Selecting TRBs for Machine Tools: Understanding Tolerance Standards

Geometric tolerance standards for precision-class tapered roller bearings (TRB) used in machine tool applications are controlled by the ISO and ABMA standards, and must be understood in order to select TRBs for specific applications. It is always recommended to consult the bearing supplier prior to finalizing bearing selection, and the information here will prove valuable when generating such a request.

The focus here is on radial TRB, including TS (TRB, single row), TSF (TRB, single row with flanged outside diameter), TDO (TRB, double row outer, with indirect mount or “O-in." arrangement), and TDI (TRB double-row inner with direct mount, also known as "X-in." arrangement) bearings. Not included are thrust TRB (TTHD and TTHDFL) or Tapered Cross Roller (TXR) bearings, although the same concepts apply to these bearing types too.

Who are ISO and ABMA? Who owns and controls the tolerance standards for bearings? The technical regulatory associations that have standards defining bearing-tolerance classes are:
• ABMA – American Bearing Manufactures Association
• ISO – International Organization for Standardization
• DIN – Deutsches Institut für Normung
• JIS – Japanese Industrial Standards

The applicable standards that control the part numbering, boundary conditions and tolerance tables for standard- and precision-class TRB are listed in Table 1.

The standard that controls Inch Series TRB is ABMA 19.2: It contains boundary dimensions, series designators, and the tolerance tables. Metric Series TRB are controlled by ISO 355, 10317, and 492. ISO 355 contains the boundary dimensions and series designators; 10317, the designation system; and 492, the tolerance tables.

Additionally, there are regional standards that incorporate potentially more part sizes or nomenclature, but align their tolerance tables with the ABMA 19.2 and ISO 492. The most notable regional standards include ABMA 19.1 (US), DIN 620/720 (Germany), and JIS B 1534 (Japan): their tolerance tables all align with ISO 492 or ABMA 19.2.

What these standards have in common is that as the tolerance class increase, the allowable geometric deviation from nominal decreases. In other words, the highest tolerance class delivers the highest precision. The most notable difference between the standards is that Metric Series bearing tolerances are below the nominal dimensions (nominal minus allowable deviation), while Inch Series bearing tolerances are above the nominal dimensions (nominal plus allowable deviation.)

Each standard defines three precision olerance classes, which are, in order of Inch / ABMA metric / ISO metric:
• Good – 3/C/P5
• Better – 0/B/P4
• Best – 00/A/P2

In addition to the three precision classes, ABMA and ISO also define two standard classes (4/K/Normal) and (2/N/P6X) for a total of five tolerance classes. Timken bearings go a one step further with a sixth tolerance class — Precision Plus™ (000/AA/*.)  Precision Plus ™ bearing tolerances exceed the best precision-class tolerances that ABMA and ISO have defined.

What do ISO and ABMA control? What tolerances are controlled by the ABMA standard 19.2 and ISO 492? There are 11 tolerances that a TS bearing must meet to qualify for one of the ABMA/ISO precision classes, six ring component measurements, and five are performed on an assembled bearing or bearing sub-assembly.

Measurement is done in accordance with ABMA 4:1994 (Tolerance Definitions and Gauging Practices For Ball and Roller Bearings) for ABMA 19.2 and ISO 1132-2:2001 (Rolling Bearings Tolerances Part 2: Measuring and gauging principles and methods) for ISO 492.

Ring measurements include the cone bore, cup OD, cone width, cup width, cone face to bore runout (perpendicularity), cup face to OD runout (perpendicularity) as detailed in Figure 1. Assembled measurements include assembled cone radial runout, assembled cup radial runout, assembled width, cup stand, and cone stand, as detailed in Figure 2.

The cup stand is measured using a master plug to represent the cone assembly, while the cone stand measures using a master cup. The cup and cone stand are measured to allow for full interchangeability, so any cup can be matched with any cone to achieve the overall bearing T-width within the assembly width-tolerance value. Standard-class and precision-class 3/C/5 cones and cups are sold individually; Class 0/B/4, 00A/2 and 000/AA/* are sold only as matched sets that use the stand measurements to help meet “assembly-inch" tolerances.

