GEAR PUMPS AND EXTRUDERS WITH GEAR PUMPS

By Tsuyoshi Kojima MITSUBA MFG. CO., LTD. Page 2

(1) Since its output quantity is constant, regardless of the relationship between the friction and shearing taking place between the rubber compound and barrel wall, the gear pump has the following characteristics.
(a) Specific output (output/screw rotation speed: kg/hr/rpm) will not change, and the increase in output rubber temperature, due to heat generated by shearing, is small, even when extrusion pressure is increased,.
(b) Since the amount of heat generated by shearing is small even when the gear revolution speed is increased, the size of the temperature increase (Delta T) is small, and specific output remains unchanged.
(c) Even when high viscosity rubber is extruded, an increase in extrusion pressure at the outlet die part will not change specific output quantity, nor will it increase output rubber temperature by any significant amount.
Thus, high outputs can be attained with gear pumps, even under high revolution speed and high extrusion pressure, since the extent of heat generated and temperature increase are small.

(2) Air drawn into compounds extruded by gear pumps
Since the feed zone of a gear pump is not capable of degassing, a lot of air gets drawn into its extruded rubber. Yet, introducing a degassing contrivance inside a gear pump would be difficult at the present time. Thus, the direct feed type gear pump mentioned later which does not incorporate an extruder (including those equipped with feed roller units), is limited in use to upstream processes that precede the final extrusion molding process such as a straining process, or to applications that can tolerate a certain amount of air drawn in (or applications whose downstream process comes with a degassing scheme).

(3) Since rubber compounds have high viscosity, it is difficult for gear pumps to self-feed themselves with such materials as in the case with water or oil. In order to extrude rubber compounds without drawing air into them, pressure must be applied to the entrance side of the gear pump (although the amount of pressure differs for different compounds, it is generally above 0.1MPa).

(4) Self feeding ability
A gear pump whose gear teeth are given an undercut configuration at the inlet where the rubber is nipped into the gear pump, and has a set of two feed rollers built into it, possesses the ability to self-feed on rubber, but its self-feeding ability is inferior to that of a screw extruder.

(5) Sealed structure prevented by high pressure rubber path containing moving parts
Since the gear, which creates the extrusion pressure, is driven from the outside, a rotary seal that can withstand the high extrusion pressure becomes necessary between the drive and gear. With gear pumps for resin, a circulatory seal configuration, which returns rubber that got through the sealed part back to the gear pump inlet, is adopted as the structure of the rotary seal parts except for the drive shaft part, but in the case of rubber compounds, due to vulcanization, a phenomenon unique to rubber materials, the rubber gets scorched, making it difficult to adopt the circulatory seal structure except under special conditions.
Thus, rubber compounds that get through sealed parts are disposed outside the gear pump, and almost never recycled or reused. The quantity of rubber thus leaking from the rotary seal has been less than 0.1% of total output (during operations under 7 to 25 MPa) in actual cases.

(6) Retention phenomenon
A space that captures and confines materials forms when and where the gears get engaged, and rubber compounds captured into this space are returned to the inlet side (see shaded part of Fig.1).
Since bearings and other parts can get damaged if abnormal pressure is generated when rubber compounds are completely sealed into this space, an escape channel is fabricated at the walls facing both sides of each gear teeth.

(7) Structure of gear bearings The bearing system that supports the gear pump consists of either journal bearings that use the rubber compound as lubricant, or rolling bearings. In the case of journal bearings, the bearing itself doubles as the rotary seal. Some gear pumps come with an additional cylindrical non-contact seal assembly with counter-flow groove, built in outside the journal bearings. As the gap between the journal bearings and gear shaft is extremely small, solids contained in the rubber compound such as hard impurities or fiber-reinforced rubber, etc., that is larger than this gap in the bearing part, can compromise bearing performance. Some pumps have specially designed grooves fabricated on the bore of the journal bearing to improve load capacity. Under a given rotation speed, journal bearings may cease to function and halt operation due to its characteristics, as the lubricant rubber membrane breaks and metal-to-metal contact occurs, and thus, ample caution is required. Rolling type bearings, on the other hand, have a cylindrical type non-contact seal assembly built in between the gear and bearings, with a counter-flow groove fabricated either on the bore or outer circumference of the cylinder. As rolling bearings use lubricants such as grease or lubrication oil, they do not face any operational limitation in the low-speed range like journal bearings. Due to their seals for lubrication purposes, etc., rolling bearing types are structurally more complicated than journal bearing types. Some pumps employ a cooling hole, for both journal and rolling bearing types, placed at the bearing center to remove heat generated at the gear and rotary seal, and come with a rotary joint to enable water cooling. The gear drive system has a universal join built in to prevent excessive external loads from applying to the bearing. Some pumps use slipper spline joints instead of the universal joint.

