We sold a manually controlled multi-section directional control valve the other day - one of those easy sales when a client shows up with their old stuff and you are lucky to stock exactly the same new stuff. In that case, it was a 6-section Walvoild SDS150 which I configured and assembled as per the client's very old and battered sample. The man was doing a major overhaul to his small forest trailer, and the old valve had too much of an internal leakage to keep the crane arms up. The man even showed up the next day and bought a set of new fittings for the DCV because he wanted everything to look "new and shiny". So far so good - a happy customer and a happy supplier.
However, about a week later, the man called me and said that the new DCV was working OK, but... not as good as the old one. Very strange... He further explained that he used to be able to easily operate several functions at the same time with the old DCV, but it was very hard (virtually impossible) to achieve the same result with the new one. He was very pleased with the fact that the crane arms finally stopped creeping down, but he was wondering if there was anything else we could do to make the new valve work as the old one because it would make his work so much easier. Indeed, the simultaneous operation is extremely important for the productivity of such cranes.
As soon as I heard the complaints - I was (almost) sure I knew what was going on. I shared my idea with the owner, he followed on with a quick fix, and the problem disappeared. Don't worry, I'll tell you all about it in a minute, but let me tell you first what that "failure" brought to my mind:
It made me recall an opinion (or a myth, if you will) that I occasionally hear from hydraulic technicians and engineers (usually at the beginning of their careers) that "simple" multi-section directional control valves (by "simple" I mean not spoiled by individual flow compensators) are not good for machines that require simultaneous function operation. Why? Because you need compensators for that! Compensators!!!
Well... two things I have to say about this.
First - a skilled operator absolutely can achieve simultaneous operation of several functions with a non-compensated DCV with manual (lever) control by using and abusing the fact that the spools of modern DCVs come with throttling notches and human hands are pretty precise positioning devices, and second - there are two "secret crane weapons" that make their job of "compensating by hand" a whole lot easier! I am calling these tricks "crane tricks" because you typically find them in small and medium-sized loading cranes that need to work fast (hence the imperative need for simultaneous operation). But, I suppose, the same "secret principles" can be applied anywhere you want.
The first, and most obvious "trick" is using single-acting spools and actuators where single action is enough (very often the boom lifting cylinder). Single action means you can lower the boom without wasting any of the pump's flow.
The second trick is using orifices. Very often - fixed orifices that are ingeniously hidden inside fittings of sections that don't require full flow. For example - for the aforementioned crane, such fixed orifices would be installed in the grapple rotation lines. Such orifices allow the operator to simply shift the spool in the desired direction, without worrying about precise throttling, and concentrate his attention on the speed control of another function.
As you can see, if you combine these two tricks - you can, for example, grab a load of timber, lift it up, and then be turning the boom, lowering it, and rotating the grapple at the same time.
Obviously, a lot of skill and muscle memory is still required for this to work, especially if you consider the fact that you will be operating six stiff levers (as oppsed to a couple of "fly-by-wire" joysticks), but still - for a small crane, a simple multi-section DCV makes a lot more sense than its much more expensive compensated sibling because it provides acceptable function at one-third of the price!
What about the case that I described at the beginning of the article? You probably already figured it out, butI'll tell anyway. It was the new fittings. The grapple rotation section used to have 2-mm orifices welded inside the "original" fittings. Salvaging the old fittings from scrap and re-installing them brought the "old feel" back.
I can give you yet another, probably less obvious, advantage of a manually throttled system for crane control. Long booms are inertial loads, and stiff flow-compensated systems are known to develop all sorts of unexpected instabilities when they are coupled to inertial loads. There are workarounds, of course, but let me tell you from personal experience - solving self-excited oscillations in hydraulic systems is a painful and tedious process. Manually throttled DCVs are much softer and, therefore, naturally much more stable.
So, to resume - two tricks to achieve "economical" simultaneous hydraulic function - single acting spools and orifices.
More on hidden orifices (and other hidden stuff) in Three (and a Half) Hydraulic Fittings That Can Fool a Technician.