Engineering Failure Analysis

[mikeleslie]

Here's one to get things started:

Brief description of the situation:During our semi-final match at BMR, we lost power to 2 of the 3 wheels on 1 side of the robot. (6 wheel tank drive) Upon further investigation, it was determined that the drive sprockets which transmit power from the trans to the middle wheel and the middle wheel to the front wheel no longer functioned. A replacement was attempted but could not be completed in time.
Components Involved:
Andy Mark 2 speed GEN 2 servo shift trans
IFI 24t aluminum sprockets
IGUS .625 aluminum shaft
#35 chain
custom turned 6061 aluminum hubs with 1.2 OD, .625 ID and .125 keyway (welded to the sprockets)
.125 tool steel keys

Failed Components
It was determined that the custom hub on the sprocket which transmits power from the trans to the middle axle failed (see photo) , allowing the sprocket to rotate on the shaft.

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. This extra load caused the keyway to get “sloppy”. This slop increased the load on sprocket 1, (interestingly enough this sloppiness was detected before the semi-finals began, but we had no replacement parts readily available). The hub failed.

Root Cause
The root cause was that hub 2 was too small to handle the load.

Remediation

1. Both sprockets should have been manufactured as 1 part. This would transfer the load from sprocket 1 to sprocket 2 through the hub and not through the key, reducing the stress on the key.
2. The outer diameter of the hub was too small to handle the shock load caused by the sloppy key, therefore the hub outer diameter should be increased, giving a larger safety factor in the event of the key getting sloppy.

Results
This new design was used at WMR without issue. There was no increase in key “slop” during the regional.