Reliability Engineering Snapshot TM

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



Lubrication - Case No. 123: Lubrication Demands of a Worm Gear and Wheel


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.

Worm Wheel Set - Worm - 30 Minutes of Operation Worm Wheel Set - Wheel After Two Weeks Operationj
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.
Wheel Gear - Tooth Face View Wheel Gear - Close Up View Right End of Tooth Face
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.
Wheel Gear - Close Up View of Another Tooth Wheel Gear - Side View of Cold Flow
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!


All Pictures and Text Copyright © 2002 - 2016 Contact Mr. Adler Adler Engineering LLC of Wyoming USA


Great care has been taken in the compilation of this article. However, no warranty, expressed or implied, including without limitation, warranties of merchantability or fitness for a particular purpose, are given in connection with this article or any article archived on this website. Although this information is believed to be accurate by the author, the author cannot guarantee favorable results will be obtained from the use of this article alone. This article is intended for use by persons at their sole discretion and risk. Since the conditions of product or material use are outside of the author's control, the author assumes no liability or obligation in connection with any use of this information. The author is not liable for special, indirect or consequential damages resulting from the use of this material.

No part of this article or any article archived in this website, or any part thereof, may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permission of the copyright holder R. H. Adler. Nothing contained in this article or any article archived in this website shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters of patent, copyright, or trademark, and nothing contained in this article or any article archived in this website, shall be construed as a defense against any alleged infringement of letters of patent, copyright, or trademark, or as a defense against any liability for such infringement.