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.
First question regards spools for open-center systems. Some Army
literature talks about open-center spools versus tandem-center spools.
Assuming a 4way with lines T (tank), P (pump), A and B (ports to
cylinder) my understanding is that in neutral an "open center" spool
connects all ports (A,B and P) to the T (tank), such that there
wouldn't be any pressure to the cylinder (i.e. it is floating in
neutral). A tandem open-center spool (in neutral) connects P to T (so
it is just passing through the spool), and closes off A and B, holding
pressure in the A/B loop.
In the first case, a loader w/ an open-center as described would come
crashing down when you let off the controls, whereas the tandem would
hold it's position. Looking at the catalogs I haven't seen a
reference to a "tandem" spool, but I have seen "open-center with load
checks", and further reading suggests that load checks will hold the
cylinder in position.
So, is what I'm calling a tandem spool the same as an "open-center
with load checks"? Does a load check in a spool close off the lines
Second, if you have a spool that is rated for say 25GPM, will there be
any problem running it at a lower GPM? Say 12GPM or 5? Will it still
How is pressure controled in the system? Is it just a matter of
setting the relief/bypass values, and the lowest one will control the
pressure (although I guess this would depend on how it was plumbed)?
Most seem to say that they function from 1000psi to 3000psi - does
this mean I can make any combination of pressure/flow rate in the
system to achieive my goals or fit my pump/equipment?
What is the difference between a welded versus a tie rod cylinder?
Finally, in parallel spool decks is the flow simply divided between
the open spools, such that everything will work, but at a lower speed?
And pressure in all A/B lines is the same (so all spools get there
input from the P line)? In series is it that the flow out of one
spool feeds into another spool (so input is from the T line), and we
get a decrease in the pressure (with the same flow)?
Thanks in advance for your help. I appreciate any and all comments
- posted 18 years ago