Centrifugal force in the washing machne

High spin speed seems to be more a selling point than anything else. It isn't necessary and shortens bearing life greatly.

I don't remember what early front loaders did, if 600 or 800 it would have been more significant then.

NT

Before spinning, washing machine run at an intermediate speed that is supposed to distribute the washing around the drum.

Sin speeds are higher because you can have a smaller washing machine.

Machine size is governed by load capacity, not spin speed. What spin speed you get is just extras.

NT

Big machines have lower spin speeds?

Sorry, I didn't mean it was linear, I meant it changes as a function of the speed to some proportion (as you say, squared)? ;-)

Cheers, T i m

Agreed, it 'probably' is.

It would be interesting to see if anyone has done a water content (weight) test on a known quantity of material and at the complete range of speeds?

So it seems. ;-(

That sounds about right .. and like many things those sorts of speeds may have been easily doable by the most basic of materials (like bearings). I know Mums old Hotpoint must be 20+ years old now (I can't remember ever changing the bearings and it would have been me if anyone) and probably has a fairly low spin speed.

Cheers, T i m

My (1987-ish) Hotpoint 1400 RPM microprocessor controlled machine would have at to 3 goes at distributing the load, after which it would spin at only 1000 RPM if unsuccessful.

Distributing the load was done by slowly cranking up the rotation speed until it got to over 1g force so clothes stopped tumbling and stuck to drum, and checking for constant rotation speed (i.e. not speeding up and slowing down as a clump of clothes is lifted and dropped each revolution). This worked reasonably well except it couldn't detect off-balance load front-to-back (i.e. half the weight one side at the front, other half other side at the back). This also requires a tachometer (which this machine had) so it can detect speed changes within one drum revolution (which was done by mounting it on the back of the motor, which spins much faster than the drum).

Some other makes used a pedulum detector on the drum for off-balance loads, but those tended to cut the spinning at that point, rather than trying to avoid the off-balance load in the first place.

A colleague bought a slightly earlier model whem max spin speed was

The makers claim various spin speeds as rpm - and never refer to the varying diameters of the drums. A big machine (large diameter) could have a lower spin speed (rpm) but a better drying performance.

not an option (on a Siemens). I do have a 5 year guarantee.

thanks for that. I'm not a mad as I thought I was.

The point is though it is really the effect that occurs when things moving in a single direction are forced to deviate from that path in a circle. Brian

One of the issues with the term centrifugal is the apparent, albeit simplistic, inference that things would literally fly out from the centre - rather than continue on their tangential path - if released.

If you take a cotton reel and thread a piece of string through it, and attach a light weight on one end and a heavy weight of the other end, and then spin the light weight around your head by holding onto the cotton reel, the light weight will lift up the heavy weight once you spin it fast enough.

This happens once m(light) x r x angular speed >= m(heavy) * g. But the direction of the force is clearly acting *away* from the centre; if it was acting towards the centre the heavy weight would fall, not rise, as the speed increased.

My physics teacher could never explain this. We gave him a hard time about it - that and also because his name was J R Hartley, at the time of the "Fly-Fishing by J R Hartley" Yellow Pages adverts on TV :-)

Rolling element bearings will take amazingly high loads. Part of the skill in designing washing machines is in the suspension (stiffness, range, damping). Obviously, you can never get perfect balance so the drum axis moves in small circles around the centre of mass, this means the loads on the bearings are much lower than you might expect from the calc which you have done.

Interesting!

It's when the seals fail that they die. Water in the bearings wrecks the lubrication, and puts the fatigue rates up by one or two orders of magnitude.

The force (tension) in the string provided by the heavy weight

As Newton said every action has an equal and opposite reaction, you could equally claim the the light weight is providing lift to the heavy weight. In one sense this is perfectly correct. The only problem comes, as said by others, that if the string breaks, the light weight continues on its path - it does not fly in the radial direction of the string.

Obviously at some point the system may be unbalanced - in which case the heavy weight will rise or fall, adjusting the radius of orbit of the light weight until the system balances or the string hits one of its limits.

hough 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 remi nder of one's nonrobustness in this life.

The calcs were someone else's, but of course suspension reduces it some. Me mory not clear enough re the top loaders, but I don't recall any suspension on them, just a lot of rust. I presume they were ex-commercial things.

NT