The pump is aligned by the coupling.
After the pump has been inspected and repaired, in order to ensure its normal operation, it is necessary to ensure that the shaft of the pump and the prime mover are in the same straight line during operation, so as to avoid additional stress on the bearings caused by the mutual deviation of the center of the shafts of the pump and the prime mover during operation, which may lead to overheating and wear of the bearing bush and overloading of the prime mover, and even cause severe vibration that stops the pump unit from operating. The alignment after the pump inspection is carried out on the coupling. At the beginning, use a level to compare the relative positions of the two couplings of the prime mover and the pump around the coupling, find out the direction of the deviation, and then roughly adjust to make the center of the coupling close to alignment. The two end faces should be close to parallel. Generally, when the prime mover is an electric motor, the center of the coupling should be adjusted mainly by adjusting the pads of the motor's foundation; if the prime mover is a steam turbine, the center should be found mainly by adjusting the pump. During the alignment process, it is easier to achieve the purpose of centering by adjusting the end face of the coupling first, and then adjusting the center. The following will be introduced step by step.
1. Preparations before measurement
Depending on the different forms of the coupling, the circular clearance α and the end face clearance b can be directly measured using a feeler gauge or a dial indicator.
During the measurement process, the following points should also be noted:
(1) Before alignment, the two coupling shafts should be connected using the special alignment bolts. If it is a fixed coupling, the two parts should be inserted properly.
(2) During the measurement process, the axial position of the rotor should remain constant to avoid errors caused by the lateral movement of the rotor when it is rotated.
(3) Before measurement, make sure to fully tighten all the anchor bolts.
(4) When calibrating, it must be done under cold conditions. During hot operation, the center cannot be determined.
2. Measurement Process
Mark and align the two coupling devices. Place the marked ones in the zero position (vertical or horizontal position). Install the special tool holder or dial indicator, and rotate the rotor in the rotational direction from the zero position in sequence by 90°, 180°, and 270°. At the same time, measure the circumferential clearance α and end face clearance b at each position, and record the measured data in the figure as shown in Figure 1. Based on the measurement results, take the average of the values of each point within the two end faces, and record them as shown in Figure 2.
1. Measurement of the clearance between couplings a and b (using a dial indicator)
1 - Wheel coupling; 2 - Adjustable bolt; 3 - Bridge gauge; 4 - Micrometer
II. Gap Record Chart of a and b
By analyzing the above data comprehensively, it is possible to determine the inclination of the coupling and the direction that needs adjustment.
3. Analysis and Calculation
Generally speaking, the states that the rotor can be in are limited to the following几种
The end faces of the coupling are not parallel to each other. Although the center lines of the two rotors are not on the same straight line, the centers of the two couplings are exactly aligned as shown in the figure. During adjustment, the 3rd and 4th bearings can be moved by values of δ1 and δ2 respectively to make the center lines of the two rotors form a straight line and the end faces of the couplings be parallel. The calculation formulas for δ1 and δ2 can be derived based on the proportional relationship of similar triangles, that is:
Coupling is concentric but not parallel.
In the formula, Δb = b1 - b2; D is the diameter of the coupling; L1 is the distance for adjusting the coupling to the 3rd bearing; L2 is the distance between the 3rd and 4th bearings.
The end faces of the two coupling devices are parallel to each other, but their centers do not coincide, as shown in the figure.
The coupling is not parallel and not concentric.
When adjusting, you can separately move the 3rd and 4th bearings by 1 d each. Then the two rotors will be concentric and in a straight line.
(3) The end faces of the two couplings are not parallel to each other, and their centers do not align either. This is a common situation.
4. Allowable error during adjustment
When adjusting the gasket, the measuring table frame should be removed or loosened. The ground feet of the gasket and the dirt on the gasket should be cleaned thoroughly. Finally, when tightening the ground bolt, the additional wedge iron or jack and other supporting objects should be removed, and the change in the reading of the dial indicator should be monitored. As for the allowable error for centering the coupling, it varies with the form of the coupling and can be referred to in the table.
Allowable error for centering of the coupling (mm)
Coupling types
Distance between points (the difference between any two of the numbers a, a2, a3, a4)
Face distance (difference between any two of I, II, III, and IV)
Rigidity and Rigidity
0.04
0.03
Rigidity and semi-flexibility
0.05
0.04
Flexibility and Flexibility
0.06
0.05
Gear-type
0.10
0.05
Spring type
0.08
0.06
Furthermore, as the operating conditions change, such as when the water pump is delivering hot water (above 60℃) or when the water pump is driven by a steam turbine, the situation where the pump and the turbine rotor expand due to heat and cause the center to rise should be considered together with the formula calculation values of the coupling center. For example, if the motor and the water pump are installed on the same base and the water temperature being pumped is 60℃, the motor needs to be raised by approximately 0.40 - 0.60mm to ensure that the center of the pump and the motor's shaft are exactly aligned during operation.
