1 The meaning and measurement method of rotating machinery alignment
Rotating machinery alignment is the process of adjusting the position of the main shafts of two or more equipment to be connected to ensure that the shafts of the equipment are in a coaxial state under normal operating conditions.
Misalignment is one of the most common problems with rotating machinery.
According to relevant industry statistics, more than 50% of equipment damage can be attributed to misalignment and misalignment. The above-mentioned replacement costs, additional energy costs and production stoppage losses of the main shaft seals, bearings, couplings and the main shaft after damage caused by excessive centering deviation cannot be ignored for any unit, enterprise, or even public environment.
The centering deviation is usually divided into concentricity deviation, angular deviation and their combined deviation. In order to facilitate engineering measurement and equipment adjustment, the alignment deviation is generally decomposed into two components: concentricity deviation and angular deviation in vertical and horizontal directions, namely horizontal concentricity deviation, vertical concentricity deviation, and horizontal angular deviation. Deviation and vertical angular deviation.
Alignment method and alignment quality are closely related to technological development. There are straight ruler feeler alignment methods, dial indicator alignment methods, and laser alignment methods. Generally speaking, any alignment method can achieve sufficient accuracy, which can reach 0.001 ~ 0.01mm, which mainly depends on the precision of the instrument and the skill level of the alignment operator.
Now, the commonly used alignment methods are dial indicator alignment method and laser alignment instrument method.
The laser alignment instrument is completely based on the theory of dial indicator alignment, combined with advanced and precise optical and electronic technology, to minimize various error factors that are prone to occur in the dial indicator alignment method, and greatly eliminate the percentage of The error caused by the measurement equipment of the Chinese method. At the same time, it automatically completes a lot of calculation work, making the centering operation simple, fast and accurate. However, the high price of this type of equipment and some inherent errors of electronic instrumentation and control components limit its promotion to a certain extent.
The dial indicator is in contact with the measuring surface through the rod, and the relative movement of the rod is amplified by the transmission gear to measure the small space position change between the two axes, so as to measure its centering state.
There are currently two commonly used methods of dial indicator alignment: radial axial method and double radial method.
The radial-axial method is to use one meter to measure the concentricity deviation, and the other (to eliminate the influence of the shaft channeling on the angular orientation, two pieces are often evenly distributed in the diameter direction) dial indicator to measure the angular orientation deviation. , which is the most commonly used method.
The double radial method is to use two dial indicators to measure the concentricity deviation at the measurement point of the opposite shaft, and the concentricity and angular deviation of the shaft system can be calculated through the two sets of data.
Whether it is the radial-axial method or the double-radial method and their evolution alignment methods, such as the double-radial method and the double-axial method of the long coupling, their geometric principles are the same, and the measurement results should also be exactly the same. They have their own advantages and disadvantages in practical applications, and good measurement results can be obtained by selecting them appropriately according to the actual situation.
2 The main error factors of the dial gauge alignment method and their control methods
The dial indicator plays an important role in the centering operation of rotating machinery, but there are many error factors that need to be analyzed and controlled.
Common error factors and solutions include the following 10 aspects:
(1) Improper setting of the initial measuring point of the dial indicator and improper selection of the range
Improper setting of the initial measuring point of the dial indicator needle and improper selection of the range may cause the probe to hang in the air or get stuck during the rotation process, that is, the upper and lower dead points of the stroke appear on the dial indicator, resulting in unreal and inaccurate measurement results.
The specific solution is to select a dial indicator with a larger range as much as possible (especially in the initial alignment), generally select a range of 3 to 10mm, and set the initial measurement point (0 point) near the midpoint of the range.
Taking multiple measurements requires overall repeatability of the data, and selecting the most stable set of data.
There is also an important rule for judging the validity of measurement data when reading in the dial indicator. That is, the sum of the data in the vertical direction (0° and 180°) is equal to the sum of the data in the horizontal direction (90° and 270°).
In actual construction, if the difference between the two is greater than 0.02mm, it can be judged that the measuring table frame is not fixed firmly or other reasons to be analyzed below, and measures to eliminate it can be taken.
This data validity rule applies to the determination of the correctness of the readings of concentricity and angular deviation.
(2) Dial indicator stuck or affected by strong magnetic field
Dial indicator hands, sticking of the stem and the influence of strong magnetic fields will cause inaccurate readings. Such errors are mainly avoided by regularly calibrating and checking the flexibility of the dial indicator's hands and keeping them away from strong magnetic fields. Data validity laws apply to checking for this type of error.
(3) Data and symbol record errors
Due to human viewing angle, different judgment ability or wrong reading, the read value may deviate from the actual displayed value, which will naturally cause deviation.
