Design clearance

Positive-displacement meters are normally rated for a limited temperature range. Meters can be constmcted for high or low temperature use by adjusting the design clearance to allow for differences in the coefficient of thermal expansion of the parts. Owing to small operating clearances, filters are commonly installed before these meters to minimize seal wear and resulting loss of accuracy. 

Radial clearance has increased more than 1 mils over nominal design clearance. 

Another leakage path occurs through the clearance between the tube holes in the baffle and the tubes. The maximum design clearance will normally be in. (0.8 mm). 

Another source of error that can become quite important is the leakage flow through the clearance between the flight and the barrel. A normal design clearance (radial) is 0.001 D, where D is the diameter of the screw. When the clearance is normal, the flow through the clearance will be quite small. However, if the screw and/or barrel is subject to wear, the actual clearance can increase substantially beyond the normal design clearance. This can cause a considerable reduction in output and it is important to know how to evaluate the effect of clearance flow. 

The viscosity in the clearance ij. is differentiated from the viscosity in the channel j, because, in reality, the viscosity in the clearance will be substantially different from the viscosity in the channel as a result of differences in local temperature and shear rate. When the flight clearance is close to the normal design clearance, the value of 4 will be very close to zero and can be neglected unless extreme accuracy is required. However, when the radial clearance is considerably larger than the normal design clearance, for instance, as a result of wear, the actual value of fL should be used in the expression for the total volumetric output, Eq. 7.224. 

Design clearance dimensions for a tall operator (74" in height). 

If nothing else, excess wear ring clearances waste horsepower. Tripling the design clearance increases motor power requirements 10%. 

Solder balls and splashes that violate minunum electrical design clearance, are not encap-snlated in a permanent coating, or are not attached (soldered) to a metal surface are unacceptable (see Fig. 52.24). 

The third example of polymer mixing due to screw design clearances is a barrier screw. All barrier... 

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