Spherical Plain Bearing Engineering

Spherical Plain Bearings:

Spherical plain bearings are designed to provide rotational motion in omni-direction. There are primarily two types of spherical plain bearings, radial and angular contact. Spherical plain radial bearings offer high load carrying capability in primarily the radial direction. Spherical plain angular contact bearings are designed for high load carrying capability in mainly the thrust or axial direction.

Radial Contact Spherical Plain Bearings:

The outer ring bore of our spherical plain bearing includes a concave spherical surface (as shown in Figure 1). The outside diameter of the inner ring, or ball, has a convex spherical surface (as shown in Figure 2). We match these two spherical diameters to offer the optimum combination of load capacity and bearing frictional torque. This attention to detail in both design and manufacturing makes NPB spherical plain bearings products of choice, best suited for applications requiring the transmissions of heavy loads under oscillatory as well as continuous rotational motions. They are also intended for applications requiring angular misalignment. Our bearing rings and balls are made of high strength bearing steel, heat treated to Hrc 58 minimum hardness for longevity of performance.

Figure 1, Bearing Outer Ring

Figure 2, Bearing Inner Ring

Sealed Bearings:

National Precision Bearing offers sealed spherical plain bearings to minimize the ingress of contaminants on to the bearing rotating surface and to help keep lubricant within the bearing (as shown in Figure 3). Seals are effective for operation within a temperature range of -10o F to +250o F. Special seal materials are also available for operation under extreme temperature beyond this range. Please contact our engineering department for more information.

Figure 3, Sealed Spherical Plain Bearing

Heavy Duty Spherical Plain Bearings:

For application requiring extra load carrying capacity, NPB offers heavy duty spherical plain bearings. With the same bore diameter options as our standard series, the heavy duty series has larger inner and outer ring diameters, as well as wider outer ring widths (Figure 4). This provides increased surface contact to support heavier loads. In general, our heavy duty spherical plain bearings have 25% more capacity than that of standard duty bearings.

Figure 4, Heavy Duty Spherical Plain Bearing

Extended Inner Race Spherical Plain Bearings:

For applications requiring additional axial space, NPB offers extended inner ring spherical plain bearings (as shown in Figure 5). This type of bearing eliminates the need for an extra spacer on both sides of the inner ring.

Figure 5, Extended Inner Race Spherical Plain Bearing

Self Lubricating Bearings:

Self-Lubricating bearings (as shown in Figure 6) do not require grease or oil for lubrication. Instead, they utilize a liner system bonded to the inside diameter of the outer ring. The inner ring or ball is made of heat treated high strength steel and is chrome plated on the outside diameter. The chrome plating provides a smooth, hard surface to run against the liner system. These bearings are also manufactured with seals to help keep contamination out of the bearing. Self-Lubricating bearings are particularly well suited when the loading is in one direction. Single direction loading prevents grease or oil from flowing into the load zone of the bearing, while the liner system provides uniform lubrication in the load zone.

Figure 6, Self-Lubricating Bearing

Angular Contact Bearings:

Angular contact bearings (as shown in Figure 7) are best suited for applications with single direction, thrust load. When thrust load reverses, angular contact bearing will separate. Therefore, for applications with reversing thrust load, it is necessary to use a pair of angular contact bearings. For applications requiring moment flexibility, arrange angular contact bearings in Face-to-Face or "DF" (as shown in Figure 8). For applications requiring high moment rigidity, arrange angular contact bearing in Back-to-Back or "DB" (as shown in Figure 9).

Figure 7, Angular Contact Bearing

Figure 8, Face-to-Face (DF) Angular Contact Bearing Pair

Figure 9, Back-to-Back (DB) Angular Contact Bearing Pair

Fractured Outer Ring Assembly Process:

As part of the spherical plain bearing manufacturing process, the outer ring is fractured to complete bearing assembly. At National Precision, we use high quality steel and a controlled heat treatment process to harden the bearing rings to Rockwell C 58. Our bearing ring at 0.2% yield strength is 320,000 psi, with ultimate strength reaching 450,000 psi. Therefore, with controlled assembly processes, the inner ring may be pressed into the outer ring (as shown in figure 10).

Figure 10, Fractured Bearing Outer Ring, for inner ring insertion.

Dimensional Tolerance:

Other than self-lubricating bearings, all outer rings and balls are phosphate coated for corrosion protection. The spherical surfaces are then Molybdenum Disulfide coated. The nominal thickness of this dry film coating is .0002 inches. Depending on the internal clearance and/or preload between the bearing outer ring and ball, the fractured outer ring may be forced open prior to installation into the housing. Therefore, all tolerances listed in our catalog apply to bearing rings before outer race fracture and application of dry film coating. NPB Spherical plain bearings meet or exceed ISO 12240-1 and/ or ANSI/ABMA Std. 22.2.

Load Ratings:

Load carrying capability of a spherical plain bearing is a function of surface area in actual contact under load. In other words, bearings with a larger contacting surface will have higher capacity. Therefore, in addition to the use of high quality steel and a well controlled heat treatment process, very close attention is given to the spherical surfaces on both the outer rings and balls.

There is no industry standard governing spherical plain bearing capacity. At NPB, we start with high quality steel that is heat treated to optimum hardness and finally ground to a stringent spherical profile. This process allows the bearings to accept load up to 47,500 psi surface contact stress. In calculating individual bearing capacity, we have excluded the grease grooves grease hole and corner radii. The dynamic load capacity is 1/3 that of the static load capacity.

Note: Ultimate loads for our spherical plain bearings are approximately 1.5 times the catalog load ratings.

Lubrication:

The primary function of lubricant in spherical plain bearings is to provide a wear resistant film, separating metal to metal contact. As a result, friction, wear and noise are reduced, extending the bearing's useful life. A secondary function of lubricant is to protect the bearing from corrosion.

At NPB, all bearings Load Series spherical plain bearings (except self lubricating bearings) are phosphate coated for corrosion protection. The spherical surfaces are also Molybdenum Disulfide coated to provide initial protection before the grease is adequately distributed by oscillatory or rotational motion between the ball and outer ring.

It is essential to lubricate all metal to metal bearings thoroughly before installation. To ensure that lubrication covers the entire spherical surface, we suggest rotating the inner ring several turns in all directions. For applications where load is in one direction, the initial distribution of lubricant is essential. We recommend regular re-lubrication at scheduled machine maintenance. This process improves bearing life by replenishing lubricant, as well as purging out containments and wear debris.

Housing and Shaft Fits:

It is important to accommodate an interference or "press" fit between the housing and spherical plain bearing outer ring. This serves two purposes. It keeps the outer ring fracture closed and it prevents the outer ring from sliding under external loads. We recommend press fit to ISO R7. Depending on the particular application, mounting fit for the bearing inner ring and shaft may either be slip (loose) or press (tight). For applications requiring slip fit, we recommend an ISO f6 fit. For applications requiring press fit, we recommend ISO m5 fit.

Note: A slip fitted bearing inner ring will rotate and/or oscillate on the shaft under load. Therefore, the shaft must have an adequate surface finish (32u-in or better) and must be sufficiently hardened to 45 Hrc minimum.

Bearing Mounting:

When installing Load Series spherical plain bearings, do not hammer on the bearing rings. It is also important to align the outer ring fracture away from externally applied load. Always push on the ring that is being mounted. In other words, push only on the outer ring while installing the bearing into housing. Push only on the inner ring while installing bearing onto a shaft (as shown in Figures 11 and 12).

Figure 11, - Push only on the Outer Ring while installing bearing into housing.

Figure 12, - Push only on the Inner Ring while installing bearing onto shaft.