Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1123345
HYDRAULICS diff erence between pressure and fl ow. It is not uncommon to hear the terms used interchangeably, as though they are synonyms. ey aren't. I often hear the complaint that a pump isn't putting out as much pressure as it should, implying that the pump is supposed to deliver pressure. A common assumption is that if the pressure is low, the pump must be bad. is is not the case. e pump doesn't pump pressure. e pump delivers a rate of fl ow. e single function of the pump is to take fl uid from one place and put it somewhere else. Pressure is the result of resistance to fl ow. In our training classes, we use the simple schematic shown on page 20 to explain this concept. A fi xed-displacement pump is the simplest type of hydraulic pump. It is turned by a primary mover, generally an electric drive motor or, in mobile equipment, the same engine that moves the machine. e amount of fl ow is determined by the displacement and speed of the drive motor. By "displacement," I mean the amount of fl uid delivered for each full rotation of the pump. On typical industrial systems, the pump is turned at a constant speed and therefore delivers a constant amount of fl ow. When the pump is started, oil is moved from the reservoir and into the system. e higher the fl ow rate, the faster the actuator will move. If you trace the fl ow from the pump, you reach a "T" in the line. Whenever you follow the fl ow on a schematic and arrive at a line split, you must track the fl ow in both directions to determine the path of least resistance. Hydraulic fl uid always takes the path of least resistance. If you trace the fl ow to the left, you encounter a relief valve symbol. e relief valve is represented by a single square with an arrow indicating the direction of fl ow. Notice the arrow does not touch the inlet or outlet port. is signifi es that the relief valve is normally closed and blocking fl ow. e "zigzag" line at the bottom of the relief valve symbolizes a spring. A good way to think of a relief valve in a schematic is to think of the spring pushing the arrow up away from the ports, holding it closed. is means that, in order to open the valve, something must push down on the arrow harder than the spring is pushing up. Also, note the dotted line. In hydraulic schematics, a dotted line usually represents a fl ow path that is somewhat smaller than that of a solid line, typically a drain line or a pilot line. e one shown in the schematics on the left is a pilot line connected immediately upstream of the valve. Whatever pressure is in When the pump is turned on, the path of least resistance is to the drum and not through the relief valve. This schematic shows a closed manual valve, which blocks fl ow to the drum. 22 | May - June 2019 | www . machinerylubrication.com