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'[EE] Lawn mower rock sensor'
2005\08\15@104605 by olin piclist

face picon face
First, I fixed the subject line.  This is a technical topic that definitely
fits in "Everything Engineering".  Multiple topic tags only gets it posted
in both topics, which is not appropriate here.  I also added a meaningful
title.  These are all things you should have done in the first place.

KY1K wrote:
> I'd like to put some sort of sensor in that would disengage the
> blades automatically when the blades hit something solid and I would
> like some suggestions from the group.

Isn't it too late by the time the blade has already hit something?  Even if
you could disengage instantly after the first hit, wouldn't the inertia of
the blade keep it rotating anyway, assuming the rock didn't stop the blade
cold?  If it did, then disengaging the blade from the engine isn't going to
protect the blade, but maybe the engine or at least keep it from stalling.


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2005\08\15@110756 by John Ferrell

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Bush Hogs (big mowing deck for rough use behind a tractor) use swing blades.
They also have a cone/dome covering the blade hub they call a "stump
jumper". The names say it all!

John Ferrell
http://DixieNC.US

{Original Message removed}

2005\08\15@145338 by Denny Esterline

picon face
> Needless to say, the blades get dull fast and rarely last more then 2
> years and have to be re-sharpened every month or so.

Count yourself lucky, I know a lady that's gone through three sets of blades
this year. (and a new quill :-(

> I'd like to put some sort of sensor in that would disengage the
> blades automatically when the blades hit something solid and I would
> like some suggestions from the group.

My first thought for an engineering solution would be a shear pin. Find a
way to put something soft, cheap, and easily repairable between the motor
and the blades. (think of lead for the key on the blade shaft) That way it
limits the damage to the rest of the parts. Or maybe you could make the
blade attachment squeeze between a couple of washers, sort of a torque
limited clutch.

> Any ideas? It has to be cheap and simple. I can handle the
> interfacing to the solenoid.but need suggestions on what type of sensor to
use.

I don't really think a technological solution is the best idea here. You
could try the solution I'm using for the lady I mentioned above, charge a
*lot* every time I have to fix it. I'm hoping it will help her to remember
to quit running over large hard objects. Hasn't worked yet, but we'll see...

Good luck,
-Denny

2005\08\15@145912 by Michael Rigby-Jones

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{Quote hidden}

I worked at an agricultural engineering company many years go. Lots of tractor driven farm machinery uses (or at least "used", not eactly up to date on modern stuff) shear bolts for protecting the transmission.  However, they are frustrating and time consuming to replace, I've seen numerous examples of farmers replacing the special shear bolt with a high tensile one...

Regards

Mike

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2005\08\15@150944 by Denny Esterline

picon face
> and time consuming to replace, I've seen numerous examples of farmers
> replacing the special shear bolt with a high tensile one...
>

Any safety system is only as good as the users that are going to bypass
it... :-)

-Denny

2005\08\15@165523 by James Newtons Massmind

face picon face
I must just be missing something here, but wouldn't the best thing be to not
drive over the rock in the first place?

How about a guard on the front of the unit that stops the blade if it runs
into something solid? It could be one unit or multiple smaller units so that
if any one were pushed up your existing blade clutch gets tripped.

It can be pretty heavy and spring loaded so that nothing short of a rock is
going to move it. If it starts to push things along in front like a dozer
blade, then mount it at an angle so the crap gets pushed off to the side, or
make it a v like a train "cow catcher" to deflect junk to either side. Even
if the rock gets moved out of the ground by the guard, it will then be
setting on the mowed grass and be quite obvious to see and remove.

After it trips, the blade stays cut out for some number of seconds then
automatically re-engages. The result is a number of little unmowed spots
which indicate where the rocks are. You go along after and remove the rocks
and then mow over those spots.

---
James.



2005\08\15@170449 by Bob Blick

face picon face
> While not directly applicable, search out that little video of the
> capacitive sensor for table saws. It can detect a finger, stop the
> blade rotation and withdraw the blade all before the finger gets much
> more than a semi-deep cut. I'm on the road at the moment (well, in an
> airport) so I don't have the link. I'm sure someone does though!

Problem is, it only works once, then you need to replace the blade and
brake set. Most lawns have more rocks than people have fingers :)

Cheerful regards,

Bob


2005\08\15@185726 by Jinx

face picon face
> Isn't it too late by the time the blade has already hit something ?
> Even if you could disengage instantly after the first hit, wouldn't
> the inertia of the blade keep it rotating anyway, assuming the
> rock didn't stop the blade cold?  If it did, then disengaging the
> blade from the engine isn't going to protect the blade, but maybe
> the engine or at least keep it from stalling.

