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.
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?
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.
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.
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.
The graphic cutaway shows how the lands and undercuts on the spool form each position.
It's also common to see DCVs with two positions, or even four positions. Like ports, more positions are possible, but relatively uncommon.
Valves can also have infinite positions, which means that the valve spool does more than simply stop at one of the defined positions. It can also be sent to any point between one of the full positions in order to achieve flow control.
There are four common valve centers for four-port valves:
An open center valve connects all four ports when the valve is centered.
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.
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.
Operator is a category that describes the mechanism that drives the valve to change its position. There are a lot of operators out there; the valve in the press circuit uses a handle (or lever), but you might also see a solenoid, or hydraulic pilot, to name a few.
Responses are similar to operators, but they are never directly powered. Instead, they provide an automatic "response" that describes the valve's behaviour when its operator is not active. The valve from the example has a spring response. This means that when you are not using the lever to push it into the straight through or crossover position, the spring will return the spool to the center, tandem position.
Now that we know a little more about DCVs, let's figure out how to troubleshoot that press.
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