Spherical Roller Bearings

Figure 1: Spherical roller bearing

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Spherical roller bearings are rolling bearings with one or two rows of barrel-shaped rollers positioned at an angle to the bearing axis. This bearing type is among the most popular due to their ability to self-align. These bearings are more robust and can handle higher loads, both radial and axial, and more evenly spread out the load on the rollers. Heavy-duty applications, such as mining, construction equipment, and industrial machinery, commonly use these bearings. This article further discusses the construction of spherical roller bearings, how they operate, their advantages and disadvantages, and compares them to other common bearing types.

Table of contents

• Applications
• Spherical roller bearing design
• Spherical roller bearings aspects
• Spherical roller bearings vs other bearing types
• FAQs

Applications

Spherical roller bearings are suitable for heavy industry applications due to their ability to handle misalignment and shaft displacement. For example:

• Conveyor systems: Spherical roller bearings support the shafts that drive the belts. Misalignment and shaft deflection occur due to uneven loading, belt tension, or temperature changes.
• Wind turbines: Spherical roller bearings support the rotor shaft and the gearbox. Misalignment and shaft deflection can happen by wind direction changes, wind speed variations, or the turbine blades' weight.
• Paper machines: Spherical roller bearings support the rollers that move and shape the paper web. Tension variations and roller speed can cause misalignment and shaft deflection.
• Construction equipment: Spherical roller bearings support rotating components, such as a crane's hook or slew ring. Misalignment and shaft deflection can be caused by uneven loading or movement on uneven surfaces.
• Mining equipment: Spherical roller bearings support mining equipment components such as conveyor pulleys or crusher shafts. Large forces generated during the extraction process can cause misalignment or shaft deflection.

Inappropriate applications

Despite their versatility, spherical roller bearings are not suitable for every application. For example:

• Precision machinery: The spherical shape of the rollers allows for some degree of play or looseness in the bearing, which affects the precision. Bearings more suitable for high precision are angular contact ball bearings, cylindrical roller bearings, tapered roller bearings, or precision deep groove ball bearings.
• Low-load applications: Spherical bearings are best suited for heavy loads. Deep groove ball bearings and cylindrical roller bearings are more appropriate for lower loads.
• Extreme temperature environments: Spherical roller bearings are not the best choice for industrial applications with high heat. Specialized bearings with high-temperature materials may be necessary.

Spherical roller bearing design

As seen in Figure 2, a spherical roller bearing has the following components:

• Outer ring (A): The contact surface of the outer ring has a spherical surface. This allows the rollers to move sideways.
• Rollers (B): The barrel-shaped rollers have a spherical contact surface with a curvature that allows for the bearing to handle axial loads. Spherical roller bearings with two rows of rollers, also known as axial spherical roller bearings, are ideal for higher axial loads. Spherical roller bearings with one row of rollers are suitable for applications that encounter minimal axial loads. The rollers in each row are angled in opposing directions to resist thrust loads in either direction.
• Cage (C): A cage holds the rollers in place.
• Inner ring (D): The inner ring's contact surface has two raceways for the rollers to move along.

Although the axial handling capacity of spherical roller bearings is less than that of an angular contact bearing, spherical bearings can handle these axial loads in both directions. Angular contact bearings can do this only in one direction unless mounted in a back-to-back or face-to-face configuration.

Figure 2: A spherical roller bearing's primary components: outer ring (A), roller (B), cage (C), and inner ring (D).

Tapered vs cylindrical bore

Spherical roller bearings can have a tapered or cylindrical bore. Choosing between the two is based on the following:

• Rotational speed: For applications with high rotational speeds, bearings with tapered bores are preferred.
• Misalignment: Bearings with tapered bores can handle more misalignment than cylindrical bores.
• Load: A spherical roller bearing with a tapered bore can handle heavier loads than those with cylindrical bores.
• Shaft design: Like bearings, shafts can have cylindrical or tapered outer diameters.
• Vibrations: Applications with heavy vibrations should have spherical roller bearings with tapered bores.

The primary reason why spherical roller bearings with tapered bores operate better on the above factors is due to the ability of adjusting their internal clearances. Internal clearance is the distance between the rollers and the raceways. If the clearance is too small, there can be extreme wear and tear; if the clearance is too large, there will be excessive vibrations, leading to reduced load capacity and life. Adjustment is accomplished using a tapered adapter sleeve (see below).

