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While bearings are made of hardened steel, th’ey are surprisingly susceptible to contamination. The reason is the very high contact pressures that are present in the small contact zones between the rolling elements and the raceway. Pressures of around 200,000 psi between the ball and the race in an angular contact ball bearing are not uncommon. Over-rolling contaminant particles with such high contact pressures results in dents in the raceway.
This article contains excerpts from the paper, “Bearing Maintenance Practices to Ensure Maximum Bearing Life” by Brian P Dahmer of SKF USA Inc. at the 2013 International Pump Users Symposium in Houston, Texas.
Particulate contaminants, whether those particles are soft or hard, large or small, will cause some damage to bearings. Even polymer particles, though quite soft, will extrude when over-rolled and cause large but shallow dents. Harder particles create smaller but sharper dents. All of these dents are detrimental but the harder the particle, the sharper the dent and the higher the stress concentrations at the edges. Therefore, typically, hard particle contamination is more damaging than soft particle contamination. Both types of particles still do damage so keeping the bearings clean during installation and operation is critical to avoid both denting and wear.
Hard particle dent and a soft particle dent (bottom) can both be damaging. The presence of contamination shortens bearing service life in two ways. Every time a rolling element passes over a dent, contact pressure increases at the edge of the dent. Higher stresses result in shorter fatigue life.
The second mechanism is wear. While balls do roll in a ball bearing, due to the curvature of the balls and races, there is some sliding that occurs as well. The sliding portions of the contact, when contamination particles are present, can result in wear of the surfaces. Roller bearings can also exhibit wear from contamination but this wear may be in different places such as the ring flanges in addition to the raceways. When bearings are being installed, they should be left in their original package as long as possible and only unwrapped just prior to being put on the shaft. If they have been unwrapped, they should be covered with their original wrapping or a clean, lint free cloth to keep dust and airborne dirt from coming into contact with the bearing.
Likewise, once installed on the shaft, the bearings should be covered or wrapped to keep contamination away from the bearing. Also, use clean tools as well as clean gloves and rags when handling and installing bearings. Other components such as the bearing housing, the shaft, locknuts and lockwashers, oil rings and seal components
should also be cleaned prior to bearing installation.
Cast housings can be a serious contributor to contamination. Cast-in recesses and channels can retain casting sand or iron particles. Even after cleaning, problems can arise. Some mold release agents may hold particles and resist normal cleaning methods only to release those particles when the housing warms during operation. This means that any cast part in contact with the lubrication being used for bearings should be checked carefully to make sure it won’t seriously contaminate the bearings once in operation. Iron particles that may be released from a casting can cause other lubrication related problems. These particles may enter the oil and then speed oxidization of the oil. This may lead to the development of black oil and the assumption that the bearings are damaged while in actuality it is particles coming from the housing that are creating the issue.
Painting the interior of a cast housing with suitable oil resistant sealing paint can be a practical solution to preventing particles remaining in the housing from coming free and entering the bearing. Once the bearing and shaft assembly is installed, lubrication must be clean as well. Make sure that oil or grease containers are not left open to the air so that they may pick up contaminants. Funnels and other items used to introduce oil into the housing should be cleaned as well so that particles adhered to them are not introduced into the housing.
Sealing of the bearing housing is critical to prevent the ingress of contaminants. Seals should be appropriate for the operating conditions and should be designed to prevent entry of all types of contaminants into the bearing system. Three types of seals that may be used are lip type contact seals, labyrinth seals and magnetically charged face seals. Lip seals, typically made of rubber lips encased in a sheet steel shell or case; provide excellent protection from external contaminant. However, the seal lip contact with the shaft must be lubricated and, even with lubrication, the seals do create added frictional heat.
The rotor provides a flinger effect throwing contaminants off of the seal when it is rotating. The axial or face labyrinth created by the nesting of the rotor and stator makes it extremely difficult for any type of contamination to pass through the seal. Additionally there are typically ports at the bottom of the seal to allow any contamination that has entered the labyrinth to then exit through these ports. Lubricant is also prevented from exiting the housing keeping leakage to a minimum. Often, the seal may integrate an o-ring that prevents ingress of moist air when the shaft is static. When the shaft rotates the o-ring then expands dynamically eliminating friction between the o-ring and the seal components. Another benefit is that when the o-ring is expanded, the housing can breath and allow warm air to exit. Since there is no contact in this seal when it is operating, no friction is created and no heat is transferred to the bearing.