Pumps & Systems, July 2008
Q.
Every pump manual clearly requires that a pump shaft must be carefully aligned with its driver shaft before operating the equipment. However, the precision of the alignment is seldom provided. Is there a simple guideline for the allowable misalignment?
A.
There is no single answer to this question. Misalignment occurs in the following ways:
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Parallel offset
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Angular offset
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Combination of both
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Axial movement
The allowable misalignment in these modes will be determined by considering the alignment capability of the driver, pump and coupling. The coupling has the greatest ability to tolerate misalignment by far
Elastomeric couplings can tolerate all forms of misalignment by the distortion of the elastomer. Most single metallic couplings such as gear, plate or grid type can tolerate only angular misalignment. To accommodate parallel offset, metallic couplings usually are supplied in pairs as a spacer coupling. The most forgiving is the universal joint when supplied as a pair with a spacer shaft. The tolerance for the metallic couplings will depend on the length of the spacer. The coupling supplier can provide the tolerance values for each type and size of coupling.
Parallel alignment can also be affected by thermal expansion during operation. Pumping hot liquids or heat from the summer sun can disturb alignment. Pumps on hot applications must be realigned while the pump is filled with hot liquid.
Room between the shaft ends must provide for axial movement of the shafts, especially with pumps in hot applications that cause expansion of the pump shaft.
Consult the pump manufacturer for the allowable misalignment between the pump and its driver if this information is not contained in the instruction manual.
Q.
I understand that radial hydraulic forces from an impeller cause a load on bearings that may affect their life. Do such forces also affect the deflection of the pump shaft, and if so to what extent can damage be done?
A.
Shaft deflection is a design criterion that greatly influences pump performance due to its effect on the mechanical seal, internal clearances and bearings.
Radial loads acting on the rotating impeller(s) are transmitted directly to the pump shaft. This force will deflect the shaft where it is applied, irrespective of the bearing configuration. The direction of the hydraulic radial load, at a given operating point, remains constant with respect to the pump casing. However, it is seen as cyclic stress reversal with respect to the rotating shaft and has a dynamic effect on the mechanical seal. The shaft must be designed to accommodate this hydraulic radial load in conjunction with the additional radial load imposed due to the mass of the impeller(s) and other rotating components. Under these conditions, the rotor must be stiff enough to limit the resulting deflection to within limits predetermined by internal clearances and mechanical seal requirements.
Dynamic deflection of the pump shaft changes the relative location of the mechanical seal faces, so it has a large impact on the overall seal life. For a mechanical seal to reach its design life, a number of requirements have to be met. From a static or dimensional point of view, the relative locations of the primary, stationary and rotating faces must be held within control limits. These limits can be met by using a combination of flexibility within the seal assembly and appropriate manufacturing procedures and processes.
Limiting the shaft deflection will also improve packing life in arrangements where a packed stuffing box is used as the method of shaft sealing.
Internal operating clearances are also governed by the anticipated dynamic shaft deflection. Adequate clearance must be verified at critical locations by using the shaft deflection equations.
Shaft deflection affects pump bearing life because shaft deflection directly affects the angular misalignment between the inner and outer rings of the bearing.
External loads such as coupling misalignment should also be considered.
Q.
Is there a simple way to determine the total head that a pump will develop when a performance curve is not available? If so, what is it and how accurate will it be?
A.
Yes, an estimate of the head developed by an impeller can be determined from the following equation:
