Bearings can be divided into two subgroups: Plain Bearings and Rolling Contact Bearings.
Both have their place in the world of machines. Each type has some obvious advantages and disadvantages, but there are subtle properties as well that are often ignored. Each type of bearing can be found in a multiplicity of places, and each can be lubricated with either oil or grease. Some bearings are lubricated by water, and some are lubricated by air (as in the case of a dentist's drill).
11.2 Plain Bearings
Plain Bearings consist of two surfaces, one moving in relation to the other. Plain bearings can be the Journal type, where both wear surfaces are cylindrical; Thrust type, where there are two planar surfaces, one rotating upon the other; and various types of Sliding bearings where one surface slides in relation to the other. All depend upon a lubricating film to reduce friction.
Unless an oil pump is provided to generate the oil film, these bearings rely on shaft motion to generate a Hydrodynamic Oil Wedge.
Advantages of Plain Bearings
- They have a very low coefficient of friction if properly designed and lubricated.
- They have very high load-carrying capabilities.
- Their resistance to shock and vibration is greater than rolling contact bearings.
- The hydrodynamic oil film produced by Plain bearings damps vibration, so less noise is transmitted.
- They are less sensitive to lubricant contamination than rolling contact bearings.
Types of Plain Bearings
(1) Journal (Sleeve Bearings)
These are cylindrical with oil-distributing grooves. The inner surface can be Babbitt-lined, Bronze-lined, or lined with other materials generally softer than the rotating journal.
On horizontal shafts on motors and pumps, oil rings carry oil from the oil reservoir up to the bearing. In the case of very slow-moving shafts, the bearings may be called Bushings.
(2) Segmented Journal
These are similar to the journal except that the stationary bearing consists of segments or bearing shoes. Each shoe is individually adjustable.
This type of bearing is commonly found in vertical hydrogenerators and large vertical pumping units. This bearing is usually partially immersed in an oil tub.
(3) Thrust Bearings
These bearings support Axial Loading and consist of a shaft collar supported by the thrust bearing, many times in segments called thrust shoes. The thrust shoes are sometimes allowed to pivot to accommodate the formation of the supporting oil wedges.
There are many different configurations of the thrust bearing aimed at equalizing loading and oil wedges. The bearing is immersed in a tub of oil. On large hydrogenerators and pumps an oil pump is sometimes used to provide an oil film at start-up.
Plain Bearing Lubrication Selection
The most common lubricants for Plain Bearings are Oils and Greases.
Traditionally, Non-Renewable petroleum based mineral oils have been generally used except in extreme hot and cold temperature applications where petroleum based synthetics provide superior performance.
Today, renewable Stabilized High Oleic Base Stock (HOBS) vegetable oils are replacing petroleum based mineral and synthetic oils as the far safer, superior performing lubricant of choice for all types of bearing applications.
Oil is used for faster rotational speeds where the Hydrodynamic Oil Wedge can be formed and maintained. It also is used in high temperature conditions where grease may melt or degrade.
Grease is used for slower rotational speeds or oscillating movements where the Hydrodynamic Oil Wedge cannot form. It is used in cases of extreme loading where the bearing operates in Boundary Lubrication conditions. The lubricating properties of greases are significantly affected by the base oil and type of thickeners used.
Viscosity is the most critical lubricant property for insuring adequate lubrication of Plain bearings. If the viscosity is too high, the bearings will tend to overheat. If the viscosity is too low the load-carrying capacity will be reduced.
Table 11-1 shows some of the important considerations regarding lubricant selection.
Lubrication Choices For Spindle Bearings
Most builders of high speed spindles today offer the same basic spindle model with either permanently Grease lubricated spindle bearings or some form of Oil lubricated spindle bearings (Oil Mist, Oil Air, Oil Jet Lubrication).
There is a trade-off concerning the Maximum Spindle Speed if the bearings used are permanently grease lubricated verses oil lubricated.
There are also pros and cons to selecting Grease verses Oil lubricated spindle bearings. Factors influencing the decision on the choice of which type of bearing lubrication to use are grouped and discussed below:
Oil Lubricated Spindle Bearings
Oil lubrication never runs out.
When properly maintained, oil lubricated spindle bearings can have a longer service lifetime than grease lubricated bearings. Oil lubricated bearings can achieve twice the bearing life of those lubricated with grease.
Oil lubricated bearings are continuously flushing themselves of wear particles and other contaminants that can damage the bearings.
For a given operating speed, Oil lubricated bearings allow the benefits of a larger bearing size (higher load capacity) and a larger shaft size (greater rigidity). The additional load capacity and rigidity may outweigh any of the negative aspects of oil lubrication for certain applications.
Oil lubricated bearings will require additional tubing lines to the spindle.
Most oil lubricated spindle start-up routines have to establish lubrication several minutes before the spindle is started as the circulating system cycles the oil into bearings.
Oil lubricated spindles increase the complexity of the spindle system.
Oil lubricated spindles increase the risk for lubrication related problems.
