Monthly Archives: February 2018
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Shaft Alignment for Rotating Machinery
By Anzar Hasan
Failure to properly align shafts results in premature failure of bearings, bearing seals, couplings, and shafts that, in turn, may lead to catastrophic machine failures and process or plant shutdown resulting in a combination of property damage and loss of business. Industry sources estimate that 50 to 70%of rotating machinery failures can be attributed directly to shaft misalignment. When a driver like an electric motor or a turbine is coupled to a pump, generator, kiln drive, blowers or any other piece of equipment, it is essential that the shafts of the two pieces are aligned. Any misalignment between the two increases the stress on the shafts and will almost certainly result in excessive wear and premature breakdown of the equipment.
Since machines are three dimensional, the misalignment of the shaft centerline is addressed by determining their projected position with respect to horizontal and vertical planes. Each position has two parameters; parallel and angular. Therefore misalignment is expressed as vertical (parallel and angular) and horizontal (parallel and angular). Parallel alignment is also referred as “parallel offset”.
Methods of shaft alignment:
• Dial Indicator Methods: The most commonly used method for shaft alignment is the dial indicator method. This method is not 100% accurate, but is considered adequate for small machines. Often alignment brackets are utilized to support dial indicators. Sometimes the brackets are specially made for application.
• Laser Method. This system utilizes a laser beam source and laser beam target in the same module and uses a reflecting prism in the second module. This is a widely used and accurate method for alignments. There are commonly two types of laser beam alignment procedures – React System and Digilaser System.
• Optical Method: This requires careful set up and technique, but has the advantage that it can be used for initial alignment, determining hot alignment, and checking cold alignment offset.
• Electromechanical Spatial Resolution Method: The unique feature of this procedure is that neither shaft must be rotated in order to determine shaft position. The procedures utilized sensor and signals can be electronically nulled to a zero calibration point, which establishes an electronic straight line reference.
• Continuous Monitoring Method: Large equipment often utilizes a continuous shaft alignment monitoring system. This method utilizes unidirectional lasers mounted on the driver and the driven machine for continuous read outs. There are several configurations systems used – Permalign, Dynalign, Acculign and Indikon system that uses eddy current proximity probes.
During inspection of critical rotating equipment, it should be the practice to establish whether or not the plant has a scheduled shaft alignment program.