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These pumps come with a straight spur, helical, or herringbone gears. Straight spur gears are easiest to cut and are the most widely used. Helical and herringbone gears run more quietly, but cost more. A gear pump produces flow by carrying fluid in between the teeth of two meshing gears. One gear is driven by the drive shaft and turns the idler gear.
The chambers formed between adjacent gear teeth are enclosed by the pump housing and side plates also called wear or pressure plates.
A partial vacuum is created at the pump inlet as the gear teeth unmesh. Fluid flows in to fill the space and is carried around the outside of the gears. As the teeth mesh again at the outlet end, the fluid is forced out. Running clearances between gear faces, gear tooth crests and the housing create an almost constant loss in any pumped volume at a fixed pressure.
This means that volumetric efficiency at low speeds and flows is poor, so that gear pumps should be run close to their maximum rated speeds.
Although the loss through the running clearances, or "slip," increases with pressure, this loss is nearly constant as speed and output change. For one pump the loss increases by about 1. Change in slip with pressure change has little effect on performance when operated at higher speeds and outputs. External-gear pumps are comparatively immune to contaminants in the oil, which will increase wear rates and lower efficiency, but sudden seizure and failure are not likely to occur.
Figure 3. Lobe pump. The lobe pump is a rotary, external-gear pump, Figure 3. It differs from the conventional external-gear pump in the way the "gears" are driven. In a gear pump, one gear drive the other; in a lobe pump, both lobes are driven through suitable drives gears outside of the pump casing chamber. A screw pump is an axial-flow gear pump, similar in operation to a rotary screw compressor. Three types of screw pumps are the single-screw, two-screw, and three-screw.
In the single-screw pump, a spiraled rotor rotates eccentrically in an internal stator. The two-screw pump consists of two parallel intermeshing rotors rotating in a housing machined to close tolerances. The three-screw pump consists of a central-drive rotor with two meshing idler rotors; the rotors turn inside of a housing machined to close tolerances.
Flow through a screw pump is axial and in the direction of the power rotor. The inlet hydraulic fluid that surrounds the rotors is trapped as the rotors rotate.
This fluid is pushed uniformly with the rotation of the rotors along the axis and is forced out the other end. The fluid delivered by a screw pump does not rotate, but moves linearly. The rotors work like endless pistons, which continuously move forward. There are no pulsations even at higher speed. The absence of pulsations and the fact that there is no metal-to-metal contact results in very quiet operation. Larger pumps are used as low-pressure, large-volume prefill pumps on large presses.
Other applications include hydraulic systems on submarines and other uses where noise must be controlled. Figure 4. Internal-gear pumps - gerotor and crescent. Internal-gear pumps, Figure 4, have an internal gear and an external gear. Because these pumps have one or two less teeth in the inner gear than the outer, relative speeds of the inner and outer gears in these designs are low.
For example, if the number of teeth in the inner and outer gears were 10 and 11 respectively, the inner gear would turn 11 revolutions, while the outer would turn This low relative speed means a low wear rate. These pumps are small, compact units. The crescent seal internal-gear pump consists of an inner and outer gear separated by a crescent-shaped seal. The two gears rotate in the same direction, with the inner gear rotating faster than the outer.
The hydraulic oil is drawn into the pump at the point where the gear teeth begin to separate and is carried to the outlet in the space between the crescent and the teeth of both tears. The contact point of the gear teeth forms a seal, as does the small tip clearance at the crescent. Although in the past this pump was generally used for low outputs, with pressures below 1, psi, a 2-stage, 4,psi model has recently become available.
The gerotor internal-gear pump consists of a pair of gears which are always in sliding contact. The internal gear has one more tooth than the gerotor gear.
Both gears rotate in the same direction. Oil is drawn into the chamber where the teeth are separating, and is ejected when the teeth start to mesh again. The seal is provided by the sliding contact. Generally, the internal-gear pump with toothcrest pressure sealing has higher volumetric efficiency at low speeds than the crescent type. Volumetric and overall efficiencies of these pumps are in the same general range as those of external-gear pumps.
However, their sensitivity to dirt is somewhat higher. Figure 5. Basic unbalanced vane pump. In vane pumps, a number of vanes slide in slots in a rotor which rotates in a housing or ring. The housing may be eccentric with the center of the rotor, or its shape may be oval, Figure 5.
