Toolbert wrote:
"don schad" wrote in message
om...

Hi all.

I am scheming to build a front-end loader for my tractor, and I have
been looking into hydraulics. I have a couple of pretty basic
questions which I have generated after reading various sources and
looking through the catalogs. Any help/insight into any of these
questions would be much appreciated.


Your best bet is the simplest type of open-center system. This has a pump
that pumps a constant volume and in the "idle" condition, with all the
spools at center position, there is free flow through the system and the
only power consumed (and heat generated) is from friction loss in the pump
and plumbing. Almost any other type of system is far more expensive and
complicated.

Open-center valves are so common that the term has come to mean what your
old text calls "tandem". In the center position there is free flow from P
to T, and A and B are shut off and blocked, so that whatever cylinder is
connected to A and B is locked in position.

There is typically only a single pressure relief in the system, built in to
the valve body, set at whatever the safe limit is for the pump, hoses, valve
body, cylinders etc. This could be anywhere from 1500 to 3000 psi for a
minimum-cost system. Won't find it in minimum-cost equipment.

If your cylinder bores and mechanical linkages are sized properly, there is
no need for separate pressure relief settings for each cylinder. Pressure
and flow limiting comes into play only in equipment where the operator can
plausibly break something, and for some reason it's easier to protect the
equipment by pressure limiting than by sizing the cylinder.

The limitation of open-center is that to get predictable response at the
cylinders, you can operate only one spool at a time. For example raise the
boom, tilt the bucket, raise the boom, tilt the bucket.

In an open-center system, with more than one spool open there is nothing to
distribute or balance the pressure between the active spools and their
cylinders, so all the flow will go to whichever cylinder is presenting the
least resistance to work. In an extreme case, the fluid might even flow
backwards through a spool. For example raising a boom (lots of effort)
while lowering a bucket (low effort) may result in the boom at first falling
instead of rising, until the bucket has tilted to its extreme.

But the cost advantage is so strong that you just deal with it by operating
only one at a time.
with all your spools

Closed-center can mean a number of different types of systems. It can be
the same constant-volume pump that in the "idle" condition is pumping its
full output against a pressure relief valve set at a full system pressure of
say 2500 psi. The advantage of this design is to the extent that the pump
can supply the required volume, multiple spools can be opened with
predictable flow in response to each's position independent of the load on
the cylinders. Burst-flow at full pressure can be had by adding an
accumulator, that is a bladder pressure tank. This is probably the
second-cheapest system design and is used for equipment that requires
precise operation of multiple spools at the same time while still keeping
the cost reasonable. Its behavior has more in common with compressed air
systems. The main disadvantage of this type is that the system is running
continuously at full power and the entire output of the engine and pump are
going into heat that must be blown off in a radiator.

More sophisticated closed-center systems use variable-volume pumps that
maintain constant pressure at all the spools while wasting far less power.
There are also more sophisticated multiple-spool valves that behave like
open-center valves when all the spools are at center.

A "load check" is a one-way flow valve that prevents backwards flow through
a spool when multiple spools are open in an open-center system. It has
nothing to do with pressure relief or with whether the valve blocks flow or
"floats" when at center.

The only disadvantage of using a spool that is rated for a much higher flow
than your application is that it may provide less of a "fine touch" control
than a smaller valve. It should not be a problem for what you describe.

"Normal" multiple-spool valves are parallel flow which has the issue
described above, with all flow going the path of least resistance when
multiple spools are active.

Bob


If I was doing this I would not use oversize valves or pump . With small
bore rams it will go up and down too damned fast . You will never be
able to feather the loader like you will need at times . That plus
smaller components will be a lot cheaper to buy . Ask any industrial
supplier and they will be able to help you size the components to where
they will work with what is already proven acceptable .
Ken Cutt