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Press Pressure Questions And A Press Design


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#1 Bern

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Posted 08 June 2010 - 11:36 AM

The pressure plate on my grandfather’s ~85 year old #40 ratchet press cracked into several pieces (things just don’t last anymore). Since then I’ve built, and last year used, a wood frame press and the basket from the old ratchet press. I’ve incorporated many ideas from the great minds on this forum but the one area that I am still not 100% pleased with is the “force generator”…6 ton hydraulic jack.

I’ve been doing some reading here and need help with some press information. I’ve gathered a box of parts that I could put together to make a pneumatic ram for my wooden press frame. I have been using the 6 ton jack but after it flew out last year during pressing and landed about 20 feet away I started looking for a better (safer) solution.

I saw the mention of an air cylinder in this thread http://www.winepress.us/forums/index.php?/topic/38817-making-a-new-hydraulic-press/page__st__15 but didn’t see a resolution or follow-up in that thread. My question is how much pressure is needed to press grapes? In one thread I saw 3.4 Bar which is about 50psi and I’ve seen higher and lower but could not find any definitive number. My question comes up from my calculations I did while penciling the design.

Here is what I came up with. I have a 2” diameter pneumatic cylinder and an air compressor that can supply 150psi comfortably. The amount of force from a pneumatic cylinder is easy enough to calculate in that it is the area of the cylinder plunger x the supply pressure so in this case it is 471 pounds of force. That sounds great until I introduce the 15” diameter press plate on top of the must. The area of the press plate is 177 sq. in. So if I divide 471 lbs of force by the area of the press plate 177 sq. in. I get 2.66 pounds per square inch of force on the grapes…this seems too low but I don’t know what the “optimum” pressure should be.

Working backwards IF 50psi is needed on the grape must to press the juice out, then an 8” cylinder would be needed…ouch.

Anyway…does this sound like it would work or am I crazy…wait…I can answer that one. And, does anybody know what the optimum pressure should be to get the juice out of a fermented grape?

Thanks…Bernie
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#2 Howie

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Posted 08 June 2010 - 12:11 PM

I don't remember where I got the information, but I believe about 30-35 psig. pressure on the grapes is optimum. Bladder presses can use city water pressure that may go as high as 60 psig. I once figured out if you could use the spin cycle on an automatic clothes washing machine to centrifuge the juice out of the grapes and the pressure of the spin cycle turned out to be about 30 psig. I haven't had the time to pursue that project.
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#3 Crazy Run Ranch

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Posted 08 June 2010 - 12:30 PM

Bernie,
What's the safety advantage of a pneumatic cylinder? Seem the opposite to me. I think your calculations are showing why a bladder press can operate on household water pressure and basket presses require massive hydraulic force, its applied to a smaller area. My bladder press has a gauge and I typically use 20-25 psi up to about 35 psi max. Above this starts to squeeze astringent wine out and very little of it.

#4 Bern

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Posted 08 June 2010 - 07:33 PM

Bernie,
What's the safety advantage of a pneumatic cylinder? Seem the opposite to me.


For me the safety advantage is in the fact that the cylinder will be attached to a 1/2" steel plate which will be secured to the cross member of the press. The other end will be attached to a plunger plate which will push on the press plate and be lowered into the basket at least 10 inches or so at the start. The problem I had using the 6 ton jack was there was no attachment on either end and at one point the cake collapsed slightly on one side - probably from having the jack not perfectly centered. While under pressure I saw the jack tilt a little but before I could relieve the pressure the jack literally flew out and about 20 feet across the driveway. That to me was pretty scary since someone could have gotten hurt pretty bad.

I think your calculations are showing why a bladder press can operate on household water pressure and basket presses require massive hydraulic force, its applied to a smaller area. My bladder press has a gauge and I typically use 20-25 psi up to about 35 psi max. Above this starts to squeeze astringent wine out and very little of it.

I do not have much familiarity with bladder presses but from what I've learned here and a bit of common sense, You are correct about not needing tons of pressure because as I showed in my first post the amount of force is based on the area of the object doing the pushing times the pressure. For example, in a rudimentary explaination, if you are using 25psi water pressure in the bladder and have 200 sq in of bladder surface then the press force would be in the neighborhood of 5000 pounds. Now, that is probably not what your grapes are seeing because of the design of the press itself but you can see that there can be significant pressure developed in this way. It basically works the same way a piston in a pneumatic cylinder or any other for that matter does. I know there are many talented people on this forum who can probably give a much more complete presentation of this information and probably more accurate.

To get to 50psi at the grape I would need an 8" cylinder...from Howie's suggestion that ~35psi is optimal then about a 6" cylinder would work...unfortunately I don't have one of those in my junk box.

Thanks...Bernie
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#5 Hammered

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Posted 08 June 2010 - 10:47 PM

I've had one episode where the jack wanted to jacknife on me, but I knew when I started that the approach was risky. Maybe share your hydraulic design that was difficult and we can suggest some easy fixes to that system without reinventing the wheel. I think you're close to having a great press.

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#6 WnCmbll

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Posted 08 June 2010 - 11:11 PM

From the standpoint of safety, hydraulics are considered far safer than pneumatics, so I'm not so sure that you are considering the best path. On the otherhand pneumatics can be safely used with proper consideration of their explosive potential. Before continuing you should have the design of your press evaluated from a safety standpoint. Otherwise, you could be changing from an unsafe situation to one that is even worse.

