BUSHINGS:
Bushings are part of the "bearings" of clock mechanisms. They are extremely important.
Clock brass is not all the same. It is important to realize how many different types of
clocks there are in existance , and how many have been made in the last 200 years. There
are literally millions of them. Thousands of different types; made of many different types
of brass. Brass is an alloy. It is unusual in that it gets harder when struck. If you stretch
a piece of soft brass it will harden. If you hammer on a brass plate it will harden. It is
important to know this when considering bushings. Usually a bushing is not as hard as the
steel of the pivot that rides in it. If the brass in a bushing is harder that the steel in
a pivot , the pivot will wear and eventually break , or stop the clock or both. If bushings have
been punched the brass will be harder. Usually this is not a problem. Sometimes the area that was
punched is uneven enough to cause abnormal wear in a pivot. Be aware of this when you check out
a mechanism. Also keep in mind that many clocks were repaired during wartime when brass and copper
were used for bullets, so none was available to make clock bushings. If you wanted to fix a clock
you used a punch. There are hole closing punches available that will do an almost acceptable job
but I cannot in good conscience recommend them. If you can , use bronze bushings.
Bronze bushings will outlast brass in most cases. Be sure to check for wear
that does not show: that is , wear underneath the oil cup. Before pressing in a new
bushing be absolutely sure that the hole is re-centered. Reamers are
available that will make a hole just the right size for the bushing to be press-fit into
the plate. I have used a drill press to push the bushings into the plate. You must be careful
not to start them crooked or you will ruin the bushing and possibly the plate. Bushing machines are
available at considerable cost. I never did use one but I am sure they work just fine. It is my
personal opinion that using a drill press is faster, but that depends on what you are used to I am sure.
I have always used a micrometer to measure the pivot sizes and it
seems to work just fine. Remember that when a bushing is pressed into the brass
plate, the hole in the center of the bushing will be squeezed to a smaller size, Between
.001 and .002 is common. Keep this in mind when choosing the bushing size for the
pivot.Relocating a worn bushing to its true center is difficult and must be done
carefully. Before disassembling the mechanism, after cleaning it, check each bushing
to find out how much wear has occurred. The bushings that are worn should be
marked, if necessary , and the amount and direction of wear should be noted , by
a mark on the plate with a non-permanent marking pen with an extremely fine point. With an
eye loop you can see on the inside where and how the bushing has worn and with practice and time
you will be able to tell very accurately where to put the new bushing. It took me several years
before I got fast at doing that. It can be done, however, and once you catch on it will be second
nature and you will be able to fit new bushings very quickly. I belive it is possible to be more
accurate sooner in your career by using a bushing machine , so if you have any doubts , maybe an
investment in one would be a good idea. They are quite expensive so be prepared to spend a lot.
I have used a small round taper file to back file the bushings to their original center. You can
tell where the original center is by looking closely at the "good" part of the busing and measuring
with the micrometer to what is the center. Then file the good part to make up for the wear pattern.
Do this by filing back as far as they have worn. Then use a drill bit to open up the hole
before using the reamer on it. After pressing the bushings in you may need to ream them out to fit
the pivots. Be sure you have polished and cleaned the pivots before you fit them to the bushings.
Depending on the type of clock you are working on the tolerance will be different for the amount
of play needed in the bushings. French clocks have very close tolerances .001 inch is not uncommon.
American time and strike clocks usually like to be somewhat looser. .003 thousandths is typical on
the number 2 wheel time or strike. This of course also depends on the size of the pivot.
On a small escape wheel three thousandths would be too loose.
Back to Notes From The Bench index
Page by:
Perpetual PC's
Copyright (c) 2002 David Tarsi.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being no invariant sections, with the
Front-Cover Texts being no Front-Cover Texts, and with the Back-Cover Texts being
no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".