Author Topic: Pully and Lever Help  (Read 1729 times)

Offline knipknup

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Pully and Lever Help
« on: April 21, 2012, 06:50:07 PM »
I apologize in advance because I know this is a commonly discussed topic.  I've done some searching and reading, some calculating and estimating and am hoping for some help with my specific setup.

To clarify the 'pulley' system, I have an eye hook and a hook in the forward post. The twine is attached to the eye hook, strung to the first s-hook, down to the hook, then up to the second s-hook and to the weight.

From my understanding, this would be a (estimated, not including the angles) 3:1 MA for the pulley system: 4 legs - 1 leg that weight is attached.

My lever forces are already calculated (more like estimated) at 25lb per 5psi.  This calculation takes into account mold diameter.

So, If I am to press at a 'medium' force of 15psi, would it be correct as follows:

15psi = 5psi * 3 = 75lb(lever) = 75lb(lever)/3MA(pulley) = 25lb final weight. 

I understand that the weight should be somewhat higher due to angles.

The mold is 4.625" diameter, so I'm using 16in^2.

Do I have this totally screwed up?

Thanks for any help.

« Last Edit: April 21, 2012, 06:58:08 PM by knipknup »
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Online smolt1

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Re: Pulley and Lever Help
« Reply #1 on: April 21, 2012, 08:09:04 PM »
Lever MA= 20/6
Pulley MA=3
Total MA = 3*20/6 = 10
25 lbs on lever arm= 250 lbs pressing weight
250 lbs on a 4.625 dia mold= 14.88 PSI

BUT 1) The tare weight ( pressing weight without weights on the lever arm) should be added to the 250 lb pressing weight.
       2) The angle of the pulleys means you need a larger weight on the lever arm, as you said.
       3) The 3 inches between the pulleys add an additional error.
My GUESS is that the errors add up to between 10 and 15 %

Hope this helps.

Offline knipknup

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Re: Pully and Lever Help
« Reply #2 on: April 21, 2012, 08:27:03 PM »
Thanks for the help.  I forgot to mention that the initial lever MA of 25lb per 5psi takes into account the tare.  I used the java app that is on the forum. It is really helpful.

The only thing I'm not sure if you confirm or assume with me is the Pulley MA being 3:1. I read a few articles and I concluded that the leg holding the weight shouldn't be included, but my brain got scrambled somewhere in the midst.
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Re: Pully and Lever Help
« Reply #3 on: April 21, 2012, 08:45:41 PM »
Yes, don't include the leg holding the weight. There is a great pulley picture somewhere on the CF. Just search "pulley"

Offline Boofer

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Re: Pully and Lever Help
« Reply #4 on: April 22, 2012, 01:50:50 AM »
Yes, don't include the leg holding the weight. There is a great pulley picture somewhere on the CF. Just search "pulley"
One of these, perhaps?

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Bread, beer, wine, cheese...it's all good.

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Re: Pully and Lever Help
« Reply #5 on: April 22, 2012, 09:07:30 AM »
Yes, a picture is worth a thousand woords!

Offline knipknup

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Re: Pully and Lever Help
« Reply #6 on: April 22, 2012, 09:27:31 AM »
Thanks for the help all!  These are very helpful.
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Offline zenith1

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Re: Pully and Lever Help
« Reply #7 on: April 22, 2012, 12:03:43 PM »
Also have a look at this website

http://www.swe.org/iac/lp/pulley_03.html
Keith

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Re: Pully and Lever Help
« Reply #8 on: April 22, 2012, 02:00:19 PM »
zenith1
Being an old physics major I love that stuff, but most of the folks who write cheese making books and sell presses don't know the difference between weight and pressure much less force and vectors. Just check out the Ebay adds for cheese press and you will find that most descriptions say that the press will press with x lbs of pressure. I get more questions on my website caused by the confusion between weight and pressure than any other subject.