Re: chain saw v. trimmer - urgent!


In article <31303030323030384358C33189@xxxxxxxxxxxxxxxxxxx>,
Jaques d'Alltrades <rusty.hinge@xxxxxxxxxxxxxxxxxxx> writes:
|>
|> > >I don't know what they claim, but a demonstration showed almost instant
|> > >stopping - less than half a second, I'd say.
|>
|> > That I would believe, but it is FAR too long.
|>
|> It's not exactly travelling at the speed of light^H^H^a bullet.

1' in 1/20th second is under 14 MPH. That is an extremely likely
speed for the business end to flip back if it hits an iron bar.

|> Nonsense! Even the torsion of a big saw (like the 36" Pioneer I learned
|> on) is tiddly when it stops, or when you accelerate from tick-over to
|> full-chat. Even the torsion from my R80 (800cc) BMW is pretty puny when
|> you gun it. The ratio of the weight of a chainsaw to the inertia of the
|> moving parts (some of which will cancel each-other anyway) is small.

Not that small. I don't know what the weight of the chain is, but
it will probably be 4-8 oz for a 10 lb saw. At 2,500 RPM (and some
go higher), stopping in 1/20th of a second is about 3 foot-pounds.
Not a problem if you are expecting it, but quite enough to make
you lose your grip if you are not.

Your statement about gunning it is irrelevant, as I don't believe
that it goes from nothing to full speed in 1/20th second.

Alternatively, using some reasonable estimates of moments of inertia
(most of the mass is pretty well concentrated), 2500 RPM translates
into the top of the blade moving at about 10 feet/sec (EXCLUDING
any effect of hitting a bar). That is 1/10th second to travel a
foot.


Regards,
Nick Maclaren.
.

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