In some designs, centrifugal force holds the vanes in contact with the housing, while the vanes are forced in and out of the slots by the eccentricity of the housing.
In one vane pump, light springs hold the vanes against the housing; in another pump design, pressurized pins urge the vanes outward. During rotation, as the space or chamber enclosed by vanes, rotor, and housing increases, a vacuum is created, and atmospheric pressure forces oil into this space, which is the inlet side of the pump.
As the space or volume enclosed reduces, the liquid is forced out through the discharge ports. Figure 6. Balanced vane pump.
Figure 7. Variable-displacement, pressure-compensated vane pump. Balanced and unbalanced vane pumps — The pump illustrated in Figure 5 is unbalanced, because all of the pumping action occurs in the chambers on one side of the rotor and shaft.
This design imposes a side load on the rotor and drive shaft. This type vane pump has a circular inner casing. Unbalanced vane pumps can have fixed or variable displacements. Some vane pumps provide a balanced construction in which an elliptical casing forms two separate pumping areas on opposite sides of the rotor, so that the side loads cancel out, Figure 6.
Balanced vane pumps come only in fixed displacement designs. In a variable-volume unbalanced design, Figure 7, the displacement can be changed through an external control such as a handwheel or a pressure compensator. The control moves the cam ring to change the eccentricity between the ring and rotor, thereby changing the size of the pumping chamber and thus varying the displacement per revolution. When pressure is high enough to overcome the compensator spring force, the cam ring shifts to decrease the eccentricity.
Adjustment of the compensator spring determines the pressure at which the ring shifts. Because centrifugal force is required to hold the vanes against the housing and maintain a tight seal at those points, these pumps are not suited for low-speed service. Operation at speeds below rpm is not recommended. If springs or other means are used to hold vanes out against the ring, efficient operation at speeds of to rpm is possible.
Vane pumps maintain their high efficiency for a long time, because compensation for wear of the vane ends and the housing is automatic. As these surfaces wear, the vanes move further out in their slots to maintain contact with the housing.
Vane pumps, like other types, come in double units. A double pump consists of two pumping units in the same housing.
They may be of the same or different sizes. Although they are mounted and driven like single pumps, hydraulically, they are independent. Another variation is the series unit: two pumps of equal capacity are connected in series, so that the output of one feeds the other. This arrangement gives twice the pressure normally available from this pump. Vane pumps have relatively high efficiencies. Their size is small relative to output. Dirt tolerance is relatively good. Piston pumps Figure 8.
Axial-piston pump varies displacement by changing angle of swashplate. The piston pump is a rotary unit which uses the principle of the reciprocating pump to produce fluid flow.
Instead of using a single piston, these pumps have many piston-cylinder combinations. Part of the pump mechanism rotates about a drive shaft to generate the reciprocating motions, which draw fluid into each cylinder and then expels it, producing flow.
There are two basic types, axial and radial piston; both area available as fixed and variable displacement pumps.
The second variety often is capable of variable reversible overcenter displacement. Most axial and radial piston pumps lend themselves to variable as well as fixed displacement designs. Variable displacement pumps tend to be somewhat larger and heavier, because they have added internal controls, such as handwheel, electric motor, hydraulic cylinder, servo, and mechanical stem.
Axial-piston pumps — The pistons in an axial piston pump reciprocate parallel to the centerline of the drive shaft of the piston block. That is, rotary shaft motion is converted into axial reciprocating motion.
Most axial piston pumps are multi-piston and use check valves or port plates to direct liquid flow from inlet to discharge. Figure 9. Radial piston pump. Low prices and a huge selection.
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Fluid Mechanics and.
Hydraulics Revised Ed.. Other results for Fluid Mechanics Hydraulics Pdf: 46, matched results. Preface B. Symbols and Abbreviations 1. Properties of Fluids 2. Fluid Statics 3. Hydrostatic Force on Surfaces 4. Buoyancy and Flotation 5. Translation and Rotation of Liquid Masses 6. Dimensional Analysis and Hydraulic Similitude 7. Fundamentals of Fluid Flow 8.
Flow in Closed Conduits 9. Complex Pipeline Systems