Since the left and right deflection of the dial indicator pointer during the measurement process represents the positive and negative movement directions of the watch stem, the leftward deflection indicates that the watch stem is a positive displacement, and vice versa, it represents a negative displacement, so the percentage should be carefully and continuously observed during the entire measurement process. The table pointer is turned and the raw data is read correctly. Once the direction is judged incorrectly, the subsequent adjustment value will have a large deviation, and the alignment cannot be completed.
In addition to the above-mentioned correct reading method, the above-mentioned data validity law can also be used to judge whether there is a recording symbol error. Assuming that the theoretical values measured at 0°, 90°, 180°, and 270° with a dial indicator are 0, 17, 22, and 5, while the actual recorded data are 0, 11, 22, and 5, respectively, it can be found that 11 +5=16≠0+22, it can be judged that there is a reading error, (read 17 as 11); and suppose that 5 at 270° is read as -5, then 17+(-5)≠0+22 (The correct expression should be 17+5=0+22) It can be determined that the data is incorrect and is invalid data. Through analysis, it can be determined that the first case above may be the recording error of the reading, and then the ? is the error of the sign judgment. If it is not found in time and accurately, it will lead to the calculation error of the adjustment amount and the repeated adjustment is not in place.
If the data is incorrectly determined, the adjusted data obtained by calculation or drawing will also deviate far from the expected result and cannot be correctly aligned. On the other hand, it shows the necessity of the validity judgment of the middle data.
(4) Bearing radial runout and excessive bearing clearance
This error shows in the measurement data that it does not conform to the principle of data validity and cannot be eliminated by improving the structure of the watch frame. From the perspective of eliminating their influence on the alignment measurement, the influence can be eliminated first by measuring the bearing runout or by pushing the main shaft radially in the same direction at each measuring point, making it close to the bearing seat.
(5) Measuring surface irregularity or eccentricity
This error will also cause the readings not to conform to the data validity judgment principle. The usual elimination method is to ensure that the two axes rotate synchronously and that the positions of the measuring points are basically fixed, so as to eliminate their influence on the alignment data. In engineering construction, this error has been fully recognized and valued. However, it should be noted that some special equipment cannot be reeled during installation or during equipment shutdown and maintenance. This situation should be treated differently. The influence of surface irregularity or eccentricity on the measured value should be measured, and appropriate methods should be taken to correct or eliminate it. .
(6) Shaft channeling
Shaft drift is often a troublemaker in alignment measurement, it will seriously affect the data measurement of shafting angular deviation. Often a circumvention approach is taken to eliminate bias. Among the two commonly used dial indicator alignment methods, the radial-axial method uses two symmetrically installed dial indicators to measure the angular deviation, which can offset the influence of shaft channeling; the double radial method is used to prevent shaft channeling. influences. So this is the main reason why the double radial method is usually more accurate than the radial axial method.
(7) The rotation angle of the shaft system is inaccurate during alignment
Theoretically, the alignment deviation of the shafting can be calculated by measuring at any 3 angles, but in order to simplify the calculation, in the actual alignment measurement process, 4 evenly distributed measuring points are generally required on the main shaft or the hub. The readings are measured at 4 positions of 0°, 90°, 180° and 360°, but they often cannot be accurately positioned at these 4 angles, and the measurement point may deviate from the theoretical position. If it deviates from 5° to 10°, the resulting percentage The relative error of meter reading can reach 10% to 15%.
The main methods to avoid the deviation of the measurement reading caused by the uneven rotation angle are: use a spirit level to measure at 4 evenly distributed measuring points, or measure and mark in advance, and try to slow down the rotation process to ensure that it can accurately stop at each time. desired location.
The deviations in the above seven cases can be judged by the data validity rule.
(8) The dial indicator rod is not perpendicular to the surface to be measured
Due to the limitation of the structure of the watch frame and the operator's cognition, in the actual measurement process, due to the structure of the watch frame, the watch rod and the measured surface may often appear non-perpendicular phenomenon. If the inclination of the watch rod is within 15°, the reading error is generally within 5%, which can be ignored. When the inclination is 15° to 30°, there will be an error of 5% to 15%, which will seriously affect the measurement accuracy.
The measurement rod is not perpendicular to the surface to be measured, resulting in the readings being greater than the actual value. In actual construction, it is a very common problem that the measurement rod is not perpendicular to the surface to be measured.
(9) Deflection deviation of the table frame
Because of the overhanging structure of the dial indicator on the Chinese-French table frame, the table frame supporting the dial indicator and its extension rod and the gravity of the dial indicator cause elastic deformation of the table frame, which will bend downwards, which is called table frame deflection. Usually, during the centering measurement of a horizontal rotating machine, during the rotation of the watch frame, since the sliding direction of the watch rod changes with the rotation direction, it is not completely consistent with the direction of gravity. The influence of deflection at different positions on the reading of the dial indicator varies, so in the subsequent data processing, if it is not eliminated, it will seriously affect the accuracy of the measured value. Relative to the alignment tolerance of rotating machinery, sometimes the deflection will be several times to ten times the actual alignment tolerance.