Spherical roller bearings aspects

This section discusses four important aspects of spherical roller bearings:

• Misalignment
• Materials
• Lubrication
• Mounting options

Spherical roller bearings misalignment

There are various reasons why bearing misalignment occurs. However, for spherical roller bearings, the primary reason for misalignment is related to them being used in heavy industrial applications. Spherical roller bearings support and guide rotating shafts, which can deflect under heavy loads, leading to bearing misalignment. Also, heavy vibrations can cause misalignment.

Spherical roller bearings can handle axle misalignment up to 2°. For applications with more than 2° deflection, self-aligning bearings may be a suitable solution.

Spherical roller bearings material

The outer and inner rings and rollers in a spherical roller bearings are most often made of chrome steel, which is the common material for many different types of bearings. This material has been standardized to some degree:

• AISI (United States)
• DIN
• 100CR6 (Germany)
• SUJ2 (Japan)
• GCR15 (China)

The common cage materials are:

• Steel
• Brass
• Polyamide
• Sheet steel

Spherical roller bearing lubrication

As with all bearings, spherical roller bearings must be adequately lubricated with grease or oil. Grease has a lower operational speed capacity than oil. For high-speed applications, oil is a better choice.

If grease is preferred, choose a bearing well suited for this lubrication type. Often, manufacturers build a grease groove into the outer raceway to make for easy application of grease using grease nipples.

If oil is the preferred choice, this can be done through a sealed bearing with an oil reservoir or oil-bath lubrication. With an oil bath, only the lowest roller should contact the oil. This allows for splash lubrication. If more than the lowest roller contacts the oil, the bearing will need more force to bring the rollers through it, losing its effectiveness and speed advantage.

Figure 3: Bearing lubrication

Spherical roller bearing mounting methods

There are three main ways in which spherical rolling bearings are typically mounted:

• Set screw
• Tapered adapter
• Direct shaft

Set screw mounted

The bearing is mounted with a collar and set screws, bolts entirely threaded across their length. As a rule, set screw-mounted bearings are designed to support light to moderate loads, but not heavy loads. The expected loads determine screw sizes if the screws are tightened to their total capacity. Choosing a screw size that is too big results in excessive ring bending. Following the manufacturers' recommendations regarding shaft fit and set screw tightening is crucial to achieving the desired bearing performance, especially when operating under heavier loads or higher speeds.

Tapered adapter sleeve

Tapered adapter sleeves can mount a spherical roller bearing with a tapered bore to a shaft with a cylindrical or slightly tapered outside diameter. The sleeve fits over the shaft and has a tapered inside that matches the bearing's taper.

A tapered adapter sleeve is generally made of steel. It is split along its length, allowing easy installation and removal. The primary reason tapered adapter sleeves are preferred is their simple installation. They do not require additional machining or special tools. The sleeve is tightened on the shaft using a nut or locking device. This compresses the sleeve onto the shaft, creating a secure and stable fit.

Direct shaft mounted spherical roller bearings

Direct shaft-mounted spherical roller bearings have a cylindrical bore and mount directly to relatively small cylindrical shafts. Direct mounting eliminates the need for additional components and simplifies the assembly process. This mounting style is suitable for applications with limited space for mounting components.

Spherical roller bearings vs other bearing types

Cylindrical roller bearings vs spherical roller bearings

The primary differences between spherical and cylindrical roller bearings are the loads they accommodate and their ability to handle misalignment. They also differ in friction coefficient and application.

• Loads: Cylindrical roller bearings handle high radial loads and small axial loads. Spherical roller bearings, though, not only handle radial loads but also handle higher axial loads.
• Misalignment: Cylindrical roller bearings do not handle misalignment, unlike spherical roller bearings.
• Friction: Cylindrical roller bearings have a lower friction coefficient than spherical roller bearings and, therefore, can handle higher-speed applications.
• Applications: Cylindrical roller bearings are found in machineries such as pumps, compressors, and gearboxes. Spherical roller bearings can handle higher vibrations and loads and are therefore suitable for mining, paper mills, and construction machinery equipment.

Spherical roller bearings vs ball bearings

The primary difference between spherical roller bearings and ball bearings is how they're applied. Ball bearings can operate at higher speeds than spherical roller bearings. However, ball bearings cannot support as much load or handle misalignment. Example applications for ball bearings are electric motors, bicycles, and conveyor systems.