VI. Straight Axis Operation
When the shaft bends, first, the entire length of the shaft should be measured using a dial indicator at room temperature, following the method described earlier, and a bending curve should be drawn to determine the location and degree of the bending (in any section of the shaft, the difference between the maximum and minimum values of the relative position's runout is 1/2 of the size). Secondly, the shaft should also undergo inspection work.
Conduct inspection work on the shaft:
(1) Check the area where the maximum bending point of the shaft is located for cracks. Use methods such as immersing in kerosene and then applying white powder or other techniques to inspect the cracks, and eliminate them before straightening the shaft. Before eliminating the cracks, determine their depth using methods such as grinding, turning, or ultrasonic testing. For minor cracks, they can be repaired to prevent crack expansion during the straightening process; if the depth of the cracks affects the strength of the shaft, they should be replaced. After eliminating the cracks, conduct a rotor balance test to compensate for the imbalance of the shaft.
(2) Measure the hardness of the shaft surface at the crack location and in the surrounding normal areas separately to understand the degree of change in the metal structure of the bent part, in order to determine the correct method for straightening the shaft. The shaft that has undergone quenching treatment should undergo annealing treatment before straightening.
(3) Inspect the material If the material of the shaft is uncertain, sampling analysis should be conducted. Only after knowing the chemical composition of the steel can the appropriate shaft straightening method and heat treatment process be determined. After all the above inspection work is completed, an appropriate shaft straightening method and tools can be selected for the shaft straightening process. The methods of shaft straightening include mechanical compression method, twisting method, local heating method, local heating and compression method, and stress relaxation method, etc. These methods will be introduced one by one below.
Twisting method (cold straightening method)
The twisting method involves using a twisting rod to perform twisting and vibration on the concave part of the bent shaft. This causes the internal cohesion between the metal molecules in the concave area (where the fibers are compressed and shortened) to decrease, thereby enabling the metal fibers to extend. At the same time, the metal surface of the shaft at the twisting point undergoes plastic deformation. The fibers thus acquire residual elongation, achieving the goal of straightening the shaft.
The basic steps of twisting and rubbing are:
Based on the measurement results of the shaft's bending, determine the position of the straight shaft and make marks accordingly.
(2) Select the appropriate twisting rod for the operation. The material of the twisting rod is generally 45# steel. Its width depends on the diameter of the shaft (usually 15 to 40 mm). The working end of the twisting rod must match the arc of the shaft surface, and the edge should be rounded without sharp corners (R1 = 2 to 3 mm) to prevent damage to the shaft surface. After the twisting rod is rolled at the top, it should be repaired or replaced in time to avoid damaging the pump shaft. The shape of the twisting rod is shown in the figure.
The shape of the twisting rod
When using the straight shaft, place the shaft concave side upwards. Support the lower part of the maximum bending section with hard wood and pad it with lead plates, as shown in the figure.
Additionally, when using the direct-axis method, it is best to place the shaft on a dedicated stand and press both ends of the shaft downward to accelerate the vibration of the metal molecules and cause the fiber to elongate.
(4) The twisting range is one-third of the circumference (i.e. 120°). This range should be marked on the shaft in advance. The axial length during twisting should be determined according to the size of the shaft's curvature, the material of the shaft, and the degree of surface hardening of the shaft. Generally, it is controlled within a range of 50 to 100 mm. The twisting sequence should alternate at symmetrical positions, with more twisting at the center and less at both sides. As shown in the figure.
(5) During the twisting process, a 1-2 kg hand hammer can be used to strike the twisting rod. The center line of the twisting rod should be aligned with the marked range on the shaft. The force applied during the hammering should be moderate rather than excessive.
(6) After each stroke is completed, use a dial indicator to check the changes in the curvature. Generally, the straightening process is faster at the beginning, but later it slows down as the surface of the shaft hardens. If the twisting at a certain bending point has no significant effect, then stop the twisting, find out the reason, determine a new appropriate position for the twisting again, and continue until the correction is achieved.
(7) After the straightening of the shaft, the shaft should be slightly bent in the opposite direction of the original bending by 0.02 to 0.03 mm, that is, it should be slightly corrected.
(8) Once the bending of the shaft reaches the required value, the twisting operation can be stopped. At this point, a comprehensive and meticulous measurement of all sections of the shaft should be conducted, and records should be kept.
(9) Finally, the twisting shaft is subjected to low-temperature tempering at 300 - 400℃ to eliminate the surface hardening of the shaft and prevent it from bending again after straightening.
