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Measuring Runout in Manufacturing: A rundown on runout

Written by:

Emily Smith

Runout is a vital dimensioning parameter that plays a crucial role in ensuring the functionality and quality of machined parts.

Source: GD&T Basics

In this article, we will cover:

  • The history of runout 
  • How best to understand runout
  • The difference in measurement methods
  • Best practice for assessing runout effectively 

All of the contents above will provide you with a high-level run down on runout and the important information you need as a manufacturer.

Firstly, let’s touch on GD&T and the numerous advantages of understanding what GD&T is and how to use it. By integrating GD&T with precision-enhancing tools like statistical process control (SPC), manufacturers achieve heightened quality and accuracy in production, leading to reduced waste and cost savings. 

GD&T plays a pivotal role in enhancing customer satisfaction by facilitating the creation of high quality parts and components, approaching perfection when strict tolerances are maintained.

Furthermore, GD&T promotes collaboration by standardising manufacturing expectations across teams, bridging communication gaps between engineering and quality managers. In essence, investing in GD&T empowers manufacturers to elevate their production pipeline, ensuring financial stability and reliability.

What is the history of Runout?

Focusing on runout in particular, let’s take a look at where it all began with the first rotating part, the crankshaft. 

According to National Geographic, and other sources, the crankshaft’s history as the first rotating part dates back to ancient times. Notably in ancient Greece and later advanced during the 9th century by engineer Al-Jazari. This innovative crankshaft-driven device set the stage for its integration in various applications. Over centuries, the technology evolved, eventually becoming crucial in the late 19th-century automotive industry. 

The crankshaft’s role in converting motion remained pivotal in mechanical systems. As engineers integrated crankshafts, ensuring their smooth rotation became crucial for reliability and efficiency. This led to the recognition of runout. Engineers devised methods to minimise runout, emphasising its importance in manufacturing and engineering for precise and long-lasting rotating components.

What is Runout in GD&T?

Runout is the deviation of a rotating object from its ideal axis; it is a critical parameter to measure and control in metrology. 

GD&T runout is a geometric tolerance that controls the amount of runout (both radial and axial) that a feature or surface can have when referenced to a datum axis or plane. In Geometric Dimensioning and Tolerancing (GD&T), two distinct types of runout are specified: circular runout and total runout.

What is Circular Runout?

Source: GD&T Basics

Circular runout is a 2D tolerance that regulates the variation of a surface as it completes one full revolution around a datum axis. This composite tolerance controls both the surface elements of a part and its relation to a datum axis. It amalgamates variations in roundness, flatness, straightness, and coaxiality into a single tolerance. 

For instance, if the circular runout of a flange on a rotating shaft is controlled, it ensures that the flange’s surface remains within the specified tolerance range as the shaft rotates.

What is Total Runout?

In contrast, total runout is a 3D tolerance that governs the entire surface of a part during a 360-degree rotation around the datum axis. This tolerance accounts for both radial and axial variations along the entire length of the surface. It is a more stringent requirement compared to circular runout, as it oversees the entirety of the part’s surface rather than focusing on a specific cross-section. 

For example, if the total runout of a cylindrical surface on a rotating shaft is controlled, it dictates that the entire surface of the cylinder must stay within the specified tolerance as the shaft rotates.

Why is measuring Runout important for manufacturers?

Runout is a measure of how much a specific reference feature deviates concerning another datum during a 360° rotation around the datum axis. It acts as a control for circular features, indicating the extent of variation with the rotational axis. The tolerance zone, a 2-dimensional circular area defined by a datum axis, ensures precision in circular features and their relationship to the rotational axis. Runout quantifies the total variation allowed when the part rotates around the datum’s true axis, providing a direct reference to the datum feature.

What is the Difference Between Runout and Concentricity?

In its simplest terms, runout can be thought of as  the combination of roundness and concentricity. While concentricity deals with the circular form of geometric dimensions and tolerance symmetry, runout incorporates both circularity and concentricity. The equivalence between runout and concentricity is achieved when the component is perfectly spherical and round. 

Concentricity evaluates how a cylindrical shape is positioned on a theoretical axis, while runout assesses how the target deviates from the dimensions of a circle when perfectly positioned on the rotation axis. Coaxiality comes into play when comparing internal and outer diameters, typically in a similar cross-sectional plane.

What is the impact of Runout on part performance?

The impact of runout on part performance is significant and multifaceted. When runout, or the deviation of a rotating part from its intended axis, is poorly controlled, it can lead to a cascade of detrimental effects. 

One of the primary consequences is the generation of high vibration within the system, which not only compromises the precision of the machinery but also contributes to increased noise levels. Furthermore, reduced mechanical efficiency becomes evident as the misalignment hampers the smooth operation of the components, resulting in energy losses and diminished overall performance. 

The wear and tear on parts subjected to poor runout control are accelerated, leading to high levels of abrasion and potential damage. This heightened wear can hasten the occurrence of early failures, jeopardising the reliability and longevity of the equipment. Therefore, meticulous attention to runout is crucial for maintaining optimal part performance, ensuring efficiency, and prolonging the operational life of mechanical systems.

Best practice for assessing Runout effectively

  • Familiarise Yourself with Runout Specifications

Prior to commencing the inspection process, it is imperative to comprehend the runout specifications applicable to your component. These specifications encompass the datum reference frame, tolerance zone, modifier, and material condition. The datum reference frame delineates the axis of rotation and reference surfaces crucial for runout measurement. 

The tolerance zone defines the cylindrical or conical area within which the feature must reside. The modifier specifies whether the runout is total, circular, or radial, while the material condition determines whether the tolerance applies at maximum or minimum material condition. It is essential to scrutinise the drawing notes and symbols for any supplementary information or requirements.

  • Select the Appropriate Inspection Technique

Depending on the dimensions, configuration, and intricacy of your component, a choice must be made among various inspection methods for runout. A dial indicator and fixture can be employed to gauge the feature’s deviation from the tolerance zone. This method is straightforward, swift, and accurate, but necessitates a suitable fixture and a calibrated dial indicator. An alternative is the utilisation of a coordinate measuring machine (CMM) to scan the part and generate a 3D model of the feature. 

Although more versatile, precise, and comprehensive, this method mandates a CMM and a proficient operator. The most advanced option involves using a laser scanner or vision system to capture the part’s image and measure runout through software algorithms. While automated and versatile, this approach requires a high-resolution scanner or camera and a potent computer.

  • Adhere to the Inspection Protocol

Once the inspection method is chosen, meticulous adherence to the inspection protocol is crucial. This involves preparing the part by cleaning it and eliminating any contaminants, such as dirt, grease, or burrs, which could impact the accuracy of the runout measurement. Additionally, it requires clear and accurate marking of the feature and datum axis. Calibration of equipment and adjustment of settings according to runout specifications are necessary steps, followed by proper alignment of the part and equipment. 

Subsequently, runout is measured at multiple points along the feature, and the results are documented. To ensure accuracy, the repeatability and reliability of the measurement should be verified through multiple iterations or different equipment.

Lastly, a comparative analysis is conducted between the obtained results and the runout specifications to ascertain whether the part complies with the specified tolerance. Any identified sources of error or variation should be addressed promptly through corrective actions.

How can The Sempre Group help?

The Sempre Group can provide in-person GD&T training, for individuals and whole teams looking to up skill and improve their knowledge. 

As a company we provide and support measuring equipment from a range of Global manufacturers, our in house experts are able to advise on the best solution to measure your components. 

Our GD&T training courses are run by our expert in house metrologist and the course content and online material are provided by our partners GD&T basics.

To find out more or to request your training course visit our GD&T page.

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