Spherical roller bearings vs tapered roller bearings

The primary differences between spherical and tapered roller bearings are how much load they accommodate and the ability to handle misalignment. The shape of the tapered rollers allows for more contact between the rollers and the bearing's races. Therefore, the tapered roller can accommodate higher loads at higher speeds. Tapered roller bearings, however, do not handle misalignment like spherical roller bearings.

FAQs

What are spherical roller bearings used for?

Spherical roller bearings are used for low to heavy loads, both radial and axial loads, and if there are misalignment issues.

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What kind of loads can a spherical roller bearing support?

Spherical roller bearings can handle radial, axial, and thrust loads ranging from low to extremely heavy.

In rolling-element bearings (also called “antifriction bearings”), rolling elements such as balls or rollers interposed between the housing and the shaft produce rolling motion instead of sliding motion. The shape of the rolling elements as in Figure 1, such as ball, tapered roller, needle roller, or spherical roller, usually describes the modern rolling-element bearings. European companies took the lead in much early bearing production, so the metric system of measurement has been widely adopted, ensuring high interchangeability of many popular bearing types and sizes.

Ball bearings are the most common rolling-element bearings and are produced in a variety of sizes and types for applications from miniature high-precision equipment such as dental drills, to huge industrial machines and jet engines.

Bearings using rollers that are tapered, cylindrical, or spherical give the machinery designer greater load-carrying capacity than ball bearings, and the geometric shapes of their rolling elements offer unique capabilities.

Making it work with misalignment

A bearing’s basic function is to reduce friction while carrying radial load or thrust load, or both. It may also have to accommodate either dynamic or static misalignment.

An example of dynamic misalignment is a bent shaft rotating in a bearing, which causes constant deflection or wobble of the shaft where it sits in the bearing. A shaft may deflect elastically under load, rather than bend permanently, but the effect on the bearing is the same.

Also, the head shaft on a heavily loaded belt conveyor might deflect, or the support structure could also move slightly under load, increasing potential misalignment at the bearing. Spherical roller bearings are designed to rotate while constantly accommodating this “wobble” and yet carry full system load.

Self-aligning spherical bearings can also compensate for manufacturing tolerances in machined housings and misalignments common in cast or fabricated equipment structures. If both the shafting and structure are rigid, the initial manufacturing tolerances of bearing housings make it difficult to align shafting perfectly perpendicular to the bearing housing, as some types of bearings require. Most spherical roller bearings can accommodate ±2-deg misalignment. On bearings set at 10-ft centers, this can equate to as much as 8.3 in. of misalignment, Figure 2. For either dynamic or static misalignment, spherical bearings provide the “forgiveness” necessary where economics or the real-world environment compromise the perfect alignment that ball, tapered, or cylindrical bearings may require.

Types of spherical bearings

Spherical roller bearings come in two types of self-aligning rollers, Figure 1. The Swedish-designed “barrel-shaped” roller is common and adjusts for misalignment as the roller operates around the spherical surfaces of the inner and outer ring. Self-alignment can be accomplished also by hourglass-shaped rollers, a unique United States design patented by Julius Shafer. As shown, the hourglass roller misaligns relative to the inner and outer spherical raceway surfaces in a fashion similar to the barrel shape.

Spherical design characteristics

Figure 3 shows the relationship between diameter and width on standard spherical bearing series for a given bore. Load capacity increases as the boundary envelope enlarges. The increasing mean diameter of the boundary envelope permits larger rollers.

For the best dynamic ratings, designers tend to use as large and as few rollers as possible. For the best static load-carrying capability, more rollers, and therefore smaller diameters, are best. ID, OD, and width of these units are standardized metric dimensions, with the last two digits in the bearing’s nomenclature representing the bore size. For example, if the last two digits are 12 (say the bearing number is ) then multiplying 12 by 5 indicates the bore is 60 mm. The smallest spherical bearing bore diameter interval that has evolved is 5 mm. Thus, standard spherical designs are offered in even multiples of 5 mm, and it has become industry convention to use a bearing nomenclature where the bore is listed as the quantity of 5-mm increments. This permits a two-digit value to span a bore range from 20 mm (04) to 480 mm (96). This numbering system is used also on other types of rolling-element bearings, such as ball bearings and cylindrical roller bearings.

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Spherical roller bearings are usually designed with two rows of rollers tilted at opposite angles to resist thrust load in either direction. Thrust loads are common when deflections and misalignments are sufficient to require a spherical roller bearing.