The above-mentioned cold straightening method is a commonly used direct shaft alignment method in work. However, it is generally only applicable to shafts with relatively small shaft diameters and a bending degree of approximately 0.2mm. The advantages of this method are high straightening accuracy, easy control, less stress concentration, and no cracks occurring during the shaft straightening process. Its disadvantages are that there are residual compressive stresses in a small section of the shaft material after straightening, and the straightening speed is relatively slow.
2. Internal Stress Relaxation Method
This method involves heating the entire curved part of the pump shaft to a temperature that causes the internal stress to relax (lower than the tempering temperature of the shaft by 30 to 50 degrees Celsius, typically 600 to 650 degrees Celsius), and the entire shaft must be fully heated. Then, an external force is applied to cause the shaft to undergo an elastic deformation opposite to the original bending direction to a certain extent, and this process is maintained for a certain period of time. In this temperature range, the metal material undergoes a spontaneous relaxation phenomenon of stress reduction under the action of high temperature and stress, converting part of the elastic deformation into plastic deformation, thereby achieving the purpose of straightening the shaft.
The steps for straightening are:
(Measure the bending of the shaft, and draw the curve of the shaft's bending.)
(2) Clean the entire circumference of the largest bending section and check for any cracks.
(3) Place the shaft on a specially designed platform equipped with a rotating device and a pressurization device. Position the bent part of the shaft with the convex side facing upwards. Install a micrometer on the side of the heating area. The heating method can be through electromagnetic induction or by using a resistance wire electric furnace. The heating temperature must be 20-30℃ lower than the tempering temperature of the original steel to avoid changes in the steel's properties. During temperature measurement, the temperature of the shaft surface at the heated area is directly measured using a thermocouple. When straightening the shaft, do not rotate the shaft during the heating process.
(4) When the temperature at the bending point reaches the specified relaxation temperature, maintain the temperature for 1 hour, and then start applying pressure in the opposite direction (the convex surface) of the original bending. The point where the force is applied should be as close as possible to the maximum bending point, while the support point should be as far away as possible from the maximum bending point. The magnitude of the applied external force should be determined based on the degree of shaft bending, the heating temperature, the relaxation characteristics of the steel, the duration of maintaining the pressure, and the internal stress of the shaft caused by the applied force.
(5) The internal stress within the shaft caused by external force should generally be less than 0.5 MPa, and at most not more than 0.7 MPa. Otherwise, the maximum deflection of the shaft should be determined based on a stress of 0.5 to 0.7 MPa, and external force should be applied in multiple stages to finally straighten the bent part of the shaft.
(6) After pressurization, a stable period of 2 to 5 hours should be maintained, during which the temperature and pressure should not be changed. The external force applied should be perpendicular to the shaft surface.
(7) After maintaining the pressure for 2 to 5 hours, remove the external force, keep it at a constant temperature for 1 hour, and rotate the shaft every 5 minutes by 180 degrees to ensure uniform temperature distribution on the upper and lower parts of the shaft.
(8) Measure the changes in the bending of the shaft. If the requirements are met, then a stable annealing treatment after straightening the shaft can be carried out; if the shaft has been straightened too much, it needs to be straightened back. In this case, the required stress and deflection should be reduced by half compared to the values required during the first straightening process.
When using this method to straighten the shaft, the following points should be noted:
(1) When applying force, do it slowly and ensure the direction is directly towards the convex surface of the shaft. The point of application should be padded with aluminum or copper sheet to prevent scratching the surface of the shaft.
(2) During the pressurization process, a dial indicator should be installed on the left and right (lateral) sides of the shaft to monitor the lateral changes.
(3) At the heating area and its vicinity, insulating materials should be wrapped with asbestos layers for insulation.
(4) When heating, it is advisable to use two thermocouples for temperature measurement, and at the same time, an ordinary thermometer should be used to measure the temperature near the heating point to verify the temperature readings from the thermocouples.
(5) When using a straight shaft, the initial heating rate should be 100 to 120℃ per hour. Once the temperature reaches the maximum, apply pressure; after the pressure application is completed, cool down at a rate of 50 to 100℃ per hour. When the temperature drops to 100℃, allow it to cool naturally at room temperature.
(6) The shaft should be cooled down while in rotation. Only in this way can the cooling be uniform and the contraction be consistent, and the bending apex of the shaft will not change its position.
(7) If the straightening operation is performed more than twice, and if it is certain, the last straightening can be combined with the annealing treatment. The internal stress relaxation method is applicable to any type of shaft and has good effects, is safe and reliable, and is widely used in practical work. Regarding the calculation of the external force applied in the internal stress relaxation method, this will not be further introduced here. When applying it, one can refer to the calculation formulas in the relevant technical books.
