Expanding Tolerance Analysis for a Robust Product Design Expanding Tolerance Analysis for a Robust Product | Page 6

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Expanding Tolerance Analysis for a Robust Product Design

Cost of materials and manufacturing plays an important
role as the design is altered. Often, it is necessary to review
the original requirements given the new information
discovered during the iterative process.
Changes in the tolerance are often an early step in
adjustments to a design. However, when a design is reviewed
using analysis software, an engineer may be able to determine
that changes in a nominal measurement will increase
robustness without prior to adjust tolerance which can increase
the overall costs.
In the example below, a shift in the nominal of a
measurement increases the yield from 2.2% to 87.8%

0.23
shift
require
d

which could exceed the original product requirements.
Therefore, changes in requirements may be necessary and
creativity on the part of the engineer to solve complex
problems in necessary.
3.6. Document Assembly and Part Requirements
Documentation of the assembly and part requirements is
necessary to communicate the design intent.
Industry
standards for Geometric Dimensioning and Tolerancing are
used in both the 3D models and the 2D drawings as shown in
the example below.

2.2%
Yield

Fig. 9. Before Adjustments Distribution

Fig. 12. Documentation Example

4.0. Case Study – Engine Cylinder Head

87.8 %
Yield

Fig. 10. After Nominal Adjustments

Further adjustments in the tolerance value result in a further
increase in the yield to a desired result for this example of
99.9%.

An engine cylinder head is a complex assembly with many
precision fits and many sources of variations. Temperatures
can vary by hundreds of degrees, dust and particles can
interfere with movement, wear occurs over time, and
variations occur during the manufacturing of the components.
With such a highly interactive system tolerance chains can be
complex, and requirements critical. In this case study, we
review the method of creating a robust design by managing
tolerances to adjust for the wide variety of variations.
4.1 Valve Tappet Clearance

99.9 %
Yield

Fig. 11. After tolerance adjustments

Of course, the changes in the tolerance values may result in
a requirement for more expensive manufacturing processes,

One example of a critical requirement is the tappet
clearance, or valve lash, which is the gap between the top of
the valve train and cam (Fig. 4.1). Its purpose is to allow for
thermal expansion of the valve train as the engine warms to
operating temperature. If insufficient clearance exists in the
engine when cold, the valve may not fully close after the
engine warms up, or even worse mechanical failure of the
parts may occur. If the clearance is too large, the engine will

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