I'd agree that by the time "a sensor" has reacted, it's too late. A
couple of experiences -

I once hit the edge of the concrete path with a small domestic
mower. The blade stopped dead, the crank broke, the mower
got thrown away. Had no chance

The land between my house and the railway was developed a
few years ago. After the big pine trees had been carted away
(boo) they brought in a huge grubber. This was an attachment
on a back-hoe arm for grinding out the stumps. Very heavy,
very fast, very loud (boo). Did a great job of the pine stumps,
but did not so good when it hit a short piece of buried railway
track (which BTW is now my #1 lump of metal to bash things
on).  The horrendous graunching sound of tearing metal was
probably heard miles away. We never saw that grubber again

2005\08\16@040524 by Ling SM

picon face

> My first thought for an engineering solution would be a shear pin. Find a
> way to put something soft, cheap, and easily repairable between the motor
> and the blades. (think of lead for the key on the blade shaft) That way it
> limits the damage to the rest of the parts. Or maybe you could make the
> blade attachment squeeze between a couple of washers, sort of a torque
> limited clutch.

Not on rock sensor, but on the blade.  I am not sure is this practice
widely in the state.

Here, there used to broken blades flying out and cut passer by.  Now the
grass cutter uses a cable-tie type of "string", that replaces the blade.
 One end with knot is inserted through a hole in the rotor.  The knot
keep the "string" in place when cutting, countering the centrifugal
force.  If it does break off, due to the small mass it cannot fly far.
Needless to say, it is cheap.

Ling SM

2005\08\16@061633 by Alan B. Pearce

face picon face
>My first thought for an engineering solution would be a shear pin.

This is usually the way to do it.

>Find a way to put something soft, cheap, and easily repairable
>between the motor and the blades. (think of lead for the key
>on the blade shaft) That way it limits the damage to the rest
>of the parts.

The Flymo electric mower I have use the plastic spacers that adjust the
height of the blade as the shear pin item. Each spacer has a pair of "pins"
about a 1/4" diameter on each side, and a matching pair of holes for the
"pins" to sit in when two are mated up.

>Or maybe you could make the blade attachment squeeze between a
>couple of washers, sort of a torque limited clutch.

This is exactly what happens with the plastic spacers mentioned above when
the pins shear. The rough surface where they shear from has enough friction
against the other plastic face to allow you to keep mowing. Replacement
spacers are readily available from the local DIY shop.

2005\08\16@062213 by Alan B. Pearce

face picon face
>The land between my house and the railway was developed a
>few years ago. After the big pine trees had been carted away
>(boo) they brought in a huge grubber. This was an attachment

This wasn't along Pooks road in Swanson? Drove along where we used to live
when last back home, and it is now all filled in between the road and
railway.

2005\08\16@063353 by Jinx

face picon face

> >The land between my house and the railway was developed a
> >few years ago. After the big pine trees had been carted away
> >(boo) they brought in a huge grubber. This was an attachment
>
> This wasn't along Pooks road in Swanson? Drove along where
> we used to live when last back home, and it is now all filled in
> between the road and railway

No, near Fruitvale Station in New Lynn. Not that it makes a
difference - seems every last little space in Auckland has ended
up with a few "boxes" on it. Sub-dividing is rampant. Not too
hard to find stories in The Herald about future ghettos/slum areas

One question for the OP, just to keep it on topic !!

Isn't some kind of ground inspection in order before mowing ?

2005\08\16@161423 by Peter

picon face

On Mon, 15 Aug 2005, James Newtons Massmind wrote:

> I must just be missing something here, but wouldn't the best thing be to not
> drive over the rock in the first place?
>
> How about a guard on the front of the unit that stops the blade if it runs
> into something solid? It could be one unit or multiple smaller units so that
> if any one were pushed up your existing blade clutch gets tripped.

There is a much easier way, it is used in agricultural machines. A rake
or rigid wires forms an apron at the front of the machine, practically
ploughing the ground. Each rake finger is springed (strongly). When a
few of those move (it takes a couple of kgf to move each) then the whole
working head lifts (hydraulics) and full wheel brake is applied. Big
harvesters use such mechanisms afaik. I don't know what it costs to
replace a lawn mower blade but think about what it costs to fix a
harvester. There must be some well-thought systems already out there.

Peter

2005\08\16@182804 by James Newtons Massmind

face picon face
> > How about a guard on the front of the unit that stops the
> blade if it
> > runs into something solid? It could be one unit or multiple smaller
> > units so that if any one were pushed up your existing blade
> clutch gets tripped.
>
> There is a much easier way, it is used in agricultural
> machines. A rake or rigid wires forms an apron at the front
> of the machine, practically ploughing the ground. Each rake
> finger is springed (strongly). When a few of those move (it

That is what I was trying to get to....


2005\08\16@183443 by Paul James E.

picon face

How about mounting "Ground Penetrating Radar" unit on the front of the
mower, with a video monitor mounted to the handle so as to be visible by
the operator.   This way all objects can be seen and avoided and the blade
would get hit at all.

:')        
                                           Regards,

                                             Jim



{Quote hidden}

> --

2005\08\16@185715 by marcel

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face
And you could mount some Sidewinder missiles on the lawn mower to destroy any
interfering objects... Or high-powered lasers! Or some sort of high-energy ray
thing.
Or, y'know. A cowcatcher or mechanical detector, as has been suggested.
- Marcel


"Paul James E." <jamespspamKILLspamintertex.net> wrote:

{Quote hidden}

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