Undersized Rods

Using standard size packing rings with an undersized rod can still result in an effective seal as long as the rod is truly circular and is without taper. However, this generally results in an extended break-in period, with leakage greater during breakin. In lubricated applications, there is an additional possibility that lubricant may be blown away at gas pressures in the packing. Contact surfaces become dry and subsequently overheat. Some packing materials, such as Teflon, will degrade rapidly if they become dry after once having been lubricated. Under this condition, an abrasive paste or small, hard beads are formed in the packing area. Beads and abrasive paste can quickly cause deterioration in rod and packing contact surfaces.

For normal applications, standard size packing rings can be used successfully on rods which are not undersized by more than approximately 0.002 inch per inch of rod diameter. For high-pressure applications (1000 psi), experience indicates that standard size packing should be used only if rods are no more than 0.003 inch undersize. When rod undersize exceeds the above guidelines, specially bored packing can be purchased. The major problem with the use of special bore packing is the chance that the wrong size may be installed. Additionally, the use of various bore packings creates stocking problems.

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