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Troubleshooting Directional Hydraulic Valves

Troubleshooting Directional Hydraulic Valves

Valves blocked at all ports are often used in machines with a flip-up gripper design in which the gripper moves vertically at any given time.

“The clamp die does not close.”

This is the phone call I received from a client a few months ago. The company’s bender is a hybrid machine that uses hydraulics to open and close clamps with directional valves.

Hydraulic directional valves are fairly simple devices that are commonly used on tube benders to control the movement of a particular device. As their name suggests, they control the direction of movement. They are sometimes called “Bang-bang” – bang-bang valves because they are either open or closed. Turn the valve on in one direction and bang, you have movement in that direction. Energize it in the opposite direction, and bang, you’re moving it there.

In a hydraulic cylinder, this means that the valve will cause the cylinder to extend or retract.

A hydraulic cylinder is just a tube with sealed ends with a piston in the center. The cylinder is filled on one side of the piston, pushing the cylinder toward the opposite end and creating the desired movement of whatever device it is attached to. When we want it to move in the opposite direction, the cylinder is filled on the other side of the piston.

Most CNC tube benders use direct-acting solenoid valves. In simple terms, this means that when voltage is applied to the solenoid, it creates a magnetic field that pushes a coil, allowing hydraulic oil to flow to the device it controls.

Some machines with very large devices can use pilot-operated valves. When the solenoid is activated on these valves, a spool is similarly moved to allow oil flow, but this flow is used to create pressure to move the spool on a much larger valve.

Valve configurations

There are many types of valves with many different functions, but most hybrid CNC benders use four-port valves, most often in a three-position configuration, and these are bolted to a manifold or series of collectors.

The four ports are:

  • P – Pressure port. This is the inlet port of the valve. Hydraulic oil enters the valve through this port at the pressure set by the system.
  • T – Tank port. This is the low pressure side of the valve. Oil is returned from the device to the hydraulic system storage tank.
  • A – A port. This is one of two output ports. It is connected to one side of the device being moved.
  • BB port. This is the second output port. It will be connected to the other side of the moved device.

The manifold allows multiple valves to share single pressure and reservoir lines, simplifying connection to the entire hydraulic system.

You can manually check if a directional valve has pressure by plunging it with a small screwdriver or Allen wrench.

Although there are maintained directional valves, in which the spool remains in the position it was last moved to, I have never seen one on a tube bender.

Instead, tube benders use momentary directional valves. When voltage is applied to the solenoid, a magnetic field is created around an armature which then pushes the coil, allowing flow to one of the ports. When the tension is removed, the springs return the valve to the neutral or center position.

A three-position valve has two solenoids: Solenoids A and B correspond to the port that will be connected to the tank port when control voltage is applied.

When voltage is applied to solenoid A, the spool moves, creating a path for oil to flow from port A to port T while allowing flow from port P to port B. This allows the The pressurized oil pushes the piston away from where the tube is connected to B as it fills one side of the tube. As the piston moves, oil on the opposite side of the piston is returned to the reservoir as the valve allows flow from A to T. Although there is a path for oil to flow from P to B, a connection is made with allowing the flow of oil from A to T. Likewise, if solenoid B is energized, a path for flow from P to A and a path for flow from B to T are created.

When there is no control voltage on either solenoid, springs return the coil to the center position. This center position is usually how the valve is set. The valve configuration is usually represented by a diagram on the valve cover.

There are dozens of center position configurations available, but on a tube bender the two most common are:

  1. All ports blocked – In this configuration, when no voltage is applied to the solenoids, all four ports are blocked. There is no path for flow from P to either outlet port, and there is no path from A or B to the reservoir lines. Since there is no path for oil to flow, the connected device will remain stationary when voltage is removed from the solenoids.

  2. This is useful for devices that must be positioned somewhere other than the full travel of the cylinder to which they are attached. This type of valve can be used in a retractable gripper design, in which there is a vertical component to the movement of the gripper. When the voltage is off, the clamp does not open due to gravity. It is important to remember that it is possible for A and B, or both, to have residual pressure. So use caution when removing hydraulic lines for maintenance or troubleshooting, as rapid and unexpected movement could occur.