3. Local heating method
This method involves rapidly applying localized heating to the convex surface of the pump shaft, artificially creating compressive stress on the shaft that exceeds the elastic limit of the material. When the shaft cools down, the metal fibers on the convex surface are compressed and shortened, resulting in a certain degree of bending, thereby achieving the purpose of straightening the shaft. The specific operation method is as follows:
(Measure the bending of the shaft and draw the curve of the shaft's bending.)
(2) Clean up and inspect the cracks on the entire circumference of the maximum bending section. Make sure to check and record properly.
(3) Place the shaft convex side upwards on the special stand, and install dial indicators on both sides close to the heating area to observe the changes after heating.
(4) Wrap the largest bending area with asbestos cloth, and center the asbestos cloth to form rectangular heating holes around the maximum bending point. The length of the heating holes (along the circumference) is approximately 25% to 30% of the shaft diameter at that point. The width of the holes (along the axis) is related to the degree of bending and is approximately 10% to 15% of the diameter at that point.
(5) Use smaller 5, 6 or 7-sized welding nozzles to heat the axial surface at the heating hole. When heating, keep the welding nozzle about 15-20mm away from the axial surface. Start from the center of the hole and then move to both sides, moving the torch evenly and periodically. When the heating reaches 500-550℃ (the axial surface turns dark red), immediately cover the heating hole with asbestos cloth to prevent rapid cooling which could cause the axial surface to harden or develop cracks.
(6) When correcting the pump shaft with a smaller diameter, the heating time can generally be controlled by observing the thermal bending value. The thermal bending value is the difference in the dial reading of the vernier caliper between the axis' straight position before heating and its position after heating (near the maximum bending section), when the protruding part of the shaft is heated by a torch. The general thermal bending value is 8 to 17 times the straightening amount of the shaft, that is, when the shaft is heated and protrudes by 0.08 to 0.17 mm, it can be straightened by 0.01 mm after cooling. The specific situation depends on the shaft's length-to-diameter ratio and the material. Regarding the relationship between the thermal bending value after the first heating of a shaft and the shaft's elongation,
(7) After the shaft has cooled to room temperature, use a dial indicator to measure the shaft's curvature and draw the bending curve. If it does not fall within the allowable range, it should be re-aligned. If the maximum bending point of the shaft does not respond to reheating, the shaft should be moved in the axial direction by a certain position at the original heating point, and then heated and corrected in sequence using two welding nozzles.
(8) The shaft should be slightly over-bent, that is, it should have a bending value of 0.01 to 0.03 mm in the opposite direction to the original bending. After the shaft undergoes annealing treatment, this over-bending value will disappear.
When using the local heating method, the following issues should be noted:
The straight-axis operation should be carried out in a room with dim lighting and without any air circulation.
(2) The heating temperature must not exceed 500 - 550℃. When observing the color of the shaft surface, color glasses must not be worn.
(3) The magnitude of the stress required for the straight shaft can be adjusted in two ways: one is to increase the heated surface; the other is to increase the depth of the metal layer of the heated shaft.
(4) When there is local damage to the shaft, if there is a locally high hardness surface at the straightening section, or if the pump shaft material is alloy steel, generally, the local heating method for straightening the shaft should not be used. Finally, the straightened shaft should undergo heat treatment to prevent it from bending again in a high-temperature environment. However, for the shaft operating at normal temperature, no heat treatment is necessary.
4. Mechanical pressurization method
This method involves using a spiral press to press the convex part of the bent section of the shaft downward, thereby compressing the metal fibers in that area and straightening the shaft, as shown in the figure.
Mechanical pressurization method for straightening shafts
5. Local heating and pressurization method
This method is also known as the thermal mechanical alignment method. It has the same heating parts, heating temperatures, heating times and cooling methods as the local heating method. The difference lies in that before heating, a pressure tool is used to apply force near the bending point, causing the shaft to undergo elastic deformation opposite to the original bending direction. After heating the shaft, the metal at the heated area is unable to expand freely and reaches the yield limit prematurely, resulting in plastic deformation.
This direct-axis method is much faster than the local heating method. Each heating cycle yields better results. If the bending after the first heating and pressure treatment does not meet the standards, a second heating can be carried out. The heating time for the second heating should be determined based on the effect of the initial heating, but it should be noted that the maximum number of heating times for a certain part should not exceed three. Among the five methods of straightening axes discussed in this section, mechanical pressure method and twisting method are only applicable to axes with smaller diameters and smaller bends; local heating method and local heating and pressure method are suitable for axes with larger diameters and larger bends. These two methods have better straightening effects, but there are residual stresses after straightening, and the surface quenching is prone to occur at the axis straightening area, which is easy to cause bending again during operation. Therefore, they are not suitable for correcting alloy steel and axes with hardness greater than HB180-190. The stress relaxation method is suitable for any type of axis, and it is safe, reliable and effective. However, the operation time is slightly longer.
