Gearless Motors: A Sealed Spherical Roller Bearing With Improved Performance
This Product Spotlight describes the multiple improvements to the bearing, now designated with the -RS suffix.
This paper was presented at Madrid 2016, the International Congress on Vertical Transportation Technologies, and first published in IAEE book Elevator Technology 21, edited by A. Lustig. It is a reprint with permission from the International Association of Elevator Engineers (website: www.elevcon.com).
Installed in the world’s latest gearless traction machines, SKF’s sealed Explorer spherical roller bearings (ELEVATOR WORLD, February 2016) support the inherent space- and energy-saving aspects of these elevator motors. Compared to the previous-generation bearings, new sealed bearings can reduce bearing friction by up to 20% and contribute to lowering building energy consumption. With this new seal design, the bearings reduce operating temperatures significantly, resulting in a limiting speed up to twice as high as previous designs. Because they can run cooler, relubrication intervals can be up to twice as long, depending on the operating conditions, reducing grease consumption or even enabling relubrication-free solutions, further reducing the motor’s environmental impact and maintenance costs.
In new building construction, elevator manufacturers are moving to machine-room-less (MRL) and gearless traction-machine designs that enable high speeds and greater energy efficiency in a more compact solution. The MRL design represents more than half of the new elevator units and is dominating the market.
The SKF sealed spherical roller bearing, as shown in Figure 1, is able to answer the needs of the elevator and traction machine manufacturers. Using a roller bearing type instead of a ball bearing, it allows the motor designers to increase the load-carrying capacity of their gearless machines, while maintaining or even downsizing the bearing arrangement.
Recent years have also seen an evolution in the development of suspension systems. Two examples are Aramid rope from Schindler and the flat belt from Otis. Such products have different bending properties and a smaller bending radius compared to conventional steel wire ropes. This had a direct impact on the bending ratio between the sheave and the rope, allowing use of the smaller sheave, and, consequently, an increase in the speed of the elevator traction motors.
A more recent elevator manufacturer trend is to optimize the range of gearless traction machines, similar to a “one-size-fits-all” approach. The main scope is to add flexibility in the application by using, for example, the same motor to cover a wider range of speed at constant load, or even the same motor to cover wider ranges of speed and load.
Both trends mentioned above can have a significant impact on the function and performance of the bearing.
The use of sealed spherical roller units that support misalignment and shaft deflection is characterized by a line contact running condition, instead of a point contact for ball bearings. This makes the spherical roller bearing an optimum solution in terms of compactness and load-carrying capacity. However, increasing the speed of the motor could have the following effects:
- Increased friction, which could increase energy consumption
- Increased bearing temperature, which could shorten bearing life
- Reaching the bearing limiting speed
Development of the New Seal Design
The main scope of the R&D done in collaboration with the SKF Sealing Solutions Department and SKF Development Centre for self-aligning bearings was to create a new design aiming to optimize the seal friction and anchorage, while maintaining the main seal properties, like seal ability and grease retention. By reducing the friction, product performance improved in terms of lubricant life, limiting speed and energy consumption.
The design criteria was clearly defined at the beginning of the project. Starting from that baseline, SKF Sealing Solutions optimized the seal lip design accordingly by means of different analysis, like the Finite Element Model. Figures 2 and 3 show the modeling results for the lip contact pressure and seal stress.
Balancing several parameters allows the optimization of the seal lip shape to maintain the seal ability and reduce friction. Lowering the friction would decrease the bearing running temperature, and, as such, improve grease life and overall life of the bearing component.
Apart from the optimization of the seal lip shape and pressure, the seal head itself and outer-ring groove shape had to be redesigned. SKF decided to introduce a three-lip design not linked to any metal-forming operation. The contact between the groove and seal was done by means of a rubber-to-metal contact. Several iterations were conducted to fine-tune the final solution. A conservative model was created and used, considering a safety factor 2. In a second step, retention requirements were defined to validate the seal head and outer-ring groove design.
When developing the new seal type, SKF also standardized the seal lip design, going from three different designs (with the suffix “–CS”) to only one (with the suffix “-RS”), as described in Figure 4.
This change results in an optimization of production capability and a reduction of the number of variants and components in production. Moreover, using a single seal anchorage design will also reduce the anchorage types and anchorage variation when assembling the seal head into the outer-ring groove.
The new -RS design is valid for the range of small sealed spherical roller bearings having mainly a bore diameter between 25 and 120 mm. For larger bore diameter, the -CS design will still be the standard for sealed versions of spherical roller bearings.
Validation Test and Results
An overview of the test validation can be found in Table 1.
