What are Ball and Roller Bearings?
Ball and roller bearings are machine elements that are used in all kinds of machines and devices with rotating parts. Their properties have frequently contributed greatly to technical and economic progress in different branches of engineering. Standardization of bearings has made it possible for designs to be chosen directly from bearing tables in the manufacturers’ bearing catalogs. Companies like SKF, NTN, NMB, TPI, Timken, FAG, Schaeffler, INA and many others are natural sources for selection of information. Much of the time the designer is able to select stock bearing products for his design directly from the catalog. Choosing popular and widely used designs facilitates the best availability and provides the most economical solution.
What is the Difference Between Ball and Roller Bearings?
Generally speaking ball and roller bearings may be divided into two main groups: radial bearings and thrust bearings. The basic difference between roller and ball bearings lies in their basic design.The main difference between roller and ball bearings is the rolling element being either a roller or a ball. Roller bearings use tapered, needle, spherical and cylindrical rollers. Radial ball bearings use a round ball as its rolling element. Both roller and ball bearings may have a separator or cage.
Since most types of radial bearings carry some thrust load and some thrust bearings can carry radial load, no apparent division line between the main groups exist, however, one main difference is that for bearings considered as radial ball bearings the carrying capacity is given in catalogs as pure radial load. For Roller or Ball thrust bearings the carrying capacity is given as pure thrust load. If your application calls for pure radial loads then your first choice is a radial ball bearing. If your application has both radial and thrust loads it is then possible to choose the optimal bearing based on basic ratings given.
Different Types of Ball Bearings
There are two basic types of ball bearings . Ball bearings are classified as either single or double row. The most popular is the single row deep groove type. Single row consists of one row of balls traveling in a single grooved pathway. Double row consists of two rows of balls in two sets of grooved raceways. Both single and double row ball bearings consist of an inner race, outer race, balls and cage . Both types of bearings can support high radial and axial loads. Deep groove ball bearings are used in both low and high-speed applications depending on tolerance class, lubrication, internal clearance, and type of seals or shields selected.
The cage separating the balls spaces them evenly around the periphery. Due to the cage, a ball bearing usually becomes a self-contained unit so that it can be easily mounted or dismounted. In the design of deep groove ball bearings, the grooves are relatively deep and the degree of osculation between ball and raceways is very high. Osculation occurs when two smooth curved surfaces touch. In bearings, the degree of osculation is said to be high if a principal radius of curvature of Body 1 is in the same plane and has the same direction, and almost the same length, as a principal radius of Body 2. If these radii differ appreciably in length the degree of osculation is said to be low.
To assemble the bearing the balls are placed between the rings in an eccentric position relative to one another. The balls are then spread evenly around the circumference, the cage is inserted and riveted. Due to the relatively large size of the balls and their high degree of osculation with the rings, the bearing has a comparatively high load-carrying capacity in both radial and axial directions. The ball bearing performs well in applications with combined loads and especially in high-speed designs and is often more satisfactory for carrying thrust loads than thrust bearings.
A radial ball bearing referred to as the “Conrad type” is made with a filling slot cut axially through the shoulders into the grooves. This arrangement allows the introduction of more balls as permitted between the rings as much as the pitch circumference allows. Since the filling slot must reach to the area of the center of the groove, the bearing cannot be subjected to any large thrust loads as the balls could come in contact with the edges of the filling slot. What is the advantage of adding extra balls in this design? The added balls in the Conrad type ball bearings are used to provide an extra load carry capability in the same envelope dimension.
Roller bearings are generally designed with a cage separating the rollers. There are various types of roller bearings: cylindrical rollers, spherical rollers and tapered rollers, journal roller and needle rollers. Roller bearings have many application uses but generally are used for slower, heavier loads however some cylindrical roller bearings are used in high-speed applications when they meet a high-precision standard. Roller bearing kinematics and low friction make cylindrical roller bearings very suitable for high speeds.
What is a Spherical Roller Bearing?
Spherical roller bearings are particularly suitable for carrying high loads. The common type is a double row design with both rows of rollers having a common spherical raceway in the outer ring so the bearing is completely self-aligning in the same manner as is the self-aligning ball bearing. The rollers are barrel shaped with one end smaller than the other. Consequently, they are pushed against the center guide flange with a force that though small is sufficient to hold the larger end surface of the rollers in continuous contact with the flange, thus making the guiding effective.
What are Cylindrical Roller Bearings?
