Expanding Tolerance Analysis for a Robust Product Design Expanding Tolerance Analysis for a Robust Product | Page 7

Expanding Tolerance Analysis for a Robust Product Design

not operate at peak performance as the timing of the opening
and closing of the valves will be off. Furthermore, the impact
between the cam and top of the tappet may also lead to longterm damage if left unadjusted.
Over time mechanical wear causes an increase of tappet
clearance, a situation usually characterized by a ticking sound
in the engine. As this happens the thickness of shims between
the tappet and the valve stem must be “adjusted” through the
use of larger shims.

Tappet
clearance

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Such evaluations often highlight conditional constraints when
one group of features are controlling part interaction in some
instances and another completely difference set of features
control the interaction in other scenarios. Such situations are
typically avoided as it makes troubleshooting either during
manufacturing or in the field much more difficult.
One must also consider how clearances between controlling
features, such as shaft in bearings, will be treated within the
analyses. In some cases there is a bias of location and
orientation (e.g. tangential contact due to gravity). In other
cases the solution should assume that the position of such parts
is completely random.
Some of typical joints between the camshaft and the tops of
the valves would be as follows (Fig. 15),

Shim

Fig. 13 (a) Tappet Clearance Drawing (b) Shim photo
Fig. 15. Typical Joints (a) Concentric (b) Tangent (c) Coaxial (d) Point

4.2 Measurement of Space for Tappet Shim
4.4 Dimensional Network
The desired tappet clearance is generally between 0.15 mm
and 0.20mm, or a total tolerance range of 0.05mm for both
intake and exhaust systems. Meeting these requirements
necessitates many different thicknesses of shims to be
provided for the clearance adjustment (Fig.14). A tolerance
analysis can determine the cost effective range of shim
thicknesses expected to be needed.

The three-dimensional network (chains) is a graph showing
parts connected to each other by joints and which links those
relevant features with dimensions and tolerances (Fig. 16). It
can be used to verify the modeled relationship of the parts and
features to each other. It is the 3D equivalent of the vector
loop diagram common to 1D tolerance studies.

Fig. 14. Tappet Clearance Drawing

Fig. 16. Three-dimensional network graph

4.3 Build Simulation Models

4.5 Tolerance Assessment

To accurately predict what will happen during
manufacturing the tolerance analysis solution must account for
the effects of part imperfections between interfacing parts.

To improve the design based on the results of the analysis
the engineer should consider many factors including the
centering of the nominal analysis results within the required

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