View Full Version : Impeller design for yard mulcher / chipper shredder leaf vac
11-10-2010, 03:03 PM
Does Anyone have any experience calculating air flow CFM, hp requirements for the impeller + mulching loads, strength requirements of the impeller, etc?
large metal impeller:
typical hand held leaf blower plastic impeller:
magnesium impeller for toro blower / vac:
http://cgi.ebay.com/Toro-Vacuum-Magnesium-Impeller-Blower-Fan-100-9076-NEW_W0QQitemZ330433696921QQcategoryZ159930QQcmdZVi ewItemQQ_trksidZp5197.m7QQ_trkparmsZalgo%3DLVI%26i tu%3DUCI%26otn%3D4%26po%3DLVI%26ps%3D63%26clkid%3D 4860892303298981496
As you can see if you follow the links, the first steel impeller looks like something welded up in the garage + some minimal machining. I am considering building an impeller + some sort of scatter shield to deal with leaves and acorns. I have a 1.5 hp electric fan motor from an HVAC system, and a 10 hp gas side shaft motor as my options to power it. I also have two electric hand held blower vacs and a 2 cycle blower only unit, so suggesting I just buy a cheap $70 blower is not the answer.. I want to build something primarily, having it be safe and actually work decently would be great too :)
gives me the force of a spinning disc, so I thought that could be a good starting point.
But what do I do with the results of this? What would be a safe force? Assume mild steel, welded together, is 35 lb unreasonable for the disc listed in the first ebay auction?
If so, it seems like that thing would need to be seriously geared down to avoid exploding...
11-10-2010, 04:20 PM
Just a design note:
the impeller on my 9hp leaf vacuum/chipper has what I would call flails on its tips.
I'll be opening the cover in a few weeks for a modification of my own, and I'll see if I remember to take some pics, but the general idea, is that anything that hits the tips of the blades gets hit by these instead of the blade itself. I'm not sure if this is to aid the mulching effect (for compaction), or to protect the blade from damage (as they would have some give on impact), but it looked like an interesting idea.
11-10-2010, 05:08 PM
yeah I see these inmpellers on ebay for 50-100 also. I was not sure how much they came into play as the inner part of the impeller makes first contact with leaves. I have no plans to chip/mulch branches, so I was hoping to just emulate the hand held combo vacuum / blower designs. I see the "finger / flail" design usually has a replaceable cutting blade as well where the small branch feed chute directs the branches into.
Any input /pictures as to the flails purpose would be great too! What are your planned modifications?
Do you know the rotation speed of your impeller? Is it direct drive or does it use belts to reduce its speed relative to the 9hp motor?
11-10-2010, 05:26 PM
The little gas blower/vacs run at high speeds, so it's possible to have impellers that spin fast with no gear down The first link reminds me more of a lawn mower blade - those have a 'maximum allowable tip speed' - which is determined by blade length and rpm. Not sure if there might be similar regulations for other equipment, or if it's just for mowers because people put their feet under running mowers...
If you only have leaves to shred, an impeller like the first link may work - just be sure the housing around it is close enough to create the suction you need and can handle the impact of any rocks or other hard thigs it may pick up. If your leaves have "other" stuff in them, you may need a bigger 'hammer' to chop it up.
If you want a 'walk behind" vac that goes over the yard, then the 10HP would be a good choice because it can also drive the wheels to make it self-propelled. For a stationary unit that you bring the leaves to, the electric motor would make a unit that is lighter and easier to move, but that motor may not be enough power for debris thicker than dry leaves. Even Harbor Freight lists their electric shredder as 2 1/2HP 15 amps.
