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Tech
Tips Index
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Flexible Couplings
Drive couplings on horizontal pumps must perform two basic functions. First, it must transmit the torque available from the driver to the pump. Second, because the driver and pump are separate pieces of equipment, there will always be some degree of misalignment between their shafts. Therefore, the coupling must have the additional capability of allowing for a small degree of shaft misalignment. To accept this, couplings must have some degree of flexibility and, as such, are called "flexible couplings." There are two basic types of flexible couplings. The first is a gear type which uses gear teeth to make them flexible. They are normally grease lubricated, but on larger sizes can be continuously oil lubricated. The second is called a "multiple disc" or "diaphragm type." These couplings do not require lubrication of any kind. The flexibility is created by a series of multiple discs or a diaphragm, which are made of spring steel and flexes as the shafts rotate. Although the word "flexible" leads one to believe that misalignment is acceptable, that is not the case. A misaligned coupling will cause premature pump and motor bearing failure, as well as damage to the coupling itself. Therefore, it is very important to align the coupling correctly. In order to properly align the coupling, you will need a taper gauge or set of feeler gauges, and a straight edge or dial indicator. There are two types of misalignment encountered with flexible couplings: 1)Angular misalignment, in which the shafts are not parallel; and 2)Parallel misalignment. Where the shafts are parallel but not on the same axis. Angular Alignment To check angular alignment, insert a feeler gauge or taper gauge at any four placed 90 degrees apart around the coupling halves. Insert shims under the driver feet until the same reading is obtained at all four checkpoints. The pump and driver will then be in angular alignment. To check angular alignment with a dial indicator, clamp the dial indicator to the pump coupling half so that the ball just rests on the face of the motor coupling half. A chalk mark should be made at the point where the ball contacts the coupling half. Both the pump shaft and the motor shaft should be rotated an equal amount so that the reading is taken at all checkpoints with the ball on the check mark. Insert shims or adjust motor as required. Parallel Alignment To check parallel alignment, a straight edge should be held against the edges of the coupling halves at any four places 90 degrees apart. The straight edge should be parallel to the pump and driver shafts at all times. Insert shims until the straight edge lies flat against both coupling halves at all four checkpoints. The pump and driver will then be in proper parallel alignment. To check parallel alignment with the dial indicator, the ball should rest on the periphery of the motor coupling half. A chalk mark should be made at the point of contact, and the shafts rotated equally so that the reading is taken with the ball on the chalk mark at all check points. Insert shims as required. Special training is needed in the use of a dial indicator. If there are questions, contact the pump manufacturer. Note Any adjustment to correct one direction of alignment may affect the other direction. It will be necessary to recheck both angular and parallel alignment after each adjustment. As is the case with all rotating equipment, a periodic check of the coupling alignment should be performed. |
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