There’s a lot of terminology in hydraulics. Some of it descriptive. Some of it misleading. Pressure Relief Valve. No confusion here. Directional Control Valve. Same. It says what it means and it means what is says. So let’s use this test: it says what it means and it means what is says, as the gold standard for descriptive terminology.

What about Pressure Override? It’s an operating characteristic of direct acting relief valves. But a hydrostatic transmission pump can have pressure override (POR). Really? Yes, but there are some words missing: Load Pressure Override of the Pump’s Displacement Command. As you can see, there is a significant loss of meaning in the shortening. But at least the abbreviated version is not wrong, even if it can cause confusion.

Load Sensing Pump. No such thing. Okay, okay, it’s the type of pump matched to a load pressure sensing DCV. But the pump is not load sensing. The DCV is. The pump only receives a pressure signal from the DCV. And the design of the DCV ensures this pressure signal is the highest load pressure of all the actuated functions.

On this type of variable displacement pump, load pressure is the input and regulated flow is the output. But the controlled parameter is delta-P. Don’t believe me? What is the one adjustment, other than pressure cut off and sometimes volume stops, that is available on this type of pump? Delta-P. Why? Because delta-P is the controlled parameter. This delta-P controlled pump is also called a constant flow or flow regulated pump. But this magic only happens because delta-P is the controlled parameter. Directional Control Valve. Delta-P Controlled Pump. It says what it means and it means what it says.

How about Open Center Load Sensing? Complete misrepresentation. Apart from the open center bit, that is. Because load pressure is not sensed anywhere in this system. One machine OEM calls it Negative Flow Control. Wow. How does the flow go negative? It doesn’t. Negative Flow Control is a reference to the type of pump control used in this system, which is: Negative Hydraulic Displacement Control. The word ‘Negative’ here means the hydraulic command signal and pump displacement are negatively correlated. So when the hydraulic command signal increases, pump displacement decreases. And the hydraulic command signal controlling pump displacement in this system is generated by its open center DCV. This magic happens through clever deployment of fixed and variable orifices in series in the DCV’s bypass gallery. It means when flow in the DCV’s bypass gallery decreases, pump displacement increases. This roughly matches pump displacement to actuator demand. There’s no load pressure sensing here.

So what should this NON load sensing system be called? Well given a load sensing system gets its monica from its load pressure sensing DCV, why don’t we do the same here? Respecting the gold standard for descriptive jargon, this would be: Bypass Flow Sensing. It says what it means and it means what is says.

What about a Hybrid Load Sensing system? I am going to scream. Really. These systems are comprised of a delta-P controlled pump, which as already explained is NOT load sensing, and a closed center DCV which is NOT load sensing either. This system is a hybrid. But not a hybrid of a load sensing system. In this system, it’s the DCV that’s a hybrid. The type of DCV used here is: mobile hydraulics closed center. Meaning an open center, 6/3 DCV with the pressure gallery isolated from the bypass gallery. This blocks P in the spool’s center position. This then allows the bypass gallery to be used to signal a delta-P controlled pump. And this magic happens through clever deployment of fixed and variable orifices in series in the DCV’s bypass gallery. This system roughly matches pump displacement to actuator demand. No load pressure sensing here either. So what should this NON load sensing system be called?

One OEM calls it Open Center, Variable Pump. And another calls it Variable Flow, Open Center. Now I’m pulling my hair out. As already explained, the DCV used in these systems is closed center. Okay, so they started with an open center DCV, isolated the pressure gallery from the bypass gallery to make it closed center and then did some pressure signalling trickery with the now vacant bypass gallery. Yes, there is a small, parasitic loss of pump flow through the bypass gallery when all spools are centered but this doesn’t meet the criteria for an open center DCV. Based on any reasonable interpretation of the accepted jargon, the DCV is closed center, not open center. So why use the term Open Center to describe this system? It’s wrong. And it’s misleading. So here’s my naming suggestion for this system, again considering what’s happening at the DCV: Bypass Pressure Sensing. It says what it means and it means what it says.

Now, with all of the above in mind, here are a few of my suggestions for a new lexicon:

Load Sensing System (formerly known as Closed Center Load Sensing): A load pressure sensing DCV with P-blocked spool centers, signalling a delta-P controlled pump.

Bypass-Flow Sensing System (formerly known as Open Center Load Sensing, Negative Flow Control): An open center, bypass flow sensing DCV, signalling a negative, hydraulic displacement controlled pump.

Bypass-Pressure Sensing System (formerly known as Hybrid Load Sensing, Open Center Variable Pump, Variable Flow Open Center): A closed center, bypass pressure sensing DCV, signalling a delta-P controlled pump.

Why is any of this important? Well imagine for a moment that what we all know as a Directional Control Valve, was called a Pressure Distributing Valve instead. When asked to explain its function, you’d have to start with something along the lines of: “Yeah, nah, it’s not really a pressure distributing valve, we just call it that”.

When I explain the operation of the above systems in The Mobile Hydraulics Handbook and The Definitive Guide to Mobile Hydraulics, I use the terms Closed Center Load Sensing, Open Center Load Sensing, Negative Flow Control and Hybrid Load Sensing. But only because you have to meet people where they are in order to take them where they need to go. In other words, a new lexicon doesn’t have much utility if you’re the only one using it.