Evidence suggests that as much as 50% of machine breakdowns can be directly attributes to incorrect shaft alignment.
What is shaft alignment?
Shaft alignment is the process whereby two or more machines are positioned such that at the point of power transfer from one shaft to another, the axes of rotation of both shafts should be collinear when the machine is running under normal conditions.
Important points to note:
- All shafts have some form of catenary due to their own weight, thus shafts are not straight, therefore the location where the alignment of two shafts can be compared is only at the point of power transfer from one shaft to another.
- “Shaft alignment” is not “Coupling alignment”, the coupling surfaces should not be used to define alignment condition since they do not represent the rotation axis of the shafts.
- The alignment can change when the machine is running due to thermal growth, piping strain, machine torque, foundation movement and bearing play.
Misalignment types
Misalignment, just like unbalance, is a major cause of machinery vibration. Some machines have been incorporated with self-aligning bearings and flexible couplings that can take quite a bit of misalignment. However, it is not uncommon to come across high vibrations due to misalignment.
There are basically two types of misalignment:
- Parallel misalignment: the shaft centerline of the two machines is parallel to each other and have an offset.
- Angular misalignment: the shaft centerline of the two shafts meets at angle with each other.
- Or a combination of parallel and angular misalignments
Parallel Misalignment
Parallel misalignment results in two hits per cycle and therefore, 2X vibration in the radial direction is dominant. Parallel misalignment shows high radial vibration that approaches a 180° phase difference across the coupling in the radial direction. Thus, we will see both the 1X and 2X peaks. Coupling construction will often significantly influence the shape of the spectrum if misalignment is severe.
Angular misalignment
Angular misalignment primarily subjects the driver and driven machine shafts to axial vibrations at the 1X, high 2X, and 3X in addition to 180 deg phase difference across the coupling in the axial direction.
Alignment tolerances
The suggested tolerances shown on the following slide are general values based on years of experience. They should be used only if no other tolerances are prescribed by existing in-house standards or by the machine manufacturer.
Consider all values to be the maximum allowable deviation from the alignment target.
*Rigid couplings have no tolerance for misalignment, They should be aligned as accurately as possible.
Alignment methods
Preparation
Eyesight - Straightedge
This method involves placing the straight edge on the pump and coupling, then visually checking to see if the components are aligned. Feeler gauges are used to measure the gap at the top and bottom of the coupling. Alignment is then done through a process of trial and error.
While the easiest and least expensive method of alignment, straight edge is the least accurate and least likely to achieve manufacturer’s alignment specifications. It’s usually used for small pump/motor combinations that don’t allow enough room to use more accurate methods.
Dial indicators
Dial indicators are used to measure the relative position of machine shafts. They have a spring-loaded plunger that causes the dial to move clockwise or counterclockwise depending on if the plunger is pushed in or let out.
To measure misalignment, dial indicators are mounted on fixtures attached to the shafts. Readings are taken in multiple positions to determine offset and angularity. Bar sag from the weight of the fixtures must be measured and accounted for in the readings.
The rim-face method uses two dial indicators, one on the rim and one on the face, to measure offset and angularity and determine the relative positions of the shafts. This method implements either a trial and error basis or a calculation system.
Advantages of Using Dial Indicators:
- Precision: High accuracy in measuring misalignments.
- Versatility: Can be used for various types of machinery.
- Reliability: Consistent results when used correctly.
Considerations:
- Skill Required: Proper use of dial indicators requires training and experience.
- Initial Setup Time: Setting up the indicators and taking measurements can be time-consuming.
Laser alignment
Laser alignment is the most accurate method available. This method employs state-of-the-art lasers to determine shaft positions. It then relates this information to the computer, allowing very precise recommendations for adjustments. While more expensive, laser alignment offers significant advantages over time, and initial costs can be offset by reduced friction and energy use, lower levels of vibration and noise and longer lasting bearings, seals, shafts and couplings.
Advantages of Using Laser Alignment:
- High Precision: Extremely accurate measurements, often within thousandths of an inch.
- Ease of Use: User-friendly interfaces and real-time feedback simplify the alignment process.
- Speed: Faster setup and measurement compared to traditional methods.
- Data Storage: Ability to store and analyze alignment data for future reference.
Considerations:
- Cost: Higher initial investment compared to traditional alignment tools.
- Training: Operators may require training to use the system effectively.
- Maintenance: Regular calibration and maintenance of the laser equipment are necessary to ensure ongoing accuracy.
U-joint alignment
Benefits of cardan shaft alignment. Why is it important?
- Vital part in drives for process machines, breakage of cardan shafts causes down time.
- Badly aligned cardan shafts set limitation in production capacity and product quality.
- Stands for a considerable cost when repair is needed or a breakdown occurs.
- Badly aligned cardan shafts reduces lifetime in gearboxes and motors.
- Break down of a cardan shaft in running condition could be extremly dangerous.
Cardan shaft alignment tolerances
There currently is no standard for all Cardan shafts but in general:
- Angular error within 0.25 mils/in
- Offset within specifications.
*Remember: U-joints must be kept in phase. Small changes in phase = big vibration problems.
