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Selecting TRBs for Machine Tools: Going Beyond Tolerance Standards

Sept. 30, 2020
The second of two reports studying precision-class tapered roller bearings looks at the factors not addressed by ISO and ABMA standards, but that contribute to performance, reliability, and service life for TRBs.

As detailed in part 1, ISO 492 differs slightly from ABMA 19.2; however, both standards focus primarily on the geometric tolerances and form of bearing rings. Because machine tool spindles are subject to one or more of the following requirements, features outside of the ISO and ABMA standards need to be taken into consideration when selecting a bearing for applications that:
• Operate at high speeds;
• Are subject to high loads;
• Have a small runout budget; or
• High stiffness requirement.

What is not defined by ISO/ABMA? The ISO and ABMA precision-class standards are a good starting point for selecting a bearing, but they are incomplete: Some of the critical bearing features not covered in the ISO/ABMA standards include raceway roundness, wall thickness variation, rib geometry, roller/raceway surface finish, roller profile, roller size variation, roller roundness and steel cleanliness/heat treat. These features, if not controlled properly, can contribute to elevated levels of noise, vibration and runout, and in some cases to premature bearing damage.

TRB features that differentiate precision-class from standard-class products include:
Raceway/roller roundness and surface finish;
• Roller geometry and size variation; and,
Steel cleanliness, dimensional stability and hardness.

Roller and raceway roundness and surface finish may contribute to noise and vibration, especially when the bearings are set with preload. Preload creates a 360° load zone where all the rollers are firmly seated between both inner and outer raceways and act to amplify the noise vibration.

Roundness and surface finish is determined primarily by how the raceway and rollers are finished. Grinding is a common method for finishng roller and raceway surfaces. The speeds, feed rates, grinding media, and stiffness of the fixtured part all contribute to the quality of the surface. Grind chatter can cause excessive waviness (roundness well beyond two undulations per revolution) and surface roughness.

Roundness and surface roughness requirements need to be in place to help guarantee precision bearings operate quietly.

The roller profile or crown can have a measurable impact on a spindle’s stiffness, as well as its ability to carry load. Selecting a bearing with the best profile will impact the spindle performance. Selecting a roller profile can increase the bearing stiffness up to 25% over a standard-class bearing, depending on the bearing design.

The roller size variation (RSV) among the rollers in a bearing contribute to the Assembled Radial Runout, as defined in the ISO and ABMA standards, and yet there is no limit for RSV in either standard.

Runout is defined by two factors:
1) synchronous (repeating) runout; and,
2) asynchronous (non-repeating) runout.

RSV is a primary contributor to asynchronous runout. Reduced RSV will help to improve total spindle runout, especially if synchronous runout is minimized by finish-grinding the spindle shaft with bearings installed.

Streel cleanliness refers to inclusions in the steel, which may act as sub-surface initiation points for spalling (fatigue cracking) under cyclic rolling contact loading. Clean steel can improve the bearing life, especially for spindles that are required to carry heavy loads. Setting cleanliness limits for the quantity and type of inclusions are up to the bearing manufacturer: no industry standards exist.

Most suppliers do not have the means to measure cleanliness and rely on the steel supplier to provide certification sheets. Some with experience in steel manufacturing have these capabilities and can guarantee the rings and rollers are sourced from clean steel.

The hardness of the rollers and raceway directly impact the static capacity of the bearing. The static capacity is the radial load at which a roller will plastically (permanently) deform the raceway. In general, the higher the hardness the higher the static capacity. Hardness is achieved through heat treating. The rings are quenched from an elevated temperature that achieves a microstructure that will yield roller and raceway hardness typically between 58 and 62 Rockwell C, depending on the steel and bearing size.

The dimensional stability is the ability to maintain its original size and structural under temperature and load cycles. If the bearing rings and rollers do not maintain original size or geometric form over time they can “fall out” of their tolerance class. Dimensional stability is achieved through tempering the steel, this is a process that drives out or stabilizes the retained austenite. Tempering is done well below the quench temperature so it does not change the hardness.

"Beyond standard" precision. Timken differentiates its precision-class tapered roller bearings by exceeding the best standards set by ISO and ABMA (00/A/P2).

Precision Plus product (000/AA) is available for TRBs with ODs up to 315 mm for Metric Series, or up to 12 in. for Inch Series, and have a maximum radial runout of 1 micron (0.001 mm), which is half that of the best ISO and ABMA classes.

When an application requires a bearing with features not available in the ISO or ABMA precision classes, special performance codes can be applied to customize the bearing performance to meet the application’s needs. This is common for large TRBs where the ISO and ABMA standards only have tolerancing for Class 3 above 12-in. OD and Class C/P5 above 315 mm. It is not uncommon to see TRBs of 24 in. and above with 0/B/P4 runout.

Any TRB can be made in a precision class, but not every TRB will work in a machine tool application. The internal geometry of the bearing is critical to how the bearing will perform. There are more parameters than can be practically listed. Selecting the bearing with an optimized design for an application is probably one of the most important factors outside the ISO and ABMA standards that can guarantee the success of an application.

With more than 125 years of bearing design experience, Timken offers an extensive bearing selection, including precision TRBs. To help navigate your options, Timken Customer Engineering can work with you to select the best bearing and precision class for an application, using field experience and analytical modeling. Timken Field Service Engineering also can assist with installation and setting of the bearings. Both services are further examples of how Timken goes beyond the ISO and ABMA specifications to deliver better performance.

In summary, the ISO and ABMA tolerance standards address basic component and assembled geometry. These tolerances decrease as the precision level increases.

The standards control the basic features that are critical to bearing fitting and performance; however, there are numerous tolerances critical to bearing precision and performance that are left to the bearing suppliers discretion. So, choosing a quality bearing supplier is critical to ensuring the performance of an application.

In addition, there are applications that require tolerancing better than the standards define, and bearings can be customized to meet the needs of a particular application. For all these reasons it is important to partner with a bearing supplier that goes above and beyond the ISO and ABMA standards.

Eric Faust is an Application Engineering Specialist at The Timken Company, providing sales and customer support with technical bearing expertise and helping to resolve customer performance concerns.

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