[python] Bronze bearings
- From: Ralf Weigel <Ralf.Weigel@xxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Fri, 30 Jul 2004 10:37:26 +0200
Hello,
I am following this bearing discussion for quite some time already. I do
not want to sound negative, but I think I have to dissuade you from the idea.
The reason for my opinion is a little number called "friction coefficient".
It means, broadly speaking , the ratio between the load on a sliding part
and the force nessescary to move it. For your common plain bearing, you
will need a force about 10% of the load to make it move. In the case of the
greased bronze-steel pair, it goes down to 3-6% once it moves. This could
cause an effect called stick-slip-motion. Industrial plain bearings are
designed to build up an oil film that separates the two metals. This is
achieved by careful choice of tolerances (not too tight, surprisingly) and
oil inlet (under NO CIRCUMSTANCES in the zone that bears load). The oil
film builds up a pressure that bears the load. It also works only for
continous motion and from a certain speed on. For this slow movements forth
and back as in a steering pivot one would have to resort to solid
lubrifiants. This brings you back to the nylon bushings on a child's
bicycle mentioned before. Or you go for very large bearing surfaces, tight
tolerances and keep a consistent oil film by capillary forces.
Ball- and roller bearings, on the other hand, are virtually free of
stick-slip motion and have a friction coefficient of about 0.2 -0.1 %.
Their behavior does not depend on a consistend oil film (though it helps to
extend their lifetime). That's why they became so popular as pivot bearing.
I will do the calculations for a Phyton pivot bearing that supports me (110
kg) and post the results there, probably in a couple of weeks. I will aim
for low cost and the absence of exotic parts.
Greetings from Karlsruhe, Germany
Ralf
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