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Dave Palmer
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The case of the breaking brakes
Dave Palmer   6/24/2011 3:03:42 PM
Powder metal parts can cut costs, but (as with any material or process) it's important that there be a good match between the application and the material.  The award-winning parts seem to be good examples of this.

Unfortunately, I've also seen examples of parts which never should have been powder metal.  For example, I was once involved in a failure analysis for a mechanical brake actuator which consisted of a hub with a long shaft.  Since the compaction direction was along the axis of the shaft, and the length of the shaft was more than twice the diameter of the hub, it was difficult to achieve a high density in the shaft.  The shaft was subjected to torsion and bending loads.  Depending on the exact size and location of porosity, fatigue cracks would initiate at the base of the shaft.  With repeated actuations, the shaft would snap off, rendering the brake inoperable.

Making the part out of powder metal may have saved money in the short term, but this could have been a very expensive problem if anyone had been injured or any property had been damaged (fortunately, this never happened).  As it was, a significant sum of money needed to be spent on this issue.

Of course, there are PM techniques which could have been used to solve this problem.  For instance, double press / double sinter would have made it possible to achieve a higher density.  Powder forging would have allowed an even higher density.  But once the part was already in production, either of these options would have required significant new tooling expenditures.  Ultimately, after examining a number of options, it was found that it was cheaper to machine this part from an inexpensive forging.

There are many applications for which PM is a great option.  However, design engineers need to make sure that it is a good fit for the application.  PM suppliers should also be careful to bring up any potential areas of concern during the design review process, so that any potential issues can be resolved before the part goes into production.

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