Evaluating a ComplEx mEChaniCal assEmbly to minimizE tolEranCE risk By Paul Thomas, M.E.

Thirty-nine dimensions were involved in calculating the vertical offset distance for this project.

Andrews-Cooper was engaged by a medical device manufacturer to evaluate the design of a complex mechanical assembly. The client’s R&D reliability expert was extremely concerned that an important vertical travel distance between an upper working pad and its base might have a large, and possibly unacceptable, manufacturing variation. This variation could potentially cause improper operation during use.

The client’s manufacturing team was concerned that the component variations would create an overly difficult assembly procedure and require a highly-trained assembly staff specific to this product line. The end result would be a device-specific calibration requiring a high level of technical aptitude on both the assembly line as well as during the course of preventive mainte-nance or service in the field.

First stEp: Understand the 3D CAD Model and Proof-of-Concept Prototype The first step of the analysis was to understand how the device worked from its 3D CAD model and the proof-of-concept prototype provided – put simply, “what drove what?” in the assembly. Once the contributing components were isolated, the drawings were pulled and evaluated in-depth to determine the tolerance stack up for all contributing features.

The investigation showed that 39 dimensions were involved in calculating the vertical offset distance. We understand the fundamental step of adding and subtracting nominal dimensions and adding tolerances. However, this mechanism was highly complex and included some interesting and unique challenges.

ChallEngEs: Dimensions, Directions, Orientations and MechanismsThese challenges are outlined below and the steps taken to address these aspects during the analysis are explained in detail.

1) Some dimensions had no tolerances documented on the component drawings.

2) The directions of forces effected clearances in the assembly.

3) A toothed coupling had “n” teeth giving it “n” discrete angular orientations.

4) Part of the assembly included a long ball screw drive mechanism.

For the purpose of this analysis, the dimensions without tolerances were assigned industry standard tolerances based on the material used. This provided a starting point for a preliminary answer. The missing tolerances would need to be applied to the specifications of the various features going forward.

FurthEr EvaluationsClearances were evaluated between mating parts based on the directional forces applied to the mechanism. Depending on the rotational position, cer-tain portions of the mechanism were in compression while other were in tension. The clearance moved from one extreme to the other depending on whether a par-ticular joint is in tension or compression.

The mating halves of the toothed couplings were also evaluated. The client specified that this particular cou-pling pair have teeth with sharp edged tips. This being the case, the teeth (under load) would slip either clock-wise or counter clockwise depending on the rotation cycle. The worst-case angular tolerance – half the tooth pitch – was analyzed and included.

Worst-CasE analysis Although the ball screw was relatively long, it was tightly toleranced. The ball screw’s effect on the overall assembly tolerance was minimal in both cycle directions.

A worst-case analysis revealed that the vertical distance between the upper working pad and its base was 167.2mm +-6.1mm. The RSS tolerance value was within an acceptable tolerance limit. Even the worst case tolerance of 6.1mm was determined to be acceptable for this particular application. This analysis also provided the client with confidence that the assembly procedure could be completed by trained technicians and did not require extensive or excessive constraints.

How can we use our expertise to solve your complex engineering problems?

Call Jay Tucker, business development manager, at 503-565-2007 or email jay.tucker@andrews-cooper.com.

adding valuE by solving your EnginEEring problEms This Tolerance Analysis illustrates how we help our clients overcome difficult engineering challenges. By using our expertise to review a complex subassembly, our client gained assurance that the assembly procedure could be completed by technicians who were already on staff without implementing an expensive training program.