Wood Chippers for Small Tractors

Wood Chippers for Small Tractors
Fall 2005

Opinions/mistakes are mine alone;
your mileage may vary.
© 2005 BEK/Wolfeel/Woolfield

For several years I've rented 6" (or larger) hydraulic feed/trailer mounted wood chippers (like this) to deal with the piles of hawthorn and chokecherry trees/trimmings I seem to accumulate every year. A 6" chipper costs about $200/day to rent in my area. The last chipper I rented had a crapped out hydraulic feed unit (feed quit when the fluid got too hot); the next-to-the last chipper wanted to throw the drive belt if it wasn't engaged ever-so-carefully. And then there is the delay to pick up the chipper (while the counter clerk has his coffee & boots up his computer) and the need to quit a little early to gas it up before returning it to the yard....

Anyway, I've looked for a small wood chipper for my Kubota BX-22 for about a year. The BX-22 is a "sub-compact" tractor. It delivers 16.7 hp at the standard 540 rpm rear PTO, but has a smallish cat. "1" 3 point hitch with somewhat limited 3 point lift capacity. Hydraulic flow at the rear auxiliary of this tractor is too low to power a hydraulic feed chipper.

Small chippers I've investigated come in two basic types: chipper/shredders and straight wood chippers.

Chipper/shredders typically have a rotating drum holding a number of internal swinging hammers or knives. One end of the drum is a built-up flywheel with chipping knives mounted against slots in the flywheel. The drum is usually belted up to run at about 3x the tractor PTO of 540 rpm. See a diagram here.
To use a chipper shredder you drop small stuff (usually up to about 1.5" in diameter) in the top; the swinging knives in the drum grab it and work it over. Stuff bigger than ~1.5" in diameter gets put into a chute on the side of the machine; the chute guides this material against the chipping knives mounted on the flywheel end of the drum.
There is a plate or grate below the drum that sizes what can fall through. Material stays in the drum until it is small enough to fall through the grate and out the bottom. Usually this grate can be changed out to control the size of the chipped/shredded material.
Chipper/shredder Advanages:

Disadvantages:

Chippers usually have a heavy flywheel spun in a housing by the tractor's PTO. There are slots in the flywheel and chipping knives mounted to the face of the flywheel adjacent to the slots (like this). Material is put in a chute that directs it against the knives. Chips cut from the end of the material fly through the flywheel slot and are blown out a chute by vanes attached to the back of the flywheel.
"Classic" PTO chippers are direct-drive from the tractor's PTO: their flywheels run at ~540 rpm. "Classic" PTO chippers need heavy flywheels (100 lbs or more is common) to store energy between knife cuts and so even out the load on the tractor's engine.
In the last few years there have appeared PTO chippers with smaller, lighter flywheels belted up to run at higher rpms like the drums of chipper/shredders. Energy storage in these chippers can be nearly that of classic chippers with heavy flywheels, but the units themselves can be smaller and lighter, and often require less horsepower to run.
To run a PTO chipper you drop small stuff or insert larger stuff into the intake chute. If well designed, PTO chippers will aggressively "self feed" 1" or 2" (or larger) continuous material. Leafy branches will feed until the branch gets too twiggy and leafy; the remainder of the branch just sits in the chute waiting to be pushed in by the next branch.
Most chippers (like chipper/shredders) have reversible/resharpenable chipper knives. Many units chip against a bed knife that can be adjusted to somewhat control chip size.
Chipper Advanages: