When a hydraulic pump is in operation, it fulfills two primary roles. Firstly, its mechanical movement generates a vacuum at the pump inlet, allowing atmospheric pressure to push liquid from the reservoir into the pump’s inlet line. Secondly, this mechanical action transfers the liquid from the pump outlet into the hydraulic system.
It’s important to note that a pump facilitates the movement or flow of liquid; it does not create pressure. Instead, it generates the flow needed to build pressure, which arises from the resistance to fluid flow within the system. For instance, if an Automotive Lift Repair Orlando pump is not connected to a system (load), the pressure at its outlet will be zero. When the pump is connected, the pressure will increase only to the extent necessary to overcome the load’s resistance.
Classification of Automotive Lift Repair Orlando Hydraulic Pumps
Hydraulic pumps can be categorized as either positive-displacement or non-positive-displacement, with most pumps in hydraulic systems being positive-displacement.
Non-positive-displacement pumps provide a continuous flow but lack a positive internal seal against slippage, resulting in significant variations in output as pressure changes. Examples of these types include centrifugal and propeller pumps.
If a non-positive-displacement pump’s output port were blocked, pressure would rise while the output would drop to zero. Although the pump’s moving component would continue to operate, flow would cease due to internal slippage.
Conversely, in a positive-displacement pump, slippage is minimal compared to its volumetric output. If the output port were obstructed, the pressure would quickly rise to a level that could lead to catastrophic failure of the pump or its Automotive Lift Repair Orlando components, unless the drive shaft breaks first.
Positive-Displacement Principle
A positive-displacement pump delivers a consistent volume of liquid for each cycle of its pumping element. This constant delivery is made possible by the precise fit between the pumping element and the pump casing. The amount of liquid that escapes past the pumping element is negligible compared to the maximum possible delivery. Thus, the volume delivered per cycle remains largely unaffected by changes in the pressure the pump faces. If significant fluid slippage occurs, it indicates that the pump is malfunctioning and requires repair or replacement.
Automotive Lift Repair Orlando Positive-displacement pumps can be categorized into fixed and variable displacement types. Fixed displacement pumps maintain a constant output during each cycle at a given speed, whereas variable displacement pumps allow for changes in output by modifying the geometry of the displacement chamber.
These pumps can also be referred to as hydrostatic for positive-displacement pumps and hydrodynamic for non-positive-displacement pumps. “Hydrostatic” indicates that the Automotive Lift Repair Orlando pump converts mechanical energy into hydraulic energy with relatively low fluid quantity and velocity. In contrast, a hydrodynamic pump relies on high liquid velocity for output pressure.
Types of Positive-Displacement Pumps
Several hydraulic pump designs utilize the positive-displacement principle.
Reciprocating Pumps
The reciprocating pump exemplifies the positive-displacement principle. As the piston moves outward, it creates a partial vacuum in the pump chamber that draws liquid from the reservoir through the inlet check valve. This vacuum effectively secures the outlet check valve. The volume of liquid entering the chamber is determined by the pump casing’s geometry, such as in a cylindrical design.
When the piston retracts, the inlet check valve closes, and the piston’s force opens the outlet check valve, expelling the liquid into the system. Each cycle of the reciprocating movement forces out a consistent volume of liquid.
All Automotive Lift Repair Orlando positive-displacement pumps, regardless of whether they are reciprocating or rotary, deliver the same volume per cycle as a fundamental characteristic. This output is independent of the driving speed; however, increasing the pump’s speed will result in a greater total volume of liquid delivered.
Rotary Pumps
In rotary pumps, liquid is transferred from the inlet to the outlet through rotary motion. These pumps are typically classified based on the mechanism that moves the liquid, leading to designs such as gear, lobe, vane, or piston rotary pumps.
Automotive Lift Repair Orlando External-gear pumps can be categorized into external and internal types. An external-gear pump operates with two meshing gears, one driven by the drive shaft and the other as the idler gear. The space created between the gear teeth forms a chamber that fills with fluid as the gears unmesh, creating a partial vacuum at the inlet. As the gears mesh again at the outlet, the fluid is expelled.
Under optimal conditions, gear pumps can achieve volumetric efficiencies of up to 93%. However, running clearances and gear tooth wear lead to a consistent loss at a fixed pressure, making their performance at lower speeds and flows less efficient. Thus, gear pumps are best operated close to their maximum rated speeds.
Although the loss of fluid through clearances, or “slip,” increases with pressure, it remains relatively stable regardless of speed and output. External-gear pumps can tolerate contaminants in the oil, which can affect wear rates and efficiency, but they are less likely to experience sudden failures.
Lobe Pumps
Lobe pumps are rotary external-gear pumps where both lobes are driven by external gears. In screw pumps, which operate similarly to rotary screw compressors, there are three variations: single-screw, two-screw, and three-screw. The fluid in a screw pump moves axially, with the rotors pushing it uniformly along their axis without generating pulsations, resulting in very quiet operation.
Internal-Gear Pumps
Internal-gear pumps consist of an internal gear and an external gear. The lower relative speed between these gears, due to the difference in the number of teeth, minimizes wear. The crescent seal design uses an inner gear with one fewer tooth than the outer gear, creating a seal that allows hydraulic oil to flow smoothly.
Vane Pumps
Vane pumps feature vanes sliding in rotor slots within a housing that may be eccentric or oval-shaped. As the rotor turns, a vacuum is created that draws oil into the chamber, which is then forced out as the volume decreases.
These pumps can be designed in balanced or unbalanced configurations. Balanced vane pumps counteract side loads, while unbalanced pumps may incur additional strain on the rotor and drive shaft.
Vane pumps maintain efficiency over time, automatically compensating for wear. They are available as double units, containing two pumping units in one housing, and can also be arranged in series to enhance pressure output.