Geometric Dimensioning and Tolerancing (GD&T) is a system used in engineering and manufacturing to precisely define the size, shape, orientation, and location of features on a part. Circular runout is an important GD&T concept that helps ensure the quality and proper functionality of cylindrical parts. Circular runout is represented by a specific GD&T symbol, which is crucial for maintaining precision and minimizing oscillations and vibrations in machinery.
What is Circular Runout in GD&T?
Circular runout is a geometric tolerance that controls the variation in the circular profile of a part as it rotates around an axis. It measures the total of any out-of-roundness, eccentricity, or other deviations from a perfect circular form as the part rotates around its rotational axis. In simple terms, it tells you how much a circular feature deviates from a theoretical perfect circle as it spins.
For example, think of a car wheel. The circular runout specification would help determine how much the actual shape of the wheel deviates from a perfect circle as it rotates, which is crucial for a smooth and safe ride. Circular runout is affected by deviations in circularity and axis offset, emphasizing the importance of these factors in the performance and alignment of rotating components.
Two Types of Runouts
Radial Runout
Radial runout measures the variation in the radius of a circular feature as it rotates around an axis. It is concerned with how much the distance from the center of rotation to the surface of the part changes during rotation across circular cross sections.
For example, if you have a cylindrical shaft, the radial runout would tell you how much the actual radius of the shaft varies as it spins. If the radial runout is too large, it can lead to issues such as vibration, uneven wear, and poor fit with other components.
Axial Runout
Axial runout, on the other hand, measures the variation in the position of the circular end – face of a part along the axis of rotation. Consider a pulley. The axial runout would describe how much the face of the pulley (perpendicular to the axis of rotation) moves up and down or wobbles as the pulley rotates. High axial runout can cause problems like misalignment of belts and improper force transmission.
Runout in GD&T
GD&T provides a standardized way to specify and control runout through runout control. The use of GD&T runout helps manufacturers communicate the acceptable levels of variation to ensure that parts function properly and fit together as intended. By using a feature control frame, engineers can precisely define the runout tolerance and the associated datum (reference) axis.
GD&T Runout Example
Let’s say we have a motor shaft. The engineering drawing might specify a circular runout tolerance of 0.05 mm for the shaft’s outer diameter with respect to a central datum axis. This means that as the shaft rotates, the maximum deviation of the circular profile from a perfect circle (in terms of the radius) should not exceed 0.05 mm.
Feature Control Frame of Circular Runout
Circular Runout Symbol
The symbol for circular runout in GD&T is a circle with an arrow around it. This symbol is placed in the feature control frame to indicate that circular runout is being specified.
Circular Runout Tolerance limit
The tolerance limit is the maximum allowable deviation specified in the feature control frame. It is usually given in linear units such as millimeters or inches. Total runout tolerance, unlike circular runout tolerance which measures variations at individual cross-sections, controls variations across the entire surface of rotating parts, ensuring better performance in high-speed applications.
Datum axis
The datum axis is the reference axis around which the circular runout is measured. It provides a consistent reference point for evaluating the runout of the feature. The datum axis also defines the tolerance zones for circular runout, ensuring that any variations in the cylindrical feature’s surface remain within acceptable limits during rotation.
How to Measure Circular Runout?
Here is a step – by – step guide to measure circular runout:
Step 1: Preparation
Select the appropriate measuring equipment. A dial indicator is a commonly used tool for this purpose. Ensure that the dial indicator is calibrated correctly to provide accurate readings. Calibration may involve checking the zero – setting and the accuracy of the graduations on the dial.
Mount the part whose circular runout you want to measure on a suitable fixture or a lathe chuck. The mounting should be secure and allow the part to rotate freely around its axis. For example, if you are measuring the circular runout of a cylindrical shaft, it should be mounted in such a way that it can rotate smoothly without any wobbling caused by a loose or improper mount.
Step 2: Set up the Dial Indicator
Position the dial indicator so that its contact tip touches the circular surface of the part. The tip should be perpendicular to the surface to ensure accurate measurement. For radial runout measurement, the tip should touch the outer diameter of the circular feature. In the case of axial runout, the tip should touch the end – face of the part.
