The harsh conditions and energy-intensive nature of mining and cement applications pose special challenges for equipment systems and their components. Selecting the right bearings for mineral-processing machinery is crucial.
The engineering basis for selection of rolling bearings comes from ISO 281:2007 Rolling Bearings – Dynamic Load Rating and Rating Life, which defines the Dynamic load rating (C) for a bearing based on its type, size and internal macro geometry. The Standard considers the application conditions of applied load (P), type of bearing and operating speed (n) and now considers the lubrication condition (κ) and cleanliness (ηc) condition to calculate the L10mh rating life.
The L10mh rating life, therefore, considers the two most common causes of rolling-bearing failure in mining and cement machinery—poor lubrication and contamination (poor cleanliness). These two factors are culprits in 50% of all rolling-bearing failures. That’s why it’s so important to use the L10mh rating life when selecting and replacing these bearings.
The true criterion for selection of rolling bearings should be quality and rating life, including considerations of the lubrication and contamination condition, as well as correct selection of cage type, internal clearance, precision, etc., for specific operating conditions. Bearing dynamic load rating (C) from a catalog is a far too simplified criterion.
Unfortunately, Dynamic load rating and low cost is still the way many bearings are selected by purchasing departments and offered for replacement by MRO distributors. Such decisions are based on the assumption that a bearing with a greater dynamic load rating is the best choice—with little consideration as to whether the bearing is well-made, fits well on the shaft and in the housing or performs at a satisfactory cool operating temperature. This type of selection process leaves much to be desired when it comes to boosting equipment reliability and optimizing energy efficiency, especially since a range of innovative roller-bearing technologies are now available to do both.
Self-aligning spherical roller bearings & improved rating life…
Heat treatment has great impact on a roller bearing’s service life and performance in poor-lubrication-related and contaminated conditions. By necessity, there’s a need to balance the bearing’s material properties, i.e., high hardness and surface residual stress profile for longer bearing fatigue life, fine microstructure for wear resistance and toughness for crack resistance. Bearings generally are subjected to a through-hardened martensitic or bainitic heat treatment, or they’re case-hardened.
An improvement in bainitic heat-treatment process technology has led to higher hardness and finer microstructure, yet still maintains toughness. The result is a near doubling of bearing rating life in poor lubrication conditions (κ<1) and high-contamination conditions (ηc<0.5) for Upgraded SKF Explorer Performance Class bearings compared with existing SKF Explorer Performance Class spherical roller bearing. The finer mircostructure translates into considerably less internal wear. Compared to a normal spherical roller bearing, the Upgraded SKF Explorer Performance Class units have a much longer rating life and show less wear.
Sealed spherical roller bearings…
Typically, medium- and large-size conveyor pulleys use spherical roller bearings mounted in split housings sealed by a contact, labyrinth or Taconite seal. The Taconite seal is used to provide extra protection against liquid and particle contamination ingress. Often, large quantities of grease are fed to bearings to purge the contamination. Still, because of wear and fatigue from contamination, the service life of the bearings often falls shorter than the pulley life (lagging).
A recent innovation is the factory-sealed Upgraded SKF Explorer spherical roller bearing mounted in the sealed split housing. In this type of solution, a sealed spherical bearing is fitted with steel-backed contact seals and lubricated by a good-quality bearing grease at the factory. Mounting inside the sealed housings results in three barriers to the ingress of contamination: the housing seal, the grease volume inside the housing and the new sealed spherical bearing itself. Experience has shown this solution to increase Mean Time Between Failure (MTBF) of the conveyor pulley bearing by two to three times—which, in some cases, has exceeded the lagging life of the pulley.
The sealed bearing still can be relubricated as needed, depending on the operating and ambient conditions. A 90% reduction in required grease consumption also has been documented. That’s because only a small quantity of grease is needed to lubricate the sealed spherical bearing inside the sealed housing compared with the large quantity of grease needed to purge contamination from an open, unsealed bearing.
This solution improves MTBF and, accordingly, Mean Time Between Repairs (MTBR). Unlike other approaches (such as split bearings) that are aimed only at decreasing Mean Time To Repair (MTTR), the Three Barrier Solution reduces the Total Cost of Ownership, TCO, by extending MTBR and reducing maintenance costs.
