Today we'll look into the details of orbital steering valves with classic (a.k.a. static) load sensing. To be more precise - we'll be exploring the most common rotary spool design with the help of our "friends" from Danfoss and M+S Hydraulic (chosen at random, simply because they chanced to turn up at the shop this week):
We'll see how the rotary spool secures the classic load sensing function, and we'll also see why these assemblies are asymmetrical (meaning there's only one correct mounting position for the spool).
In order to benefit from this post in full, you do need to have a clear understanding of what the holes in the rotary spool valve assembly and the steering valve body do, for which I strongly recommend going through my previous posts on orbital steering valves (if haven't done so already):
A Non-Reaction Orbital Steering Valve With... Reaction?!! - Part 1
A Non-Reaction Orbital Steering Valve With... Reaction?!! - Part 2
Orbital Hydraulic Motor Principle Explained
How a Danfoss Reaction Orbital Steering Valve Works
Ok, going back to the classic LS now. Everybody understands that a classic load-sensing arrangement has to do the following:
When a valve is in a neutral position:
it needs to block the P port and vent the LS port to tank.
When a valve is commanded:
it needs to direct the LS to a work line downstream of a controlled restriction point (or a variable orifice, if you will).
I modified the "more correct diagram" from the post on open-center steering so that it would reflect the classic LS arrangement:
Like I said - the good old classic load-sensing system at its best! So, let us see now how the engineers achieved all that function in this very compact rotary spool package.
First of all - let us examine the bodies and their connections.
You can see that the main connections are the same as what we saw last time (the P, T, A, and B), and the only difference is the additional LS port, which is located on the side of the body and is directed to a small hole located at the back, and if we look at the spools, we'll see that it connects to the respective narrow circular slot of the sleeve:
We can also tell that despite being different, the Danfoss and M+S spools have similar features:
Both of the spools block off the P port when centered (just like on the diagram):
Both of the spools have three blind holes drilled inside the sleeve, three notches, cut at the top of the spool, and three pairs of small diameter holes, drilled in the circular LS slot:
The rest of the spool valve assembly works in the same way we saw before - it directs the P port and the respective work port to the orbital section for the volumetric metering, and it vents the opposite work port to the tank. But the "LS secret" lies in the intricate cooperation between the three blind holes, the three pairs of LS holes, the six notches of the spool P ring, and the three notches cut in the top of the spool. Once again, for the sake of telling what is what:
If we measure the parts and put all of the "participants" in a dimensionally accurate drawing, things become crystal clear, and you immediately see two things: first of all, you can tell how the LS line is cleverly vented to the tank via the blind holes when the spool is centered, and second - you can also see that nothing good happens if you assemble the spool/sleeve combination incorrectly (turn the spool 180º, which is totally possible)
Actually, if you read Danfoss service manual for the steering valves, you'll see that they do mention the correct position of the spool on page 20 - however, they don't explain why. And I believe that if you know why the blind holes are there - you're way better off because knowing why things are where they are makes you a better tech, and this is what all this insane-hydraulics blog business is about!