Adjust the dial indicator to set the zero – point. This is usually done by rotating the part slightly until the indicator needle shows the lowest (or highest, depending on the setup) reading and then setting the dial to zero. This zero – setting provides a reference point from which the runout measurements will be taken.
Step 3: Measuring Radial Runout (if applicable)
Slowly rotate the part by hand or using a motorized spindle. As the part rotates, observe the movement of the dial indicator needle. The difference between the maximum and minimum readings on the dial indicates the radial runout.
Make sure to rotate the part through a complete revolution (360 degrees) to capture all the variations in the radius. Record the maximum and minimum values and calculate the radial runout by taking the absolute difference between these two values. For example, if the maximum reading is +0.03 mm and the minimum reading is – 0.02 mm, the radial runout is |+0.03 – (-0.02)| = 0.05 mm.
Step 4: Measuring Axial Runout (if applicable)
If you need to measure axial runout, the setup is similar, but the focus is on the movement of the end – face of the part. As the part rotates, the dial indicator placed against the end – face will record the axial variations.
Again, rotate the part through a full revolution and record the maximum and minimum values. The axial runout is calculated as the difference between these two values. The same principle of taking the absolute difference applies as in the case of radial runout.
Step 5: Documentation and Analysis
Record the measured values of circular runout (both radial and axial, if measured) accurately. These records are important for quality control and to ensure that the part meets the specified tolerances.
Compare the measured runout values with the allowable tolerances specified in the engineering drawings or manufacturing standards. If the measured runout is within the tolerance limits, the part is considered acceptable. If it exceeds the tolerance, the part may need to be re – machined, adjusted, or rejected depending on the severity of the deviation and the requirements of the application.
Cumulative TIR in Machining
Cumulative Total Indicated Runout (TIR) is a critical consideration in machining, as it affects the overall accuracy and quality of the final product. TIR represents the total amount of variation in the surface of a rotating part as it spins around its central axis. When multiple tools are used to machine a part, the cumulative TIR is the sum of the TIR values from all these tools.
Managing cumulative TIR is essential to ensure that the final product meets the required specifications and tolerances. Excessive TIR can lead to parts that do not fit together properly or function as intended. By carefully monitoring and controlling the TIR during the machining process, manufacturers can produce high-quality parts with precise surface characteristics. This is particularly important in industries where even minor deviations can lead to significant issues, such as in aerospace and medical device manufacturing.
Why Use Circular Runout?
Using circular runout in design and manufacturing helps to improve the quality and performance of products. It ensures that rotating parts have a consistent circular shape, which reduces vibration, noise, and wear. Runout controls are essential for maintaining proper alignment and fit between mating parts. For example, in the case of a gear system, controlling the circular runout of the gears is essential for smooth power transmission and to prevent premature failure.
Avoiding Excessive Runout
Excessive runout can lead to severe problems, such as bearing failures or permanent shaft bending, which can compromise the functionality and safety of mechanical systems. To avoid excessive runout, manufacturers must implement stringent controls throughout the machining process. This includes using high-precision machinery, such as CNC (Computer Numerical Control) or EDM (Electrical Discharge Machining) machines, which offer superior accuracy and consistency.
Additionally, implementing strict quality control measures is crucial. Regular inspections and measurements using tools like dial indicators or coordinate measuring machines (CMMs) can help detect and correct runout issues early in the production process. Using GD&T symbols, such as the runout symbol, in engineering drawings provides clear guidelines for acceptable runout tolerances. These symbols help ensure that all parts meet the required specifications, reducing the risk of excessive runout and enhancing the overall quality and reliability of the final product.
Circular Circula Runout Applications
Circular runout is widely used in many industries. In the automotive industry, it is used to control the quality of engine components such as crankshafts, camshafts, and wheels. In the aerospace industry, it is crucial for components like turbine shafts and propellers, ensuring all points on the entire surface meet the necessary specifications. In the machinery manufacturing industry, it is applied to shafts, spindles, and other rotating parts to ensure their proper operation and the accuracy of the overall mechanical system. Total runout is important for preventing issues like vibration and oscillation in applications such as automotive and industrial machinery.