For the special situation with only radial load, bearing designers can use the same boundary to contain only one row of even-larger-diameter rollers. This maximizes the bearing’s radial load-capacity but minimizes thrust capacity.

For the special situation with only extremely high thrust load such as a rock crusher, you can use a spherical thrust bearing. It consists of one row of rollers positioned between special bearing races so that high axial shaft load can be transmitted through the bearing to the mounting structure and yet accommodate the types of misalignment described previously.

You can get spherical roller bearings with selected inner and outer ring tolerance variances for special shaft or housing fits, and manufacturers’ catalogs usually provide details. Many factors influence proper shaft or housing fit, such as magnitude of the load, shock, vibration, and temperature. Furthermore, housing fits must consider axial movement requirements for shaft expansion.

Internal operating clearance of a spherical roller bearing — the space between the rollers and the outer ring, Figure 4 — can also be critical to the application. Internal operating clearance is also encoded in the nomenclature. For example, a suffix could be shown as C2, C0, or C3, with C0 representing normal tolerance; C2, less clearance than normal; and C3, more. Be sure to check nomenclature designations on a given bearing with a catalog from the maker of that bearing to assure meanings of all suffixes in the nomenclature. There are some variations among manufacturers.

As an example of the usefulness of operating clearance suffixes in bearing nomenclature, high-temperature or highspeed applications provide better service if additional clearance (C3) is provided. Lower clearance (C2) may increase service life in a heavily loaded, low-speed application, because more roller surface is in contact with raceways.

Mounting arrangements

There are three general mounting arrangements for spherical roller bearings. When a manufacturer provides the bearing housing, such as a pillow block or a flange block, the bearing is often mounted with a collar and setscrews. These lock the inner ring to the shaft. Proper shaft diameter tolerance and mounting collar setscrew torque are important and let these factory-assembled and prelubricated units be mounted and dismounted quickly.

Setscrew-mounted bearings are ordinarily designed to accommodate light to medium loads, but not heavy loads. Most bearing makers define a heavy load as one greater than about 25% of the bearing rating. (Bearing dynamic capacities are reference values for longevity calculations and do not represent allowable load limits for normal use.) Bearing manufacturers have established shaft-to-inner- ring fits that allow the convenience of slip-fit installation, while still providing adequate support for the expected loads. With setscrew mounting, the “bite” of the screw is a function of its diameter and tightening torque. Bearing makers select screw sizes that resist the expected loads if the screws are fully tightened. Over-designing the screw size has a detrimental effect: excessive ring bending. Especially with larger loads or speeds, it is important to follow manufacturers’ recommendations for shaft fit and setscrew tightening to achieve the desired bearing performance.

A second mounting arrangement common in split pillow blocks is the tapered adapter sleeve, Figure 5. In this arrangement, the tapered sleeve is drawn into the bearing, clamping it tightly to the shaft. When a locknut is tightened, the tapered adapter compensates for shaft clearance, but the mounting procedure requires more skill and care to ensure that the bearing adapter is tightened enough and the operating clearance of the bearing has not been altered. Manufacturers’ instructions usually outline proper mounting procedures — be sure they are followed.

Direct shaftmounted spherical roller bearings may be either slip-fit or press-fit, depending on loading considerations and service life expectation. As with tapered sleeve mountings, housing and shafting tolerances determine resultant fit — make sure manufacturer’s suggestions are followed.

As with ball and cylindrical roller bearings, spherical bearing bore and outside diameter tolerances are in accordance with the system of tolerancing established by the International Organization for Standardization and adopted by the American Bearing Manufacturers Association and the American National Standards Institute. Housing fits, for example, are designated by a capital letter and a number. The letter indicates the location of the housing bore tolerance limits with respect to the nominal bearing OD. The number indicates the size of the tolerance zone. A similar system, using a lower case letter and a number, applies to shaft bearing seat diameters.*

Most modern ball and roller bearings provide exceptional value for their price. Unusual problems in application or machinery design — especially misalignment problems — may be better fulfilled by the unique capabilities of spherical roller bearings, When properly selected and mounted, they can solve problems and boost service life.

*For more about tolerancing of bearing bores and ODs, see PTD, “Shafts and Housings for Radial Bearings: Picking the Right Fit,” 3/90, p. 47.

William D. Schroeder is Marketing Manager, Link-Belt Bearing Div., Rexnord Corp., Indianapolis. When this article was written, he was Product Manager.

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