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GENERAL CHEESE MAKING BOARDS (Specific Cheese Making in Boards above) => Problems - Questions - Problems - Questions? => Topic started by: staples on November 24, 2012, 01:13:34 AM

Title: Cheese press with pulleys - help with weight calculations
Post by: staples on November 24, 2012, 01:13:34 AM
Hello,

I started off with a home-made cheese press (one that you stacked weights on top of, so you knew how much weight was going onto the cheese), but was given a dutch cheese press as a gift last year. The press has two pulleys, in addition to the lever system - I've posted a photo, to give a better idea of what it looks like. The problem is that I don't know how much weight the press is putting on the cheese (my recipes all use weight, not pressure). If anybody knows the correct way to calculate the weight the press puts on the cheese, I would really love to hear from you.

Thank you!
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Schnecken Slayer on November 24, 2012, 01:40:39 AM
I think we need to know the distances from the fixed pivot point to a) the pressing rod and b) the pulley.
We may also need the distance to the second pulley with the weight attsched, my maths is rough after a couple of red wines.  :D
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: hoeklijn on November 24, 2012, 04:23:18 AM
I think we need to know the distances from the fixed pivot point to a) the pressing rod and b) the pulley.
We may also need the distance to the second pulley with the weight attsched, my maths is rough after a couple of red wines.  :D

True. With my press the distance from pivot to pressing rod is 7 cm and the distance from pressing rod to the pulley is 35 cm. 35/7 = 5.
This means that without pulleys every 1 lbs will be 5 lbs on the pressing rod. When you use pulleys like on the picture, you multiply that by 2, so 1 lbs will be 10 lbs. Beware: This is not the same as PSI.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: margaretsmall on November 24, 2012, 01:58:13 PM
Thats a great photo of your press which I intend to show to my woodworking husband. (Fortunately he was a science teacher in a former life so he will be able to work out the weights for me). I don't yet have one like this, so have 2 questions which might be naive -

- The peg under the arm on the right hand upright - is that a removable peg to hold the arm up when you are not pressing?
- How do you stop the whole thing from toppling over to the right when you put the weight on - is it fixed to the floor?

Margaret
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Tiarella on November 24, 2012, 03:32:19 PM
Margaret,  you might want to show your husband the press my dad made for me.  It's in the equipment board list under something "cheese press my Dad made for me".  It has a couple of cool ideas that could be combined with other designs.  I think I could get his full plan if you wanted. 
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Boofer on November 24, 2012, 05:07:21 PM
Staples, nice-looking press.

I see you've made the same mistake I made in placement of pulleys. :P

Take a look at this thread (http://cheeseforum.org/forum/index.php/topic,4026.0.html) to get some ideas for proceeding.

I've attached a recent pic of my press in action, pressing Beaufort #5 (http://cheeseforum.org/forum/index.php/topic,10316.0.html).

-Boofer-
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Al Lewis on November 26, 2012, 01:31:33 PM
Okay, the pulley advantage should be two to one.

The fulcrum/lever calculation can be calculated like this...

A force (weight) of 1 pound is exerted at a distance of 2 ft from the fulcrum.

The effort force at a distance of 1 ft from the fulcrum can be calculated as

Fe = Fl dl / de

    = (1 lb) (2 ft) / (1 ft)

    = 2 (lb)

Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Boofer on November 26, 2012, 06:48:09 PM
Hey, Al, could you go back and MODIFY your message and reattach the pic as a JPEG? I don't think the forum software likes PNG.

-Boofer-
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Al Lewis on November 26, 2012, 07:12:38 PM
Sorry about that.  Got it fix now.  Still learning the ropes here.  ;D
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on November 30, 2012, 01:43:15 PM
Thank you everyone for your help.

Just to make sure I have understood:

dl = 520 mm, de = 150 mm, (I'm measuring to the centre of the pressing rod, rather than the edges - is this correct?) so:

dl/de = 520/150 = 3.47

with the pulleys, 3.47 x 2 = 6.93

so, for every pound I hang off the end, I get 6.93 lb weight on the cheese?

Also, what is the rule with the pulleys? If I was to move the one down the bottom so that it is almost underneath the top pulley, what would the increase in mechanical advantage be?

Margaret -

- The peg under the arm on the right hand upright - is that a removable peg to hold the arm up when you are not pressing?
Yes, it is. It is really useful if you just want to hang a cheese to off the end to drain.
- How do you stop the whole thing from toppling over to the right when you put the weight on - is it fixed to the floor?[/i]
It isn't fixed to the floor - the stand on the bottom seems to give it sufficient balance, at least with the weight ranges I use.

Thanks again for all your replies - it's great to know what weight I'm putting on the cheese!

Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Al Lewis on November 30, 2012, 01:52:46 PM
Rather than spend all of that time on the math why don't you hang a 1 pound weight on the pulley and put a scale under the foot?  That will tell you for certain exactly what teh ratio is. ;)
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on November 30, 2012, 02:12:31 PM
I've tried - the scales count backwards when put under the press, no matter what the weight is. They work fine with anything else.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: bbracken677 on November 30, 2012, 02:52:56 PM
What do you mean by "count backwards"?  As in for a 1 lb weight, they register (for instance) a negative 7 lbs?
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: mightyMouse.tar.gz on November 30, 2012, 03:01:46 PM
What do you mean by "count backwards"?  As in for a 1 lb weight, they register (for instance) a negative 7 lbs?

Heh heh, we're gonna be RICH! CF has discovered the next diet trend! The amazing fat burning cheese press. Mwah ha ha ha ha ha!!!
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: mightyMouse.tar.gz on November 30, 2012, 03:05:03 PM
I've tried - the scales count backwards when put under the press, no matter what the weight is. They work fine with anything else.

How are you placing the press exactly? Could you post a pic? What kind of weight are you hanging off?
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Sailor Con Queso on November 30, 2012, 03:09:51 PM
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 ??? ::) ;)
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on November 30, 2012, 03:50:56 PM
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.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Mike Richards on November 30, 2012, 04:35:02 PM
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.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on November 30, 2012, 05:11:35 PM
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
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: H-K-J on November 30, 2012, 06:43:47 PM
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
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: hoeklijn on December 01, 2012, 04:13:38 AM
LOL, and I need a press that acts like a time machine. When do I need it? That's irrelevant....
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: mightyMouse.tar.gz on December 01, 2012, 01:05:36 PM
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.....
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: bbracken677 on December 01, 2012, 02:16:34 PM
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! 
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on December 01, 2012, 06:35:33 PM

[/quote]

And the vat full of milk?

Hmmm.....
[/quote]

Some days the goat is very insistent that we have the milking bucket upside down...
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: mightyMouse.tar.gz on December 01, 2012, 09:08:40 PM
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....)
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Mike Richards on December 02, 2012, 09:52:53 PM
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.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: Al Lewis on December 02, 2012, 10:14:34 PM
Congratulations!!!
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: smolt1 on December 02, 2012, 10:50:33 PM
Start thinking of a name for your family basketball team!

Here is an approximate solution.

(http://)

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.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: staples on December 03, 2012, 12:07:12 AM
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.
Title: Re: Cheese press with pulleys - help with weight calculations
Post by: smolt1 on December 03, 2012, 12:51:21 AM


"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.