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Shanghai Mitsubishi Elevators has long been China’s leading elevator manufacturer, supplying the elevators in many of the high-rise buildings that now populate this fast developing country. Development is so fast here that in 2007 the Shanghai Mitsubishi factory was actually the world’s leading volume producer, outperforming all the bigger-name international companies in this business.
With a wide range of lift designs and sizes, the company caters for many different customer requirements. But one requirement consistent with all their lifts is to have quick, quiet and safe elevators. And this is something that has become a particular characteristic of their range.
With this “guide” in their mind, they embarked on a new high-load, high-speed, elevator drive design, switching from a geared motor, used in all their earlier models, to a new direct drive, gearless, design. The new drive would continue the quick, quiet and safe tradition, and would be more compact and more cost effective. The elevator running speed would be 4 meters per second and the expected life of the drive system could be 20 years or more depending on operating conditions.
Fang Kai, section chief at Shanghai Mitsubishi explains: “The first designs met many of our specifications and performance factors, but one feature was a concern to us. In our geared designs, we used open bearings to support and rotate the gear shaft that assists in lifting and descending of the elevator cabin, and these bearings were lubricated by oil from the gearbox. But in the direct drive design, we have no gearbox and no gear oil, so the bearings needed to be greased. The use of open bearings would require regreasing, and we were concerned that minor grease leakages would find their way onto the brake mechanism. While this would not prevent the basic brake function, and regular maintenance would ensure safe braking, this feature was undesirable when measured against our ‘guide’ criteria.”
Solutions involved either a new design or to investigate if using a sealed bearing would meet their stringent design demands. The sealed bearing would need to contain the correct lubricant, in the correct quantity and could last the entire lifetime of the elevator depending on operating conditions, which was set at 20 years.
The most cost-effective solution was the sealed bearing but the only company producing such a bearing suitable for the application was SKF, and Shanghai Mitsubishi had no experience of using this bearing execution. So, Shanghai Mitsubishi installed an SKF sealed spherical roller bearing in their in-house test tower and ran endurance and function test for three months.
Impressed by the performance of the sealed bearing, the company switched the entire production to include this type of bearing and entered into deeper development talks with SKF. The direct drive design, as an engineering concept, was not new to SKF, which has been involved with such designs from its inception.
Kai comments: “SKF brought its extreme knowledge and sophisticated calculation programs to look at the complete shaft system, including bearings. Our engineers appreciated the confidence that the calculations showed and this resulted in confirming the life of the sealed bearing and especially the grease, which would not be changed or replenished for 20 years depending on operating conditions. Such was the confidence of our engineers that we also decided to replace the other bearing in the system with an SKF bearing. This project has brought Shanghai Mitsubishi and SKF much closer than just a supplier and customer relationship. We have seen excellent technical support from SKF, together with speed of action. As a consequence we are partnering on the shaft system and bearing requirements for the development of our newest elevator design which will be the fastest in China, descending at 6 meters per second.”
Technical details of SKF Explorer sealed spherical roller bearing:
· SKF Explorer sealed spherical roller bearings can accommodate very heavy radial and heavy axial loads in applications where misalignment or shaft deflection can occur.
· The efficient contact seals and factory-filled high quality grease make them ready-to-use units that are lubricated for life in most applications. The well proven double lip seals also provide excellent protection against solid contaminants and moisture.
· The SKF Explorer range of bearings has proven in tests to outperform by large margins competitive spherical roller bearings under typical heavy duty conditions.
· The bearings are made of patented SKF Xbite steel. This is an extremely clean, reduced-oxygen level steel that provides greater strength. Surface hardness and dimensional stability at high temperatures. And improved raceway surface finish increases lubricant effectiveness for a smoother, cooler running bearing; all features that contribute to extended service life.