Misalignment of machine shaft centrelines cause up to 50 per cent of machine vibration problems and are the result of two common conditions. Parallel offset misalignment is a condition where the centrelines of the two shafts do not meet, whereas angular offset occurs where the centrelines of the shafts are not parallel to each other. Parallel and offset can also exist at the same time. In order to ensure that rotating machinery is reliable, productive and efficient, it is critically important that misalignment problems are eliminated.
Test your alignment knowledge by first answering the questions and then considering the validity in the accompanying solutions to ensure first-rate outcomes.
1. How do you recognize a misalignment problem through a visual inspection?
Logic: Symptoms of misalignment include powdered rubber or lubricant directly below the drive coupling (depending upon the coupling type), broken foot bolts, oil “shimmering” on the base plate or near the foot bolts, higher than normal energy consumption, cracked or broken foundation, premature or frequent bearing or seal failure, higher than normal operating temperature of one or both machines, or at the coupling itself, high vibration conditions – usually at both machines, and intermittent or continuous leaks caused by pipe strain.
2. How do you determine if a misalignment problem actually exists?
Logic: Using various diagnostic methods, such as vibration analysis, misalignment is characterized by a 180-degree phase difference across the coupling. Using a stroboscope, a misalignment is often indicated when the reference mark is unsteady or rotates intermittently in both directions and measuring temperature will indicate that a very minor misaligned coupling or shaft can cause dramatic temperature increases.
3. Once a misalignment is confirmed, why should you review the machine files?
Logic: Machine history will confirm any previous alignment problems, bearing or seal failures and past maintenance activities that have been carried out. In addition, understand that thermal growth will affect the resulting alignment, so it is very important to know what the proper operating temperatures of the machine components are, such as at the coupling and bearings. Thermal growth is affected by the type of material used for the machine housings. For example, the coefficient of thermal expansion of aluminum is twice that of cast iron, so aluminum housings will grow twice that of cast iron with the same temperature increase. Thermal expansion is about .01 mm (.000394 inch) per metre (40 inches) for each Celsius degree increase in temperature. Therefore, it is essential to have known and recorded the temperature measurement at each bearing and at the machine housing near each foot during normal operation. The thermal growth must be added when shimming the feet during the realignment process. Also, couplings come in all types and each has its own vibration frequencies and operating temperatures. These should be listed in maintenance files.
4. Are you certain that the correct coupling is in use?
Logic: The causes of coupling failure (depending on the application) are excessive misalignment, improper, inadequate or insufficient lubrication, harsh environmental or operating conditions and excessive speeds or loads, but a primary cause is improper coupling selection for a particular application. For example, a serpentine spring metallic grid coupling is generally recommended for variable high torque machine trains operating at moderate speeds, the transmission of high torque at low speeds should use a chain coupling, while the transmission of high torque at both high and low speeds should use a gear coupling. Once satisfied the correct coupling is in use inspect it for correct lubricant (if used), proper bolts (note length, machining and weight), eccentricity, loose components such as grids, elastomers and disks, proper shaft fit, worn teeth or grid members and correct setscrew length and tightness.
5. Why should you carry out a pre-alignment inspection?
Logic: there are other conditions that should always be considered before carrying out the realignment procedure; these include a review of the vibration analyses reports to determine if pipe strain is a concern because this condition can cause resonant frequencies that will trigger resonance in other components of either the drive, the driven machine, or both. If pipe strain exists, these distortions can be determined by mounting dial indicators, then tightening and loosening pipe flanges while noting dial indicator readings. Soft foot is a condition where a void exists between a foot of the machine, the shim pack and the base. This condition can cause distortion, reduced internal clearances, binding rotors and preloading of bearings and seals. Satisfactory alignments cannot be achieved if soft foot conditions are not corrected. Inspect the shaft keyways for wear and never replace a worn key with a key of lesser length or difference in weight.
6. Are your technicians trained to use effective alignment methods?
Logic: There are several methods in use, among them the reverse dial method used when both shafts can be rotated freely, the face-rim method that requires the use of true couplings, the electro-mechanical method that will calculate for bar and bracket sag and the laser alignment method, which is considered the most precise, but airborne dust can affect accuracy (two popular systems are the Optalign and Combi-Laser).
7. Do you ensure that the completed alignment meets acceptable standards?
Logic: There is a common perception that flexible couplings can accommodate misalignment, however depending upon the coupling type, these components can only accommodate from .25 to about 2.5 degrees misalignment, and high speed, high load applications should be aligned to closer tolerances.
Allowable misalignment tolerances are provided below as a guide.
Machine speed RPM Parallel- offset Misalignment Angular Misalignment
< 500 5 1.5
500 – 1,250 4 1
1,250 – 2,000 3 .5
2,000 – 3,500 2 .3
3,500 – 7,000 1 .25
> 7,000 .5 .2
8. Do you know what types of lubricant to use in lubricated couplings?
Logic: There are three coupling types that require lubrication, namely, gear, chain and “combination” mechanical/material grid couplings. The lubricants recommended include ISO 460 compounded oils containing tackiness agents (to withstand high centrifugal force due to high-speed rotation), ISO 100 oils for machines operating at temperatures at or below -20°C, while NLGI grades 1 and 2 grease containing tackiness agents are also available for these couplings.