Runout is a manufacturing error that occurs specifically to rotodynamic components. As nearly every industry relies on machinery like pumps, gears, axles, etc., it is inarguably an important error to look out for.
In this article, we will cover the runout definition, its types: radial runout and axial runout, management of runout in GD&T, and some tips on how to measure runout.
What is Runout?
Before we dive into the nitty-gritty details, let us take a quick look at a general runout definition. Runout is a specific type of manufacturing error that applies to rotating parts like motor shafts.
It measures the deviation of these parts as they rotate about a fixed reference axis. Ideally, the part’s axis and the reference axis should be colinear, but this is not the case in real life.
In simpler words, runout is a measure of the amount of wobble in rotodynamic components. Understandably, it is an undesirable error that must be kept under control. If the runout in machining is out of bounds, it can cause catastrophic problems like bearing failures or permanent shaft bending.
Two Types of Runouts
Above that we have discussed a runout definition, we will explain its two types: axial runout and radial runout.
Radial runout occurs when the axis of the rotating component is offset from the axis of rotation. In this scenario, the axes are still parallel, but just not colinear.
Imagine a drill bit fixed slightly off-center in a chuck. It would rotate with the chuck but not about its axis. Rather, the axis of rotation would be the central axis of the chuck. In this case, the drilled hole would be larger than the drill size by an amount equal to the radial runout.
Axial runout is defined by the angle between the component axis and the reference axis. In this case, they are not parallel to each other. Due to the angle, the deviation between the part and the reference axis gets larger the farther you move away from the point of intersection between both axes.
A good example is a loose car tire. You can see its wobbly motion from behind. The wobble is less severe near the axle and more pronounced at the circumference of the tire, as we just explained.
Runout in GD&T
Geometric dimensioning & tolerancing (GD&T) is a sophisticated system to characterize engineering tolerances and relationships between parts. It serves as a guideline for manufacturing and a standard for quality assurance. Since rotary machines are very common, all GD&T systems have provision for it.
As per the ASME Y14.5 standard, runout in GD&T is controlled by the runout symbol shown below. It is a 2D feature that establishes a circular tolerance zone centered on the reference axis. GD&T experts also use the term ‘datum axis’ for the reference axis.
All points on the control surface must lie within this tolerance zone for the part to pass necessary quality checks.
A 3D version of the runout is also available. If the total runout symbol is called on a round surface, the entire surface must be measured and should fall within a 3D cylindrical tolerance zone.
These two symbols: the runout symbol and total runout, are effective tools in a design engineer’s hand to convey their manufacturing requirements to production and quality.
Now, to drive the point home, we will demonstrate the use of the runout symbol through a simple example.
GD&T Runout Example
It is very easy to deal with runout in GD&T. Firstly, let us look at the runout symbol example shown below. Runout is called on the smaller cylindrical surface as seen by the arrow beside the callout.
The ‘A’ in the callout refers to the datum feature, which is the axis of the larger cylinder. The 0.03 value is the radial runout tolerance. The illustration on the right shows the measurement as well as the +/-0.03 tolerance zone. The measurement shows that the profile (solid black line) lies within the tolerance zone (dashed line). Thus, it is good to go!
Next, we will discuss the total runout symbol. Let us extend the previous example by replacing the runout symbol with the total runout. The illustration below reflects this change.
The measurement remains the same as before. This time, however, the tolerance zone is a 3D cylinder. The entire surface of the smaller cylindrical feature must fall within this tolerance zone. Consequently, the technician will sample multiple measurements at different locations along the axis of the feature. This requires some extra work but imposes a stricter tolerance for the geometry.
How to Measure Runout: Step by Step
The GD&T example above discusses runout measurement. In this section, we answer the question: how to measure runout?
Dial gauges are by far the most common measurement device to measure both radial runout and axial runout. They are intuitive to use and very convenient to set up. Furthermore, they are remarkably accurate and produce repeatable measurements.
A step-by-step summary of the measurement procedure is as follows:
1. Fix the Datum
With reference to the example above, this would mean mounting the datum (large cylinder) firmly in a rotary device like a chuck or spindle. By doing this, it is ensured that the datum itself will not wobble during measurement.
2. Mount the Dial Gauge
Use a fixed frame of reference. For example, the floor.
3. Press the Pin Against the Feature
Set the dial gauge in a convenient position and gently press the pin against the face being measured. For radial runout, this is the round surface. For axial runout, set it against a surface perpendicular to the central axis. Refer to the axial vs radial runout illustration for more clarity.
4. Set the Dial to Zero
On a standard dial gauge, you can rotate the dial frame to set it to zero.
5. Perform Measurement
Rotate the part for a full revolution and record the deflections seen on the dial gauge. Furthermore, you may repeat the process multiple times to minimize errors.
In addition to dial gauges, modern measurement tools like laser measuring devices or coordinate measuring machines (CMM) are also widely utilized by engineers for high-precision applications. These machines are automatic and can measure errors with much higher accuracy.
Why is Avoid Runout Important in Manufacturing?
Runout, like all manufacturing errors, is crucial to avoid in certain applications. Typically, it is important for machinery like motors, axles, pulleys, flanges, gear shafts, etc.
One critical reason for avoiding runout is chatter. Runout in a rotary component will cause it to vibrate about its axis, generating centrifugal forces and unwanted motion. This vibration increases with the amount of runout and rotational speed. In extreme cases, this vibration can damage the entire assembly, cause permanent deformations in the part, exciting dynamic modes of the assembly, and decrease the component’s fatigue life.
Furthermore, another reason to avoid runout is motion accuracy. Imagine a car with a wobbly wheel. The wheel’s runout t poses a safety hazard, and it will surely damage the car as well. Thus, for smooth and safe driving, the total runout must be minimal.
Runout in machining is another important issue. Many machining operations involve rotating tools. For example, turning, milling, and drilling. If the tool is mounted incorrectly (with excessive runout), the operation will produce inaccurate results. Moreover, it will damage the tool and machine.
Precision manufacturing is all about optimizing your processes for minimal errors. It is directly linked to industrial productivity and quality. Runout error is the most important consideration when it comes to rotating components, which are the lifeblood of modern industry. Managing runout in machining is undoubtedly a small part of the manufacturing industry’s efforts toward ultra-precision manufacturing.
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What is the definition of runout？
Runout is a manufacturing error characterizing the deflection of rotating parts from their true forms. A large runout means that the part wobbles when spun about its axis of rotation.
What is the difference between circular runout and total runout?
Circular runout is a GD&T term representing the 2D circular tolerance zone established by the runout symbol. Total runout is another GD&T symbol, but with a 3D cylindrical tolerance zone applied to the entire round surface.
How to control runout in rotating machinery?
Manufacturers typically use high-precision machineries such as CNC or EDM machines to minimize runout errors. Post-processing methods like filing and sanding are also used after the primary manufacturing process to bring a part within the defined tolerance limits.