Re: Precision Ground Bar Stock

On Mon, 13 Aug 2007 09:26:37 -0500, Mr Crane <Mr.Crane.2v9dsn@xxxxxxxxxxxx>
wrote:


Andrew Mawson Wrote:

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

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 the
boat took off so he didn't set a record. The boat is powered by 32 nimh
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't
have the money or access to a big enough lake!).

@Chris:
The prop shaft will only be about 60mm in length, running in ptfe
bearings. I'm principally concerned with roundness - straight isn't a
problem on a small length like this - to minimise vibration at high
rpm.

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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