(8) Adoption of geared constant output mechanism to create extrusion pressure Although it is possible to achieve high precision outputs without being selective about rubber type, because of the constant output characteristic, the pulsation in the extrusion, whose frequency is proportional to the number of gear teeth ("number of gear teeth" x "rpm"/60:Hz) poses a limitation to extrusion accuracy. The pulsation may cause a continuous, irregular pattern called "gear mark" to be formed on the surface of the extruded product. To minimize pulsation, a helical gear with curved teeth, or a double helical gear with both left and right gears curved are used. While the curvature of the helical gear causes a thrust force to be generated against the sides of the gear teeth, the thrust force of double helical gears are canceled out and do not materialize. If the overall curvature angle of the teeth is too large, rubber will get blown from the outlet side toward the inlet side of the gear engagement, leading to reduced accuracy of extrusion and fluctuations in specific output. Some gear pumps adopt an assembly that utilizes the mechanism in which the following side of the double helical gear is also driven, and thus, no thrust force is generated. While precision of extrusion can be categorized into relative output precision (%) and absolute output precision (mass/time), it is important to chose the criterion carefully. When output is great, even if the relative output precision (%) is good, there will be a drop in absolute output precision (g/sec). This becomes a problem when conducting small output extrusions with a gear pump that has a large specific output quantity. Since the output of gear pumps, unlike that of extruders, is not influenced by the rubber compound's viscosity characteristics and coefficient of friction, it is not sensitive to the temperature control accuracy of the various components, but is directly influenced by the gear's rotational accuracy.

(9) Retention time and heat generated by shearing Since the length of a gear pump's pathway between its inlet and outlet, through which the rubber flows, is short, and the areas with shearing generated heat is limited to the gear engagement part, and the tips (addendum) and sides of the gear teeth, the amount of heat it generates is small compared to screw extruders that have large amounts of heat generated along the entire screw channel. Since the rubber's flow path is short, scorching of rubber does not occur easily either. A gear pump's specific energy, which is a measure of the amount of heat generated by shearing, is 0.01 to 0.03 (kW/kg/hr) as compared to the rubber extruder's 0.06 to 0.21 (kW/kg/hr). Since the rise in temperature is small even under high-output and high-pressure extrusions, compared to extruders (whose mesh size is generally around #60), the gear pump can be equipped with a very fine mesh size screen (#200 mesh size is possible).

(10) Kneading effect Since a gear pump's rubber flow path is short, and it does not have a serious kneading zone, not much of a kneading effect can be expected from it.

(11) Self-cleaning characteristics and changing of material Because gear pumps retain rubber compounds at the root (deddendum) of its gear teeth, it does not have a self-cleaning capability, and the gear teeth often need to be exposed and cleaned every time the material is changed. A purging method, that purges the current residual material with the next material to be used, without exposing the gear teeth and cleaning them is available.

(12) Extrusion head An extrusion head with the same structure as that of a rubber extruder can be used, so there is no particular reason to reconfigure the structure to one that is geared specifically for a gear pump.
(13) Rise time when commencing operation Because of its constant output characteristic, the gear pump can start and stabilize in a short amount of time.

(14) Temperature control While gear pumps for plastics are temperature controlled solely by electric heaters, gear pumps for rubber are equipped with hot water jackets in their main body. Some pumps also come with a water-cool hole with rotary joint at the center of their gear shafts.

(15) Endurance Since ultra hard materials are used for critical parts such as gears and bearings, endurance is not considered to be an issue with gear pumps. Gear pumps are already widely used for resins, and judging from actual cases of gear pumps for rubber used abroad, it would be safe to conclude that they pose no problem in terms of endurance.

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