Looking for a sanity check. If the washing m/c spins at 1400 rpm and has a drum radius of 25 cm. my dozy calculation is that the centrifugal force is around 672g.
Just sounds a little high.
rusty
Looking for a sanity check. If the washing m/c spins at 1400 rpm and has a drum radius of 25 cm. my dozy calculation is that the centrifugal force is around 672g.
Just sounds a little high.
rusty
But only a little, I make it
((2? * rpm/60)^2 * r)
((2? * 23.33)^2 * 0.25)/9.81 = 547g
w = 1400 * 2 * pi / 60 = 146 rad/s
r = 0.25
F = m x w^2 x r
given g is 9.81, I make the force 550g, same league as your calculation.
Have I gone wrong as well?
Well, according to this
To know the force, you also need to know the mass that you're hurling around. A mass of 10kg requires a centrifugal force of 5479.4 kg-force.
Of course, as it points out in that link, centrifugal force is fictitious. There is no such force. What you calculate is actually the centripetal force, i.e. the force required to accelerate the object towards, in your case, the centre of the drum.
Yes there is: In Newtonian mechanics, the term centrifugal force is used to refer to an inertial force (also called a 'fictitious' force) directed away from the axis of rotation that appears to act on all objects when viewed in a rotating reference frame. Where the name has also been used to refer to the reaction force to the centripetal force.
If it's not in any dictionary then I might agree the word was fictitious.
Er, no. In fact if you spin a flywheel up to the point where it shatters, that failure is a direct result of the opposition of centrifugal and centripetal forces within it.
I very much doubt it. When it comes to calculations, you're the TOP DOG here.
One of the secrets of any calculation is to have a feel for the result. I thought the result was too high hence my concern.
The circumferential stress on the drum must be significant, as must be the bearing and outer case that has to deal with any imbalance.
No wonder they don't last long! Perhaps best to stick to a slower spin speed!
[...]
Yes, the technically minded often have a woeful understanding of language. There is nothing fictitious about centrifugal force, it is knackering the main bearing of my washing machine even as I type (terrible noise coming from back corridor). It is just a misnomer.
Tim W
The forces on those bearings are huge. What I was always more wary of thoug h were the large top loaders. With far more weight & diameter, and lots of rust etc, being near those when they got upto a roaring spin was a reminder of one's nonrobustness in this life.
NT
Centrifugal force is as fictitious as the earth going round the sun, when everybody knows they are merely following the line of least resistance through curved spacetime.
Vocabulary - corridor, passage. Be thankful.
When we salvaged a 13 month old, written off as (financially) irreparable Zanussi washing machine because of shot bearings (and made it run another 7 years), the max spin rpm was 1400 but on the plastic tub it was clearly marked 'Max 1100 RPM'.
Maybe part of getting more than 7 times the life out of it post salvage than from the factory was as you suggest, the fact that we used a max 900 RPM spin speed? (Given that is was still 200 RPM better than the machine it replaced).
I'm pretty sure it's only the spin cycle that puts a real strain on anything?
Cheers, T i m
Dunno, could be more than that depending on how wet the stuff inside is I suppose. My machine does not spin at full rate until the clothes get a lot drier so obviously there has to be some monitoring going on or the poor thing would shake itself to bits.
Anyway, I had a science teacher who maintained that we should in fact call it centripetal force.
Brian
I'd venture to say that "real strain" on the system comes from unbalanced loads. These things would last a hell of a lot longer if the load were evenly distributed (which of course it invariably isn't).
Didn't we all?
If you look at wikipedia definitions centripetal force is the force exerted outwards through rotation, and centrifugal force is that opposing it.
Common usage ignores the sign of the force, and mixes the two up. YMMV which term you use although centrifugal is the more common used word and perhaps the most understood.
Hence we were told to think of it as *negative* centripetal force, the text books all called it centrifugal force though, and I think the physics master knew that, for most of us, it was enough that we knew both names.
even as I type
lol!
True, but aren't they managed / limited on most machines and then only really impacting when trying to rotate them at speed?
True, I don't know how well they have to be distributed other than I've seen our machine having several 'goes' at re-distributing (randomly of course) the clothes to try to get something acceptable.
So, IMHO, a clump of clothes being thrown about when in the std washing cycle wouldn't be too much of an issue. Trying to minimise the clumps (and the weight of the water they may contain) whilst going for a high speed spin is / causes harder work.
If you look at the g-force calculations seen elsewhere in this thread it's proportional to speed and when in the washing cycle that speed is very very low.
10kg of load in the drum is only going to put 10kg worth of 'load' on the bearings. As you say, even 1kg's worth of imbalance is going to weigh a lot more than that at 1400 rpm! ;-(Cheers, T i m
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