Articles tagged Tolerance analysis

The calculation of the allowable deviation from a standard or nominal value that maintains fit, form, and function. Tolerance analysis provides a means to account for the natural variation of parts and fittings.

by Hemant Urdhwareshe Leave a Comment

Case Study in Tolerance Design of a Spring using Monte Carlo Simulation

One of the weak areas in designing parts is deciding tolerances of various parts. We have shared a video of statistical tolerancing for assembly of parts. Many viewers have expressed that we should also post a video of application of Monte Carlo Simulation for tolerance design when there is a transfer function that relates the input parameters to an output variable. We therefore present in this video an application example of designing tolerance for a helical spring using Monte Carlo Simulation. The video explains this procedure step-by-step using Simular software. I have used Simular software to demonstrate this with a practical example of spring. Simular is a free software (emailware) which can be downloaded from https://www.simularsoft.com.ar/. However, one can use other software such as Crystal Ball, @Risk etc. for such analysis. Tolerance design is usually an essential step in Design for Six Sigma (DFSS) projects.

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by Hemant Urdhwareshe 2 Comments

Statistical Tolerancing using Monte Carlo Simulation

One of the weak areas in designing parts is deciding tolerances of various parts. Most engineers are familiar with Arithmetic Tolerance stack up analysis wherein they check impact of maximum and minimum values of various tolerances on assembly of parts. However, this can often result in high manufacturing cost. Thus, it may be more appropriate to analyse tolerances using statistical tolerance stack up approach. This can be performed using Monte Carlo Simulation. In one of the previous videos, I had shown how to predict reliability using Monte Carlo simulation. In this video, I will explain how to perform statistical tolerance stack up analysis using Monte Carlo Simulation. I have used Simular software to demonstrate this with a practical example. Simular is a free software (emailware) which can be downloaded from https://www.simularsoft.com.ar/. However, one can use other software such as Crystal Ball, @Risk etc. for such analysis. Statistical tolerance stack up is usually an integral part of Design for Six Sigma (DFSS) projects. I hope viewers will find this video useful. Feedback is welcome!

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by Fred Schenkelberg Leave a Comment

Tolerance Specifications Impact on Reliability

Mechanical drawings and electrical schematics communicate the design.

They provide information sufficient to create a product or build a system.

They provide the necessary details that allow others to construct something that originally was only imaged.

We no longer rely on a single craftsman to build a chest of drawers from concept to delivery.

Instead, they may be a team scattered across many organizations relying on the drawings.

Included on the drawings and schematics are dimensions along with tolerances. A keypad will be a specific width, plus/minus some amount. A resistor is nominally 100 ohms, yet anything between 90 and 110 ohms is fine.

Tolerances acknowledge the variability between individual components or caused during the construction process.

Tolerances also impact the reliability performance of your products. [Read more…]

by Fred Schenkelberg Leave a Comment

Inputs to Consider When Setting Tolerances

Technical drawings and electrical specifications often include tolerances.

The range of values allowed for a particular aspect of the part.

Reliability engineers do not set the tolerances, yet we should both understand and influence them.

A critical element of design for reliability is the creation of a robust product.

It is not just robust in the end use, your product should also be robust to the variability that will occur during manufacturing and transportation. [Read more…]

by Mike Sondalini 3 Comments

The Importance of Fit, Tolerance & Clearance

Many equipment breakdowns and stoppages occur because of improper clearance between holes and shafts.

The shaft is too tight in the hole; the center of the hole is not at the center of the shaft making it off-center; one part is loose on another and slips out of place or does not seal as it should. [Read more…]

by Fred Schenkelberg 4 Comments

Tolerance Intervals for Normal Distribution Based Set of Data

This is not the same as a confidence interval. For a mean or standard deviation, we can calculate the likelihood that the true parameter is within a range of values — confidence interval concerning a parameter.

A tolerance interval applies to the individual readings, not the statistics. The interval contains a certain proportion of the values within the distribution of individual data points. The endpoints are tolerance limits. [Read more…]

by Fred Schenkelberg 2 Comments

Reliability and Monte Carlo Determined Tolerances

In the Monte Carlo method, one uses the idea that not all parts have the same dimensions, yet a normal distribution describing the variation of the part dimensions is not assumed.

Although the normal distribution does commonly apply, if the process includes sorting or regular adjustments or if the distribution is either clipped or skewed then the normal distribution may not be the best way to summarize the data.

As with any tolerance setting, getting it right is key for the proper functioning of a product. Monte Carlo method allows you to consider and use the appropriate models for the variations that will exist across your components. [Read more…]

by Fred Schenkelberg 2 Comments

Reliability and Root Sum Squared Tolerances

The root sum squared (RSS) method is a statistical tolerance analysis method. In many cases, the actual individual part dimensions fall near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits.

This, of course, assumes the part dimensions are tightly grouped and within the tolerance range.

Setting tolerances well, using the best available data about the part(s) variation, allows creating designs that function well given the expected part variation. This is better for reliable performance. Also, the same method can be applied when the loads and stresses are normally distributed.

Check that assumption with you data first, of course. [Read more…]

by Fred Schenkelberg Leave a Comment

Reliability and Worst Case Analysis

Worst-case tolerance analysis is the starting point when creating a tolerance specification.

It is a conservative approach as it only considers the maximum or minimum values of part variation—whichever leads to the worst situation. Setting tolerances such that the system will function given the expected variation of manufactured components improves that ability of the system to perform reliably.

In the worst-case method, you simply add the dimensions using the extreme values for those dimensions. Thus, if a part is specified at 25 ± 0.1 mm, then use either 25.1 or 24.9 mm, whichever leads to the most unfavorable situation.

The actual range of variation should be the measured values from a stable process. It may be based on vendor claims for process variation, industry standards, or engineering judgment. [Read more…]

by Fred Schenkelberg 24 Comments

Root Sum Squared Tolerance Analysis Method

The root sum squared (RSS) method is a statistical tolerance analysis method.

In many cases, the actual individual part dimensions occur near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits. This, of course, assumes the parts are mostly centered and within the tolerance range.

RSS assumes the normal distribution describes the variation of dimensions. The bell-shaped curve is symmetrical and fully described with two parameters, the mean, μ, and the standard deviation, σ. [Read more…]

by Fred Schenkelberg 2 Comments

Why do Tolerance Analysis

The short answer is, everything varies.

The longer answer involves the agreement between what is possible and what is desired.

If we could design a product and it could be replicated exactly, including every element of the product, we would not need tolerances. Any part would work with any assembly. We would simply specify the dimensions required. [Read more…]

by Fred Schenkelberg 2 Comments

Worst Case Tolerance Analysis

Worst case tolerance analysis is the starting point when creating a tolerance specification. It is a conservative approach as it only considers the maximum or minimum values of part variation, whichever leads to the worst situation.

The worst case method simply adds the dimensions using the extreme values for those dimensions. Thus if a part is specified at 25mm +/- 0.1mm than use either 25.1mm or 24.9mm, whichever leads to the most unfavorable situation. [Read more…]

by Fred Schenkelberg 1 Comment

Purpose of Tolerances

The short answer is, everything varies.

The longer answer involves the agreement between what is possible and what is desired.

Instead, variation happens.

Widths, lengths, weights, roughness, hardness, and any measure you deem worth specifying will vary from one part to the next. Manufacturing processes impart some amount of variation between each item produced. In many cases, the variation is acceptable for the intended function. In some cases, the vacation is unacceptably large and leads to failures. When the design does not account for the variation holes will not align, components will not fit, or performance will be poor.

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