The Problem

Congratulations! You are the proud owner of a small custom cabinet shop.

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You've just finished unboxing and setting up a new and shiny press! The purpose of the press is to hold veneer evenly against a plywood sheet until the glue bonding the workpieces cures.

But…

The new press is only great until you try to use it in production. The cylinder behind the press extends and retracts correctly, but when you release the valve handle, the cylinder does not maintain the pressure against the workpieces, and the veneer starts to separate from the plywood.

It's a brand new system! What could the cause of the problem be? And how will you find it and fix it as efficiently as possible?

The Circuit

Let's get started by quickly reviewing the major components in the press circuit.

This press uses a simple gear pump to push fluid to a tandem-center directional control valve, or DCV. When the valve is in neutral position, pump flow returns to tank. When you operate the lever, you extend or retract the press cylinder. A relief valve and pressure gauge are tied into the line from the pump.

Directional Control Valves (DCVs) are usually described by their ports, positions, centers, and operators. For example, if you were to describe the DCV in the press circuit, you would say that it is a 4 port, 3 position, tandem center, handle operated, spring centered valve.

Here's the same valve, shown as a schematic symbol.

PTAB

It's a lot to say, but it's the only way to accurately describe a valve. Even if you happen to have its part number handy, it's best to know the complete description because manufacturers can (and do!) change part numbers without warning.

Ports refer to the number of lines into and out of the valve. The press circuit DCV has four ports to connect the valve to the pump, both sides of the cylinder, and to the tank. While four ports are very common, it's also easy to find examples of valves with 2 ports, 3 ports, and 6 ports. It's less common, but certainly not impossible, to find 5 and 7 port valves. There is no real limit, aside from practicality, on how many ports a valve could have.

Also Known As

Some people may refer to ports as "ways". For example, you may hear someone say that they have a 4 way, 3 position valve. This term is older, but still correct.

Step One
Define the Problem

The first question to ask yourself is whether this is mostly* a flow, pressure, or directional problem.

Step Three
List Suspect Components

Make a list of all of the places where a failure could be causing the cylinder to release its hold on the workpieces. In Step 1 you determined that this is a pressure problem, so that will help to inform your choices here. You don't need to have a theory about what the exact failure is, just ask yourself whether any issue with a specific component could cause a pressure problem.

Step Four
Isolate Sub-Circuits

This step does not always apply. Some simple hydraulic circuits do not have subcircuits that can be easily isolated. This circuit is extremely small and simple - there really isn't a practical way to isolate any part of it.

You've probably noticed by now that we are using a rather rigorous approach to troubleshooting a problem in a very simple circuit. While it might be overkill in this example, practicing good troubleshooting habits when working with simple problems will keep you on track when working with larger and more complex systems.

Step Five
Determine Order of Checking

You've shaved the list of suspect components down, which is certainly progress. But which of the suspect components should be checked first?

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Many people would start with the most likely culprit, but that might not be the most efficient troubleshooting strategy. Instead, start with the easiest item(s) to check externally, without removing them from the circuit.

Checking the hoses, fittings, and adaptors is really easy to do, so why not get it out of the way first and cross it off your list?

Examine the exposed hoses and fittings for wetness/leakage. While this step is very simple, it can be dangerous if you don't follow some basic safety precautions. Wear your PPE and do not touch fittings or hoses when system is under pressure. This means:

• No flexing a suspicious hose to see what happens.
• Don't run your hand over hoses or fittings to look for leaks, unless you have completely depressurized the system. Gloves won't protect you from an injection injury!
• Don't wiggle fittings to see if you can make them leak.

Cylinder

If flow is able to creep over the piston seal from the blind end to the rod end, there might be enough clearance in the valve spool to allow that flow to escape back to tank.

You visually inspect the cylinder for signs of a leak. You listen carefully as the cylinder is extending against pressure, but cannot detect any suspicious hissing that would indicate flow passing around the piston seal.

Suspect Components1. Hoses & Fittings2. Directional Control Valve ??3. Cylinder

Conclusions

If a leak in the cylinder is the problem, it's unlikely that the line from the valve to the blind end of the cylinder would suddenly and completely depressurize in the way you have observed. A slow, creeping change would make more sense if a leak was the problem.

It seems more likely that the pressurized fluid in the blind end of the cylinder is finding an easy path back to tank the moment the valve has centered. It's time to move on to the next step: it's time to test and hopefully confirm this theory.

Confirm your observation that pressure on the A line is, indeed, dropping to zero (or near zero) almost immediately.

You remove the blind end line from the cylinder, and attach a gauge to get an objective measure of what's going on in that line. You find that the pressure drops from about psi to near zero in a split second as you allow the valve to recenter.

This should not be happening - the tandem center in the valve should hold the pressure when the valve centers. There is clearly a problem with the valve.

Step Ten
Why Did it Happen?

Finding the Root Cause

Now that you are certain that the DCV was the source of the problem, it's time to devote a little more brainpower to the problem, and understand why. Since it's a brand new piece of equipment, it shouldn't have failed at all, and understanding the "why" will help you decide whether it is likely to fail again in short order. Should the whole system be boxed up and returned?

The easiest way to learn more about the valve is to take it apart. As soon as you do, you notice that the spool doesn't look quite right. The wrong valve was bundled with the system; you're looking at an open center, not a tandem center. So while there was nothing wrong with the valve itself, it was the wrong valve for the system. Now that you have installed the correct valve, there is no reason to expect any further problems.

Put your schematic away in a safe place, and get to work making cabinets!

One of the most fundamental and important components of any fluid power system is the directional-control valve. As the name suggests, directional control valves are used to direct the flow of fluid through the system. Directional control valves control when and where the fluid in the system flows. These valves serve to direct fluid flow in a system.

How Directional Control Valves Work

Directional control valves are selected to handle the pressure in a system. Valves will shift from fully open to closed or proportionally. This occurs instantly, causing fluid to rapidly accelerate and decelerate, or in the case of a proportional valve, it is modulated to ramp acceleration and deceleration of actuators. This is done either manually or automatically with settings to cycle valves. Directional control valves keep fluid in a standby mode which prevents its flow from within the system until it’s needed to move and perform its designed function. When called into action, directional control valves shift to perform the operation and then shift back to the neutral position when completed. This action occurs instantaneously, causing fluid to accelerate and decelerate.

The simplest directional control valve is a 2-way valve. These simply stop flow or allow flow. As the name suggests, a 2-way valve has two ports called the inlet and the outlet. A water faucet is an excellent illustration of a 2-way valve and its simplicity. A water faucet allows flow or stops flow by its manual control.

When selecting a DCV, the designer is looking for two primary characteristics; fluid ports and the number of positions. Valve ports provide a passageway for hydraulic fluid to flow to or from other components. The number of positions refers to the number of distinct flow paths a valve can provide. IE: Forward – Neutral – Reverse

Selecting a Directional Control Valve

Directional control valves are classified according to their various characteristics such as the max flow rate, max rated working pressure, the number of ports, number of positions, actuating method used, the fluid path, leakage rates and so on as follows:

• Max Flow Rate/Max Rated Working Pressure: Maximum pressure to perform the work in the process
• Fluid Path: Check valves are an example of 2-way 2-position valve, actuated by line pressure to free flow fluid in one direction or blocking flow in the opposite direction. Shuttle valves is an example of 3-way 2-position allowing switching from two ports into a one common circuit
• Positions: There are typically two or three positions – Forward – Neutral – Reverse
• Ports: The number of flow paths through which fluid can flow into and out of the valve.
• Actuation: How the valve is cycled