Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Machine Design - Case No. 135: Spur Ring Gear Excessive Load


When an old gear tooth has metal rolled over its edges it means only one thing - operating conditions have changed.

When a new gear has material rolled over it's edges it could mean that the gear was not properly heat treated, or that there was an application mismatch, in other words, someone miscalculated. In both cases the gear tooth is not strong enough for the operating load. But when a gear that has been operating for years suddenly shows deformation, such as that shown in the figure below, the indication would be that an operating condition has changed. You are looking at "cold flow" of the driven gear. It is a condition where the stronger gear, in this case the pinion, literally pushed through the weaker driven gear. The amount of material that had moved is highlighted in the picture.

Some gear teeth are not through-hardened. They have a hard outer case and a softer interior. Once the harder outer case is pushed aside, or worn away, the amount of distortion will continue at a faster pace. Some gear material will work harden and maintain a certain amount of resistance to this type of over load problem. The key word though is "resistance." The gear will still wear at a faster rate because as the tooth profile changes in shape so too do the loads imposed upon the gear. The profile will experience more rubbing wear as opposed to rolling wear. Rubbing wear is harder to control than rolling wear.

Gear design theory has one tooth in contact at any point in time and calculates the tooth size based upon that notion. However, in reality there is one point in time when there are two drive teeth in contact with two driven teeth. That point in time is when the first drive tooth is about to complete its engagement with the driven tooth. At this point in time the next drive tooth begins to engage the next driven tooth. In this manner the load is evenly distributed and both the drive tooth tip and the driven tooth tip evenly split the entire load. The sequence of events is as follows: Tooth engagement begins with the thicker section of the drive tooth engaging the thinner section of the driven tooth and ends with the thinner section of the drive tooth engaging the thicker section of the driven tooth. As the contact between these two teeth approaches the thicker section of the driven tooth the next drive gear tooth begins its engagement with the next driven tooth. The load is momentarily shared by two pairs of teeth. That is not the case when the gear teeth are worn down so much. The picture above shows the tooth tip of the driven gear sheared off. This can only happen as the first drive tooth is leaving its engagement with the first driven tooth. This makes no sense except if the tooth profile is so severely worn that the engagement is completely off. Seeing is believing though as the picture above shows one of many driven gear teeth that were sheared off.

This gear behaved well until one point in time the operating conditions were changed. For the same amount of load the speed was decreased and this resulted in a significant increase in torque. The thought was that the gear was turning the same amount of product but at a slower speed and thus there should be no problem. They were incorrect. Torque is inversely proportional to speed. As the speed increases the amount of torque required to turn a given load decreases. Conversely, if the load stays the same and the speed is decreased then the required torque increases. Working the equations this increases the amount of tooth load.

To give you an idea of the significant change in gearing, this gear is being upgraded to handle the lower speed conditions. The width of this gear went from 8" up to 12". That's a big difference.


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