Roger Gibbs wrote:
>
> Ken, (Michael?),
>
> I have been considering replacing the bushings on my Sunbeam Alpine in the
>next year
> or two with Delrin and so read with interest your statement below. I have an
>Atlas
> lathe and was considering turning my own bushings. A couple of quesitons:
>
> 1. What did you use for the interference fit dimension (how tight a press
>fit)?
The Delrin is a plastic, so it deforms a bit. It's been a while since I
turned them, but as I recall, I machined the OD to about -0.000" to
about +0.003". The eyes in the A-arms weren't perfectly round (as one
would expect of a part to receive a rubber bushing), so there's probably
a bit of distortion which would tend to hold the plastic bush in place
in the eye of the A-arms. The tool steel bushes (5% chromium to
minimize corrosion) were another matter. I straightened the receiving
lugs on the frame as close as I could to perpendicular, then cut the
length of the inner bushes to each eye, so that when the bolts were
installed, the lugs were solidly compressed against the inner bush. I
felt this was important, because with the standard rubber bushings, any
attempt to torque the mounting bolts enough pinch the metal sleeves in
the rubber bushes badly distorted the mounting lugs.
I did the Delrin bushes in halves with about 0.100" clearance between
the halves when they were installed, because I wanted to put a shoulder
on the bush to take up the front-to-rear end play and to eliminate a
center on which the bushes could rock and wear. I probably should have
machined the shoulders for enough clearance to install wear shims, but
decided against it, because that would cause too much noise with
suspension travel. We'll see how the shoulders wear eventually.
> 2. How did the Delrin turn relative to brass or aluminum ?
I've machined straight Delrin on a milling machine with an eight-blade
cutter, and it cuts a nice, clean chip. I used Delrin AF for the bushes
because I kept reading about bushing noise in non-rubber bushings. The
AF stands for "Added Fluoroelastomer", i.e., alloyed with Teflon.
Thought that might reduce or eliminate any squeaking. On the lathe,
with continuous turning, the material cuts very easily, but produces a
long, stringy, continuous chip, so one has to stop and clean up
frequently. With very light cuts, a dull tool wants to slip on the
surface a bit, so a very, very sharp edge on the tool helps to make a
uniform cut.
> 3. How are the results ? Did you think it worth it for a street driven car ?
>Do you
> lubricate the Delrin ?
As far as machining and fitting went, no trouble at all. And no bushing
noise while moving the chassis around, but I haven't had the car on the
street yet, as I have many more things to do to it. And that work is
somewhat complicated by local code enforcement gendarmes forcing me to
move the frame from my backyard into storage (no "inoperable vehicles"
allowed on the premises in my town). I have a city councilor who runs
two auto repair shops, so she goes looking for people in her district
working on their own cars in the yard, and reports them to code
enforcement, because she thinks that such is taking money out of her
pocket... oh, well. So I can't give you a definitive analysis of how the
bushings behave on the street. Before I made the bushes, I did talk to
a plastics engineer where I worked, asked him about the choice of
material, and he thought it was a good choice, but we'll just have to
see. As for lubrication, I'm hoping the added Teflon alloy is
sufficient to minimize noise.
> 4. Where did you purchase the Delrin ?
Since I live in a small town with no outlets for specialized plastics, I
got a couple of feet of round bar from McMaster-Carr. Probably more
expensive than through a dedicated plastics house, but still available.
I think two feet of 1-1/2" dia. round bar Delrin AF was fifty or sixty
bucks through them a few years ago.
Cheers, Roger.
--
My other Triumph doesn't run, either....
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