On Fri, 30 Jan 2004 11:16:56 -0500, "don schad"
vaguely proposed a theory
.......and in reply I say!:
OK. You think even the theory's simple. I am a RAG (Rough as GUTS)
dirt breaker "engineer" who uses old "big boys toys" to try to make my
property behave itself, but hydraulics fascinates me, and I needed to
look at a few things, because I wanted to make up strong things to fit
on buckets etc.
I am reasonably bright, even though I often hide it well. But I am not
qualified and I am ony fiddling around the fringes.
So even at my level I am constantly coming up against inertia,
pressure, area, physics, geometry, just in design. Then when I started
welding the steels used in these machines, along came chemistry,
metalurgy.....and in some cases quite a bit of bloody inertia, when
trying to move the bits about.G
Then there is bearing mechanics, lubrication, wear steels etc etc.
You don't _need_ to be qualified. But remember that many years' worth
of though has gone into machines that continually evolve as new
mistakes are made.
Hi all,
Thanks for all the help/suggestions. It's very much appreciated.
I'll take the liberty to ask another question while we are he
There has been a lot of talk about having things sized properly such that
the system is controllable. Is proper sizing just a question of making some
assumptions and doing the math?
Yes. Provided you make the right assumptions. From what you say you
are not even looking at tractors with loaders on them.
Assumption #1. You do NOT knwo what you are doing
Assumption #2. Others do things for a reason.
For example, lets say that I wanted the loader to go from the ground to the
full-up position in 3 seconds (is this too fast? too slow?).
WooHoooo! Too fast. Just sit there and imagine you are in your tractor
(or sit in the tractor if it's available) and imagine a tonne or so of
stuff raising to right above your head from the ground in 3 seconds!
Most tractor-based loaders lift very high towards over your head to
get the height for trucks etc.
Speaking of which, you would not want to go _too_ far. The rams should
not be too long or too short.
Also, imagine the force on the back of the tractor. You would probably
bite the dirt right under the bucket.
8-15 seconds is more common, with lowering a lot faster. For a farm
tractor, head toward the 15 seconds IMO.
You should need to keep the revs up to get a fast lift, so that you
have a slow lift when needed. To give you an idea of how important
this is, a commercial loader often has a brake pedal that also
disengages the auto transmission, so you don't need to be in neutral
when raising the bucket.
Assuming:
Two hydraulic cylinders with a bore of 2", and a stroke of 14" to go from
down to full up.
3 seconds to extend ram 14"
To compute flow:
Speed (in/min) = flow (in3/min) / area (in2)
flow = speed (in/min) * area(in2) = 14"/0.05min * 3.141in2 = 880in3/min *
1g/260in3 = 3.4gpm
and since we have two cylinders, we need 6.8gpm?
Nope. To go from full down to full up will take more like 2 x 36"
cylinders, maybe 2.5" working diam. A lot of this is neeed not for
lift but for strength.
This estimate seems to be reasonable based on what I have seen for other
loaders regarding pump size. Is this (basically) all there is to it? And
from there should I pick the lowest pressure which will (a) operate all the
parts and (b) give me the force which I need to do what I want? So if I
want to have the loader capacity be 1000#, I would have two cylinders which
could produce (minimally, since I guess there is geometry to consider and
the loss of usable lifting force - haven't gotten there yet 
) 500# each
at a given PSI (seems like 1500 is the lowest common on, and this will
produce a force 500#)?
Easy...so what am I missing?
shudder....wrong attitude.
In answer to your question....what you are missing is that geometry
that you "have not got to yet".
This is off the top of my head. I may get a few details wrong. but I
think I am close. For 10 to one maybe say 8:1 etc. But the errors will
cut both ways, I am sure. Prove me working and you are on the way to
designing G
Your 14" cylinders above will have to move the end of the bucket maybe
12' full down to full up. So to lift 1000lbs _at the best angle_,
firstly we need 144"/14" = 10,000 lb. Then the cylinders are often at
an angle (to the _direction of effort required_) of maybe 45 deg.
Divide by the sin of 45 deg. 14500 lbs. If the angle changes to 30 deg
than divide by the sin of 30 deg. I admit that I did quick diagram
that showed 14" rams at 90 deg. So we could arrange that to go. But as
soon as the arms begin to lift, the angle changes. best check at many
locations.
Remember the weight of the loader arms and bucket. Probably a good
100lbs in themselves, spread in quite a complex manner. So let's say
the arms are weight spread evenly. You immediately add another 1000lb
with COG at 4' from the fulcrum. In other words you have to multiply
all your forces by 1.5.....say 21,000 lbs. The beam strength at the
rams attachment point would need to be impressive with such short
rams, as they need to be attached right near the fulcrum. The fulcrum
pins would be impressive as well.
etc
It can be quite complex, and is very dynamic as the loader cycles
through its actions.
There is no way that two 500lb pushes will lift a 1000lb bucket in a
loader. Not even near the mark. What will probably happen is that with
the pump you are using in your calcs, you will need 3 times the ram
area - volume and get the 10 seconds' lift I want you to get above
G.
For instance I have a loader that I did some calcs for. The crowd
cylinders are able to do 21 tonne. The bucket tip actually comes to
about 7 tonne....at the best angle. This constantly changes as the
bucket moves. The Lift rams are 32 tonne. The lift theoretically is
about 6.25 tonne, again at the max situation, which continually
changes....and yes I _can_ lift the back of the bloody loader off the
ground!
Immediately the system changes from that best angle, it all goes
haywire, in both directions. You don't want to get the load halfway
up, and then find that you simply cannot lift any further.
You will also be surprised at eh length of ram needed to do a job,
even with all the mech disadvantage that hydraulics allows.
In all of this, remember that most tractors are not designed for
really heavy loads at the front. When you hang the bucket out there,
allow for the moment of arm of the weight, especially when driving
around and going over bumps. loaders wreck tractor front ends.
Are you going to make the bucket self levelling? That's law here,
because a few guys have dropped things on their heads as they raise
the load.
etc.
See if there are any plans on the net, or buy secondhand. Believe me,
making one from scratch, the excitement could be only _starting_ when
you have finished the loader.
While you may well be able to work out all the answers, the hardest
part is knowing all the questions.
And I am not even very good with hydraulic circuits in practice,
although the _theory_ is not that difficult.
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