I have a 5HP (gas) Craftsman chipper/shredder that barely changes speed on a load of leaves, but does an audible slow down when pine cones go through. This is a "drop in" shredder for stuff under 1/2", and a "sweep in" for leaves - the chute can be laid horizontally. It has a multi-layer cut/shred wheel, consisting of a blade at the start of the wheel with a series of metal flails behind the blade. The blade cuts things to less than 3" long and the flails (4 sets of 4, I think) beat the cut segments into smaller pieces. Dry leaves come out about the size of corn flakes. There's a picture of a similar impeller/flail assembly about 1/3 of the way down the left side of this page:
This chipper has a separate port for branches up to 3", but a previous owner tried to put too much in too fast. The obvious damage was the sheetmetal of the port, which was brazed together. The less obvious damage was to the chipper blades which are on the opposite side of the wheel from the flails. I haven't yet pulled the impeller/flail unit to see whether the blades (1"x4'x1/2" blocks of metal) are damaged or missing - that is on my "round tuit" list, but fall yard cleanup has lots more leaves than branches - and the leaves just keep coming down...
11-10-2010, 05:38 PM
My previous one was a 4.5hp vertical shaft motor, and I believe the impeller was direct drive underneath it. My new one has a 9hp horizontal shaft, and has a rear wheel drive that works like my snowblower, so there is probably a drive belt in there, but I'll have to look.
The nozzle has three holes in its rim that mate with studs on the volute, and you need to tighten three wing nuts to put it on. The problem is that in storage, I need to remove the nozzle, and this gets time consuming, so I got a few cam clamps that I plan to weld onto the volute to lock the nozzle on quickly.
Leaves do indeed get pulled into the middle, but (although I haven't looked to be sure) I believe there is something that stops them from being sent to the bag before they are sufficiently broken down, and the flails beat up this stuff against the outside of the volute (like a garbage disposal).
I had a handheld vacuum that shredded leaves with what looked like blender blades at the center of the impeller. Worked kind of well, although I would expect that the faster moving outside of the impeller is more capable of reducing leaves, but that would also require more torque, which the handheld didn't have to spare.
My buddy's got an old tow behind yard vac for his old Simplicity tractor at the shop. I know I replaced the impeller years ago when it died. I'll try hard to remember the camera and take some picts if I can still get to it in the barn on Friday.
11-11-2010, 12:39 AM
Have a look at the chipper Steve Bedair built a few years back.
He goes into good detail on building his impeller. He simply used angle iron bolted to his flywheel.
11-11-2010, 10:54 AM
forhire -excellent link, thankyou!
21" diameter 150 lb cutting disc spinning at 2000 RPM, with bolted on construction... I dont think I will have much concern then for a much smaller disc at only slightly higher RPM. The calc shows my tension in the original smaller diameter lighter faster impeller would be much much less than he has.
How scared should I be of using a 25lb olympic plate as the base of the impeller then.. It could actually be drilled and tapped for angle iron...
11-11-2010, 11:39 AM
I wouldn't be comfortable with a cast iron flywheel. I would cut that thing out of a steel plate, and do everything I could to get it as balanced as possible.
11-11-2010, 11:43 AM
olympic plates are typically cast iron. I would use a mild steel plate if it were me.
11-11-2010, 11:51 AM
most car flywheels are cast iron and 12-13" + in diameter..
11-11-2010, 11:57 AM
But they dont take impacts... a leaf mulcher should not either.. if I was adding cutting blades and chipping it would be a bad idea I bet.. but for just a wheel with some angle iron bolted to it? Maybe It would be better to use some scrap auto flywheels from the pile.. a 20 lb Subaru plate maybe? comes pre drilled and threaded, pre balanced... I think the auto tranny flex plate would be too weak even for only leaf duty. plus if I recall, it has a lot of holes in it for weight reasons...
11-11-2010, 12:02 PM
Machinery's Handbook has tables and other information about flywheels of various construction; some of it would surely be applicable to your problem. If nothing else, it gives some idea of the amount of engineering that can go into these 'appliances'.
11-11-2010, 12:20 PM
The blades on a vertical engine lawnmower are high quality steel, both sharpened and with curves, to function as fans. Suppose you ganged three or more together, using short spacers between them and perhaps a multi-blade fan on the top to get even better suction, using a vertical shaft as with the mower. The blades would not need to be as close to the ground as on a mower, so bearings could be used both top and bottom and it could use a top pulley driven by a twisted belt from the horizontal engine shaft.