The friction test was performed on seals only. The scope was to look at the impact of the new seal design compared to the current one.
New limiting speeds were defined based on the result of the seal friction test. The new limits depend on the seal size and are linked to a new maximum seal lip speed limit of 10 mps. Figure 5 illustrates the relative seal friction between -CS and -RS designs at 200 rpm and at limiting speed for one of the test samples.
The new seal design reduces friction by up to 50% on the seal only.
The friction reduction on the complete bearing was also validated by calculation. Figure 6 is an example of the calculation performed.
The bearing friction can be reduced by up to 20%.
Additional temperature tests were conducted on test rigs on the complete bearings to have a better view of the benefits in application. A load of C/P = 10 (where C is the basic dynamic load rating of the bearing in kilonewtons and P is the equivalent dynamic bearing load in kilonewtons) was applied on the bearings. Tests were run at different speeds. This typical speed test is able to show the effect of lower friction at different speeds; results are shown in Figure 7.
The bearing running temperature can be reduced by up to 20°C depending on the application conditions. This reduction will positively impact the grease life and potentially multiply it by up to two times.
To ensure that the new seal design maintains the same contaminant exclusion feature as the current design, a typical Arizona dust test was performed. The test specifications are as follows:
- Speed: one-third of the limiting speed
- Dust type: Arizona dust grade A2, accredited by ISO 12103-1
- Dust quantity: 10% of the chamber volume
- Test duration: 168 hr.
- Test sequence: Run 8 hr., stop 16 hr.
The test conclusion shows that the seal ability of the new design is maintained.
Grease retention has been evaluated in two different tests: outer-ring rotation test and high-speed test.
Three different bearing sizes were tested, and the grease leakage evaluated. The result shows that the grease retention is maintained, with the grease leakage quantity observed under 1 g.
The current permissible misalignment for the self-aligning spherical roller bearing is 0.5°. With the new seal profile being different, it was necessary to check that the misalignment capability is maintained. Validation was performed by calculation and testing to verify the performance. The results show that misalignment capability is maintained to a maximum of 0.5°.
Three different tests were performed as described in Figure 8 to ensure optimum seal anchorage of the seal head in the outer-ring groove.
During handling, the bearing could potentially see misalignment over 0.5°. The scope of this test is to check the effect of the force to which the seal could be exposed during normal handling and compare it to the SKF acceptance criteria. Results are shown in Figure 9.
The new seal anchorage complies with the SKF acceptance criteria.
The scope of this test is to check the forces needed to rotate the seal in the groove. To verify the impact on the retention torque, tests were performed with dry groove, grease and/or preservative in the groove. An example of results is illustrated in Figure 10.
The new seal anchorage complies with the SKF retention torque acceptance criteria.
The push-out force is used as a criterion to ensure that, if the bearing is overfilled with grease, the seal will not pop out, but instead release the grease between the bearing inner ring and seal lips.
The grease was pushed through one of the holes in the bearing outer ring, the other two holes being plugged. All excessive grease leaked between the seal lip and the inner ring and, as such, fulfilled the SKF acceptance criteria in terms of push-out force.
With the recent development of alternative suspension systems having different bending properties compared to conventional steel wire ropes, the elevator market has generated a new elevator traction motor concept using smaller sheaves and running at higher speed. Moreover, to optimize their gearless motor range, elevator manufacturers are aiming for higher flexibility in application with motors covering a wider range of speed and loads.
Taking both trends into account, SKF had to adapt its own products and add more flexibility in application. This was realized by developing and validating a new sealed spherical roller bearing with improved performance by means of a completely new low-friction seal design. The resulting operational features and customer benefits are listed in Table 2.
By reducing the friction and, potentially, the operating temperature, the grease life could be multiplied by up to two, and, as such, the relubrication intervals extended by up to twice as long, compared to the previous generation. This brings a significant reduction in grease use — less environmental impact — and maintenance needs. Depending on the operating conditions, most of the application could be then considered maintenance free.
In terms of energy savings, friction reduction translates to significantly lower CO2 emissions. For a typical gearless traction motor in a heavy-duty elevator, replacing two sealed SKF Explorer bearings with the newly optimized design could cut CO2 emissions by up to 100 kg annually. Over the motor’s 20-year lifecycle, that adds up to a 2-T. reduction in CO2 emissions.
The suffix -RS characterizes the new bearing generation and will concern the range of small spherical roller product having a bore diameter between 25 and 120 mm.
Special thanks to Henrik Wickberg, SKF AB, for his support during the different development phases of this project.