Giant vacuum

[robbie]

I have a giant market garden, about an acre, and I need to add a lot of organic materials to it every year to increase fertility. Rather than buying stuff, I would rather use leaves from my own land, because I have at least 50 acres of oak trees. Most of this wooded area has no grass and little undergrowth, so it's just a giant carpet of leaves this time of year. An oak tree's job is to bring minerals up from the earth for the wimpy plants to use, so I need set up my farm to work with nature's own plan. I just don't feel like raking tons of leaves every fall.

What I want to do is build a machine to collect lots of leaves efficiently. I have a Briggs vertical shaft 8 hp that needs a little bit of work, or I could get a 99 cc or larger engine from Harbor Freight. Attached to the shaft would be a centrifugal impeller fan like the one in this photo:

Around the fan would be a shroud shaped like a large cookie tin. Come to think of it, maybe I could even use a cookie tin for the prototype, just as long as it has the correct aspect ratio, shaped like a tuna can rather than a can of beans..... anyway, if you imagine a little hole in the bottom of the tin, the crankshaft would go through that. There would be a large hole in the center of the lid for the air inlet, and a hole in the side for the air outlet. The final machine would be built with medium grade sheet metal for durability.

Attached to the inlet would be a 4 or 6 inch corrugated plastic drain pipe about 10 or 20 feet long, for the vacuum hose. The business end of the vacuum hose would be mounted to the center hole of a lawnmower deck. The grass outlet chute would be blocked, and the wheels would be raised all the way to lower the deck to within an inch or two of the ground. This would allow the lawnmower deck to act as an efficient vacuum head, like the ones used in swimming pools, only bigger. This unit could be rolled around the forest floor easily, and I'm guessing it would be about the best way to suck up oak leaves. Before you say "riding lawnmower", I'd prefer a push mower because of logs, holes, rocks, etc.

The outlet hole on the fan housing would be attached to a smooth, solid vent pipe, such as what you use on a drier or wood stove. I'd probably opt for the larger size stove vent, maybe 6 inch pipe. This would lead to the top of a trailer to catch the leaves. The engine would of course be mounted on the tongue of the trailer.

The trailer would be built on the axle of an old pop-up camper. I'd build a lightweight frame with a gate on one end, and enclose the top, bottom and sides with hardware cloth or chicken wire. The leaf discharge chute would blow into the top of this, so it could fill the whole thing before getting clogged up. I figure the whole trailer would be about 12 x 6 feet, and maybe 6 feet tall.

I have a couple of questions for you crazy metal fabricating, engine tinkering backyard engineers.

There's an aluminum centrifugal impeller at the scrap yard. I saw it last year but I didn't buy it because I wasn't ready at the time, so I stuck it under an old boat, so I'm pretty sure I could go back over there and get it. It's about 16 inches in diameter, and I think it has 6 or 8 vanes about 3 inches tall, and it has a circle in the middle where the air would enter, with no vanes.

If this were you, would you sharpen the edges of the vanes? Would you expect leaves to pile up on them otherwise? Should I expect this to be a major vane-clogging nightmare if I don't shred the leaves first?

If you can imagine a 6 inch drain hose sucking up leaves, you might get a mental image of a wad of leaves under a mower deck clogging the hose. I did. One possible answer to such a problem would be "more suction". If I want to increase suction, the obvious thing to do is spin the impeller faster, but I suppose there's a limit to that before I'd have to go to a bigger engine. Maybe I should use the 8 hp to start with and build it with such ridiculous overkill that I would never have to worry about it.

Here's another idea. What if I mount the 8 hp on the mower deck, with a separate belt drive going to a standard mower blade. That would shred the leaves, which would then be sucked up by a massive vacuum and sent to a hose attached to the top of the trailer. The only problem with this idea is that it would make the vacuum head heavier.

Wow, sorry for the long post, but do any of you have any ideas? Maybe I've overlooked something.

 

[Doc Sprocket]

I had a 5hp (or was it 6) Crapsman leaf mulcher/bagger that I acquired in a trade, and subsequently traded off. The impeller was probably about 14" diameter, and directly mounted to the crankshaft. The impeller had a "mulching screen" at the back of the housing, before exiting to the bag. The screen consisted of a plate that basically had triangular shaped windows of bout 3 square inches cut out of it. There really wasn't much "plate" left so speak of. I guess the (dry) leaves just blew apart, passing through.

