Type 75 and Type 75HR Pneumatic Air Relays

The Marsh Bellofram Type 75 and Type 75HR Pneumatic Air Relays use signal pressure to accurately control output pressure over a wide range of flow and supply pressure variations. Under these varying flow conditions, output pressure is maintained by use of an aspirator tube, which adjusts the air supply valve opening in accordance with flow velocity. A balanced supply valve, utilizing a patented Marsh rolling diaphragm, makes the relay virtually immune to changes in supply pressure, with reliable operation over millions of cycles.

The basic Type 75 relay offers excellent precision, along with high forward flow rates. An optional high-relief version (75HR) is also available, which provides extra fast “blowdown” for rapid release of output pressure. This relief feature makes the relay suitable for use in applications requiring fast exhaust. In addition, units are available with an internally spring mounted fixed negative bias, with a formula that is particularly useful for obtaining zero pressure from pneumatic devices such as I/P transducers, which cannot normally be adjusted this low, as well as obtaining higher outputs from such devices. The design and construction of the Type 75 relay allow for simple maintenance, with units able to be serviced without removing it from the line, thus reducing downtime. Signal to output pressure ratios of 1:1, 1:2, 1:4 and 1:6 are available. Maximum output is 150 PSIG (10.3 BAR).



  • Balanced Valve Design
  • High Flow Capacity
  • Field Serviceable
  • Multiple Output Ratios
  • Negative Biasing Option


  • Volume Boosting
  • Dancer Roll Loading
  • Calendar Roll Loading
  • Cylinder Bucking Control
  • Clutch and Brake Controls
  • Gas Flow Control
  • Tensioning Control
  • Valve Motor Loading
  • Cylinder Return Stroke Actuation (75HR)
  • Air Hoists (75HR)
  • Electronic Control Applications (Negative Biasing Version)