For just leaves and acorns, you don't need much 'flywheel' effect, as a chipper would. The blades (and maybe extra fan) would provide enough mass. The blades wouldn't need to be as sharp as for cutting grass either; dry leaves shatter and green ones are macerated so loose their rigidity, even when hit at high speed by a blunt flail.
Do you put this material in the garbage, or compost it? Just curious.
11-11-2010, 01:38 PM
I have a 1/3rd acre lot within city limits with 10-12 oak trees that are about 2 foot in diameter that drop a enough leaves and acors that I usually spend about 6 weekends each fall raking / blowing / mowing them to the curb or to a large mulch / compost pile in the back yard. I typically tie a knot on the end of a 10x20 tarp, rake leaves onto it, and then drag them to the compost or into the street where the city vacuums them up in two separate collections. The acorns usually get raked up with a large dust pan and thrown in which ever locatiuon is closer. I usually fill a 50 gallon trash can 18 inches or so with acorns and then drag these barrels around as they end up over 100lb with just that little bit of the can filled. My pile in the back is pretty neglected, I add to it, but I don't turn it , water it , monitor nitrogen / ph whatever, so it degrades / collapses very slowly. I figured if I could collect more than the 10% or so of the leaves I currently do into the pile, and be diligent for one season and actively try to compost it all, I should generate a large volume of good soil to use around the yard.
So quick answer, I throw most of them out, but I want to keep it..
11-11-2010, 04:38 PM
There is a book in the "workshop practice series" on designing impellers that is very thorough on the math. You can get it thru Amazon.com. I've bought several of the series used for just $7.00 or so and it should answder all your questions about cpm, sizes etc. Good luck.
11-12-2010, 02:47 AM
Here's the impeller from a drpower. It looks simple enough but a much better design then the one on ebay.
11-12-2010, 08:48 AM
looks straight forward..
another dr power - pretty much same design, just shows the "shark teeth" better ( their branding, not my description)
Looks like the engine is an 8hp B&S and it looks to be direct drive. Picts for you...
11-13-2010, 04:04 AM
Maybe these calculations will help you. Not sure.
Fan Affinity Laws
Affinity laws can be used to calculate volume capacity, head or power consumption - when speed or rpm, and wheel diameters are changed.
11-14-2010, 10:43 AM
Pressure fans are funny things, you can build them and then make them suit the horsepower by restricting the intake/exhaust size.
The first ebay impeller is not a garage job (looks like and old Billy Goat fan), it can move more air than you would care to buy gas for. Also it looks to be no-phase balanced (where a large weight is added and various smaller trim weights are tried and added at 180 degrees to settle it down). I'm not a big fan of this but it works better than nothing.
If you study the link forehire posted you can see how quickly small changes in fan size or speed uses power. So if you build something that you're 5 horse motor can't handle you can restrict the intake until the fan begins to "slip" and this will have the benefit of allowing trash to get more churned up inside.
I have a shredder that has the flails inside and the fan blades are similar in size to the chopper Steve Bedair made (forehire's first post). It blows as much air at just off idle as it does at full speed. The amount of air it can push is restricted by the exhaust tunnel and If I were to enlarge the exhaust the 8 horse motor prolly wouldn't be able to reach full speed.
A true pressure fan works by sizing the clearance from the fan and the housing. In my shredder the clearance to the fan blades is about 3" on the face and more on the diameter so it will move a lot of air but at very low pressure.
11-15-2010, 11:27 AM
Oh, pictures as promised.
The entire impeller, the output screen, with the flails that fit through it, teeth on the impeller housing that mate with the same flails for improved grinding, and my addition of clamps to install and remove the nose faster than using wingnuts.
11-15-2010, 11:54 AM
thanks for the pics and responses guys! This forum is great!
I didn't realize how the "flails" worked until those pics!
12-08-2010, 09:06 AM
what ar eyou doing with that White 950????