I designed my press to withstand applying at least 80 psi to the grapes with the intent of not going over 35 psi. I found that I got very little increase in flow as pressure to the must increased above 30 psi. Therefore, I suggest limiting the pressure to 35 psi for red wine, and use somewhat less for white as is typically recommended for bladder presses. Design the press to safely withstand at least twice the maximum force you intend to apply, or are capable of applying.

#7 Crazy Run Ranch

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Posted 08 June 2010 - 11:12 PM

That's what I was getting at, you either have small pressure over a large area (bladder press), or lots of force on a small area (ratchet or basket press).
Your calculation seem good but commercial cylinders seems to have more output. This one only weighs 40 lbs. yet delivers 8 tons:
http://www.northerntool.com/shop/tools/product_200349249_200349249

#8 Bern

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Posted 09 June 2010 - 10:24 AM

OK...lots of ground to cover...I'll try to do it in one response.

Hammered
I know my hydraulic portion of the design was not the best at the time but had to get something in place so I could finish pressing after the ratchet mechanism broke. I basically have a 2x10 and 2x8 wooden frame very similar to what several people on this forum built. I'll try to get some pictures to post. I placed the oak press plate from the ratchet press on the grapes then stacked 4x4's in a tower to reach to the bottom of the 6 ton jack. A bigger problem I had was that I had nothing in place to hold the basket from moving so when the whole thing shifted the basket moved a bit off center which really caused things to happen fast. I don't remember if it was in the pictures of your press or someone elses but Iit appeared that the 6 ton jack was attached to the top cross bar. I was wondering what method was used to do that. I am not opposed to using a hydraulic jack but I would like for it to be captured between two stable mounts.

WnCmbll
I do understand the dangers of pneumatics. I work around them almost everyday as an automation programmer and electrical designer...now I don't claim to know a lot about it I do know some and what I don't know I will usually trade a bottle of wine for some information from some of the engineers at our office...sometimes I think that I make wine just so I can learn things. smileycheers.gif The press frame is pretty beefy and I know can withstand at least a 6 ton jack but my problem is getting something into place that will adequately press the grapes and be safe. The info of 30 to 35psi on the grapes is good and what I was looking for...thanks.

Crazy
Hope you don't mine if I call you by your first name. roflmao.gif That is a very interesting cylinder and a consideration for this project. The only two drawbacks I see with it are the weight and maybe the length. I have to do some measuring this weekend to see how that would fit up with the frame and basket. Appreciate the suggestion.

Now if anyone knows where I can get a replacement pressure plate for the ratcheting mechanism for my old press I would just replace that and be done with the rest of this. I tried getting one machined...way too expensive...tried to get a one off done at a foundry...way, way too expensive...and tried to have this one welded back together...pretty expensive. For the most part any one of these methods would have cost me as much as or more than a whole press and finding replacement parts from the manufacturers was just as futile.

On a separate note...is it my system or is anybody else having difficulty with the response window on the forum. When I want to type a response the Styles and Fonts bar is directly across the typing area which blocks about half the typing window so I have to keep previewing the message if I want to check what I wrote.

Thanks...Bernie
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#9 Brett C.

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Posted 09 June 2010 - 11:07 AM

OK...lots of ground to cover...I'll try to do it in one response.

Hammered
I know my hydraulic portion of the design was not the best at the time but had to get something in place so I could finish pressing after the ratchet mechanism broke. I basically have a 2x10 and 2x8 wooden frame very similar to what several people on this forum built. I'll try to get some pictures to post. I placed the oak press plate from the ratchet press on the grapes then stacked 4x4's in a tower to reach to the bottom of the 6 ton jack. A bigger problem I had was that I had nothing in place to hold the basket from moving so when the whole thing shifted the basket moved a bit off center which really caused things to happen fast. I don't remember if it was in the pictures of your press or someone elses but Iit appeared that the 6 ton jack was attached to the top cross bar. I was wondering what method was used to do that. I am not opposed to using a hydraulic jack but I would like for it to be captured between two stable mounts.

WnCmbll
I do understand the dangers of pneumatics. I work around them almost everyday as an automation programmer and electrical designer...now I don't claim to know a lot about it I do know some and what I don't know I will usually trade a bottle of wine for some information from some of the engineers at our office...sometimes I think that I make wine just so I can learn things. smileycheers.gif The press frame is pretty beefy and I know can withstand at least a 6 ton jack but my problem is getting something into place that will adequately press the grapes and be safe. The info of 30 to 35psi on the grapes is good and what I was looking for...thanks.

Crazy
Hope you don't mine if I call you by your first name. roflmao.gif That is a very interesting cylinder and a consideration for this project. The only two drawbacks I see with it are the weight and maybe the length. I have to do some measuring this weekend to see how that would fit up with the frame and basket. Appreciate the suggestion.

Now if anyone knows where I can get a replacement pressure plate for the ratcheting mechanism for my old press I would just replace that and be done with the rest of this. I tried getting one machined...way too expensive...tried to get a one off done at a foundry...way, way too expensive...and tried to have this one welded back together...pretty expensive. For the most part any one of these methods would have cost me as much as or more than a whole press and finding replacement parts from the manufacturers was just as futile.

On a separate note...is it my system or is anybody else having difficulty with the response window on the forum. When I want to type a response the Styles and Fonts bar is directly across the typing area which blocks about half the typing window so I have to keep previewing the message if I want to check what I wrote.

Thanks...Bernie

RE: the edit window - I have the same issue when using IE6. My machine with a newer browser works just fine.

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#10 Bern

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Posted 09 June 2010 - 06:31 PM

RE: the edit window - I have the same issue when using IE6. My machine with a newer browser works just fine.

Running IE7 on both home and work computers...probably need 8 to make it work...Thanks.
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