Research on Measuring Large Diameter Accuracy by Raising the Roller Method

Large-scale mechanical equipment, such as turbines, steam turbines, large generator sets, and large bearing rings, often requires high-precision measurement of their large diameters to ensure accurate control of size during the machining process. Measuring large diameters presents a series of unique challenges, including small-scale measurements and online measurement capabilities. Currently, several methods are used for this purpose: 1) The π rule is commonly used due to its simplicity, but it suffers from significant measurement errors and limited accuracy improvement. 2) Using a large caliper can be problematic, as the device becomes impractical with increasing diameter. 3) Electronic calipers offer higher accuracy, but their measurement range remains limited. 4) Scale-based methods are more advanced, but large scales are expensive and not easily suitable for online use. 5) Rolling method involves measuring the circumference of the object and calculating its diameter. This method is flexible, capable of measuring both small and large parts, and allows for easy online measurement. Despite its theoretical accuracy, the rolling method faces challenges, particularly in preventing slippage between the roller and the workpiece, which can introduce errors and limit precision. Solving the issue of slippage at the contact point is key to perfecting this technique.

1 Roller Measurement Principles and Measures to Improve Accuracy

The principle of measuring large diameters using the roller method is illustrated in Figure 1. A roller is used to measure the circumference of the object under test, and the relationship between the circumference and the diameter is used to calculate the diameter of the measured device:

D = L / π

Figure 1 Roller diameter measurement

Here, L represents the perimeter of the object under test. It is clear that the accuracy of the measured diameter D depends on the precision of the perimeter measurement. Therefore, when performing the measurement, the roller should be pressed against the object with appropriate pressure to ensure pure rolling without slipping. When the object rotates, the transmission relationship between it and the roller is given by:

n π d = N π D

D = (n/N) d (1)

In this formula: n is the number of revolutions of the roller, N is the number of revolutions of the object under test, d is the diameter of the roller, and D is the diameter of the object being measured. This method solves the problem of small-scale measurement and facilitates online inspection. However, the critical condition is that there must be no slippage between the object and the roller during rotation. Unfortunately, slippage has always been a challenging issue in the rolling method. To address this, several measures have been taken to reduce slippage.

1.1 Aligning the Axes of the DUT and the Roller

During installation, the axes of the object under test (DUT) and the roller must be parallel, as shown in Figure 2(a). This ensures that they rotate in the same plane across all radial sections. If no pressure deformation is considered, the contact between them is point contact, as seen at point A in Figure 1. Since both rollers rotate at the same linear speed and in the same plane, the relative speed at point A is zero, and the arc lengths they cover are equal, indicating no slippage. However, if the axes are not strictly parallel, as shown in Figures 2(b) and 2(c), the rotation speeds differ. As a result, the roller's rotation speed is reduced, causing slippage and an inaccurate measurement. By designing a fine adjustment mechanism that allows the roller shaft to swing closer to the position parallel to the DUT axis, the angle θ between the two axes can be adjusted. When θ = 0, the axes are parallel, and the measured diameter is maximized. Adjusting the angle in the opposite direction reduces the measured diameter. Only when the measured diameter is maximum does it indicate that the axes are parallel, ensuring accurate measurement.

1.2 Reducing Friction in the Roller Shaft

When the roller rotates, friction between the roller shaft and the bearing affects its motion. Minimizing this friction helps reduce slippage. An air bearing with minimal friction is ideal, so we designed an air flotation top structure, as shown in Figure 4. The top is hollow and ventilated, filled with high-pressure gas at the upper and lower ends. The hole throttling method is used, where compressed air passes through the center hole and exits through the gap. The top seat and the top part are slightly ground, creating a taper gap under static conditions. When gas flows in, the top ends separate slightly, forming a wedge-shaped gap. High-pressure gas enters the air chamber through the throttle, and the same structure is used at the top and bottom. The pressure difference between the upper and lower chambers allows the roller to float, minimizing friction and further reducing slippage compared to conventional bearings.

2 Data Acquisition

Data acquisition involves measuring the number of revolutions of the object under test (N) and the roller (n). A mark is placed on the side of the object, and when it passes a photoelectric switch, a pulse signal is generated. Each revolution produces one pulse, which is amplified and shaped before being sent to the computer for counting. For the roller, Moiré measurement technology is used. A circular grating is mounted on the roller shaft, and a light bulb, lens, and indicator grating are placed in front of it. A photoelectric receiver is installed behind the grating, creating a moving Moiré fringe. With a grating perimeter line count of 10,800, each rotation generates 10,800 pulse outputs. The computer counts these pulses to determine the number of rotations. Signal amplification uses an inverting proportional amplifier circuit, and a Schmitt trigger is added for noise reduction. Counting is done using a microcontroller, with T0 and T1 timers recording the revolutions of the object and the Moiré fringes, respectively. The calculated diameter is then displayed on an LED via a parallel interface.

3 Programming

With the hardware in place, the system counts the number of revolutions of the object and the Moiré fringes, calculates the diameter, and displays it on the LED. The main program flowchart and counting program flowchart are shown in Figures 5 and 6, respectively. These diagrams illustrate the sequence of operations, from starting the measurement to displaying the final result. The software ensures accurate counting and processing of signals, improving the overall performance of the measurement system.

4 Conclusion

The traditional roller method, when enhanced with effective accuracy improvements, can achieve an order-of-magnitude increase in measurement accuracy and repeatability. Thus, using the roller method for online measurement of large diameters remains a convenient and reliable technique, especially in industrial applications requiring high precision and real-time data.

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