DISTRIBUTION SYSTEMS - PRIMARY HYDRAULIC POWER SYSTEM BASIC PRINCIPLES OF THE SYSTEM Combines use a combination of Pressure Flow Compensated (PFC) and open-center hydraulics. In a PFC system, oil flow is minimal unless there is a hydraulic demand. In an open-center system, oil is constantly pumped through the system regardless of hydraulic demand. Flow Across a Restriction The hydraulic system of the combine uses the principle of flow across a restriction for some functions. It is important to understand this basic principle in order to understand how the system works, or more importantly, why the system may not be working. 1.
When oil flows through an unrestricted passage, the pressure in this passage, if any, will remain constant as long as pump flow remains constant. M133599 1 2. When oil in a passage flows across a restriction , the pressure after the restriction will be less than the pressure before that restriction . Flow must exist for this to happen . A restriction can occur by any component causing a resistance to flow. M133699 2 3. When oil in a passage is fully restricted from flow (no-flow), the pressure in the passage will build until it reaches the relief valve setting. This relief pressure will be maintained as long as the flow is blocked and the pump is functioning normally.
This is true regardless of what component is blocking flow. No flow will create constant pressure in the passage based on the relief valve setting. M133799 3 A.10.A / 5