[Stoves] [Gasification] Chip Guillotine was Re: Wood heating in the UK - whole log gasification

[Richard Stanley]

Andrew,

You raise good points, as ever !  I have not measured directly or intuitively the difference in energy expenditure with different shear layouts such as you you mentioned but what I do know is that a gradually induced  and consistent loading of the blade through the cut is much easier on the machine, which translates to a lighter frame and weight than a direct full contact shear design..These design concerns relate directly  to cost and transportability which are premium considerations in our arena. 

Richard Stanley
www.legacyfound.org



On Jan 4, 2014, at 1:46 PM, Paul Anderson wrote:

Andrew,

Thank you for your excellent scientifically supported explanation.

I found the comment on the short fibers allowing the slanted blade to cut through (verses 90 deg cut that hits each short fiber) to be especially illuminating.

Also the comment about the "glue" made me think of the difference between cutting green wood verses dry wood.   Dry "glue" lack lubricating function and becomes a hardened obstacle to cut.   BUT dry "glue" allows for pieces to be snapped / broken cleanly whereas the green wood bend and is hard to get a full break. (This might be all wrong, but it just seems logical.   Can anyone confirm or refute the comments?)

And I thank Tom Miles and Tom Reed who many years ago explained to me the "beaver angle" that the beavers use when cutting down trees.   I think it has been measured at 27 degrees (but do not quote me on that.).

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 1/4/2014 5:45 AM, ajheggie at gmail.com wrote:
> [Default] On Thu, 2 Jan 2014 20:31:27 -0500,"Crispin Pemberton-Pigott"
> <crispinpigott at gmail.com> wrote:
> 
>> There is some merit in the slicer when looking to cut woody things. The
>> guillotine is much more difficult to operate, in my experience. If it bends
>> the material away before slicing, things get even better. You can try it
>> with a pen knife: cutting directly at 90 degrees or slicing into wood at 45.
> Sorry for the delay in replying Crispin
> 
> This comment is pertinent: if you have cleft material for things like
> wattle fencing you will have learnt to steer the split by putting
> tension on the side of the piece you want the split to go toward. This
> must be because wood is short fibres glued together with widely
> differing  properties in shear, bending and tension. So in cleaving we
> are mostly shearing the glue between fibres and this does not require
> much force.
> 
> We need to separate the force to comminute the wood from the energy
> necessary. With efficient gearing force is not a problem, though the
> device becomes more costly. For a man powered device we need to
> minimise the energy required to produce a particle of the desired
> size.
> 
> Rolf has commented on a hacker that minimises the number of cuts and
> cuts at an angle, Richard mentions the need for the cutter to maintain
> an optimum angle. Has anyone measured the forces in these differing
> scenarios. Energy is force moved through distance and even though the
> force may be lesser on the angled cut as it is cutting a larger face
> the distance is increased.
> 
> One of the principles of the teardrop chunker was that the cut pieces
> dropped under gravity and the cutting did not impart any kinetic
> energy to the particles, this differs considerably from commercial
> chippers which blow particle a long way (principally to load a truck),
> this pneumatic delivery is energy costly.
> 
> Another thing about commercial chippers to be avoided is making
> unnecessary cuts or producing small particles because each of these
> particles has needed energy to produce.  I noticed early on that the
> chippers that cut at an angle would not feed at all well if slightly
> blunt, all out modern chippers cut at right angles and will continue
> loading the truck even if the blades are dull, albeit the chips then
> become stringy with a lot of fines ( i.e. a lot more energy has been
> expended making them).
> 
> AJH
> 
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