What is very small? It is important for the reader to have a solid understanding of the magnitude of the tolerances associated with precision-class products. To illustrate the scale of precision-class deviations, let’s start with a human hair. On average a human hair is roughly 100 μm (= 0.100-mm, = 0.004 in., see Figure 3.)

When you compare one ten thousandths of an inch (= 0.0001-in., = 2.5 μm) or one micron (= 0.001 mm, = 0.000039 in.) to a human hair, the hair looks large. A micron may seem very small measurement, but it is the maximum assembled radial runout for a class 000/AA precision-class TRB.

Standards for bearing selection. Next, we will look at how the ISO 492 and ABMA 19.2 standards influence bearing selection. To accomplish this, two examples have been provided. The first will be the impact of the cone-bore tolerance on the shaft tolerance and resultant fit-up; the second is the potential effect of the bearing assembled radial runout on the spindle error motion.

Example 1. Determining the shaft fit-up. The first example studies the effects of the cones' "Deviation of Mean Bore Diameter in a single plane" (see Figure 1, left most image) on the shaft fit-up. Let’s look at how ISO and ABMA control the "Deviation of Mean Bore Diameter", which is simply how far the average measured bore diameter can deviate from its nominal value. Table 2 provides this tolerance for precision-class Inch Series cones and Table 3 for Metric Series cones.

Figure 4 is a side-by-side comparison of the Inch versus Metric Series for 12-inch and 300-mm bores. Standard-class values also are included in the graph, to put the precision-class tolerances in perspective.

It is worth noting that a 12-in. Class 3 and 0 Inch Series bearings have smaller allowable deviations than a Class C/P5 and B/P4 300-mm Metric Series bearing. However, as the bore size decreases so does the maximum allowable Metric Series deviation, whereas the Inch Series does not. This is something that should be taken into consideration when selecting a precision-class TRB.

Consider a 300-mm bore, Metric TRB for the four ISO 492 Tolerance cone classes, N/P0 through A/P2. The suggested shaft seat deviation from ABMA 19.1 for a rotating inner and a ground seat are shown in Table 4. As you will see, as the tolerance class increases the suggested seat tolerance decreases, resulting in the N/P0 fit-up range being four times greater than the A/P2 range. Recommended fitting practices for the full-size range of cups and cones for all tolerance classes can be found in ABMA 19.1 (Metric) and 19.2 (Inch.)

Example 2. How much runout? The next example looks at the effect of the bearing "Assembled Radial Runout of the Inner Ring" on the error motion of the spindle, (see Figure 2, left most image.) As in the previous example we need to look at how ISO and ABMA control Assembled Radial Runout, which is the maximum amount of radial movement of the inner ring when the cup is held stationary and the inner is rotated. Table 5 provides the tolerance for precision-class Inch Series and Table 6 for precision-class Metric Series TRB.

Figure 5 is a comparison of the Inch versus Metric Series for a 12-inch/300-mm cone. Standard Class values were included in the graph for comparison.

It can easily be observed from Figure 5 that a 12-in. Class 3, 0, and 00 Inch Series bearing has a smaller allowable runout than a Class 300-mm C/P5, B/P4 and A/P2 Metric Series bearing. However, like the bore-diameter deviation, the Metric Series runout table is extended to show more cone bores than the Inch Series table, so smaller cone bores will have small allowable runouts. Again, this is something that should be taken into consideration when selecting a precision-class TRB.

When mounting the cones onto the shaft it is important to identify the high spots on both the bearing cones and the shaft seats so that the cone high spots can be oriented (clocked) 180 degrees from the bearing-seat high spot. This helps to minimize the shaft error motion, including tilt. If done correctly, the maximum spindle runout due to the bearings will be the Assembled Radial Runouts as listed in Tables 5 and 6.

To reduce the runout even further, the work-end of the shaft can be ground after the bearings are installed — and this will be discussed in more detail in part 2 of this report.

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.