Sealed spherical roller bearings are appropriate for lower-speed applications such as conveyors, bucket elevators, etc. Combining a standard housing seal with a sealed spherical roller bearing can provide adequate contamination protection and eliminate the need for Taconite seals. This could result in a substantial dollar and space savings since two Taconite seals—which add cost—can be difficult to install and align, and increase space requirements of the assembly.
Keep in mind that a sealed spherical roller bearing must be mounted in a good-quality housing with good shaft seals on a good-quality adapter sleeve. Otherwise, the bearing won't be supported and sealed properly. Modern housings are designed and tested for higher break-loads than older designs. This means a standard cast iron (ASTM A48 Grade 35) material can be used in most cases, the exception being larger-sized housings (shaft diameters greater than approximately 300 mm [12 in.]), for which ductile iron is recommended. It's an easy engineering evaluation to determine if a standard modern split housing of good-quality cast iron can be used in place of a ductile iron model, particularly if the housing is oriented in the conveyor with the belt load into the base support. The adapter sleeve should have oil-injection grooves to aid in the safe, quick, easy mounting and dismounting of the bearings.
Typical sealing options for pulley-housing shafts include lip seals, Posi-Trac Plus seals and Taconite seals. The use of the Three Barrier Solution means that Taconite seals can be avoided except in cases where pulleys have extreme-pressure water washdown. (Again, Taconite seals can be expensive and make alignment difficult. They also add additional axial length to the assembly.)
In smaller conveyors, the SKF ConCentra Roller Bearing Unit can be used instead of bearings in split housings. Assembled, sealed and lubricated by the bearing manufacturer, this type of unit is “shaft ready.” It slides onto the shaft, whereupon axial screws are tightened to move the bearing up its mounting sleeve. The secure axial-screw mounting allows a near 360° fitting (with no damage to the shaft by the set screws), as well as easy dismounting.
Spherical roller bearings for vibrating screens…
The vibrating screen is intentionally made to operate at high acceleration to increase the sorting efficiency of the ore. This acceleration places higher forces on the bearing used in the vibration mechanism. Depending on the design, spherical roller bearings are used for shaft-type mechanisms and mostly cylindrical roller bearings for exciter-type mechanism screens. For both bearing types, the dimensional precision, internal radial clearance and cage design must be suitable for the screen operation. In the case of spherical roller bearings, the features listed in the following blue table are needed for vibrating-screen applications:
Analysis (and experience) show that a two-piece hardened pressed-steel cage with an outer-ring-centered guide ring produces the lowest internal friction. (Figure 1 is a calculated comparison of sliding/rolling friction for various cage types.)
Fig. 1. Comparisons of slide/roll friction in vibrating-screen bearings
Energy-efficient deep-groove ball bearings…
In a conveyor system, there are typically three idler troughing rollers per every one or two meters (3 to 6 feet) of belt length, plus one return idler roller every two, three or four meters. This means possibly 10 bearings every two to three meters of belt length. In each idler roller, there is bearing and seal friction that causes a rolling resistance. The contact seals of the idler roller or those in the bearings (2RS1) contribute the most significant portion of the individual idler roller friction. Aside from the seal friction in the idler rollers, there remains a bearing rolling friction in each roller. Of course, the rolling or anti-friction bearing is designed to have low rolling friction. Multiplying this idler bearing friction times the length of the belts, however, contributes to increased torque and power requirements to drive the belt.
Innovative product development has shown that the internal rolling friction torque of ball and roller bearings can be reduced by an average of 30% with special internal bearing geometry, cage design and grease selection. This has been verified by torque measurement tests (refer to Fig. 2).
Fig. 2. Comparisons of power loss in standard and energy-efficient ball bearings
Furthermore, the service life of these energy-efficient bearings measurably increased due to lower friction, optimum grease selection and resulting lower operating temperatures. These bearings can also be used in electric motors and fans to reduce energy use. Today, they’re generally available in the small to medium sizes and recommended for light- to medium-duty (C/P > 8) applications. In other cases, SKF Explorer Performance Class deep-groove ball bearings can be used. (Energy-efficient bearings are also available in tapered roller and spherical roller bearing types.)