  3. AB tank – In this configuration, when no voltage is applied to the solenoids, P is blocked but both A and B will have a path for the oil to flow to T. As there is a path for the oil to flow flows from A and B to T, a cylinder attached to this valve should theoretically be able to move freely. Most devices on a tube bender will have an AB tank valve, but this may be necessary with a pressure die assist that can be pulled forward during bending without power being applied at all to the tube bender. the valve.

There is little chance that residual pressure will remain in either port, but before removing any hydraulic lines, be sure that movement of any device will not cause damage to the machine or a person.

Troubleshooting

For a cylinder (or other device) to move in a direct-acting directional valve, three things are required: pressure, flow, and voltage to the solenoid.

When something doesn’t move as expected, I first look to see if the control system is not allowing movement. For example, for a gripper die to be closed, the control system must verify that the rest of the machine is correctly positioned. On a multi-stack bender, the tube must be aligned vertically with the tools because the machine is on the correct vertical stack, and it must be horizontally in the correct position for the tube to fit into the bending die. Otherwise, if the clamp die closes, damage to the tube or machine may occur.

AB tank valves like this create a path for oil to flow from ports A and B to port T (tank).

The same applies to the pressure die, with the addition of checking that the carriage is not likely to be damaged in the event of the pressure die closing. If the control system cannot confirm these positions, it must not allow the device to move in a direction that could cause damage or send voltage to the solenoid. Most modern control systems display a message stating why this device cannot be moved.

If there are no conditions for the control system to prevent movement, I then check to see if voltage is being sent to the solenoid. When checking this, it is important to know what the correct voltage is. On most tube benders this is 24 V DC or 110 V AC, although some machines use 220 or even 480 V AC. This is usually stamped directly on the solenoid body itself, but you may need to check the schematics or the valve part number to be sure.

Using a multimeter set for the correct voltage, you can check this at the valve by removing the cover to access the connections inside. If you can’t find control voltage at the valve, something in the circuit is preventing it. This could be a faulty output board, relay or fuse. Check the machine’s electrical schematics to troubleshoot this circuit.

If voltage is applied and still no movement occurs, test the mechanics of the directional valve by dipping it. On the end of the solenoid there is usually a small button that you can press with a small screwdriver or Allen key. If there is pressure and you plunge the valve manually, you will create a flow path and the device should move. Most hybrid benders have high and low pressure settings. By testing a valve by manually plunging it, you can create enough pressure when the machine is at low pressure to move a device, albeit very slowly. However, if it doesn’t move, you may need to set the machine to high pressure and test the valve again.

If voltage is applied to the solenoid and it allows movement when manually plunged, it is likely that the solenoid is failing. Some valve manufacturers’ designs allow replacement of just the solenoid, but we typically replace the entire valve.

If the valve cannot plunge at all, the valve mechanics are failing and the valve must be replaced.

If the valve can be plunged at high pressure and nothing moves, the valve mechanics may be failing. However, before replacing the valve, check the mechanics of the device being moved to ensure there is no mechanical constraint preventing it from moving. It is also possible for a cylinder’s internal seals to fail, allowing oil to flow freely through the piston, so movement does not occur or occurs very slowly.

Sometimes, as was the case with the customer who called because they couldn’t close the collar, the valve spool can become stuck. Plunging the valve releases it and the customer resumes production.

However, this hinted at the possibility of a more serious problem and a few weeks later the store had the same problem. It turned out that this machine’s hydraulic system cooling system was in desperate need of preventative maintenance, and its hydraulic oil was getting very hot during long production runs. This caused the oil to become gummy in the valve when the machine sat idle overnight, eventually causing the valve to become stuck. Ultimately, the shop repaired the cooling system, replaced all the oil in the machine, and the machine is running smoothly today.