Here is a description of how the "fast dump" feature works. It is really a regen circuit in dump spool of the loader valve. This disertation was taken from a post a few years back by Kubmech who was responding to the same question in one of the Kubota forums. The theory and implementation is the same for all makes of loader valves.
<font color="blue"> The 3 position valve and the 4 position valve, both have what is known as a regenerative circuit built into the bucket dump circuit.
First let me explain what "regen" is and why.
When dumping a loaded bucket (with a good old fashioned loader control valve) the weight of the material in the bucket tends to pull the cylinders down faster than the hyd. fluid can fill the other side of the bucket cylinders. Therefore, an air pocket will develop in the cylinder causing the bucket to be a little "floppy" unless held against relief for a few seconds at the end of the dump cycle. Which creates an extra step if you want to back-drag, etc.
To solve this problem we now add a regenerative (regen) circuit into the dump function of the loader control valve.
What is "regen"? The dump function of the loader control valve applies hyd. "power" to both sides of the bucket cylinders at the same time. But Kubmech, how the heck is that going to work? Wont the cylinder just lock up and not move? No, because on one side of the cylinder the rod itself takes up space, decreasing the volume of the cylinder, on the other side of the cylinder piston there is, no rod, creating more volume. PSI=pounds per square inch, the more volume the more square inches to apply pounds, get it? O.K. so the back side of the cylinder overcomes the front side of the cyl. due to more volume and at the same time does not allow an air pocket to develop becuase of the added resistence of "hyd. power" to the front side of the cyl. allowing the cylinder to "stay tight" for subsequent operations. And as an added bonus it tends to dump faster due to the higher amount of flow/pressure to perform this operation. (Hence the "fast dump" mode the salesmen love to tell you about) In actuality the "fast dump" mode is just derivative of the regen circuit.
Soooo..... What's the difference between 3-pos. and 4-pos. valves, besides about 35 bucks.
The 3-pos. valve has regen in the dump mode period. The problem with regen is if you want to add, say a snow plow, with power angle running off of the loader control valve, the power angle works off of single acting cylinders. With "hyd. power" being supplied to both sides of the cylinder, you will only be able to angle in one direction and that's it. (back to the whole volume-vs-psi thing again).
The 4-pos. valve has a detented position. When moving the control lever to the right, to dump (or power angle right in the case of the plow) if you choose the first detented position this is the non-regen position and will work just like the good old fashion loader control valve. If you apply a little more pressure on the control lever you will push past the detented position and move into the regen position. Thus, if you have the 4-pos. valve and want to avoid the floppy bucket syndrome you have to push full right to engage regen.
So the bottom line is; both the newer 3-pos. and 4-pos valves have the "fast dump" option. The difference is, do you want to add options to your front loader without having to add an aux valve to you're tractor or not?
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