[python] Bronze bearings
- From: Jürgen Mages <jmages@xxxxxx>
- To: <python@xxxxxxxxxxxxx>, <Flevofan@xxxxxxxxxxxxxxx>
- Date: Fri, 30 Jul 2004 16:26:13 +0200
Hello Flevo and Python Mailinglist,
I am not an engineer, but the following two postings, one from
Ralf Weigel (python) and the other from Hanno Hirsch (flevo)
show, what might be the disadvantage of bronze bushings for
the steering pivot.
What Hanno called the "brake-loose-momentum" and Ralf
calls the "stick-slip-motion" might be very disturbing, when
riding such a bike.
Regards from Jürgen.
PS: Sorry for the crossposting, but I think, in this case it might
be helpful to gather some expert opinions.
----- Original Message -----
From: "Ralf Weigel" <Ralf.Weigel@xxxxxxxxx>
To: <python@xxxxxxxxxxxxx>
Sent: Friday, July 30, 2004 10:37 AM
Subject: [python] Bronze bearings
> 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
>
----- Original Message -----
From: "Hanno Hirsch" <superhanno@xxxxxx>
To: <Flevofan@xxxxxxxxxxxxxxx>
Sent: Tuesday, July 27, 2004 7:29 PM
Subject: Re: [Flevofan] centre pivot again
> Hi Olaf & all the excited Flevobikers,
>
> >Let us talk some more about bushings.
>
> >Erik's mail about bushings being used in the steering pivot instead of
> >the original tapered roller bearings got a couple of us here on the
> >list excited. This solution bring about hopes that one can make a
> >steering pivot that is considerably lighter than the original.
>
> that was my idea since begin of this year.
>
> If I weren't so lazy, i would have competed in the design contest with
Twist
> Mk II, equipped with a iglidur bushing center pivot. I already have all
the
> parts of such a center pivot, including the bushings.
> I discussed this with a lot of "experts", some think for a steering axle
the
> forces necessary to get the axle into movement (what this called?
> brake-loose momentum?) will be uncomfortably high.
>
> but since a Flevo you are steering mostly with foot force I don't think
this
> will be a big problem.
>
> I will soon try all this and then report to you.
>
>
> >How does these sintered bronze bushings compare with composite ditto?
>
> sinter bronce bushings are the traditional, old-style bushings.
>
>
> >Here is a link showing iglidur® Q type F
>
> >http://www.igus.de/Main_fs.asp?KATALOG=XIGLIDUR&GO=STAN&FILE=P160200d-
> >htm
>
> exactly theese I got myself, application sheets sound good for this
> application.
>
> >These bushings and bearings are made of plastic and are used as wheel
> >bearings and in other heavy duty applications. They require no
> >lubrication and are promised to be very durable.
>
> one fear is that dust and sand may enter the pivot and sand-paper the
> plastic off. but data sheets talk about apllication in dusty or sandy
> atmosphere.
>
> >I have not the brains to understand the formulas and the data sheets
>
> nor do I, at least I've gotten no time/envy for it.
>
> >... perhaps someone else could make a fair judgement whether the
> >iglidur really is a realistic option in the search of the holy graal.
>
> I will take the engineers approach: just try it,
> unless nobody tries it, we will never know.....
>
> soon I am going to try it.
>
> happy Flevobiking
>
> Hanno
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