Lateral Deflection of the Screw

There are various causes that will tend to deflect the screw. The most obvious cause is the force of gravity acting on the screw. If the drive support of the screw is considered rigid and the supporting function of the polymer and barrel is neglected, then the sagging of the screw by its own weight can be represented by Fig. 8.5. 

Consider a 150-mm extruder screw with a density of 7850 kg/m and an elastic modulus of 210 GPa (= 210E9 Pa) the sag for this example is  

It is clear that the unrestrained sag starts to exceed the standard radial clearance (= 0.2 mm) when the L/D ratio exceeds 10. At normal L/D ratios of 20 to 30, the unrestrained sag is about one to two orders of magnitude larger than the standard radial clearance. From these simple considerations, it is obvious that the polymer between the screw and barrel must play a considerable support function to prevent contact between screw and barrel. The supporting force necessary to counteract the sagging by the weight of the screw has to increase very strongly when the L/D is increased. 

Another mechanism that can cause lateral movement of the screw is buckling. The collapse force required to cause buckling of a uniform cantilever is  

the head pressure necessary to cause buckling is  

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Another possible cause of lateral deflection of the screw is non-uniform pressure distribution around the circumference of the screw. Figure 8.8 shows a possible pressure distribution that will result in a considerable lateral force on the screw. 

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