Thin Section Bearings for Precise Machinery Applications

Introduction

Thin section bearings are specially designed with a constant cross-section, even for larger bore size bearings. This contributes greatly to saving space and weight, especially when designing complex products based on shaft diameter and power requirements. Large diameter, small-cross-section bearings also permit the use of larger hollow shafts as opposed to smaller solid shafts. These hollow shafts can accommodate components like air and hydraulic pipelines or wires and slip rings, resulting in a tidy and well-organized assembly.

Thin Section Bearing Features

Thin section bearings typically have a square cross-section, varying from 0.1875 inches to 1 inch on each side, regardless of the bore size. The bore diameter ranges from 1 inch to 40 inches. For a 10-inch bore diameter, thin section bearings have a cross-section as small as 0.25 inches on each side. In contrast, the cross-section of traditional extra-light bearings is 1.625 inches on each side. Further comparison with extra-light bearings reveals that thin section bearings are considerably lighter. For a 4-inch bore diameter, the weight of thin section bearings is approximately 2.5 pounds lighter than XL bearings. However, for a 12-inch bore diameter, the difference is nearly 80 pounds, and for a 32-inch bore diameter, the difference exceeds 900 pounds.

Thin section bearings are commonly used in places where space constraints or design configurations pose problems for traditional bearings. Industrial applications include robotics, optical positioning stages, harmonic drive devices, rotary-indexing tables, cable drums, textile machinery, printing machines, papermaking machines, and machine tools. Non-industrial applications include X-ray scanners, antenna bases, helicopter rotors, and turrets.

Bearing Configurations

Thin section bearings are available in radial contact (Type C) and angular contact (Type A) configurations, as well as four-point contact (Type X), which can support loads in multiple directions, similar to Type C bearings. Tapered roller bearing versions (Type KT) are also available.

Four-point contact bearings have the same ball set and cross-sectional area as radial contact bearings. However, their raceways have a Gothic arch shape, resulting in two contact points on the outer and inner rings. This geometry enables four-point contact bearings to support radial, thrust, and overturning moment loads simultaneously. For many applications, a single Type X four-point contact bearing can replace two Type A angular contact bearings (duplex pair) mounted on a single shaft, reducing costs and space requirements.

However, Type X bearings are not suitable for low torque or high speed applications. These bearings experience some ball skidding, which generates friction and heat at the contact points, making their operating speeds lower than Type A bearings. Furthermore, Type X bearings have greater frictional forces, requiring larger torques under a given load compared to a pair of Type A bearings.

Torque-sensitive applications usually require duplex pairs of Type A angular contact ball bearings. These bearings are typically preloaded but can provide axial clearance. They can be installed back-to-back or face-to-face, whichever suits the application best.

Bearing manufacturers have established precision grades for thin section bearings, similar to those set by the American Bearing Manufacturers Association (ABMA) for traditional ball and roller bearings. The bearings have four precision grades: 1, 3, 4, and 6, differentiated based on bearing diameter, runout, and radial clearance tolerances. The ABMA is adopting these classifications in a new standard 26.2, "Thin Section Ball Bearings."

Torque Considerations

For bearings, torque is required to rotate the rolling raceway relative to the stationary raceway. In many thin section bearing applications, the inertia of the driven parts is small, and the amount of "work" to be done is significant. In these cases, it is useful to know the exact driving force that must be provided. For example, aircraft and robotics applications require lightweight driving components, and knowing the exact torque required can help to select the smallest, lightest motor that can accomplish the task.

In some applications, torque uniformity is just as important as the torque size. In these cases, both external and internal friction sources can significantly affect the amount of torque required. When the maximum friction source is the applied load, three factors become particularly important:

• Internal bearing clearance. 

• The distortion of mounting surfaces.

• Bearing fitting practices.

Many factors can contribute to the rotational resistance of lightly loaded bearings, and most of this resistance comes from unpredictable sources — separator resistance; viscous resistance of the lubricant; tiny geometric deviations of the balls, raceways, and mounting surfaces of the bearings, shafts, and housings; the internal fit of the bearings; and contaminants.

Distortion of mounting surfaces also increases friction. Thin section bearings themselves are flexible, so when mounted on out-of-round shafts or uneven shoulders, the bearing races are likely to conform to the distortion, increasing the friction that must be overcome. To avoid this problem, control out-of-roundness within the bearing radial runout tolerance and flatness within the bearing axial runout tolerance, as recommended by the manufacturer.