I just bought one 2 weeks ago to make a franken-vac (1/3 white, 1/3 Exmark and 1/3 unknown)
what are the clamps on the blower housing used for???
do you have your own thread with your project???
12-08-2010, 09:23 AM
Just vacuuming leaves. Nothing special. No thread, you already saw it all.
I had a 4hp craftsman vacuum for my house before, but it clogged too easily, and the bag was too small and hard to empty. This machine fills the bag in a little less time, but one full collection bag fills a lawn & leaf bag now, instead of two, the compaction is better, its easier to unclog the mouth (because it is right in front, instead of underneath the machine), and the impeller just won't clog (because with that much power, if it fits in the mouth, it IS going through).
I've seen the exact same machine painted red, with the Troy-Bilt name on it on CraigsList too.
The clamps, are so I can get the nozzle on and off faster. I haven't had to clean the impeller (plastic bags are no match with this one, and I haven't picked up a rope yet), but because of lack of space in my shop, I need to take the nozzle off each time I park it (or else the door won't open, and I won't be able to get to my welder, etc.). The factory wingnuts are plastic and won't spin on (so they don't spin off from vibration), and just took too much time to turn and turn and turn. I left the studs for alignment, put the nozzle on them, and just clamp. I didn't even have to modify the safety kill switch (which stops the machine without the nozzle).
12-08-2010, 09:38 AM
Yea, I'm in the same boat with about 1 acre of leaves. .aboutt 3 days over 3 weekend to get the yard looking good. I think i've tried about everything out there to pick up the leaves....I came across a couple of site with DIY lawn Vac and I've been hooked.. this is my project for this year.. I picked up and old exmark 2-bin hopper for $20.00 and the white 950 for 150.... i plan to fab the hose connector and plates..
what size hoses are you using. I'm thinking of a 7in for the intake and 8in for the dischange. (note I have to fab a rectangle-to-round connector to make it work..) Since this thing is 9hp is should have pleanty of Power to suck leaves regardless of the pipe size.. I've seen 5in plate and hose kits for sale but nothing bigger. I'm afraid that 5in will be too small for the takes at hand....
12-08-2010, 11:22 AM
I'm just using the supplied nozzle, but a hose or modified nozzle has been something I'm thinking about.
I would think that the intake needs to be larger than the discharge hose. Once everything is shredded, I can't see the discharge clogging if the hose is relatively short. Also, if you increase the discharge hose diameter, it will slow the airflow, and I would think that the velocity itself prevents clogging (but I'm no expert on this), so I'd think 5" would actually work fine on that side.
I would size the intake hose to be just under the size of the opening in the front.
Yes, 5" is too small for this machine. I see larger hoses out there, but the prices are pretty steep (think $20+ a foot for clear urethane 8" for JUST the hose).
Honestly, I'm not so sure a hose will help me much. My biggest problem this year, was that the nozzle would clog with clumps of wet leaves. I had someone help by advancing the machine, and I used a small rake to agitate the leaves as they went in, but I was thinking that a straighter nozzle with a more oval opening would be able to attack a pile better. I just don't want to compromise the safety of the machine.
Hummm, I can just imagine the first stage auger of a snow blower feeding into this. :)
12-08-2010, 12:38 PM
How big is the hose on the 950??I guess is the question I have???? I did not get one with mine otherwise I would use it..
I read somewhere that the discharge has to be bigger than the intake because a lot of air is created and it has to go somewhere. Thus if the discharge is too small then it will start pushing air out the intake-negating the vacuum effect. The old Exmark hopper has a 6in discharge, but it’s powered by a 6hp engine...the 950 , or course, has a 9.5hp motor. I guess I could the area of the rectangular discharge and convert that on the round tube diameter as a starting point.
The cyclone rake site like to touts big tubes… here’s that they say;
5.5hp = 7 in tube
6.5hp = 8 in tube
9.0hp = 8 in tube
10hp = 10 in tube
It’s hard to judge the size of the discharge tube
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