Therefore, in the process of using the dial indicator for centering, the installation of the dial indicator frame and the extension rod should pay attention to reducing or even eliminating the influence of the deflection of the indicator frame. Since the dial indicator is fixed with deflection in both the horizontal and vertical directions, the results have an effect on the usual concentricity and angular deviation measurements.
According to the same or similar parameter state on the device to be tested, install and fix the watch frame on a horizontal circular tube (round rod) with sufficient rigidity and the fixing position of the watch frame and the measuring point should be as smooth as possible. Rod) as the mandrel benchmark, the main parameters (l and a and the size, quality, etc. of the dial indicator) should be exactly the same, and should be firmly fixed or ensure the same tightness. The radial deflection is measured by contacting the watch hand with the annular surface of the circular tube in the radial direction, and the axial deflection is measured by contacting the watch hand with the specially arranged end face of the circular tube that is perpendicular to the axis of the circular tube in the axial direction. Set the dial indicator to zero at the top 0°, then slowly rotate the whole device 180° to the bottom, and read the dial indicator reading. Half of this value is the vertical deflection of the watch frame.
In actual operation, if this error is not considered, the deviation between the measured data and the real value is very large, and the adjustment amount of the outrigger in the vertical direction determined by this data is also useless, and will be far away from the real value. Since the deflection of the concentricity is generally between 0.10 and 1.00mm, especially in the fine alignment stage, this error will occupy the main range of the dial indicator, which may lead to measurement overtravel.
On the other hand, the following measures can be taken to reduce the numerical value of the deflection error of the stand: shorten the distance from the fixed point to the measuring point as much as possible, thereby shortening the span of the stand; optimize the selection of the correct cross-sectional size and material of the stand to enhance the resistance Bending ability; try to use a small dial indicator; fix the watch stand correctly and firmly.
(10) Theoretical error of dial indicator measurement method
Since the dial indicator measurement method usually uses the formula in Appendix 15 of the national standard GB50231-1998 to calculate the actual deviation, it can be known from analysis that the formula is based on the approximation of the angular deviation and the concentric deviation which are small and exist alone. However, in actual engineering practice, especially in the initial alignment, the deviation may be relatively large, and it often exists in the form of comprehensive deviation, and there are angular deviation and concentric deviation at the same time. The existence of degree deviation will affect the measurement of concentricity deviation to varying degrees. When the influence of angular degree on concentricity is considered, the dial indicator for centering deviation measurement is very complicated. There are many related articles that describe the theoretical analysis of centering in detail. Generally, it takes at least 4-5 Only one parameter can be accurately expressed, and it includes the solution of the transcendental equation, which is difficult to handle in the actual measurement process. In actual engineering, it is impossible to measure and process many unknown parameters in the dial indicator alignment method. Even if there is an advanced microprocessor in the laser alignment instrument, the actual algorithm is mostly simplified alignment. Algorithms are theoretically based.
The overall solution to this treatment is two-fold.
(1) In the initial alignment stage, that is, when the angular deviation and concentricity deviation are relatively large (for example, the angular deviation is between 1/100 and 1/1000, and the concentricity deviation is between 0.2 and 2 mm), according to the simplified The measurement method and the corresponding adjustment value and the actual value of the theoretical value are deviated, and the deviation rate may be relatively large, but the change trend of the error is convergent, that is to say, as the number of adjustments increases, the error will become larger and larger. When the angular deviation is close to 1/1000, the influence of the angular deviation on the concentricity measurement can be basically ignored, and high accuracy can be achieved. Generally, a more accurate state can be achieved through 2 to 4 adjustments. Therefore, in actual construction, do not expect to be able to measure accurately and adjust in place at one time.
(2) Since the angular orientation directly affects the concentricity measurement accuracy, it is recommended to adjust the angular orientation first and then adjust the concentricity.
3. The deflection deviation of the table frame itself cannot be completely eliminated by the dial indicator measurement method, but it can be reduced by increasing the stiffness of the table frame above, and the influence of deflection on the centering measurement data can be basically eliminated by methods such as calculation or actual measurement.
Although the accuracy of the dial indicator is 0.01mm, the usual measurement error may be between 0.1 and 1.0mm, which is 5 to 10 times the concentricity tolerance of 0.02 to 0.10mm. The actual measurement results will deviate significantly from the true value, and there will be huge deviations. According to the survey results of an international well-known rotating machinery technical organization, the proportion of shaft alignment that actually meets its tolerance requirements is less than 7%, which is enough to show the importance of correct shaft alignment.