Pressure Switch

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How the Pressure Switch works:

The machine control must know when the brake is engaged or disengaged to prevent damage to the braking system. The pressure switch is used for this purpose. This switch uses a form C-contact and changes state at 45 psi. The normally open side of the contact is used to signal that air is present verifying that the brake is engaged. The normally closed side of the contact is to signal that no air is present verifying that the brake is disengaged.

Symptoms of failure:

The machine control will show an air clamp or unclamp failure alarm. This alarm will disable the table and will not allow it to rotate.

Typical causes for failure of the pressure switch:

  1. Age or exceeding the designed cycle life of the switch.
  2. Corrosion on the internal gold plated contacts caused from improperly filtered air.
  3. Air pressure drops below 45 psi indicating either loss of facility input air pressure or loss of air pressure via worn or failed quad rings in braking system.
  4. Improper wiring of pressure switch consisting of 3 leads. Black. Green. Red.
    a. The BLACK lead is the common.
    b. The GREEN lead is for the normally closed (unclamped) contact. Continuity between the black and green leads should exist when there is no air present at the switch.
    c. The RED lead is for the normally open (clamped) contact. Continuity between the black and red leads should exist only when there is air present above 45psi at the switch.
  5. The switch has failed if continuity exist on both the RED (normally open) and the
    GREEN (normally closed) contacts on the switch.

Pressure Switch replacement:

  1. Turn off machine power and disconnect all electrical cables and pneumatic lines from
    the Rotary Table.
  2. The pressure switch is mounted to the inside of the motor enclosure lid. To gain access,
    free the lid by removing the 6 phillips head screws then pull the lid away from the
    enclosure paying special attention not to damage the gasket seal.
  3. Troubleshoot the switch using the above instructions on causes of failure.
  4. If switch failure is determined replace it by first noting the location of the leads on the
    terminal strip then disconnect the black, green and red leads.
  5. Remove the physical switch from the brass fitting by rotating it C.C.W. until free.
  6. Thread the new switch into the brass fitting then connect the 3 leads to the previously
    noted positions on the terminal strip.
  7. Test the switch for proper fit and function.
  8. Inspect gasket seal then replace the motor enclosure lid and six phillips head screws.