Cylindrical roller bearings are guided between flanges either on the inner or outer ring. They are held together with the guiding ring by the cage even when the free ring is removed. In the most common type of cylindrical roller bearing, the free ring has no flanges so that within certain limits the bearing rings may be displaced axially in relation to each other. If the bearing is rotating this displacement takes place without resistance provided the rate of displacement is relatively slow (e.g. displacement as a result of temperature expansion of the shaft). The low friction of cylindrical roller bearings make them suitable for relatively high speeds and often used in conjunction with high speed Angular Contact ball bearings in high speed machine spindles.
What is the Difference Between Tapered Roller and Ball Bearings?
As with all roller bearings the tapered roller bearing has a line contact versus a ball type bearing with point contact. Line contact has higher load carrying capacity whereas the ball bearing has a higher speed capacity.
Tapered roller bearings consist of a cup and cone whereas a ball bearing consists of an outer and inner ring. Tapered roller can be assembled as a cartridge but in its simplest form is two separate components while a ball bearing can be separable it is most often a self contained unit.
Tapered roller bearings use straight tapered rollers whose end surfaces contact the guide flange on the inner ring. The taper of the rollers and inner ring roller path have a common apex on the bearing axis. The outer ring in the design has a slightly curved roller path generatrix in order to prevent edge loading of the rollers. In spite of the reduced degree of osculation between roller and outer ring, the outer ring is less strained than the inner ring. The guide flange of the inner ring has a spherical surface against which the spherical end surface of the roller presses. As with a spherical bearing, this pressure against the flange effectively guides the roller. Some manufacturers like Timken, FAG, NTN and SKF make the end surface of the roller flat and the guide surface of the flange conical. The roller end then contacts the roller flange at only two points.
Since tapered roller bearings are separable they are usually mounted opposed so that one bearing is adjusted against one another. After this adjustment a certain very small clearance always is left in the bearing. Temperature variations in the shaft influence this adjustment and thus also the bearing clearance. For this reason single row tapered roller bearings are generally used only where the spacing is short. A tapered roller bearing has a high-load carry capacity radially, as well as axially in one direction. The thrust capacity is dependent upon the angularity of the rollers. As a measure of this angle (contact angle) is usually given as 12 to 16 degrees. Some heavy duty tapered roller bearings have contact angles of 28-30 degrees.
In many applications tapered roller bearings are used in back-to-back pairs so that axial forces can be supported equally in either direction. Tapered roller bearings are widely used in automotive vehicle wheel bearings handling radial and axial loads. Tapered roller bearings are often used in heavy duty applications. Many different industries including agricultural, construction and mining equipment, axle systems, gear box, engine motors and gear reducers, propeller shafts, railroad axle-boxes, differentials, wind turbines and trailers of all types.
Tapered Roller Bearing
Design consideration and selection are dependent upon a number of considerations whether to specify roller or ball bearings into your design.
- Boundary dimensions are a determinate in selection of a rolling bearing whether ball or roller type. With standardized ready to mount roller and ball bearing designs the designer must determine the required size based upon required envelope dimensions. Standard ball and roller bearings have met a universal international standard and are easily identifiable in manufacturers catalogs. Manufacturers like SKF, NTN, NSK,FAG, INA,NMB, TPI are some of the best resources having reliable quality.
- Consideration and understanding of the ambient and internal temperatures of your application is key. The operating temperature of a bearing arrangement results from its own friction, the friction of seals and possibly, from extraneous sources as well as heat dissipation from adjacent components. The operating temperature of a rolling bearing at medium speed and low load is not high because the bearing friction is small. However in many cases heat in roller bearings can be quite high due to other extraneous heat sources in the application. Some application examples include axle boxes of locomotives, hammer mill, wire mill, vibrating screen, cutter shaft of planing machine, bench drill spindle, horizontal boring spindle, circular saw shaft, blooming and slabbing mill, and backup rolls of hot strip mills.
- Determining loads and speeds required in the application. Whether you need a roller or ball bearing it is necessary to consider dimensions, loads and speeds required for every application. Dimensional constraints can be determined by review of standard catalog part numbers. Once dimensions are determined, load ratings, speeds and operating environment must be considered. Different materials used in manufacturing both radial ball bearings and roller type bearings can be specified for each operating circumstance.
Data in the catalog will be used to select which bearing is best suited whether roller bearing, ball bearing or tapered roller bearing. Life calculations can be made and optimal bearing selection can be accomplished by contacting your bearing supplier.
There are many considerations in finding the right roller or ball bearing and with the aid of a competent bearing specialist a proper bearing can be selected. It is important to find a bearing supplier that has a (CBS) certified bearing specialist on staff. Contact PIB bearing specialists today to help with your selection.