Precision Ground Bar Stock

Hi,
I'm looking to source some precision ground stainless steel round ba in 2mm, 1/8" and 3/16" sizes. I'm making some propshafts for mode
powerboats which would need to turn possibly in excess of 50,000 rpm hence the need for bar stock which is as round as possible.
I've contacted a local metal stockists who can get material ground t order, trouble is I do not know what tolerance to specify. I'm assumin the tighter the tolerance, the more it costs? Anyway, does anyone kno of a model engineer's supplies that stocks what I'm afte off-the-shelf?
Many thanks,
Pau
-- Mr Cran ----------------------------------------------------------------------- Mr Crane's Profile: http://www.rcgroups.com/forums/member.php?u 978 View this thread: http://www.rcgroups.com/forums/showthread.php?tr710
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wrote:

Paul
    Regular drawn stainless rod in these sizes is, of course, readily available from the usual ME stockists, Blackgates, College Engineering, Mallard Metals and lots of others.
    Conversely, precision ground mild steel is also readily available. Although I can't quote the tolerance but it must be at least as good as a thou.
    What particular feature of your design dictates this need for exceptional tolerance - a gland a bearing??? Or are you suggesting the dynamic imbalance of regular drawn stock would be unacceptable. Have you tried it? If so, can we solve the problem any other way without getting stuff specially ground? What is the unsupported length of the shaft? --
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
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Don't know if this helps, but...
A friend made a stainless steel prop for a class R hydro, it was one inch diameter stainless, 9 foot long from memory, turning at 12000rpm with 900Horse on one end. The only crazy thing was having to have the shaft through the kitchen window to mill the keyway for the prop...I assume the prop was balanced, but don't think the shaft was.
I don't recall any special grade of material or close spec being used. The shaft turned in Tufnol bearings (no they didn't last long!)
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wrote:

    Thinking about this a bit more, I'm not even sure that precision ground *necessarily* means straight, which I suspect is what worries you. --
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
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bar
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I hate to think how much cavitation a prop running at 50,000 rpm in water would produce, and how small a percentage of the power would transfer to the water !
AWEM
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On Mon, 13 Aug 2007 14:23:49 +0100, "Andrew Mawson"

Yep - I was wondering that myself. Maybe the OP can give us an idea why such extreme RPMs are needed? Is this a gas turbine drivibg a reduction box or something?
Regards, Tony
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See:
http://en.wikipedia.org/wiki/Supercavitation http://en.wikipedia.org/wiki/Supercavitating_propeller
--
Charles Lamont

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On Tue, 14 Aug 2007 00:52:22 +0100, Charles Lamont

Interesting links - thanks.
Regards, Tony
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Andrew Mawson Wrote:

Take a look at this video:

http://www.youtube.com/watch?v=J_edHP4mUvw

This guy has done a one way pass at over 150mph. On his return run th boat took off so he didn't set a record. The boat is powered by 32 nim cells turning a brushless motor in excess of 100,000 rpm.
The sort of thing I want to build is not as extreme as this (I don' have the money or access to a big enough lake!).
@Chris: The prop shaft will only be about 60mm in length, running in ptf bearings. I'm principally concerned with roundness - straight isn't problem on a small length like this - to minimise vibration at hig rpm
-- Mr Cran ----------------------------------------------------------------------- Mr Crane's Profile: http://www.rcgroups.com/forums/member.php?u 978 View this thread: http://www.rcgroups.com/forums/showthread.php?tr710
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wrote:

Paul
    If it's of any use - I've just measured precision ground mild steel stock (PGMS), standard 404 stainless steel and regular bright drawn rod (BDMS), all of which I just happen to have. In the little table below the first figure is the actual measured diameter for PGMS, followed by the stainless and then regular bright drawn, all of the same nominal diameter. I don't have any PGMS smaller than a quarter inch diameter.
1/4in (0.2500)      PGMS = 0.2507 Stainless = 0.2492    BDMS = 0.2483
3/8in (0.3750)     PGMS = 0.3752 Stainless = 0.3745    BDMS = 0.3745
1/2in    (0.5000) PGMS = 0.5006 Stainless = 0.4995    BDMS = 0.4982
    Measurement was with a M&W micrometer and an average of three measurements were made in each case. My lay feeling is that the difference in ovality and straightness, two of the key elements likely to produce vibrations, are unlikely to be significantly different regardless of which material you eventually use.....the 'law of diminishing returns' and all that. I would be interested to know how you can measure and quantify any vibrations produced and at what point and in what way the vibration would impact performance during the short high speed burst shown in your very impressive video.
    Keep us posted! --
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
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I would have thought your concerns ought to be the exact opposite. Anything that's been ground will be round to tolerances that won't have the slightest impact on balance or vibration. How straight it is is another matter and small diameters like this would be susceptible to damage by even mildly rough handling.
--
Dave Baker - Puma Race Engines



