In lubricated services, a certain degree of rod taper can be tolerated. Lubricating films tend to block small passages through which gas can escape. A tapered rod combines both the effects of oversized and undersized rods in that packing rings constantly try to adjust to the variations in surface profile. Excessive amounts of taper, however, will rapidly destroy the packers’ ability to seal.
Generally, the worst taper condition occurs at one end of the stroke. Reasonable leakage rates and packer life can be expected if the degree of taper does not exceed approximately 0.0005 inch per inch of stroke. For non-lubricated and highpressure, low molecular weight gas services, acceptable taper will be less than this value.