Engineering Failure Analysis

[indieFan]

Originally Posted by mikeleslie

Failure analysis
There are 2 sprockets mig welded to 2 hubs on the center axle. Sprocket 1 transmits power to the axle via a .125 key. Sprocket 2, which drives the front wheel is powered by the same key. The total length of the key is 1.0, with the hub on sprocket 1 having a depth of .6, and #2 being .4. The design did not specify 1 or 2 hubs. It was built with 2, as #1 is the same sprocket used throughout the robot. The key specified was designed to handle only the load of 1 wheel, not 2.

You say that the key was designed to handle 1 wheel. Can you explain why or how you know that? Were you using a resource like The Machinery's Handbook to determine the key to use?

My team ran into a similar situation several years ago. We had a steel sprocket attached to an aluminum hub via bolts and lock-nuts. These were then put on a 5/8"(?) steel shaft. These components were being driven by a Fisher-Price motor. All of the design and machining of the key and depths of keyways had been done using The Machinery's Handbook as a resource.

During the course of the competition, the key was literally tearing the aluminum away until there was a catastrophic failure in which the keyway on the shaft also gave out.

Our solution was to never use an aluminum hub where there's going to be a large amount of stress on the area of a key/keyway. The coach of the team and I were so serious about this solution that the following year, a college student was saying, "Let's use aluminum for the hub, it weighs less." We told him several times to use steel. We finally said that it was a lesson learned from last year. Then he gave in to using steel.

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