Conical Screw Chipper - Theory of operation

There's quite a bit of mention here, and elsewhere, of conical screw chippers. Ben posted a photo of his prototype, but I'm having some trouble sorting out how a conical screw chipper actually makes chips. Can anybody chime-in to help me past this roadblock?

Thanks!

Tags: chipper, conical, screw

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Permalink Reply by ben on October 27, 2009 at 7:36am: The screw chipper pulls the fuel in and slices it into chips, digging deeper and deeper the farther it is pulled in. This is made possible by the cone shape. The length of the chip is determined by the distance between the discs.; ▶ Reply to This

Permalink Reply by Tony A on October 27, 2009 at 8:05am: So looking at this photo...

the log is shoved onto the screw after which it is sucked-in and reduced to chips about as thick as the spacing between screw wraps and up to a length <= to the radius at the widest part of the screw? Sound right?; ▶ Reply to This

Permalink Reply by Radam on October 27, 2009 at 11:52pm: I would reckon that besides one dimension which is set by screw spacing, the other two are set by type of wood. Smaller of the two probably by shear strenght and larger one by diameter of the log.; ▶ Reply to This

Permalink Reply by Pierman on October 28, 2009 at 8:19pm: Anyone know what RPM range the shaft needs to turn at?; ▶ Reply to This

Permalink Reply by Radam on October 29, 2009 at 7:52am: http://en.laimet.kummeli.fi/index.html?n=4468

Their biggest turns at about 100rpm, smallest has 200 to 1000 range. Although I am not sure what is the reason behind it.; ▶ Reply to This

Permalink Reply by Pierman on October 29, 2009 at 8:01pm: What diameter screw would you consider large and small? I want to fab up one, maybe starting with a 8" helical disk and progressing to 18" in 2" increments with a 3" pitch or space between the flights. Probably would cut it out of some 1/2" - A572 plate. If may need to fabed out of Abrasion Resistan plate.

I would like to use a 2.875" sch 80 pipe for the shaft. If the rpm range is 100 or so, precision balancing should not be needed. Much more than that, I would believe that it would need to be balanced. I saw the pictures that are in Ben's Link page, but does any one have any idea(or picture) of inside the feed end of these units. The shape and how it relates to the conical shaped blade.

Thanks for you help.; ▶ Reply to This

Permalink Reply by Radam on October 30, 2009 at 12:06pm: This is my idea of how it works. As the log gets between guide and blade, the latter grabs it. As the blade is rotating, it appears to be moving to the right/down. Pitch of the screw determines how fast it goes to the right, and cone angle determines the speed of cutting down trough the log. Depending on how sharp is the blade, it may take longer before the chunk breaks away from the log, thus thicker chip. But the log may not always paralell to the axle, the guide at least helps hold it at one angle. At shallower angles the log is moved more forward which results in a bit longer chips, not really sure about this one.

The blades Laimet is producing seem to be around 30 degree cone (actually it even seems to be the same angle on their logo), are more bent to the right, and tip is even more forward.; ▶ Reply to This

Permalink Reply by john wayne blount on October 31, 2009 at 4:49am: I think it was the fluidyne "Andes" unit that showed the differences in chips using the same chipper and different woods. Very big difference in chip sizes. Each wood splits differently, more or less.

Radam said:
I would reckon that besides one dimension which is set by screw spacing, the other two are set by type of wood. Smaller of the two probably by shear strenght and larger one by diameter of the log.; ▶ Reply to This