Why motor shaft alignment is necessary ?
The main purpose of shaft alignment is to make sure that the center line of the motor rotor shaft coincides with the center line of the driven machinery i.e., pump or a generator. Shaft misalignment is responsible for as much as 50 percent of all costs related to rotating machinery breakdowns. Accurately aligning shafts can prevent a large number of machinery breakdowns Thus, alignment is necessary because
- Increased friction, resulting in excessive wear, excessive energy consumption, and the possibility of premature breakdown of equipment
- Excessive wear on bearings and seals, leading to premature failure
- Premature shaft and coupling failure
- Excessive seal lubricant leakage
- Failure of coupling and foundation bolts
- Increased vibration and noise
The ship’s engine room is full of such rotating equipment such as pumps, generators and so forth. It is therefore necessary for a marine engineer to have thorough knowledge about shaft alignment methods, techniques and concept
Describe the types of misalignment in motor couplings?
Generally there are two main types of misalignments
- parallel (also known as offset)
- angular (also known as gap)
In parallel misalignment, the two shafts to be aligned have centerlines that are parallel to each other but are in offset condition.
Parallel misalignment can be further subcategorized as either horizontal or vertical misalignment. Horizontal misalignment refers to misalignment of the shafts in the horizontal plane, while vertical misalignment refers to misalignment of the shafts in the vertical plane. Parallel horizontal misalignment results when the motor shaft is moved horizontally away from the pump shaft, but both shafts still operate in the same horizontal plane and parallel. Parallel vertical misalignment occurs when the motor shaft is moved vertically away from the pump shaft, but both shafts still operate in the same vertical plane and parallel.
In angular misalignment the axis of the two shafts are located at an angle to each other.
angular misalignment can be further subcategorized as either horizontal or vertical misalignment. Angular horizontal misalignment occurs when the motor shaft is at an angle with the pump shaft, but both shafts still operate in the same horizontal plane. Angular vertical misalignment occurs when the motor shaft is at an angle with the pump shaft, but both shafts still operate in the same vertical plane
Describe different methods for checking alignment of motor couplings?
Traditional alignment methods still in common use today include visual inspection combined with a straightedge or ruler. The straightedge is positioned on two bearings supporting one or more shafts, while the maintenance inspector visually assesses whether or not the components are properly aligned. While such rough alignment methods have the advantage of being quick and relatively easy, they are also highly inaccurate and do not produce the exacting degree of accuracy required by today’s precision machinery.
Dial indicators represent another traditional method of measuring misalignment. But while dial indicators do offer a higher degree of accuracy, they also present certain problems. Not only do they require a high level of technical skill to be used properly, but the effort is also generally quite time-consuming
Laser-guided tools, are quick, accurate, easy-to-use, and require only a single installation. In addition, they deliver consistently better accuracy than dial indicators and they do not require special skills to obtain accurate results virtually every time. Shaft alignment laser-guided tools typically consist of two units, each capable of emitting a precise laser beam and detecting a laser beam from its mate, plus a handheld control device.
The units securely mount to shafts via magnetic brackets and/or a magnetic chain. To run them, the operator activates the instrument via the handheld control device and each unit emits a precise laser line projected onto the other unit’s detector. A display on the handheld 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. In addition, the laser system tool documents the values, which can be downloaded to a computer and used as a benchmark for future alignment inspection
Describe the process of checking shaft alignment of couplings/shafts ?
Alignment of the shaft is possible only if the two ends of the shaft are not bent or crooked but faces each other in perfect symmetry. Now these two ends when to be aligned can be done in two main methods.
- Offset alignment
- Angular alignment
Offset alignment or radial alignment is checked using a straight edge ruler. This is done by placing the ruler at various parts of the coupling halves, that is to say at the following sides
Any gap between the shafts will indicate that the shaft is offset or misaligned slightly and it needs to be rectified. This misalignment can be rectified by adjusting the motor or by raising the platform of the motor or driven machine using shims.
This method can be used by inserting a feeler gauge and here are the steps listed below.
- Insert the feeler gauge between the coupling faces of the shafts
- After the feeler gauge is put, both couplings are rotated half way simultaneously
- The readings on the feeler gauge are checked at four points on the shaft coupling
- Any difference in the reading indicates that there is an angular displacement between the shafts
- The misalignment can be corrected by lifting the machinery or by using shims underneath the platform
In order to prevent any kind of misalignment from re-occurring, the motor and the driven machinery should be solidly bolted to the base of the machine after the final positions of both, the motor and driven machinery are decided