For the size of it, it did pretty well. I don't know how fine your mulch needs to be, maybe you can experiment with screen size.

Obviously, I suggest you fabricate a blower housing that fits the impeller pretty closely to maintain velocity and efficiency. I also suggest overbuilding the housing- If the impeller is of unknown origin and you're going to spin it up to 3600RPM and maybe even subject it to chunks of debris, stones, etc- well- you get the picture.

Have a look here- http://www.diygokarts.com/vb/showthread.php?t=11897

 

[robbie]

The leaves don't have to be ground up at all. I just want to pile them up and let them rot, which they will do without any grinding at all. Do you suppose an air impeller would pick up and throw leaves without chopping them and without clogging?

You make a good point about rocks. Maybe I should build an impeller out of steel just to be on the safe side.

The machine in your photos is a good reference point. How much air movement did it produce? Enough to allow me to go to the end of a 20 foot hose and still get leaves into a trailer? I think air movement is the key to the whole thing. That's another thing I learned from model airplanes. Sometimes a bad needle valve setting makes the difference between a plane flying nicely and barely staying in the air. Just a little more air movement changes everything.

I had a giant Vermeer wood chipper last year. (I put a government surplus 100hp Perkins turbo diesel in it, then traded it off when I decided not to mulch with wood chips.) Its main function was to grind up logs and branches, but it's also interesting to note that it generated a lot of air flow to keep the chute from clogging. It did this with four pieces of angle iron bolted to the chipping wheel to act as centrifugal air vanes. The wheel was about 4 feet in diameter, and the vanes were about 20 inches long and only 1.5 to 2 inches wide. I'm pretty sure the belt reduction ratio on that thing (from vague memory) was about 2 or 2.5 to 1, and we're talking about a diesel engine spinning about 2500 RPM, right? Not a lot of speed, but it made enough wind to keep the chips blowing. That probably has a lot to do with the fact that the wheel was so big...

Anyway, how much air was your chipper/vacuum moving?

 

[Doc Sprocket]

I can't really quantify it into CFM, but it WAS pretty dang healthy. IIRC, the vanes were about 3" tall, and there were six. Wheel diameter was 12-14". With a 20' long flex hose, that's quite a bit of static pressure to overcome- corrugated hose doesn't flow particularly well. You've got to find the right balance between impeller size, RPM, hose diameter, and length. If the hose is too big, velocity will drop. Too small restricts. Too long, and the SP will cause too much of a vacuum loss, too short, and it's impractical. If the wheel or vanes are too large, engine will not be able to produce enough power.

Also- whether it clogs would also have to do with how much material you are trying to pick up at one time- take smaller "bites" to allow airflow to "flush" the material through if need be.

Design notes- looking at the pics I posted, see the front? It's actually got a "vacuum nozzle" at the front, OR you could block that off with a plate and attach a hose to the round port.

 

[fowler]

our ride on mower did a dam good job of picking up the leaves

but it would chop them up of cause

 

[robbie]

The only reason to use corrugated hose is to prevent it from collapsing on the low pressure side. The high pressure side could use just about anything, including canvas.

 

[robbie]

The trailer would have to be pulled by something, probably the tractor. I was just thinking today that the blower could be mounted on the tongue of the trailer and driven by the tractor PTO.

 

[Doc Sprocket]

With standard tractor pto speed of, what, 600RPM- I think you'd have to find a way to gear up for a more suitable wheel speed.

 

[robbie]

Yeah, that's what I was thinking. Nominal tractor PTO is 540 RPM, so it would require a couple of pulleys to gear it up, maybe about 10 to 1.

 

[fowler]

pto speed is usually switchable between 540 and 1000

if it is a real tractor with a pto shafg u can usually take out a little cir clip slide the shaft out and turn it around so the side with 60 splines is faceing out
that changes the pto speed to 1000

 

[robbie]

My tractor is a McCormick International B275, made around 1960, which is before the new PTO options came along. There are tons of tractors available around here that all match the same specs, going all the way back to the 1930s.

 

[fowler]

our 1950s chamberlin has changeable pto?

 

[robbie]

I didn't know they had those options so long ago.

At any rate, it would probably be more practical to design any new piece of equipment to match everything else that's meant for the same speed.