By Allan R. Budris
Experience shows that many pump distress events (failures) have their root cause in the misalignment of the pump to motor. Misaligned pumps can even consume up to 15% more energy input than well-aligned pumps. Even small pumps can generate big losses when shaft misalignment imposes reaction forces on shafts, even if the flexible coupling suffers no immediate damage. The inevitable result is premature failure of shaft seals and bearings. Performing precise alignment, therefore, pays back through preventing the costly consequences of poor alignment. Indeed, using precise alignment methods is one of the principal attributes of a reliability focused organization. Good alignment has been demonstrated to lead to:
- Lower energy losses due to friction and vibration
- Increased productivity through time savings and repair avoidance
- Reduced parts expense and lower inventory requirements
Further, in order to insure good alignment, the alignment must be checked and correctly set when:
- A pump and drive unit are initially installed (before grouting the baseplate, after grouting the baseplate, after connecting the piping, and after the first run).
- After a unit has been serviced.
- The process operating temperature of the unit has changed.
- Changes have been made to the piping system.
- Periodically, as a preventive maintenance check of the alignment, following the plant operating procedures for scheduled checks or maintenance.
Hundreds of technical articles and presentations have elaborated on the serious problems that are caused by incorrect alignment between the pump and driver, such as:
- Coupling overheating and resulting component degradation
- Extreme wear in gear couplings and component fatigue in dry element couplings
- Pump and driver shaft fatigue failure
- Pump and driver bearing overload, leading to failure or short bearing life
- Destructive vibration events. Harmful machinery vibration is created whenever misalignment exists. Excessive pump vibration can shorten bearing and mechanical seal life.
Pump Alignment Basics
Pump shafts exist in three-dimensional space and misalignments can exist in any direction. It has been found to be most convenient to break this three-dimensional space up into two planes, the vertical and the horizontal; and to describe the specific amount of offset and angularity that exists in each of these planes simultaneously, at the location of the coupling. Thus, we end up with four specific conditions of misalignment, traditionally called Vertical Offset, Vertical Angularity, Horizontal Offset, and Horizontal Angularity. These conditions are described at the location of the coupling, because it is here that harmful machinery vibration is created whenever misalignment exists.
Basic issues that must be taken into account regarding pump alignment are:
- Alignment equipment sag (with dial indicators)
- Cold, hot or running alignment
- Where to make shimming adjustments (Align the motor to the pump by shimming the motor feet)
- Soft foot problems
- Type of alignment equipment
Alignment accuracy is critical to pump and driver longevity as stated above, and generally the better the alignment the longer the pump and driver bearing life. Figure 1 shows the best alignment that can expect from the three most prevalent alignment methods practiced in the industrial, worldwide.
- Straight Edge and Feeler Gauges
- Dial Indicator
Straight Edge, Feeler Gauges
This is the easiest and least expensive method of doing alignment but it is also the least accurate. Used primarily for very small pump / motor combinations where there is not enough room to use more accurate but larger alignment methods. The straight edge is laid across the flanges of the coupling hub and the feeler gauges are used between the faces of the coupling hubs. Shim changes are estimated and the alignment is attained through a process of trial & error. It is more difficult to attain the equipment manufacturer's alignment specifications through the use of a straight edge and feeler gauges.
There are two basic dial indicator methods. The Single Indicator Method uses a single dial indicator to take both the rim and face reading. You can then calculate shim changes for the motor feet to correctly align the unit. The Reverse Indicator Method uses a dial indicator on the pump shaft to read the motor shaft, and a dial indicator on the motor shaft to read the pump shaft. You can then use mathematical formulas to calculate shim changes to correctly align the unit. Although better then the "straight edge and feeler gauge method", the dial indicator method does have a few shortcomings, such as:
- Sagging indicator brackets
- Sticking/jumping dial hands
- Low resolution rounding losses
- Reading errors
- Play in mechanical linkages
- Tilted dial indicator (offset error)
This state-of-the-art system emits a pulsating non-hazardous laser beam that automatically determines relative shaft positions and conveys this information to its microprocessor. The advantages of modern laser-optic alignment devices far outweigh the possible initial cost advantages of other, more conventional methods. Reliability-focused pump users employ this state-of-art laser optic alignment determination method, even though it is somewhat more complicated to set up, but it can be more accurate if properly used. The laser is especially helpful when aligning shafts that are separated by more than a few inches. The laser systems also have software that is capable of calculating the shim changes required. Once familiar with it, the laser operator can align a pump/motor combination fairly quickly and accurately. The primary drawback of the laser systems is cost, and in some cases their size.
Specific advantages of laser alignment tools are that: they do not require as much operator skill; center-to-center pump alignment can be achieved without paying attention to thermal growth, since it is possible to feed in the thermal growth data for compensation; and laser alignment can allow the operator to check the pump when it is running and up to temperature, this is not possible with dial indicators. Other advantages for the laser are:
- It is free of gravitational hardware sag
- It can work with the couplings in-place or uncoupled
- It is fast & easy to mount
- It can detect & measure the extent of a "soft foot"
- It feeds misalignment data to a microprocessor for horizontal and vertical corrections.
Acceptable alignment tolerances are a function of shaft speed, coupling type/geometry, and the distance between the driver and pump shaft ends. The question then becomes, just how close should the pump and driver shafts be aligned? How much vibration and efficiency loss will result from the misalignment of the shaft centers? It should be noted that high-quality flexible couplings are designed to tolerate more misalignment than is ideal for the machines involved. Bearing load increases with misalignment, and bearing life decreases as the cube of the load increase, therefore, a doubling of the load will shorten the bearing life by a factor of eight.
There is generally little consensus among machinery manufacturers and users as to the allowable, and/or preferred alignment tolerances. As a minimum, the recommendations of the coupling or pump manufacturer should be followed.WW
Reference: "Pump User's Handbook, Life Extension" by Heinz P. Bloch & Allan R. Budris, Third edition, 2010, by Fairmont Press, Inc.
About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, NJ, he can be contacted via e-mail at email@example.com.