Extruder thrust bearing

The ability of the thrust bearings to handle the thrust load on the screw is reflected in the B-10 life of the thrust bearings. The B-10 life is the munber of hours that 9 out of 10 identical bearings last at a certain load and speed. The B-10 life for the extruder thrust bearings is usually given at a head pressure of 35 MPa (5000 psi) and a screw speed of 100 rpm. If an extruder is operated 24 h a day, the B-10 life should be at least 100,000 h to get a useful life of more than 10 years out of the thrust bearing. 

Fig. 1. Parts of an extruder A, screw B, barrel C, heater D, thermocouple E, feed throat F, hopper G, thrust bearing H, gear reducer I, motor J, deep channel feed section K, tapered channel transition section and L, shallow channel metering section (15).

The extruder screw is designed to develop the pressure required to pump the molten polymer through the die. This pressure also acts on the screw. Since the thrust bearing mechanism supports the drive mechanism into which fits the shank of the screw, the thrust-bearing mechanism resists the axial thrust exerted by the molten polymer on the screw. Pressures of up to 2000 psi can be developed in many extruder operations. 

All data obtained from the tests with the ESM (table 2) are processed in a mathematical model. By adopting similitude models, the data can be used as a design basis for extruders, e.g. the L/D ratio of the auger, the auger geometry, the gearbox torque, and the size of the thrust bearing, etc. 

The output of the gearbox is connected directly to the shank of the extruder screw. A thrust bearing is located at this junction (Fig. 2.3). The thrust bearing absorbs the backwards push of the screw generated by the pressure of the polymer at the output end of the screw. 

Thurst load n. In an extruder or screw-injection molder, the rear-directed force in reaction to the forward buildup of pressure in the screw, culminating in the heard pressure acting over the whole screw cross-section, and equal to nif P/4. This force is much less in an injection molder of the same screw diameter because, while the screw is turning, the head pressure is low and when the screw stops, the injection rams take up the thrust. Therefore, their thrust bearings can be much smaller than those of equal-size extruders. 

Extruder screws fit into the barrel and are supported by the thrust bearing. The screw s shank length fits into the thrust bearing, while the flighted length contacts the plastic. Extruder screws are specified by their outside diameter (D) and the LID, which is given by... 

Figure 3.9 shows a typical thrust bearing arrangement for a single screw extruder. 

Figure 3.9 Thrust bearing assembly for single screw extruder...

Figure 3.10 shows an example of a thrust bearing assembly of a counter-rotating twin screw extruder. 

Fluid film thrust bearings have been applied to extruders on a few occasions. Their load carrying capability at low speed is generally poor and a loss of fluid film would have disastrous results. If a hydraulic drive is used to turn the screw, application of hydraulic thrust bearings may deserve some consideration. Reference 28 describes an extruder with hydraulic drive that incorporates a patented thrust bearing assembly with hydraulic axial screw adjustment. By measuring the pressure of the hydrostatic chamber of the thrust bearing, the pressure in the polymer melt at the end of the screw can be determined. 

Polymeric material (in the form of pellets) is fed into the extruder s hopper and thence to the screw channel. The screw, driven by a motor through a gear reducer, rotates in a hardened barrel. A thrust bearing absorbs the rearward thrust... 

---