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. what do I need to know about hydraulics? Answered

I'm designing a compactor, it needs to be capable for compacting bubble wrap, carboard etc
hydraulic diameter?
general help and information will be much appreciated


I tried searching for an honest resource on the physics of crushing trash, and I did not come up with much.  There are machines designed for compacting trash on a residential scale, machines small enough to fit in your kitchen.  In particular a company named Broan guys must be good friends with Google, because pdfs for their products keep coming up the search results.



They seem to have something that compacts a volume starting at approximately 45cm x 30cm x 45cm, and then that last dimension gets smaller when the compressing happens.  That is to say the height, 45cm, gets squished down to 20cm? 10cm?, and the ram/pusher plate is a 45cm x 30cm rectangle.  Also importantly they say that this trash compactor is capable of 5000 lbs, or about 20 kilonewtons (KN),  of crushing force.

BTW, the point of this exercise is not to serve as a product endorsement for these guys, but just to try to take a look at what they're doing with their machine, and assume that these are sort of typical numbers involved in crushing ordinary garbage.

If you divide that 20 KN by the area of a 0.45m by 0.30m rectangle, you get like 150 kilopascal (KPa) of presure.

How much work does it take to compress trash? If I assume that the garbage is constantly pushing against the ram with this maximum 5000 lbs, or 20 KN, of force, and if I assume the ram travels a distance of 30 cm (just a guess, they don't say how far the ram travels), then that is (20 KN)*(0.30 m) = 6.0 KJ for the whole load of trash, which had a starting volume of about 0.060 m^3 = 60 liters.  Thus the amount of work per liter or trash that you start with is about 100 J/liter.

Also note that if you've got 6 KJ of work to do, and you can throw 100 watts at it, then that's 60 seconds to crush your trash. If you can throw 600 watts, it will only take 10 seconds.  And those numbers seem to be in the same ballpark as the ratings for the motor Broan is using. They say 120V * 5 amperes max. Anyway that calculation just serves as a sort of sanity check, to make sure the previous results of 100 J/liter, and 6 kJ for the whole 60 liter load, seem believable.

Another interesting fact you take away from studying this schematics of this kitchen compactor, is that they are not using hydraulics. 
(See: http://en.wikipedia.org/wiki/Hydraulic_machinery)
They're getting their monster mechanical advantage using some kind of jackscrew.  Thus it stands to reason that maybe you could do the same thing, perhaps by using a scissor-jack, like the kind intended for changing a tire on a car.  Or a wine-press might work too, if you can find one of those.

Anyway I think these numbers, specifically:  150 KPa of pressure from trash pushing against your pusher, and 100 joules per liter (that's per liter of starting volume) for the work required, I think those are some good estimates that you can apply to your trash crushing design.  E.g. if you have a pusher plate of such-and-such an area, then just multiply by 150 KPa (150 KN/m^2) to get the amount of force needed, and similarly multiply the number of liters per batch by 100 J/L to estimate the amount of work needed per batch. 

my design ended up being a raised box with latches on the door and a locking system on the jack which had a plate with small spikes...i designed several designs but they where more for development, they took up too much space and gave out too much power for what i wanted
thanks for your help people

Educational videos from YouTube provide a nice, illustrated, explanation B-) So pretty much your input would be an aquarium pump, connect that to some larger piston, then let it pump! (I've known this for quite some time, it's just that I don't know how to make the piston. You might as well grab a huge syringe of sorts then hot glue the tube to it) Oh, make sure the pump pumps to the larger piston, not the other way around. I don't get what you mean by "distributors" and "hydraulic diameter". I'll assume the hydraulic diameter is the surface that does the crushing. Should you use aforementioned system and made the crushing surface smaller, you pretty much have a spear that isn't sharp yet cannot be stopped. Should you make it larger, then you distribute the pressure throughout the surface of the crushee. Are you really willing to do all this stuff? Really, I think leverage using levers, rack and pinion, and more gears is a lot better.

Bubble-wrap will need shredding, if you try to compact it you'll have a hard time popping all the bubbles and allowing the gas to escape quickly.


Come on - A bubble wrap compactor !!
That takes all the fun out of it.

It's much better to do it bubble (POP!) by bubble (POP!) by bubble (POP!) . . .

leverage is what you need http://en.wikipedia.org/wiki/Lever

And over what stroke ? An off the shelf car "bottle jack" can lift many tons over say 200mm


Pressure ? Area of compactor ?

When I have the chance ill try upload a few basic skecthes to give an idea

re-designing, so its hand powerd...any ideas of joints/mechanisms i can use to increase pressure. I want a simple, push handle down and compactor comes down mechanism. area is 600mm by 900mm