Butterfly-type inlet valve

Rotating disc inlet valve used on larger and modulating packages, driven by an air cylinder or a lever actuator.

Also called: throttle valve · butterfly unloader

Spring returns the disc closed; separate non-return flap or check keeps oil in on shutdown.

How it works

An air cylinder rotates a disc through a shaft and lever. Because the disc position is proportional to actuator pressure, this design also supports modulation (part-load throttling) when fed by a modulation valve rather than a simple on/off pilot.

Specifications

Actuator

Single-acting air cylinder, 30 – 90 PSIG

Wear items

Shaft bushings, shaft seal, disc seal, cylinder seals

Adjustment

Linkage set so the disc is fully closed with zero pilot

Parts you can order

• Aftermarket / OEM

  Inlet-valve air cylinder / actuator

  By bore and stroke (1-1/2 to 3 in. bore typical)

  $90 – $380

Field test procedure

1. With the machine locked out, move the lever by hand — sticky or notchy travel means worn bushings.
2. Check the linkage stop settings: a disc that no longer fully closes shows as high unloaded power draw.
3. Watch amps at unload — a healthy package drops to roughly 25 – 35 % of loaded kW.

Symptoms & probable causes

• High unloaded amps
  Disc not closing fully — linkage, bushing wear, or a leaking cylinder.
• Hunting during modulation
  Sticking shaft or an unstable modulation valve signal.