The concept of a gear pump is straightforward: it consists of two identical gears rotating in sync within a tightly sealed housing that resembles the shape of an "8." The outer edge of the gears and the inner walls of the housing are precisely machined to minimize clearance. As the fluid from the extruder enters the space between the two gears at the suction port, it gets trapped and carried along the housing as the gears rotate. When the teeth mesh again, the fluid is forced out through the discharge port.
In technical terms, a gear pump is classified as a positive displacement device, similar to a piston in a cylinder. As one gear tooth enters the fluid space of the other, it displaces the fluid due to its mechanical movement. Since fluids are incompressible, the fluid must be pushed forward, creating a continuous flow. This process repeats with each rotation, ensuring a consistent output volume per revolution. As the drive shaft turns continuously, the pump delivers fluid without interruption, and the flow rate directly correlates with the pump's speed.
However, no pump can achieve 100% efficiency. A small amount of fluid is inevitably lost due to internal leakage, which is necessary for lubricating the bearings and gears. Additionally, it's impossible to create a completely gap-free housing, so some fluid may remain inside the pump. Despite this, most gear pumps still operate efficiently, achieving 93% to 98% efficiency depending on the material being processed.
Gear pumps are well-suited for fluids with varying viscosities or densities. Even when there’s resistance at the outlet—such as a screen or restrictor—the pump continues to push fluid through, maintaining a steady flow until a mechanical limit is reached. Many models include a torque limiter to protect against overloading.
There is a practical limit to how fast a gear pump can operate, primarily determined by the properties of the fluid. High-speed operation is feasible with low-viscosity oils, but when handling high-viscosity polymer melts, the maximum speed decreases significantly. It's crucial that the suction side of the pump is fully filled with fluid; otherwise, the pump cannot maintain its precise flow rate. The PV value (pressure times flow rate) also plays a key role in determining performance limits.
To address these challenges, manufacturers offer a range of gear pumps with different sizes and displacement volumes. These pumps are tailored to specific applications, balancing system capacity, cost, and performance.
The PEP-II gear pump features a solid construction with integrated hardening, ensuring long service life. The "D" bearing includes a forced lubrication system, allowing polymer to flow through the bearing surface and return to the inlet, preventing buildup and degradation. Precision machining ensures accurate alignment of the gears and shafts, reducing wear and extending longevity.
The pump uses a water-cooled seal design with PTFE components. This seal doesn’t contact the shaft directly but instead cools the polymer to a semi-molten state, forming a self-sealing effect. An alternative Rheoseal option uses reverse spiral grooves to redirect pressure back into the inlet. For easier installation, the pump is designed with a ring bolt mount compatible with standard flanges.
A built-in heating element allows for rapid and controlled thermal management, minimizing damage to individual components. Unlike traditional heating methods, this feature isolates heat to a single board, improving reliability.
Driven by a separate motor, the gear pump effectively isolates upstream pressure fluctuations and flow variations. Outlet pressure pulsations are kept below 1%, contributing to smoother extrusion processes. By integrating a gear pump into an extrusion line, operators can boost output, reduce shear stress on materials, lower residence time, and improve overall product quality.
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