Pedal-powered belt press: design concepts

Cabin fever is in full swing on a rainy Saturday here in the north country.  I took a walk up the hill this morning; little ephemeral waterfalls emerged here and there, but the snow is still knee deep in the darker parts of the woods.  On returning I made some whole wheat blueberry pancakes for breakfast (Tassajara recipe, plus wheat germ and milled flax), and now I’m settled in by the fire in a rocking chair to put down some notes on how to make a pedal-power belt filter press for next year’s cidering.

I’ve been toying with a couple of top-level design concepts for the geometry of the pressing components.  One is a pair of large drums, at least one of which is porous (perhaps made by assembling a series of plastic disks at intervals on a shaft, then wrapping a rectangular sheet of perforated stainless steel around the disks and welding them edge-to-edge.  A cloth-wrapped sausage-like package of apple pulp is introduced to the nip between the rollers, and pulled through like the wringer on an old-fashioned washing machine.  (My mom still has one of these, a James washer that she used to do all our laundry and diapers in, before electricity came to the North End.)  The other involves two rows of much smaller solid round rollers, arranged side by side to form slotted racks.  These racks would be arranged with a tapering gap between them, such that the juice would be gradually forced out of the pulp as the sausage is pulled through the assembly.

I think I favor the second approach, as it gives each portion of pulp a greater amount of time under pressure, and our experience with the old press indicates that it’s best not to rush the process of expelling the juice.  The gradual closure also presumably reduces the tendency for the pulp to extrude backwards through the system rather than pass through the rollers.

A number of practical considerations then arise.  First, what should the orientation of the cloth pulp package be?  My first thought was horizontal, but this leads to questions of how to effectively seal the edges.  I am now imagining that the racks of rollers are arranged in a vertical plane, with the rollers on vertical axes, and the pulp proceeding horizontally away from the grinder.  That way gravity helps keep the pulp in place and carries the cider off both sides of the cloth into the catch pan below.  Also, the cloth can be arranged in a u-shaped trough to catch the pulp as it falls from the grinder, then closed as it passes into the press region.  Pieces of rubber cord or polypropylene line could be stitched into the edges of the cloth in the manner of the bolt-rope on a sail; these could pass through polyethylene bearing blocks to keep things roughly where they belong.  A flap of cloth from one side would fold some or all of the way down the other side of the pulp sausage, to provide sufficient friction to avoid herniation of the bag.

Regarding the bags, should they be continuous, or broken into chunks?  A continuous recirculating belt seems like a holy grail of sorts, but would involve a whole additional level of tensioning, pomace removal, cleaning, and the like.  I think I’m willing to deal with manual handling of the cloth for the time being.  That way the bags can be dumped into a wheelbarrow by hand, sprayed down if necessary with a pressure washer, and loaded back into the machine.  Perhaps they should be provided the means to attach to one another, head-to-tail.

Next, how should the sausage be impelled through the system?  It seems like a nightmare to drive all those rollers in concert.  A single large roller at the end could presumably pull the stuff through, though you’d probably need to give the cloth at least a quarter-wrap around the drive drum to develop enough friction, and when the package of pomace emerges from the press it is typically about 3/4″ thick and about the consistency of hard cheese – not sure it will pass happily around a roller.  Plus,  you have the startup problem of feeding enough cloth through the press assembly to get the whole thing going.  All things considered, I think I favor the relatively crude but presumably reliable approach of sewing a strip of stainless steel into the end of the cloth bag, attaching one or two pieces of small stainless aircraft cable to the strip, and winching the assembly through.  This also simplifies and greatly increases the flexibility of the drive system; pulleys can guide the pull cable back to the vicinity of the shredder where the pedal-powered shafting is.  Unfortunately, it also requires that the primary drive pulley be located as far from the press as the longest bag that we would normally put through the press.  Presumably the press could be set up indoors or between two trees, but that detracts somewhat from the elegance of the solution.  I’ll have to think about that some more – one thought is a large diameter vertical axis drum (perhaps turned from a piece of maple tree trunk)  at the end of the press to double the cloth back on itself and send it back toward the shredder.

With the basic architecture established, a number of practical questions come up.  Given the presumably highly-crushed state of the pulp, I’m not too worried about it equalizing in the cloth “hopper” that will be formed under the shredder – it should be pretty soupy stuff.  However, we will need to modulate the speed of the press, so we can build up a decent charge of pulp in the hopper before the pressing begins, and we will need to make sure that over-enthusiastic shredder operators don’t overload the pulp hopper.  Perhaps for year four we should decouple the operations entirely, and operate the press from a hand-cranked boat trailer winch, since these are usually easy to find used at the dump in a coastal town.  This would be simple enough, but hardly in keeping with the pedal-powered spirit.  Alternatively, a slack drive belt with a hand-operable idler (like on a rototiller) to engage the press would be easy enough, but given the small amount of shaft work that’s actually needed, a friction disk variable speed drive operating off of the low-speed jackshaft on the grinder would be far more elegant.

What about holding all the parts in proper relation to one another?  One good thing about the racks-of-rollers design is that the associated force circuits can be relatively short – the rollers (presumably stainless steel, maybe 3/4″?) can be turned down at the ends and housed in machined delrin or UHMW bars running the length of the press.  These bars can be backed up by structural metal (maybe T-slot extruded framing), which can be tied together at close intervals by stainless bolts.  Extra points if the assembly is spring-loaded by stiff die springs, such that the press pressure is relatively uniform despite changes in the character of the fruit.  The same thing could be arranged hydraulically or pneumatically, though at the cost of increased complexity and probably decreased style.

The biggest problem at this point seems to be that actually implementing this thing is going to take a ton of time, and it isn’t going to be cheap.  Just the stainless rods for the roller racks are probably going to cost $10 per lineal foot, or several hundred bucks to do a press section a couple feet long – maybe I’ll score a load of stainless rod on Ebay.   But, if last year was any indication, the joy of watching the thing in action will be worth the hard work.

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