The Problem

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

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.

P

T

A

B

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.

Pump

A Line

B Line

Tank

An example of a four-port valve. The four ports will always include the pump ("P") and tank ("T"). By convention, the other two lines will be labeled "A" and "B".

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.

Most directional control valves are of a spool-type construction. The spool has lands and undercuts, housed within precision-machined casing. As the spool shifts, the lands and undercuts open and close flow paths.

The example valve has 3 positions: center, straight through (P to A), and crossover (P to B). The schematic clearly represents each position.

4 Position Valve

2 Position Valve

Infinite Position Valve

T

B

P

A

An open center valve connects all four ports when the valve is centered.

T

B

P

A

A closed center valve blocks all four ports.

A tandem center blocks two ports, and connects two ports. Usually, it is used to block the A and B ports, and connect the pump to tank. This center is very common in applications like the simple press circuit, to allow the pump's flow to go straight to the tank, rather than forcing it over the relief valve.

T

B

P

A

A float center blocks a single port and connects the other three. It is usually used to block the pump, and connect A and B to tank.

Back to the Press

Now that we know a little more about DCVs, let's figure out how to troubleshoot that press.

Step One
Define the Problem

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

*We use the word mostly because there can be a lot of overlap between categories. It's common to see flow problems cause pressure problems, and vice versa. The goal of this step is to consider the nature of the problem, and the characteristics of your system. Don't get caught here; spend a few minutes and then move on!

Step Two
Consult the Schematic

If you have a schematic that is up to date, and verified for accuracy, congratulations! You are miles ahead in troubleshooting this problem. An accurate and trustworthy schematic is the key to a hydraulic system; it can provide:

• a list of all of the components in the system, often including exact part numbers, codes and system-specific settings.
• a diagram of the relationships between system components.
• the exact location of test-points.
• ways to isolate sub-circuits for testing purposes.

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?

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.

Step Six
Physical Observations

You can often discover a faulty component, or eliminate it from your suspect list, without taking the system apart.

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.

You've safely checked the hoses and fittings, and found nothing suspicious. Time to continue down the list.

The major symptom is that pressure is not held in the blind end of the cylinder once the valve recenters. If there is a flow path through the valve from A to T when the valve has centered, that would explain the pressure problem.

You observe that the flexible hose leading to the blind end of the cylinder stiffens when extending the cylinder, but relaxes and sags immediately when the valve centers. This would suggest that the hose is suddenly at atmospheric pressure.

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 Components

1. Hoses & Fittings

2. 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.

Step Seven
Test Components

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 3000 psi to near zero in a split second as you allow the valve to recenter.

Step Eight
Repair/Replace

You've done enough due diligence at this point to be confident that the valve is the source of the problem. This particular DCV is not very expensive, so it's easy to replace, and not very practical to repair. Within a couple of hours you've swapped a new one into the system.

Step Nine
Verify!

It's the moment of truth! It's time to operate the press and verify that the problem has disappeared. Make sure to test the press under load, and at normal operating conditions before declaring that you're done.

Hurray! The press operates as advertised; extending, retracting, and holding pressure correctly. The DCV was the source of the pressure problem.

Ready for the last step?

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.

Drag and drop the terms to correctly complete the description of this valve. Some terms will not be needed.