Have you guys got any suggestions on how best to motorise this soil
sifter? Picture at
I left the large flat area (size just under 50x40 cm) so that a drive
could be added but I am not sure what motor and mechanism would be best.
On 05/04/2017 19:48, email@example.com wrote:
You mean to get it to jiggle up and down? Unexpected idea! I had in mind
getting it to shift sideways - hence the runners. And the grille's bars
are "rough" in that direction. The idea is to use them to help break up
clods of soil.
Incidentally, the sieve is sized to allow other, normal garden sieves
with finer meshes to be inserted, so it doesn't need to use the rough
mesh in the photo!
You possibly need it to stop and change direction quite rapidly. Using
smaller sieves will allow them to hit the sides on the direction change
and shock out the earth that sticks in the holes. You need to mimic the
operation as if you were doing it by hand where you will find it doesn't
work on anything but 100% dry dust if you are gentle.
Last weekend I sifted the contents of a full compost bin by hand.
Agreed. I could use a pushrod to drive the sled back and forth. In the
one you saw was the electric drill providing /direct/ drive to the pushrod?
Based on manual sieving the time to send the sled a full cycle (out and
back) of roughly 1 second would be about right. So perhaps some gearing
to reduce the motor speed (and increase its power) might be better. Not
sure how best to do that, though.
ISTR it had about a 15cm disk mounted directly in the chuck with a
conrod bolted about 25mm in from the edge. That would give about 25cm
throw. With the drill running a screw-driving speed it wasn't that fast.
With clay soil you need an initial stage to squash all the lumps and break
them into a size that will fit through a suitable size mesh, otherwise
you'll throw away 90% of the soil along with the stones that you want to
If you are going to wash the soil to leave the stones behind, you need a
*lot* of water and a *lot* of space where the water-and-soil can drain away
and dry, and then you need to break up the "mud flat" once it's dried.
I have "fond" memories of digging my girlfriend's garden. The soil was nice
and sandy, but there were lots and lots of layers of sandstone which had to
be removed. Big ones were easy, once you'd prised them out of the ground,
but there were lots of small 2cm x 2 cm stones. And my girlfriend though it
was "stupid", "laughable" etc to use any form of sifting (eg through a piece
of chicken wire in a frame), so I had to pick each and every one out by hand
The soil here is also clay-based. And I know it's hard to work with.
When wet it clumps. When dry it can set hard. I have even had to take a
pickaxe to some of it where a fork could not be driven in!
However, I can be sure that there won't be a problem because the soil
can be sieved manually.
I might add diagonal bracing to the sifter to make it stronger but
thought I would see how it performed with motor drive first and only add
bracing as needed.
On that I might be tempted to make a steel 'A' shaped horizontal frame
that goes either side of the sled then joins in the middle on the flat
deck, ideally with a guide going though a bearing at that point. Then
it would be quite easy (if there was enough room for the required
travel) to couple a suitable power / rpm motor with a offset pin and a
con-rod to the end of the A frame.
I'm not sure it would need full travel of the track and finding the
sweet spot of travel and speed might be key (and could vary depending
on what was going on).
Cheers, T i m
Yes, the resonant frequency (sweet spot) will almost certainly vary
according to the mass of soil in the sieve, so you want the motor to have a
variable speed. I'd opt for a mechanism with a variable motor speed, and
start with a mechanism that has a very small amount of travel in case the
travel increases a lot at resonance: make the mechanism very strong to
withstand the forces of a few kg of soil vibrating backwards and forwards.
I wonder if it could be tested (at least) using an old / cheap-new
reciprocating saw? Whilst you may not be able to change the stroke
length (it would be far less than the 200mm or so it looks to be
available on the frame) it should be reasonably appropriate and they
are often variable speed and assuming the blade travels linearly,
could be directly coupled to the sled (via a suitable A frame /
reinforced bar etc).
Once this sifter is mechanised I wonder if it would need some diagonal
/ longitudinal support between at least two legs at one end and the
main 'chassis' to stop it 'racking' from inertia (something it
wouldn't do if manually actuated of course)?
Cheers, T i m
I bet if it was heavily laden with soil and you started moving the sieve by
hand at its resonant frequency it would rock pretty violently. Maybe
splayed-out (rather than vertical) legs and a heavy weight low down on the
chassis would be necessary to reduce this.
Outside of the dynamic load affecting how the frame sat, the only way
that would happen was if the running gear wasn't very good or you hit
the end stops? Ignoring such issues, all the inertial forces should
(would?) be produced and absorbed by the person shaking the sled
(Newton's third law of motion).
Increasing the directly coupled mass of everything except the sled
would help that's for sure (Newton's third law again), as would doing
the same thing using braces and even (ideally, low electricity) guys.
Cheers, T i m
The surprising thing there is that the drill is running slowly enough
that the drive is coupled directly. I would have thought that some
gearing would be more appropriate.
Say a reasonable rate for the sled would be 1 or 2 cycles per second.
That would need the wheel to turn at between 60 and 120 RPM. A drill
would, I guess, probably be happier at four or five times that so would
need to be geared down.
Good guess at the size. There's a max of 170mm of movement.
The oscillation rate of a reciprocating saw is higher than I had in
mind. That said, do you think such a sifter would be better with a short
travel and high oscillation rate?
Probably. I thought I would try it out before deciding whether to add
diagonal bracing. Such bracing would be easy to put on later.
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