The volume of liquid drawn into the chamber is known because of the geometry of the pump case, in this example, a cylinder.Īs the piston retracts, the inlet check valve reseats, closing the valve, and the force of the piston unseats the outlet check valve, forcing liquid out of the pump and into the system. The partial vacuum helps seat firmly the outlet check valve. As the piston extends, the partial vacuum created in the pump chamber draws liquid from the reservoir through the inlet check valve into the chamber. The positive-displacement principle is well illustrated in the reciprocating-type pump, the most elementary positive-displacement pump, Figure 1. In a hydrodynamic pump, liquid velocity and movement are large output pressure actually depends on the velocity at which the liquid is made to flow. Hydrostatic means that the pump converts mechanical energy to hydraulic energy with comparatively small quantity and velocity of liquid. Other names to describe these pumps are hydrostatic for positive-displacement and hydrodynamic pumps for non-positive-displacement. The output of a variable displacement pump can be changed by altering the geometry of the displacement chamber. The output of a fixed displacement pump remains constant during each pumping cycle and at a given pump speed. Positive-displacement pumps can be of either fixed or variable displacement. Note that if fluid slippage is substantial, the pump is not operating properly and should be repaired or replaced. The delivery per cycle remains almost constant, regardless of changes in pressure against which the pump is working. That is, the amount of liquid that slips past the pumping element in a positive-displacement pump is minimal and negligible compared to the theoretical maximum possible delivery. Constant delivery during each cycle is possible because of the close-tolerance fit between the pumping element and the pump case. Positive-displacement principleĪ positive-displacement pump is one that displaces (delivers) the same amount of liquid for each rotating cycle of the pumping element. If the output port were plugged, pressure would increase instantaneously to the point that the pump's pumping element or its case would fail (probably explode, if the drive shaft did not break first), or the pump's prime mover would stall. In a positive-displacement pump, slippage is negligible compared to the pump's volumetric output flow. Although the pumping element would continue moving, flow would stop because of slippage inside the pump. If the output port of a non-positive-displacement pump were blocked off, the pressure would rise, and output would decrease to zero. Centrifugal and propeller pumps are examples of non-positive-displacement pumps. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Most pumps used in hydraulic systems are positive-displacement.Ī non-positive-displacement pump produces a continuous flow. Classification of pumpsĪll pumps may be classified as either positive-displacement or non-positive-displacement. Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.Ī pump produces liquid movement or flow: it does not generate pressure. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. When a hydraulic pump operates, it performs two functions.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |