"don schad" wrote in message
...
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?
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?).
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?

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?

As always, thanks a lot for your help.

don

Probably most useful to work backwards from what you want to accomplish and
are willing to spend. Look at the tractor, figure out how much weight can
be cantilevered how far in front of the front wheels, and what size pump
(pressure x volume) the engine can drive. Then work out the geometry of
the linkages and cylinders to figure out what straight force is required at
each cylinder or pair of cylinders to accomplish the lift (force, not
speed). Then compute the cylinder bore to provide that force. This will
tell you the lifting speed.

If you want a different lift speed, resize the pump (and maybe the engine)
or lower the lifting load capacity.

It's probably best to go with a 2500 psi design pressure, going lower is a
waste of $ as the common pumps, valves, hoses are intended for that working
pressure.

I've never designed and actually built anything hydraulic, just lots of
pencil exercise. So this is pretty rough. A basic valve will give you
some fine control. I think you're better off to get finer control by
throttling down the engine than by going with a fancier design.

As for your geometry and mechanical design, that's more on topic I suppose,
but it's hard to quantify. A well-engineered piece of professional
equipment will be much lighter for the same strength than a quick-and-dirty
homebrew. You can use handbook references to size the main members, pins,
bolts and such but figuring out all the stresses in 3-D is much more of a
challenge. Stresses will concentrate at joints, that calls for gussets, and
you'll want to consider side forces and twisting forces from off-center
loads or digging with the corner of a bucket. Common sense and intuition
goes a long way. I've been impressed with some homebrew loader designs but
they tend to be overbuilt and work better with a large, heavy tractor. If
I was going this route with a small tractor I would literally prototype it,
cut and patch and try different geometry until it felt right and then
finish, clean and paint it.

Bob