Cheese press with pulleys - help with weight calculations

[Sailor Con Queso]

I've tried - the scales count backwards when put under the press, no matter what the weight is. They work fine with anything else.

Staples - you are "down under", so you have to turn your scales upside down. ;D ??? ::) ;)

[staples]

The scales might read, say, 4kg initially, then 3.99, 3.98, etc., etc.

I use a variety of weights - mostly tin cans, or containers (in a plastic bag if there isn't something to hang the weight with). It doesn't seem to make a difference what kind of weight I use, or whether I do/don't use a follower - the scales still count backwards.

[mjr522]

I'm going to put this problem on my statics final review...I'll let you know if any of the kids can solve it.  In the meantime, I'll see what I can do with it.

The actual force provided by the press depends on the angle of the arm--I'll try to come up with a good range and describe how the changing angle affects the final pressing force.

[staples]

Brilliant, thanks Mike Richards.

Sailor Con Queso - I suppose I ought to have the whole thing (press, scales, weights and cheese) upside down for it to work properly  :D

[H-K-J]

I've tried - the scales count backwards when put under the press, no matter what the weight is. They work fine with anything else.

Staples - you are "down under", so you have to turn your scales upside down. ;D ??? ::) ;)

Sailor, now thats funny LOL ;D

[hoeklijn]

LOL, and I need a press that acts like a time machine. When do I need it? That's irrelevant....

[Mighty Mouse]

Brilliant, thanks Mike Richards.

Sailor Con Queso - I suppose I ought to have the whole thing (press, scales, weights and cheese) upside down for it to work properly  :D

And the vat full of milk?

Hmmm.....

[bbracken677]

LOL, and I need a press that acts like a time machine. When do I need it? That's irrelevant....

Very rich!  haha   When is irrelevant! 

[staples]

[/quote]

And the vat full of milk?

Hmmm.....
[/quote]

Some days the goat is very insistent that we have the milking bucket upside down...

[Mighty Mouse]

Some days the goat is very insistent that we have the milking bucket upside down...

Oh well that's easy- just turn the goat over. Problem solved!

(Suddenly I am reminded of a classic joke involving 3 academics trying to figure out how to fill a bucket with water....)

[mjr522]

My analysis might have to wait a bit--baby #5 was born 0430 Saturday morning, and suddenly I'm reminded of all the things that my wife does that don't get done when she's not around--plus running on 2 hours of sleep each night doesn't bring out the best in my engineering skills.  If I had a time machine, I'd like to put it on "pause" to catch a few winks.

[Al Lewis]

Congratulations!!!

[smolt1]

Start thinking of a name for your family basketball team!

Here is an approximate solution.



Your pulley system has a mechanical advantage of 2 ( assuming that the pulley attached to the top of the lever arm is a jam cleat to take up the slack and does not move). Since the pulley is not pulling at a 90 degree angle to the lever arm, its pull is the total pulley system pull times the sin of angle C. That angle appears to be about 60 degrees ( the sin of 60 degrees is .81 so you only get 81 percent of the pull). With the pulley system attached to two different positions on the lever arm, the effective attachment point is somewhere between the two. Assuming that the attachment point is midway is a close approximation.

The mechanical advantage of your lever arm is (A + B) / A

So the mechanical advantage of the whole press is (2 x sin C)x(A + B)/A.

So here is a guess at the dimensions and the answer.
A=10 cm,  B=28 cm, C= 60 degrees

2 x .81 x 38 / 10 =6.1

With a mechanical advantage of 6.1,  10 kg hung on the pulley would give you 61 kg pressing weight.

[staples]

Congratulations, Mike Richards!

smolt1 - thank you, that is excellent. I really appreciate the explanation, too. Just a couple of questions:

Could you clarify what you mean by jam cleat? The two pulleys are identical (both have an inner wheel that moves). I realize this question shows my complete ignorance - sorry.

If I moved the pulley down the bottom so that it was pulling at a 90° angle to the lever, would this increase the ease (and/or accuracy) of the calculations? It should be easy enough to do.

Thanks again for all your help.

[smolt1]

"Could you clarify what you mean by jam cleat? The two pulleys are identical (both have an inner wheel that moves). I realize this question shows my complete ignorance - sorry."

The only reason that I could see for the pulley on top of the lever arm was to take up the slack in the pulley system. Maybe it works by tying it off at the base of the lever arm. A jam cleat is used on a sail boat to pull a line in tight and then jam it so it won't back off.

"If I moved the pulley down the bottom so that it was pulling at a 90° angle to the lever, would this increase the ease (and/or accuracy) of the calculations? "

Yes, the sin of 90 degrees is 1, so you get 100 per cent of the pulley system pull, ALMOST! I say almost because in all lever presses when the lever is at the top of its motion the hanging weight is not at 90 degrees to the lever arm. Then as the lever comes down the hanging weight gets to the 90 degree spot, and then as the lever continues down the angle gets away from 90 degrees again. In most presses this change is less than 10 percent, so the pressing weight might go from say 19 kg to 20 kg then back to 19 kg as the lever moves from the top to bottom as the curds compress.

If your measurement of the distances is accurate, most of the inaccuracy in a press comes from friction on the contact points on the lever and in the pulleys.