Select the right bearing

Not every anti-friction device works in every application.

By Holly Shean, NTN Bearings

You are walking the plant floor and it's just one of those days. Nothing seems to be going right. And then, your nemesis, that same ol' machine, goes down again. As usual, the bearings failed. "Why me?," you ask, "They must be bad bearings. We can't afford the down time every time this happens. I'm getting tired of this." So, you call your bearing supplier and ask for a solution to your problem.

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The supplier tells you quite emphatically that the bearings are not poor quality. However, you also learn that they could be the wrong type for the application and that you will need to analyze the required bearing functions and performance demands. The supplier starts the problem-solving sequence by collecting the following data:

  • the machine's function and construction
  • bearing dimension limitations
  • bearing mounting location (point)
  • bearing life span
  • bearing load (direction and magnitude)
  • bearing running capacity
  • vibration and shock load
  • bearing speed
  • bearing temperature (ambient and that generated by internal friction)
  • rigidity
  • friction and torque
  • environment (corrosion, lubrication cleanliness of the environment etc.)
  • allowable misalignment for inner and outer rings
  • requirements for mounting and dismounting

Arrangement selection

It is important to consider the factors that affect performance of the bearing and the machine in which it is placed.

– Holly Shean, NTN Bearings

Shaft assemblies generally require two bearings to support and locate the shaft both radially and axially relative to the stationary housing. These two bearings are called fixed and floating bearings. The fixed bearing withstands both radial and axial loads and locates or aligns the shaft axially in relation to the housing. Being axially free, the floating bearing relieves stress caused by thermally-induced expansion and contraction of the shaft. It can also allow for misalignment caused by fitting errors.

Bearings that support axial loads in both directions are most suitable for use as fixed bearings. In floating bearings, the axial displacement can take place in the raceway — cylindrical roller bearings — or along the fitting surfaces — deep groove ball bearings. There is also the cross location arrangement in which two angular contact ball bearings act as fixing and non-fixing bearings simultaneously, each bearing guiding and supporting the shaft in an axial direction only. This arrangement is used mainly in comparatively short shaft applications.

Specification determination

Because they are designed for the widest possible use, the specifications for rolling bearings are standardized. However, to meet the diversity of applications required, you can select a bearing of non-standard design. To do so you will need such specifications as the dimensional and running tolerance, internal clearance, and preload, bearing material, heat treatment, and cage design and material.

Handling methods

If bearings are to function as expected, you must select and implement appropriate methods of installation and handling — fitting methods, lubrication methods, sealing methods, shaft and housing construction, and dimensions.

Deep groove ball bearings

This type of bearing is arguably the most popular. Its application and assembly is rather simple when compared to other types. The Conrad design limits the number of rolling elements. These bearings are available with or without seals and shields. They also available with grease for maintenance-free applications. A wider cartridge design is also available that allows more room inside the bearing for addition grease.

Deep groove ball bearings are suitable for radial, axial, or combination loads. They provide very quiet operation, need minimal lubrication, and operate at relatively high speeds with less friction than roller bearings. On the other hand, their load capacity is less that of roller bearings. Debris contamination could be a concern since the point contact of the rolling elements in ball bearings is more sensitive to dirt than are roller bearings. Deep groove ball bearings are typically used in electric motors, pulley, and gearboxes.

Angular contact bearings

Select this bearing over deep groove ball bearings when the axial load is higher. Their radial capacity can be higher since their construction allows for more rolling elements as compared to the Conrad bearings. This type of bearing is not available with integral enclosures. Single-row bearings, with or without radial load, accept thrust in only one direction. They can be assembled in tandem with another bearing — DB or DF configurations — to allow thrust in both directions (see two-row angular contact bearings). They are typically used in hydraulic pumps and machine tools.

Two-row angular contact bearings

These bearings use the Conrad construction with two rows of rolling elements. Diverging contact angles result in a rigid shaft support for overhung loads. They are available with or without enclosures. The sealed design modifies the internal design to allow room for the enclosures. The result is slightly lower capacity. Some sizes are available in a design with a wider cartridge. This allows room for additional grease inside the bearing. The two-row angular contact design uses less space than an assembly of two single-row angular contact bearings.

Cylindrical roller bearings

Line contact in roller bearings allows for higher loading as compared to the point contact of ball bearings. Inner rings with no ribs allow the shaft to float axially. Inner rings with one rib allow the bearing to take axial loading in one direction. There are limitations on the amount of axial load. To ensure proper roller rotation, the axial load should never exceed 40% of the radial load. Select these bearings over deep groove ball bearings to handle more radial load, although the limiting speed may be lower.

Needle roller bearings

This is another type of roller bearing used for applications in which high load capacity is required but the radial space for the bearing is limited. Needle roller bearings are available with machined races or drawn cup races. In some applications, the shaft and housing surfaces are hardened and ground to eliminate the need for separate raceways to minimize the radial space needed for the bearing.

Tapered roller bearings

This type of roller bearing is also suitable for radial, axial, or combination loads. The taper of the cup induces an axial load in response to an applied radial load. Therefore, this type of bearing must be assembled in pairs. In addition, when assembling the unit, the bearings must be set with regard to preload or endplay. Typical applications are small diameter wheel bearings. Mill shafts and construction equipment use the larger diameter tapered roller bearings.

Spherical roller bearings

These roller bearings have a spherical outer raceway that allows greater misalignment. They are best suited for applications with high radial loads and some known misalignment from either shaft bending or assembly inaccuracies. Some sealed versions are not wider but have a smaller roller complement to allow room for the seals. Spherical bearings are commonly used with plummer block housings and with straight or tapered bores to allow for flexibility in shaft size and design. The most common applications for these bearings are rock crushers; shaker screens; shredders; steel and paper mills; and mining equipment.

Thrust bearings

Available in both ball and roller designs, these bearings are used for applications in which there is only a pure radial load. If a radial load is present, then a radial bearing must be used in conjunction with the thrust bearing. Typical applications include machine tool spindles, valves, vertical drives, and turntables.

Industrial constant-velocity joints

This product combines a drive shaft with a flexible coupling on each end. Constant velocity joints differ from universal joints — there is no speed variation within the allowable range of motion of the CV joint. Typical applications include drive units for process industries in which constant speed is required. An example might be driving multiple shafts off one gearbox.

Summary

There are so many bearings for so many applications and situations. Frequently the data required to select the bearing is not clearly specified. Thus, it is important to consider the factors that affect performance of the bearing and the machine in which it is placed. Only then is it possible to identify the correct bearing for the application of the machine.