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CNC Repair and Troubleshooting | Hydraulic Solenoid Valves and Simple Circuit Diagram

This post will be the first of several discussing hydraulics as used on a machine tool. We’ll go basic to start with.

You might have many hydraulic functions on your machine or just a few.

On a lathe you would typically see some of the following tasks of a machine actuated using hydraulic cylinders or actuators:

  • chuck open/ close
  • turret clamping
  • turret indexing (on older machines, anyway)
  • tailstock quill in/out
  • clamping of a programmable tailstock

On a milling machine:

  • spindle tool clamp/ unclamp
  • B axis clamping
  • magazine indexing (older machines)
  • gravity axis counter-balance

For this post, we will discuss just a few basics that may help you get an understanding of the hydraulic circuit diagram for your machine and a little about solenoid valves.

The heart of the hydraulic system on a machine is the pump/motor unit (or hydraulic unit). This provides pressurized fluid (through piping and hoses) to the actuators of the machine. The actuators provide the actual mechanical motion needed to perform tasks (as mentioned above).

Valves (typically solenoid controlled valves) direct the fluid to the actuators at the appropriate time to perform actions (i.e. chuck open or close). A coil is electrically energized and a magnetic field is created. This magnet shuttles the valve spool. This change of position of the spool redirects the pressurized fluid to the appropriate actuator or cylinder side.

This is a drawing of a solenoid valve:

This is a 4-way, 3-position directional valve. 4-way refers to the 4 ports or connection points on the body of the valve: P, T, A, B.

A=Fluid path to side A of an actuator
B=Fluid path to side B of an actuator

 3-position refers to the 3 possible operating positions of the valve. Each position (or circumstance) is represented by one of the 3 squares in the diagram. 

The LH square represents the circumstance when solenoid A is energized:

Port A is connected to Pressure
Port B is connected to Tank (or return to tank)

The RH square represents the circumstance when solenoid B is energized (which happens to be the opposite condition of the LH square):

Port A is connected to Tank (or return to tank)
Port B is connected to Pressure

The Center square represents the circumstance when neither solenoid A or B is energized. In this case, spring pressure returns the spoool to the center position:

P is connected to T
Both ports and B are blocked

This diagram pertains to the particular solenoid valve in our example. The diagram will be different based on the different types of valves used in a machines.  A valve will typically have this type of diagram on the valve its self, so it is possible to see what happens to the fluid path under the different circumstances than can occur.

Below is a sample hydraulic diagram. It contains the same type of solenoid valve discussed above. It also shoes symbols for some typical components found in a hydraulic circuit diagram. In this example, the valve is being used to control the rod extend/ retract motion of a cylinder.

When solenoid A is energized (LH square):

Pressure is connected to port A.
Pressurized fluid enters the rod-side of the cylinder, causing the rod to retract.
Fluid in the blind-side of the cylinder is pressed out and is provided a path to return to Tank.

When solenoid B is energized (RH square):

Pressure is connected to port B.
Pressurized fluid enters the blind-side of the cylinder, causing the rod to extend.
Fluid in the rod-side of the cylinder is pressed out and is provided a path to return to Tank.

When neither solenoid A or solenoid B are energized (Center square):

Spring pressure moves the spool to the center position.
Pressure is connected to Tank.
Both ports A and B are blocked.
The cylinder rod does not extend or retract.



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