Got a new home-made bronze bush with a steel shaft in it,
well-lubricated, but it's running hot (made two actually, one is peachy,
The shaft has slight wear markings over about half it's circumference.
I thought of re-polishing the shaft, but I've never worn-in a
steel/bronze bearing before - what should I expect?
-- Peter Fairbrother
Do you mean, it's worn from 9 o'clock to 3 o'clock over the full length, or
worn over 360 degrees but only for half the length?
Is there a radial load? Is there a bending moment on the shaft? How fast,
what clearance, how lubricated? Is it "solid" bronze, or oilite? Presumably
phosphor bronze? A drop of oil once every ten years might be well lubricated
for the hands of a long-case clock. It wouldn't do for the camshaft gear
spindle on a Velocette single, though.
The shaft is marked from 9 to 3 over about 1/3 of the length. I don't
know the cause, it looks more like a dark stain, not really a wear mark
as such. It seems to be very slightly rougher than the rest of the
I don't think the shaft is, or could be, bent - but I may have made some
kind of mistake when machining it, though I can't measure one, and can't
The other bearing seems to be working perfectly though!
There will be, a high one - it's on the spindle of a milling machine. no
axial load. But I haven't put any radial load on it yet, I'm still just
running it in.
Is there a bending moment on the shaft?
Again there will be, but not as yet; anyway it's an almost-solid steel
shaft, and I doubt it'll bend much !!
up to 3,000 rpm eventually, but only to a few hundred rpm so far.
adjustable - the bronze bush is slit and tapered, in a tapered socket.
fancy expensive high-tech light grease, so far. Will probably use
ordinary grease later.
Is it "solid" bronze, or oilite?
A drop of oil once every ten years might be
It's only been run for about a hour or so, so far, and it hasn't been
allowed to run dry at all.
-- Peter Fairbrother
Mea culpa. mea maxima culpa - I was running it far too tight, so no
wonder it bound.
I'd been getting about 10 hundredths's sideways movement under pressure,
which I had misinterpreted as the shaft being too loose in the bush -
but it was the slide, which I hadn't adjusted the jibs on, which was
giving the slop.
Down to under one hundredth movement now, :), and the bearing is running
Will repolish the shaft, grind the collet hole to final size in-situ
(TIR is about 2 hundredth's now), and away we go!!
It's a BCA jig borer/milling machine, nice bits of kit but the collets
are unobtainable (a set of collets costs more than 2 machines). I
replaced the main shaft with an ER25 straight shank collet chuck, and
also replaced the bronze bearings.
I also mounted the 650 W motor at the side, so it's more convenient and
more powerful and the head can turn 90 degrees to vertical. The box to
the side is the speed controller. not wired up yet, and it needs a guard
for the belt drive and a few more biots and bobs, eg the gib adjusting
screws need replacing, and the main.
Thanks for the suggestions,
Peter Fairbrother wrote:
Then maybe I'll never have to use full power, and it'll last longer :)
(Ha! if the chips ain't blue ... you ain't doing it right! Besides, I
work in really hard stuff a lot)
Can you think of how to change the head rotation position using a
I'm considering converting it into a full 6-axis CNC mill (three XYZ
translations, plus the main stage rotary table, another rotary table
mounted on that with a horizontal axis, and the head rotation), but I
can't work out how to do the last one.
There is a hole through the body along the head rotation axis, so I
suppose a gear and worm could be mounted behind the head, on the rear
side of the body. I'm unsure about that idea though ..
The main leadscrews are useable, but a bit worn. I have a ballscrew
suitable for the head Z axis, and have bought some replacement leadscrew
for the table X and Y axes. Got metric leadscrew, so I'll have to
replace the imperial bronze leadscrew nuts, but that's easy enough, and
they are in pairs and can be adjusted for zero backlash.
-- Peter Fairbrother
I wasn't thinking about the shaft bending (although of course it will, there
being no infinitely stiff material), the point is that a bending moment will
give you loading on opposite edges of the bearings. From the witness marks
on bearings and shafts you can work out how the thing is running. Glad to
hear it's working now anyway. Sounds to me like quite a demanding
application for grease, but you didn't explain that it was a proven design.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.