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R. Keith Mobley, in Fluid Power Dynamics, 2000
A Gear Pump develops flow by carrying fluid between the teeth of two meshed gears. One gear is driven by the drive shaft and turns the other. The pumping chambers formed between the gear teeth are enclosed by the pump's housing and the side plates.
A partial vacuum is created at the 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, the fluid is forced out. High pressure at the pump's outlet imposes an unbalanced load on the gears and their bearing support structure.
Gear pumps are classified as either external or internal gear pumps. In External Gear Pumps, the teeth of both gears project outward from their centers (Figure 3-2). External gear pumps may use spur, herringbone, or helical gear sets to move the fluid.
Figure 3-2. External gear rotary pump.
John R. WagnerJr., ... Harold F. GilesJr., in Extrusion (Second Edition), 2014
Gear pumps provide consistent polymer flow, which results in more consistent product dimensions. Potential advantages associated with gear pumps include the following:
•Higher product yield per pound of material from more consistent gauge control, allowing an overall reduction in the average product dimensions.
•Increased extruder output attributed to reduced pressure flow backward into the extruder caused by high die pressure. Eqn (36.1) shows that extruder output decreases as pressure flow from high head pressure increases:
Potential increased regrind use by eliminating extruder surging from nonuniform regrind feed.
•Start-up time reduction as the gear pump automatically adjusts the screw speed while providing the desired constant die pressure.
•Energy reduction as the extruder operates at a lower backpressure or pressure flow. The lower pressure flow generates a higher throughput at a lower screw speed and less shear heat. Lower shear heating generates less heat that has to be removed through cooling.
Gear pumps control the output of the screw to the die. Gear pumps use a set of rotating gears to control the melt pressure and output volume to the die within very tight tolerances (< 1%) with little or no pulsation of the melt flow. This isolates the die from any upstream fluctuation such as surging in the screw area due to material or machine variations.
Gear pumps can be driven by AC or DC motors but accurate speed control is essential.
•Tip – Check gear pumps regularly and monitor the pump for changes in power use as this can act as an early warning for many gear pump problems.
•Tip – Herringbone geared gear pumps tend to have a lower pressure fluctuation than standard spur gears.
Gear pumps can also be used to raise the melt pressure. This will lower the screw pressure, the melt temperature and potentially increase the output of the extruder.
Getting the right motor and drive system is critical to energy-efficient extrusion. It is the basic and most important action.
| Summary of the AC and DC drive options | |
|---|---|
| DC motors | |
| For | Against |
| ■ Accurate, fast and direct torque control. ■Good speed response. ■Simple control systems. ■High accuracy of speed and torque control. ■Flat speed versus torque profile. | ■ Low motor reliability. ■High initial motor cost. ■High maintenance costs. |
| AC motors + VSD | |
|---|---|
| For | Against |
| ■ Small, light and robust. ■Simple design. ■Lower initial motor cost. ■Lower maintenance cost. ■Reduced operating cost. ■Improves site power factor. | ■ More complex control system. ■High motor controller cost. ■Variable torque versus speed profile. |
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