used a simple dead weight which, 
ignoring friction, applies the same tension to all strings at all times and 
this tension is sufficient to keep the steel as straight as possible while 
the copper is trying to pull it outwards in a V.  This weight is 80 lb., 
which is between 1/2 and 1/4 of the ultimate tension of the strings on the 
piano. So far as I know, not a single dud string has ever resulted from 
this practice and if, every six months or so, I have a string returned for 
rattling, then insufficient copper tension (tired man) or bad flattening 
(swaging) is the cause and never the machine tension.

In a previous thread about string twisting, I mentioned residual tension in 
copper wire after it is spun on.  In respect of machine tension, I would 
talk of radial compression of the (still slightly elastic) copper covering 
in the plane of the steel wire.  There is enough elasticity in the copper 
to keep the spiral tight over a range of elongations of the steel.  As the 
copper is wound on, with a slight backward angle tending to compress the 
coils or at the very least to exclude any possibility of gaps, a further 
stretching of the string of, say, 7 mm. will not result in gaps 
appearing.  When the string is removed from the machine, the coils will be 
slightly compressed.  When the strings are coiled for postage or transport, 
a further compression will be applied (and this can indeed be 
harmful).  Within limits, the remaining elasticity in the coils will take 
up the slack and when the string is brought to tension on the piano, and 
thereby stretched say 1% of its dead length, no harm will come.

It would certainly be worth while to make a thorough study of this question 
and it would be the height of arrogance in me to assume that my own 
practice could not be improved by such a study, which I intend one day to 
make, but I am sure that certain other aspects of the stringmaker's art are 
far more critical in the production of long-lasting, well-sounding covered 
strings than the machine tension, which should be sufficient to allow the 
easy achievement of the other critical goals and low enough to avoid any 
danger of breakages under the very rough treatment the steel receives 
during the covering process.

JD