Correcting Shaft Misalignment: Three Steps to Smooth Production
- Published: Tuesday, 12 October 2010 09:52
- Written by Coal Age News
By Paul Michalicka
When shafts or belts on rotating equipment are misaligned, unplanned machine downtime probably lies ahead. In the mining industry, this can mean large conveyors and crushers coming to an unexpected and complete stop at a cost of thousands of dollars per hour in lost production. Techniques and tools exist that can spot trouble before it happens, correct the root causes, and maintain the alignment that keeps conveyors and crushers running smoothly.
Misalignment is one of the most common causes of rotating component malfunction. Misaligned shafts and belts are inherently at risk of failure and are a primary cause of bearing fatigue, which accounts for 36% of all premature bearing failures.
Misalignment also damages seals and couplings. Often, lubrication problems are traceable to seals that have been compromised by shaft or belt misalignment. Simply replacing a seal will not stop future seal failure and associated loss of lubricant. The solution lies in correcting the shaft or belt misalignment.
Moreover, the increased forces necessary to put rotating components into motion when shafts or belts are misaligned will increase the cost of energy usage by a minimum of 3%. All points considered, shaft alignment is one of the most important practices a mine can apply to an overall equipment maintenance program.
Use the Right Tools
Best practices exist for measuring and correcting misalignment, and they start with the use of advanced instruments designed specifically for the job, but first, a word about practices to avoid. One common method used to check and align both shafts and belts is visual inspection combined with a straight edge. For shaft alignment, the straight edge is simply positioned on two bearings supporting one or more shafts, while the maintenance technician relies on a visual examination to determine if the components are in alignment. Belt alignment likewise makes use of visual inspection, with the straightedge touching two opposing pulley wheels. While the practice is simple and fast, it does not provide the accuracy necessary to consistently and properly align components.
The use of dial indicators is another prevalent way to measure misalignment. Dial indicators provide a high degree of accuracy, but present certain drawbacks. For example, they require a degree of technical skill to use properly. Further, they do not provide real time values that allow technicians to simultaneously measure and attain correct alignment. Instead, the indicators need to be removed and then reinstalled after the completion of each alignment adjustment. Critical measurements include coupling and feet values. Feet values indicate whether the pedestal or footing on which the machine rests is loose or in some other way compromised. Coupling values report the integrity of a coupling connecting two shafts.
Step One: Checking for Misalignment
Laser-guided tools for both shaft and belt alignment are accurate, easy to use and require only a single installation. Shaft alignment laser-guided tools consist of two units, each capable of emitting a precise laser beam and of detecting a laser beam from its mate, plus a hand-held control device. In the mining industry, crushers may experience shaft misalignment. Typically, a crusher has a shaft and a cone, and is driven by an electric motor. To measure alignment between the two machines, the maintenance technician places one laser unit on the motor’s shaft and one on the crusher’s shaft. The units securely mount to shafts via magnetic brackets and/or magnetic chain. In operation, the technician activates the instrument via the hand-held control device and each unit emits a precise laser line projected onto the other unit’s detector. A display on the hand-held device provides real-time coupling and feet values during the alignment process, avoiding the need to remove and reinstall the measuring units after each alignment adjustment. An important added benefit: the tool documents the values, which can be downloaded to a PC and used as a benchmark for future alignment inspections.
Belt misalignment causes increased wear on pulleys and belts, increased friction and energy consumption, increased noise and vibration, and premature bearing and belt failure. Maintaining them in proper alignment is equally as important as aligning shafts. For that reason, new laser-guided belt aligning devices are an essential part of an up-to-date maintenance technician’s tool kit.
Belt alignment laser-guided tools consist of just two components, a laser emitting unit and a 3-D receiver. The tool uses V-guides along with powerful magnets that quickly attach in the pulley groove to facilitate alignment of pulleys even if they have unequal widths or dissimilar faces.
When Should These Tools be Used?
Alignment checks should be performed whenever vibration, high operating temperatures or noise lead a maintenance technician to suspect an alignment problem might exist. These are primary symptoms of shaft and belt misalignment.
Hand-held instruments have been developed that provide an overall velocity vibration reading. The reading measures a machine’s vibration signals and compares them with preprogrammed ISO guidelines. The instrument generates an alert when measurements exceed the guidelines. It can also produce two different measurements for each point of measurement on a machine. These are overall velocity and enveloped acceleration. The same instrument measures the surface temperature of each measurement location.
Also of value to maintenance personnel is a new generation of stroboscopes that allow users to measure rotational speed and examine belts, pulleys and other moving parts as though they were stationary. The stroboscopes employ LED technology that makes it easier to see the belts in operation. A display clearly shows flashes per minute, reciprocating speed and other metrics. Operating speeds that are slower than previously recorded speeds and worn belts are usually signs of misalignment.
Step Two: Correcting Misalignment
Once misalignment has been detected, procedures must be taken to correct it. “Soft foot” is among the most common causes of misalignment. Accordingly, one of the first actions to take when tackling machine misalignment is to check the equipment’s foundation and employ a suitable shaft alignment tool to identify and correct potential soft foot conditions. Some laser-guided alignment tools have a “soft foot operation” capability that guides the user through corrective procedures. Usually, shim plates, which can be sourced in packs of varying dimensions to accommodate differing adjustment needs, are capable of bringing machines back into both vertical and horizontal alignment.
It is important to note that when two machines are misaligned, one will be designated the stationary machine and the other movable. It is the movable machine, usually a motor, that will be shimmed into alignment with its counterpart, such as a conveyor or crusher.
Bearings out of square with their housings are another common cause of misalignment. Often a condition of wear, the out of square bearing and/or its housing will need to be corrected or replaced in order to bring machines back into alignment.
Step Three: Maintaining Proper Alignment
Once misalignment has been detected and its cause determined and corrected, it is important to maintain proper alignment to keep production running smoothly. As part of this maintenance, alignment checks should also be performed to document alignment conditions before a machine is removed from service. This gives users the ability to install new or rebuilt machines with the same proper alignment values. Also, a maintenance program should include periodic checks to verify conditions are within tolerance.
Taking these three simple steps to detect, correct and maintain proper shaft and belt alignment can prevent costly, unplanned downtime in the mining industry. Fortunately, today’s equipment makes these three steps easier than ever.