Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|The phrase "demanding service" is a gross understatement when it comes to worm gear lubricants. The worst of everything happens in a worm-wheel gear set. It is like no other gear service, and it commands respect.
So, how does a typical worm wheel (driven gear ) like that shown in Figure 1 disintegrate into the worm wheel shown in Figure 2? Well it wasn't from old age, and it wasn't because of a lack of lubrication, or the wrong lubricant. So, why talk about it in a lubrication article? It's simple. The failure below is a good example of what kind of environment the lube oil has to endure when everything is going well, let alone when things are going bad. The root cause of the failure was two fold, but basically the worm gear and wheel were not a matched set and the lead angle was off enough that the worm gear was shaving the wheel teeth. The tooth action is one of sliding and this made it easier to shave material away everytime the teeth contacted each other.
|Figure 1||Figure 2|
|Most gears have a combination of sliding and rolling motion, each happening in ordered sequence, never happening simultaneously. On the other hand, the worm-wheel gear set has sliding motion in two orthogonal directions happening at the same time! There is no rolling contact, or point contact, just sliding motion compounded in two directions. Add on top of that the fact that, depending upon how the gears are oriented with respect to their center distances, the sliding motion can turn into a squeegee action that wipes the oil off of the worm wheel before it has a chance to develop a rudimentary film. A good design will minimize these problems, but not eliminate them.|
|Figure 3||Figure 4|
|It's a challenge to say the least, for any oil to establish a good film. The remaining figures illustrate the sliding motion taking place. In this particular case, the gear set was not machined as a matched set. The worm gear (driver) acted like a milling machine. The result of this error in machining helps to illustrate the direction and type of contact. As I mentioned earlier, there are two components, each one orthogonal to the other. One component of the sliding contact is from left to right in the figures, while the other component, in this case the wheel, is from the tip of the wheel to the root. Note the metal shavings at the root in Figure 3. Notice how the shaving is rolled in the direction of travel of the cutting action (i.e. toward the root). If the sliding contact was simply from left to right then the metal should have moved to the end of the tooth as in Figure 6. The metal shavings were cut at the end of the contact cycle between the worm and wheel. Instead of being pushed to the end, as in Figure 6, they were left there as the gear set rotated.|
|Figure 5||Figure 6|
|Worm gearing is a tough lubrication application. That's why many applications are either flooded or have a copious supply of oil spraying on the point of contact between worm and wheel. Worm gearing has no tolerance for marginal lubrication. An oil that is specifically formulated for worm gearing has advantages. These oils have to have a high capacity for shearing action.
You better keep those gear sets well lubricated!
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