21-10-2005, 04:40 PM
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chain saw v. trimmer - urgent!
The message
from (Nick Maclaren) contains these words:
In article ,
Jaques d'Alltrades writes:
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| I don't know what they claim, but a demonstration showed almost
instant
| stopping - less than half a second, I'd say.
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| That I would believe, but it is FAR too long.
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| 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.
IME, from observation, not from it happening to me, the blade tends to
stop there and then, or the bar kick back an inch or two. True, that's
not in a hedge, but a hard thing's a hard thing. (Said the constable to
the actress, as he drew his truncheon.)
| 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.
The effective momentum of a chain stopping suddenly is zero, as all the
forces cancel. The only 'kick' you'll get is if you are pressing towards
the job, in which case the 'kick' will be in line with the bar and
towards you. Any other torque will be produced by the rotating and
reciprocating parts of the engine, most of which will also cancel
each-other.
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.
Meanwhile, the rest of the chain is going the other way, or describing
an arc, cancelling any effect on the assembly if the chain stops dead.
Mind you, it might not cancel the effects on a few teeth...
--
Rusty
horrid dot squeak snailything zetnet point co full-stop uk
http://www.users.zetnet.co.uk/hi-fi/
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