Tapered roller bearing load ratings versus performance

LOAD RATINGS

Relevance of load ratings

When comparing bearings from different manufacturers, engineers often look at the load ratings as the substantial performance parameter [1]. The dynamic load rating is also key for calculating the bearing basic rating. This can be explained most easily from the life equation.

Calculation to ISO Standard

According to ISO 281, the basic rating life of a radial roller bearing is calculated from:

where
L₁₀ = basic rating life (at 90 % reliability)
C_r = basic dynamic radial load rating
P_r = dynamic equivalent radial load.

According to ISO 281, the basic dynamic radial load rating for radial roller bearings is calculated from:

where
C_r = basic dynamic radial load rating
b_m = rating factor of bearing type (value 1,1 for tapered roller bearings)
f_c = material/geometry/dimension factor
i = number of rows
L_we = effective roller length
= contact angle
Z = number of rollers per row
D_we = mean roller diameter.

The relevance of f_c as performance factor is very obvious, for example a performance factor of 1.23 results in double L₁₀ basic rating life.

Calculations to US Standards

Sometimes, and especially in the North American market, C₉₀ load ratings can be found. These are based on a rating life of 90×10⁶ revolutions:

where
C₉₀ = basic dynamic load rating based on 90×10⁶ revolutions
M = material factor, defined by bearing manufacturer
H = geometry dependent factor, defined by bearing manufacturer
i = number of rows
l_eff = effective contact length
= contact angle
Z = number of rollers per row
D_wo = mean roller diameter.

Dynamic radial load rating C₁ for 1×10⁶ revolutions rating life:

Comparison of load ratings

From the preceding it is clear that C_r load ratings according to ISO cannot be directly compared with US C₁ load ratings. C₁ values are converted from C₉₀ load ratings and though they have a similar basic structure, the equations for C_r and C₉₀ make use of different factors and exponents [6]. This results in a different weighting of the single geometry parameters.

Benchmark tests

Extensive benchmark tests with different tapered roller bearing types have been performed. Results for the tested bearing type 32011 X are commented hereafter.

The benchmark tests have been performed for two cases: clean and contaminated operating conditions. Clean tests are performed with the bearing and oil in a newly manufactured condition, as is. For the contaminated tests, debris particles had been added to the oil and the bearing was run-in in this condition first. Thereafter, the contaminated oil was removed and the bearings were run with clean oil until failure. The test conditions are depicted in table 1.

Fig. 1 shows a comparison of catalogue load ratings from SKF and two other bearing manufacturers. The reference value 100 is SKF. Compared with SKF, the two other bearing companies list load ratings relatively 17 % and 19 % higher than SKF.

Test results

Fig. 2 shows the benchmark test results from clean operating conditions.

Fig. 3 shows the benchmark test results from contaminated operating conditions.

Result: Despite the lower quoted capacity figures, the SKF tapered roller bearings performed significantly better than the competitor bearings in both clean and contaminated test runs.

Speed capabilities temperature tests

In addition to the benchmark life tests, speed capability tests also have been performed with bearing type 32011 X. A pure axial load of 2.3 kN has been applied to the test bearing before rotation started. Then, different speed stages have been applied from 2,000 r/min to 9,000 r/min, in steps of 1,000 r/min (fig. 4).

Test results

• SKF bearings operated at the lowest temperature levels at all speeds. Temperature differences increased at higher speed levels. At the maximum speed limit the mean temperature of SKF bearings was 11.5 °C lower than the mean bearing temperature of the nearest competitor (CC 2).
• For speed levels up from 4,000 r/min, competitor bearings progressively ran at higher mean temperatures compared with SKF.

SKF TAPERED ROLLER BEARINGS, NEW CONSOLIDATED ASSORTMENT AND SKF EXPLORER

To better represent the actual performance of SKF tapered roller bearings, a new and consolidated catalogue assortment has been launched.

The main points are:

• a wide SKF Explorer assortment (fig. 5); small and almost all medium sizes all are SKF Explorer; the range is being extended continuously
• larger-size catalogue bearings, for the time being, remain SKF Basic design
• the catalogue range of inch-size bearings is being extended
• the industry preferred assortments and customized products (such as CL7C) will remain, but will not be part of the standard catalogue assortment.