Incorporates an effective oil scavenging system can be an expensive option if it has not been originally designed into the spindle.
Grease Lubricated Spindle Bearings
Grease lubricated bearings clearly offer the simpler, less expensive system.
No end user maintenance requirements.
No peripheral equipment needed.
With grease lubricated bearings, there is a finite amount of oil in the grease; once this oil is used up, the bearings will fail.
Once wear particles and debris get into the grease, they remain.
Bearings must be kept clean to achieve maximum bearing life.
Methods Used To Supply Lubricants
Table 11-2 identifies some of the methods used to supply lubricants to bearings. The lubricant should be supplied at a rate that will limit the temperature rise of the bearing to 20 °C (68 °F).
Generally, oil additives such as those noted in Chapter 5 - Renewable Lubricant Additives are not required in Plain bearing applications. Some additives and contaminants may cause corrosion, so caution should be exercised when using bearing lubricants containing additives or when contaminants may be present.
11.3 Rolling Contact Bearings
In Rolling Contact Bearings, the lubricant film is replaced by several small rolling elements between an inner and outer ring. In most cases the rolling elements are separated from each other by cages. Basic varieties of rolling contact bearings include ball, roller, and thrust.
Advantages of Rolling Contact Bearings
- At low speeds, Ball and Roller Bearings produce much less friction than Plain bearings.
- Certain types of Rolling Contact Bearings can support both radial and thrust loading simultaneously.
- Rolling Bearings can operate with small amounts of lubricant.
- Rolling Contact Bearings are relatively insensitive to lubricant viscosity.
- Rolling Contact Bearings have low wear rates and require little maintenance.
Types of Rolling Contact Bearings
(1) Ball Bearing
This bearing has spherical rolling elements in a variety of configurations. It is able to carry both radial and moderate axial loads.
A special type, called Maximum-type ball bearings, can take an extra 30 percent radial load but cannot support axial loads.
(2) Roller Bearing
The roller bearing has cylindrical rolling elements and can take much higher radial loads than ball bearings but can carry no axial loads.
(3) Tapered Roller Bearing
This type has truncated-cone shaped rolling elements and is used for very high radial and thrust loads.
(4) Double-Row Spherical
The bearing has a double row of keg-shaped elements. The inner surface of the outer race describes part of a sphere. This bearing can handle thrust in both directions and very high radial loads.
(5) Ball Thrust
This type has ball elements between grooved top and bottom races.
(6) Straight Roller Thrust
This bearing has short segments of cylindrical rollers between upper and lower races. The rollers are short to minimize skidding.
(7) Spherical Thrust
This type is also called a tapered roller thrust bearing. The lower race describes part of a sphere. The rolling elements are barrel-shaped and the outside has a larger diameter than the inside.
(8) Needle Bearing
These bearings have rollers whose lengths are at least four times their diameter. They are used where space is a factor and are available with or without an inner race.
Rolling Contact Conditions
The loads carried by the rolling elements actually cause elastic deformation of the element and race as rotation occurs. The compressive contact between curved bodies results in maximum stresses, called Hertzian Contact Stresses, occurring inside the metal under the surfaces involved. The repeated stress cycling causes fatigue in the most highly stressed metal. As a result, normal wear of rolling contact bearings appears as flaking of the surfaces. Lubrication carries away the excessive heat generated by the repeated stress cycles.
While lubrication is necessary, too much lubrication, especially with grease lubrication, results in churning action and heating due to fluid friction.
Rolling Bearing Lubricant Selection
In most cases, the lubricant type, i.e. Oil or Grease, is dictated by the bearing or equipment manufacturer. In practice, there can be significant overlap in applying these two types of lubricant to the same bearing. often the operating environment dictates the choice of lubricant.
For example, a roller bearing on an output shaft of a gearbox will probably be Oil lubricated because it is contained in an oil environment. However, the same bearing with the same rotational speed and loading would be Grease lubricated in a pillow block arrangement.
Table 11-3 provides general guidance for choosing the proper lubricant.
Grease is used for slower rotational speeds, lower temperatures, and low to medium loads. Grease is used in situations where maintenance is more difficult or irregularly scheduled. It can be used in dirty environments if seals are provided.
Oil is used for higher rotational speeds and higher operating temperatures. It is used in maximum loading situations and for bearing configurations where a high amount of heat generated in the bearing can be carried away by the oil. It is used in dirty conditions when the oil is circulated and filtered.
For moderate speeds, the following viscosities are recommended:
- Ball and Cylindrical-Roller Bearings @ 12 cSt
- Spherical-Roller Bearings @ 20 cSt
- Spherical-Roller Thrust Bearings @ 32 cSt
In general, oils will be the medium to high Viscosity Index type with Rust and Oxidation Inhibitors.
Extreme Pressure (EP) oils are required for Taper-Roller or Spherical-Roller Bearings when operating under heavy loads or shock conditions. Occasionally EP oils may be required by other equipment or system components.