RotoPrecision Miniature and Instrument Bearings

Internal Geometry and Preloads

In a ball bearing, load is carried or transferred from one ring to another through the balls, making these points of contact critical in bearing design. The size and shape of these points of contact are controlled based on numerous factors including raceway curvature, contact angle, radial play, and preloads.
 

Raceway or Track Curvature


Figure 1:
Raceway Radius and Ball
Diameter Relationship

Raceway curvature is expressed as the relationship between the radius of the ball track to the ball diameter, in percentage terms (see Figure 1). Modifying this relationship can offer larger or smaller points of contact between the balls and the raceway as the needs of each application warrant. This value is typically between 52% and 58%. A larger value is applicable where lower torque values are desired (smaller area of contact) whereas smaller values permit greater load carrying capability. Please consult our Application Support personnel for assistance with this feature.

Radial Play

Radial Play results from a certain amount of intentional looseness between the rings and the balls of a properly designed and assembled bearing. It is measured as the maximum possible displacement of the inner ring in relation to the outer ring in a direction perpendicular to the bearing axis. It is sometimes also referred to as radial internal clearance. It is one of the most critical but overlooked design decisions affecting bearing performance. Radial play specifications must accommodate such critical issues as the effect of interference fits of shaft and housing on the inner and outer rings, the effect of thermal expansion and contraction from ambient or operating conditions, housing and shaft misalignment, the degree of radial loading, and the need to create contact angle within the bearing to accommodate axial loads. Further, too much radial play can contribute to excessive noise and vibration in an application.

Contact Angle


Figure 2:
Contact Angle for a Bearing
under an Axial Load

Contact Angle is the angle of a line drawn through the points of contact of a ball and each raceway, in relation to a plane perpendicular to the axis of the bearing (see Figure 2). Contact angle is controlled by manipulating the radial play, the curvature of the raceways, the diameter of the balls and the axial load. It is created through the axial movement of the bearing rings in opposing directions to take up any existing clearance. The larger the amount of starting clearance there is, generally, the larger the contact angle and the greater the degree of axial load capacity and rigidity.   Even in deep groove miniature bearings - intended for mostly radial loads - the play that exists between the bearing components allows for some contact angle to exist, if desired. Increasing the radial play values permits a greater contact angle, assuming the radial play is not taken up with improper fits or clearances in assembly. Typical contact angles for standard clearances are 10 degrees to 18 degrees. Please consult an Application Support specialist for more details on required contact angles for your particular application. 

Preloads

Preloading is used to remove the internal clearance of a bearing and is achieved by applying a permanent thrust load in an axial direction. Preloading is used to eliminate radial and axial play, increase system rigidity, reduce run out, increase the assembly’s tolerance for vibration, and reduce operating noise. The amount of preload is measured in the amount of force applied in an axial direction, past the point where internal clearance is eliminated. This causes the point of contact between the balls and the raceways to broaden into an elliptical shape, increasing the area of load-bearing contact. While this serves many positive purposes, it can also cause excessive heat generation and early failure if the preload is not optimized. Standard offerings of RotoPrecision miniature and instrument bearings handle preloads of between 0.2 lbs and 10 lbs. The three common ways to achieve preloading in bearing assemblies are with springs, with axial adjustment, and with duplex bearings preloaded from the manufacturer. Springs are one of the simplest methods of applying a preload. They are fitted against the inner or outer ring of one of the bearings in the assembly; normally the non-rotating ring to avoid balancing issues. Springs in combination with retaining rings can be designed to maintain consistent force over their compression range, avoiding the need for tight tolerances on other components. Where a tight tolerance on the shaft and housing location is required, a solid stack preload – or axial adjustment method -can be used. This can vary from tightening methods such as threaded screws to the use of solid shims and/or spacers. Solid stacking is a challenge when dealing with the delicate components of miniature bearings, so you are encouraged to consult with our Application Support personnel for assistance with assembly methods. Finally, duplexed pairs of matched bearings can be specified that have the desired amount of preload built-in. The inner or outer ring faces of the bearings have been selectively relieved a precise amount called a preload offset. When the bearings are assembled and properly tightened into position, the rings position themselves to create the specified preload. Several advantages of duplex bearings include the ability to withstand bi-directional thrust loads, to greatly increase radial and axial rigidity and to provide minimum run out while also being simpler to assemble.


 

 


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