When one thinks of proactive maintenance for mechanical systems, the big four tasks come to mind – align it, balance it, fasten it down and lubricate it. We often think of these as independent contributors to reliability or unreliability, depending upon the precision with which they’re managed and controlled. In truth, they’re highly interactive. Let’s explore the relationship between shaft alignment and lubrication. One of our more basic concepts in lubrication is Stribeck’s curve and equation. Stribeck’s equation states that the dynamic film thickness provided by the lubricant increases as viscosity and speed increase, and decreases if there is an increase in load. Design engineers utilize their understanding of this relationship to design equipment, and specify the required lubricant viscosity at operating temperature and identify when friction modifiers, such as anti-wear or anti-scuff additives, are required. It’s all pretty important stuff.
So, what does this have to with the relationship between component life and load? In the case of a rolling element bearing, the life of the bearing is related to load according to the following general equation, where LR refers to the rated load and LA refers to the actual load. Figure 1 (at right) graphically illustrates this relationship in visual terms.
A Tenuous Relationship
In his book “Machinery Vibration: Alignment”, noted author and expert on precision alignment Victor Wowk illustrates the relationship between misalignment and loss of machine life. This relationship is illustrated graphically in Figure 2. Of course, some equipment is more or less tolerant to misalignment depending on bearing type and coupling type. Likewise, the effects of misalignment are magnified as a function of speed.
Figure 1. The Relationship Between Bearing Load and Bearing Life
For journal bearing applications, researchers found that 0.8 degree of axial misalignment doubled the effective load and reduced the hydrodynamic lubricating film thickness by 75 percent! This, of course, increases the risk of a bearing wipe and scoring, in the event that abrasive particles are imbedded into the surface of the soft bearing material and are protruding above the bearing’s surface.
Figure 2. Failure to Achieve Precision Alignment Significantly Reduces Equipment Life
Whether it’s rolling element bearings, journal bearings, gear teeth or other mechanical equipment, misalignment reduces the effective clearance in our machines. In many instances, this completely eliminates the lubricating oil film, resulting in surface-to-surface contact and wear. In some instances, it causes two-body abrasion, where the asperities on one or both of the machine’s contacting surfaces abrade one another. This is particularly true when one of the surfaces is a harder material and the other surface is softer (e.g. a worm gear). If the contacting surfaces are both steel and the loads are sufficient, the loss of a lubricating film due to misalignment produces adhesive wear. Such wear occurs when two tough asperities contact under a great deal of load. If neither asperity will yield, the metal literally welds together to form a cold juncture, or adhesion. The subsequent separation of the surfaces causes a ripping effect (Figure 3). Adhesion can very quickly and substantially deform a machine’s surface. Extreme-pressure or anti-scuff additives protect the machine against adhesive wear, but if the contacting force caused by misalignment is too great, the additive simply isn’t enough to protect the machine.
Fatigue and Abrasion
Even if the film isn’t completely lost due to misalignment, its thickness is reduced, bringing two other wear mechanisms into play – contact fatigue and three-body abrasion. Contact fatigue occurs in the rolling contacts of gear teeth at the pitch line, rolling element bearings, cam-follower contacts, etc. Rolling contact lubrication is characterized by extremely high concentration on load because the force is being transformed across such a small area.
A common cause of failure in rolling contacts occurs when clearance-sized particles get into the lubrication film and serve to localize the load transfer – say at the pitch line of a gear tooth where the load is conveyed from one gear to the next. The resulting load often exceeds the fatigue limit of the metal, which results in denting and contact-fatigue-induced pitting. Likewise, if misalignment reduces the thickness of the lubricating oil film in sliding contacts, the risk of particle and non-particle-induced abrasion likewise increases. Particle-induced abrasive wear, which is sometimes called three-body abrasion, is arguably the most common wear mechanism that leads to machine failure.
Figure 3. The Adhesive Wear Process
No Small Problem
Nature provides us with more small particles than large particles. For example, in a typical lubricant or hydraulic fluid sample, there are approximately 3.5 times as many particles greater than 6 microns in size as there are particles greater than 10 microns. Likewise, there are about 3.5 times as many particles greater than 4 microns as there are particles greater than 6 microns. So, if misalignment decreases the film thickness from 10 microns to 4 microns in thickness, you can expect to increase the number of contact fatigue and three-body abrasive contacts by a factor of 12. Making matters worse, for a given material hardness, particles get “tougher” the smaller they become. Toughness relates to the friability, or crushability, of the particle. In the battle of the lubricant film, if the particle is tougher, either because of material hardness or size, the machine will take the brunt of the damage.
Figure 4. Relationship Between Film Thickness and Relative Particle Contacts for a Typical Distribution of Particles
Opportunities are Knocking
When a lack of precision alignment further reduces the already thin film of lubricant that separates your machine surfaces, there are simply more opportunities to produce adhesive, abrasive and contact fatigue wear. So be sure precision alignment is on your to-do list for achieving lubrication excellence!
References
- Wowk, Victor, “Machinery Vibration: Alignment”. McGraw Hill Professional Engineering Series.
- Vijayaraghavan, D., and Brewe, D.E., “The Effect of Misalignment on Performance of Planetary Gear Journal Bearings”. NASA Research Publication.