As mentioned in the section regarding the Three Barrier Solution, there are many shaft-sealing options for split housings. This is true for conveyors, jack shafts, pinion shafts and fans, etc. A new development in sealing is the improvement to SPEEDI-SLEEVE, a thin, hard-surfaced, polished sleeve that mounts where the shaft seal lip rides. Made of a proprietary stainless steel, the improved SPEEDI-SLEEVE has an optimized seal counterface surface to reduce friction, seal-lip and sleeve wear and extend seal life.
Another development is the ability to machine customized seals from cylindrical blanks of seal materials. This allows the design and manufacture of a seal to solve specific sealing and contamination situations: A single seal can be made or a small number of seals can be machined to find a suitable solution.
For medium-large diameter shafts (d < 1.5 m [59 in.]), a new seal with PTFE excluder has been very effective in high dust and abrasive material operating conditions. PTFE excluder blocks the abrasive materials to protect against wear and short life. This type of sealing solution has been particularly effective in Vertical Roller Mill (VRM) roller applications.
Lubrication and cleanliness concerns
Proper lubrication is critical to the performance and service life of any machinery—including equipment throughout the mineral-processing sector. Lubricant viscosity (mm2/s) at the machinery operating temperature is the key parameter for the selection of oils or greases (grease base oil). A minimum viscosity is recommended for long service life and least wear. In the case of rolling bearings, Fig. 3 (page 36) notes the minimum required viscosity at the operating temperature based on the bearing mean diameter, dm and bearing rotational speed (rpm). The ratio between the actual operating viscosity (v) and the minimum required viscosity (ν1) is named the Lubrication factor, κ (Kappa). This is one parameter for the calculation of bearing rating life. The κ should preferably be in the range between 1.5 and 3. Too low κ (too low ν) can lead to surface distress and short bearing service life. Too high κ can cause excessive internal friction and overheating of the bearing.
In many mining and cement applications, the lubrication factor is less than one—which is why the previously mentioned improvements in roller-bearing technologies are so important.
Internal cleanliness of machinery is also vital to long service life. The reliability of the bearings, gears, seals, chains, couplings and other rolling and sliding components and surfaces depend on clean lubrication. It is recommended that OEMs, mineral processors and their subcontractors maintain dedicated clean work areas for the assembly and repair of machinery. Such work areas should be partitioned from airborne dust caused by grinding, welding and the outdoors. Dedicated benches, tools and hoists that can be kept clean for working on rolling and sliding components are also recommended.
Fig. 3. Minimum lubricant viscosity for roller-bearings
Lubricants should be stored in clean and dry areas. Containers and tools used to dispense them should be clean and dry—and be dedicated to a particular lubricant to avoid mixing of products.
Bearings, too, should also be stored in clean and dry areas. They should remain unopened in their boxes and wrapping until they are assembled into the machinery.
Oils (lubricants and hydraulic) should be checked before use and periodically during use for cleanliness and moisture content. Solid particle contamination can have a significant impact on bearing service life. Although not considered in the ISO 281 standard for rating service life of rolling bearings, it is well documented that excessive moisture content in lubricants reduces their service life and similarly for gears, seals and other components. Moisture can, of course, also cause corrosion in the machinery. Moisture content in rolling-bearing lubricants should be less than 200 to 500 ppm, depending on the oil type and properties.
The ISO 4406:1999 standard is used to rate solid-particle cleanliness in fluids. ISO 4406 classifies the solid-particle count in a fluid at three defined particle sizes (4, 8, and 14 micron). The classification ranking is based on the number of counted particles in these three sizes. Examples of the classification are 22/18/13 and 16/14/12. The lower the classification ranking, the cleaner the fluid. It is recommended that the ISO 4406 be used as part of the Predictive Maintenance oil analysis.
For rolling bearings, the ISO 16/14/12 cleanliness is recommended. This recommended cleanliness can be achieved in a number of ways depending on the situation: by filtering the oil lubricants before use, by filtering the oil before it is fed to the bearing by a circulating oil lubrication system and by filtering the oil in the sump of the reservoir using a kidney filtration system. In all of these cases, the machinery needs to have effective seals and filter/breathers.
Bearing manufacturers need to communicate and work with machinery OEMs, end-users and engineering consultants to ensure that appropriate solutions are specified and implemented in mineral-processing equipment systems. The results will be improved machinery reliability, availability and maintainability.
The bottom line is that selection of roller bearings should be based on L10mh rating life, quality, performance, energy-efficiency and environmental impact, along with consideration of any lubrication and cleanliness issues related to the application. MT