P • A • R • T • 3
Traditional Approaches to Analyzing
Mechanical Tolerance Stacks
Tolerance analysis is the process of taking known tolerances and analyzing the combination of these
tolerances at an assembly level. This chapter will define the process for analyzing tolerance stacks. It will
show how to set up a loop diagram to determine a nominal performance/assembly value and four tech-
niques to calculate variation from nominal.
The most important goal of this chapter is for the reader to understand the assumptions and risks that
go along with each tolerance analysis method.
Analyzing Tolerance Stacks
Fig. 9-1 describes the tolerance analysis process.
Establishing Performance/Assembly Requirements
The first step in the process is to identify the requirements for the system. These are usually requirements
that determine the “performance” and/or “assembly” of the system. The system requirements will, either
directly, or indirectly, flow down requirements to the mechanical subassemblies. These requirements
usually determine what needs to be analyzed. In general, a requirement that applies for most mechanical
subassemblies is that parts must fit together. Fig. 9-2 shows a cross section of a motor assembly. In this
example, there are several requirements.
• Requirement 1. The gap between the shaft and the inner bearing cap must always be greater than zero
to ensure that the rotor is clamped and the bearings are preloaded.
• Requirement 2. The gap between the housing cap and the housing must always be greater than zero to
ensure that the stator is clamped.
9-2 Chapter Nine
• Requirement 3. The mounting surfaces of the rotor and stator must be within ±.005 for the motor to
• Requirement 4. The bearing outer race must always protrude beyond the main housing, so that the
bearing stays clamped.
• Requirement 5. The thread of the bearing cap screw must have a minimum thread engagement of .200
• Requirement 6. The bottom of th