Pedal apple grinder – year four modifications

The equipment modifications for cider year four are starting to take shape in my mind. First, the press. I plan to take the basic grinder unit from last year and modify it in a couple of ways. First, I’d like to make fresh cutters with milled serrations, such that the apples are shaved into ~3/8″ wide strips rather than full-width slices. This should be a relatively simple operation on the milling machine. There’s incentive to remake the cutters anyway, since in the heat of the moment last year we didn’t realize that the sheetrock screws holding together the wooden plunger we hacked together were nicking the cutters. So, order stock (e.g. mcmaster 9524K192), cut to length, machine mounting holes and csink (I’ll put a different hole pattern in the drum rather than try to match the hand-drilled holes from last year; a drum remake in HDPE wouldn’t be a bad idea), clamp each piece at an angle and mill a row of shallow slots, then clamp with the cutting edge exposed and machine the 15 degree cutting angle (maybe order a 15 degree taper mill if there isn’t one around).

Next, we schemed about a secondary crusher to further liquefy the pulp after it is shredded by the primary cutters. Holly has some stock (originally meant for the primary cutter drum) which I think may suit the purpose. The basic concept would be to arrange a router jig on the wood lathe (or use a rotary table on a milling machine) to cut shallow axial grooves, so as to make a pair of coarse interlocking gear-like drums, as follows:

If we’re lucky, we can bore the parts on an engine lathe, broach them with Holly’s ebay 3/4″ hex broach, and press in a hex shaft with turned-down end. Probably we’re not that lucky, and the shaft will have to be shimmed out or filed down to get it to fit. But we can hope.

One of the drums gets driven straightforwardly off of the same piece of chain that drives the primary cutter. We already have a sprocket left over that will do the job. The trickier part is driving the other one synchronously. Because they aren’t a proper gear tooth shape, I wouldn’t trust them not to jam if we just let the one drive the other. I think the solution involves small 25 pitch sprockets (fairly economical e.g. McMaster 2737T134) and chain (also cheap). We’ll need to fabricate one or two idlers to assist in reversing the direction. There’s room on the end of the existing shafts to add these sprockets. We can swap out the idler shaft on the press (which is un-necessarily hex stock in the center) and use it as one of the secondary shafts, and we’ll need another piece of stainless hex bar to make the other shaft (tempting to use aluminum instead; not much cheaper but substantially easier to cut) – we’ll need to find a piece of shafting to replace the idler shaft. Both shafts can run in pillow blocks bolted to a couple pieces of T-slot extrusion slung under the existing plywood grinder box to allow for easy tensioning of the chains; next year we can make a nicer framework to hold everything.

That takes care of the grinder, so it’s on to the press. Haven’t got much feedback on the overall concept, so I’ll start fleshing out the details and see if I can convince myself whether it’s on the right track.

First let’s figure out what diameter the secondary shafts would have to be. Let’s say we want to apply 70 psi. Each shaft supports a region 16″ wide and 4″ wide, so each shaft should be able to carry 4500 lbs. The load condition will be something between a centered point load and a uniform distribution. Taking the worst case of a point load, Machinery’s Handbook tells us that the stress at the center is W*L/4Z, where W is the load, L is the span, and Z is the section modulus. The same volume tells us that the section modulus of a piece of hex bar is 0.104D^3 in the less favorable direction. Setting the stress equal to 20ksi (the value given in McMaster for stainless hex bar) we arrive at a value of 2.1″ across the flats, before any safety factor. It’s hard to believe the shafting is actually that soft, but clearly the design is weak as drawn (1″ round shafts). The quickest way to decrease the load would be to reduce the span. That’s not a bad idea really since the press grates are only 13.5″ wide in the existing press; it would be good to limit the bag width to around 13″ coming out of the press. So let’s take the loaded width as 12″ and the span as 14″ (say the roller assembly was 13″ wide), and say we use 6061aluminum round stock at 35ksi (because it’s going to be crazy heavy if it’s steel or stainless). Then we have a load of 3360 lbs, and a calculated diameter (again no safety factor) of 1.5 inches.

Just for comparison, we can repeat the calculation for a distributed load, and find that the stress decreases by a factor of two, resulting in a 1.2″ diameter shaft. So probably a 1.5″ shaft is reasonable. A trickier problem is designing the system to limit the load to a level that won’t bend the shafts. A carefully designed weak link could be put in the linkages that support the rollers, but it would have to be pretty small (it doesn’t take much metal to hold 1800 lbs) and so the system would have to have additional constraints to prevent off-axis forces on the weak link. We discussed using die springs to provide some compliance; McMaster sells die springs 2″ in diameter and 4″ long that can do 1″ of compression at 1200 lbs ($12 each); that’s a little awkward but possible. Given that the pomace is never more than about an inch thick when we’re done with it in the existing press system, that would probably be enough compliance, though there’s the issue of how to get the bags started through the nip. It would be useful to have some kind of a load measurement on the existing press, to know how much force we’re actually applying.

Anyway, the new press concept is obviously going to require some more cogitating. In the meanwhile it’s tempting to just hook up some kind of hanging weight/leverage contraption, or just use the little bobcat excavator to press on a stack of cheeses. That’s hardly in the spirit of pedal power, though…


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