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The reality with most model boats with engines/motors running at that sort of RPM is to gear them down. A larger prop turning at lower RPM has far fewer losses.
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Not if he's contemplating the sort of speed that requires a 3" pitch prop at 50,000 rpm!
Mark Rand RTFM
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I don't think a prop will work at those speeds in water, 12000 can be a problem, i think the technical term is cavitation.
But this is definitely armchair stuff from me.
Steve
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120 mph with a 3" pitch prop is 42240 RPM with no 'slip' so 50k isn't a silly number to be working with. It gives 16% slip or a 9deg angle of attack; the blades are going to be in a pretty reasonable working range depending on the blade section. That's a somewhat simplistic view, but given the data tendered it's not too far off the mark.
For say a 50mm dia prop with 20mm wide cord blades a median Re comes out around 2E6 which is pretty reasonable aero/hydronamically speaking. There will obviously be some cavitation, but it all looks 'sensible'.
'A larger prop at slower RPM.....' isn't really a proposition given the target speed. A coarser prop perhaps, but that'll have even worse low speed performance problems associated with the higher AoA and may never get past making a load of froth. Recall how the Schneider planes had problems taking off because the props were stalled at such slow speeds.
Keep the shaft as short as possible and of the stiffest material you can runnig in the smallest bearing you dare. If vibration is a worry, consider radially soft mounting the bearings so the shaft/prop assembly can rotate about its C of G (not necessarily its geometric centre) without transmitting the vibration to the boat. Reasonable isolation will be achieved with a 2:1 resonant frequency ratio ie soft mounts to have a resonant freq. not more than 25kHz for the rotating masses concerned.
Be aware, however, that any significant bearing clearance will exacebate a phenomenon called half speed whirl which is when the shaft orbits within the bearing at half the rotational speed and in the opposite direction - so probably best to keep the soft mounts at half of that ie 50k/4. This is the ethos behind 'foil' bearings where the shaft is supported in foil strips and they can acheive staggering speeds - a couple of million RPM I seem to recall, but it was 30 odd years ago I was studying it so I could be wrong.
Loads of fun and much better than working for a living eh!
Let us see some pics when you've got it going :-))
Richard
On Mon, 13 Aug 2007 21:15:03 +0100, "Steve W"

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1501 Wrote:

Not with modern brushless motors. The highest speeds are set wit directly driven small props turning at high rpm. With 3 pole 540 bugg motors you had to use a gearbox as their torque was vanishingly small
-- Mr Cran ----------------------------------------------------------------------- Mr Crane's Profile: http://www.rcgroups.com/forums/member.php?u 978 View this thread: http://www.rcgroups.com/forums/showthread.php?tr710
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Given the use and your requirements I'd have thought Silver Steel would have been better. Yes, it will rust, but for the amount of time it's going to be wet I'd have thought drying it out would be a reasonable proposition.
Silver steel is readily available in close tolerance sizes, is much stiffer and quite a bit harder than SS so you could potentially go down a size and have less drag from a smaller bearing and maintain structural performance vis a vis SS. Altogether a much better drive shaft material except the corrosion aspect.
Richard
On Mon, 13 Aug 2007 09:26:37 -0500, Mr Crane
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Have you had a look at mould tool ejector pins they are normaly nitride hardened to a depth of about 10 thou but accuracy wise they are normaly very true and round they are also normaly about 1 thou down on nominal diameters see if you have a mould shop local to you Cheers Colin
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Mr Crane Wrote:

Have you thought about this sort of thing ,they do state they ar available in imperial sizes, not certain of the diameter tolerances. 'http://www.automotion.co.uk/pdf/dowel-pins-A2-stainless-steel-ISO2338.pdf (pins
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