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allowable bending stresses if the movement is small (less than 1 "). The movement must be perpendicular to the "straight run" so that the bending is similar to the bending of a diving board (a cantilever beam). These conditions can exist above tanks where the backfill and/or pipe above the tank will settle or rise with slight movement of the tank. If there is a chance that the pipe will not be free to bend, a commercial flexible hose connector is recommended (See Fig- ure 1.6.26). Note: The use of swing joints is not recommended. 1.6.8 Inspecting for potential causes of joint failure 1. joint backout When assembling a bell and spigot joint, a bead of adhesive is nor- mally formed at the edge of the bell. If the joint is not locked up and backs out before the adhesive cures, the bead will no longer be next to the edge of the bel I. NOTE: To form a good adhesive bead, it is best to apply an excess of adhesive at the edge of the unmachined surface of the spigot. Excess adhe- sive in this area forms a good bead at the edge of the bell but will not hinder joint make-up. 2. Cocked joint If a joint is cocked or misaligned, there will usually be a large gap between the bell and spigot on one side. The opposite side will usually have a smaller gap or no gap. Misalignment is easier to detect if one sights down the line and views the joint from a dis- tance. NOTE: When using heat assist, the pipe may bend if it is under stress while curing. In this case, it is extremely difficult to determine if the pipe is bent because of heat- ing or if it is a cocked joint. Avoid making up pipe and fitting joints when pipe is under stress. 3. Improperly cured joint If the adhesive bead is soft or flexible, the adhesive is not suffi- ciently cured. If the bead is cured, it is hard and brittle when checked with a knife. When the bead is not hard after exposure to the recom- mended temperature and time cycle, the adhesivewas not mixed properly. In some cases, the sur- face layer of the bead will be white and may actually be tacky and soft on the surface. This is caused by water absorption on the surface of the adhesive bead. Use a knife to remove a section of the adhesive bead and check the bottom side for the color change (i.e. white color). If the entire bead is white and tacky through- out, then the adhesive was con- taminated with water before it was applied to the joint. This will also produce an improperly cured joint. Caution: The adhesive bead will cure faster than the adhesive in the joint. It is important that the joint not be pressurized until ithas been subjected to the proper time - temperature cure cycle. A tem- perature versus time -to -pressure chart is included in the instruc- tions packed with each adhesive kit. Although not all-inclusive, the condi- tions mentioned above are the most common indications of failure to achieve a properly assembled joint. All damaged or improperly as- semb led joints must be replaced. See Section 8. 20 GREATER THAN 30 DEGREES + rIFT�4 . MINIMUM - Figure 1.6.26 M