Even a small amount of internal preload (negative clearance) resulting from deformation can produce surprisingly large ball loads, generating high torque for applications with limited torque required by rotating bearings. To minimize torque, ensure that the roundness of the housing or shaft is sufficiently small compared to the internal clearance to ensure that this clearance is not completely lost.

Thin section bearings are often mounted in lightweight structures with significantly different thermal expansion rates than bearing steel. The resulting differential expansion between the housing and bearings can cause bearing deformation, reducing internal clearance and increasing friction. To minimize this effect, maintain the shaft and bearings at the same temperature, or adjust bearing clearance to compensate for expected variations.

If an application requires minimal torque for rotating bearings, consult the bearing manufacturer. If necessary , the manufacturer can provide bearings with specific maximum torque levels. In most cases, however, ensuring that the bearing seat and shaft mounting surfaces are free of burrs or ridges will result in satisfactory torque levels, particularly when the bearings are kept clean and well lubricated.

Improper lubrication and the resulting resistance can also increase torque. The main factors affecting lubrication resistance include operating temperature and speed, as well as the type, viscosity, and quantity of the lubricant.

Customization for High-Tech Applications

There are over 300 sizes and types of standard thin section bearings available to suit various applications. However, increasingly complex machinery often requires customized bearings made from specialty materials, surface coatings, or lubricants. Key application requirements for such equipment include:

• Extremely low or uniform torque.

• Exceptional positioning accuracy. 

• Ability to operate at extreme temperatures. 

• Corrosion-resistant environments. 

• Compatibility with ultra-clean or vacuum environments.

When it comes to special application requirements, consult the bearing manufacturer. The pending ABMA standard for thin section bearings only covers raceway tolerances. Unspecified potential key parameters include ball size, grade, and quantity; raceway consistency; lubricant type; and cleanliness procedures. Bearing manufacturers have already developed an array of design and manufacturing options that allow customization of bearings based on the precise and stringent requirements of complex systems. For example, they can manufacture bearings with bore diameters up to 52 inches or precision grades higher than standard bearings.

Special manufacturing techniques can maximize positioning accuracy, smoothness, stiffness, and reliability, while minimizing the torque requirements of driven bearings, vibration, and noise or. For example, manufacturers can customize bearing stiffness or torque characteristics as per specific requirements. Additionally, they can provide special materials and surface coatings to minimize outgassing under high-vacuum conditions.

In many cases, using an integrated bearing assembly (IBA) can simplify installation, improve system performance, and reduce costs. IBAs can also incorporate additional features, such as finger elements, protrusions, grooves, extended raceway sections, and through-holes or threaded holes in bearing raceways.

Other customization options include special materials, separators, and lubricants:

1. Materials: Rolling elements and raceways for standard thin section bearings are typically made from 52100 vacuum arc remelted steel, which has a high load-carrying capacity. However, alternative materials are available for special requirements such as corrosion resistance and low magnetism. These include raceways made of 440C martensitic stainless steel, 17-4 PH stainless steel, or M50 tool steel. Optional ball materials include 440C stainless steel, silicon nitride, borosilicate glass, or M50 tool steel.

2. Separators: Various designs and materials are available for rolling element separators. Standard brass "snap" and "round pocket" separators are suitable for most applications. In other cases, special separators might be required to: • Improve load capacity and stiffness. • Decrease torque by reducing friction resistance. • Increase speed capability. • Enhance temperature range. • Reduce noise. • Provide specific environmental compatibility. Options include 17-7 PH wire separators, 300 series stainless steel spring separators and rings, as well as PTFE (polytetrafluoroethylene) and Vespel ring separators, blocks, and bands.

3. Lubricants: The three most common types of lubricants are oils, greases, and dry films or surface treatments. Oils generally provide better lubrication as they cover critical surfaces and dissipate heat more efficiently. Greases are more easily retained, allowing for simpler bearing housings and seals. In addition to standard greases, bearing manufacturers can supply vacuum-compatible greases and dry film lubricants such as graphite, molybdenum disulfide, tungsten disulfide, silver, lead, or PTFE. Since there's a wide range of selection and costs, consult lubricant manufacturers when choosing a lubricant for specific applications.