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Belts lift performance

Belts lift performance

Since 1853 when Elisha Otis invented an elevator that incorporated the very first safety mechanism allowing for the transport of people, mechanical elevator technology has remained relatively unchanged. The basic design required bulky machinery and burdensome space requirements. This year Otis Elevator will introduce a radically new elevator design that strongly challenges the conventional technology. Named Gen2, for the next generation of elevators, the system achieves an astonishing 70% reduction in volume while providing the public with a more reliable, and smoother ride.


The Gen2 elevator left, save space by eliminating the need for a machine room on a building's roof.

The heart of the design lies with the patented cables used to hoist the elevator car. Instead of using traditional woven heavy steel cables, Otis engineers adopted a design composed of a flat 1 3/16 x 1/8 inch polyurethane belt embedded with 12 steel cables. Although each belt cable is only 1.5 mm in diameter, when arranged in parallel and coated with polyurethane it achieves the same tensile strength as a traditional steel cable capable of carry loads of up to 8,000 lbs. (3,600 kg). The smaller cable also enabled engineers to use a permanent magnet motor, which would have been too expensive with traditional cable design because of the higher torque that would have been required

Elevator mechanics have remained relatively unchanged for over a hundred years for two main reasons: according to Tom Saxe, Project Engineer for Gen2, the technology simply "worked well;" and codes until a few years ago required manufacturers to abide by strict rules which stifled innovation. The conventional way of thinking began to be challenged when the European government introduced a new law allowing manufacturers to install a system if the design is better than what is required by code. Gradually, ideas created by this new law migrated to the U.S., prompting Otis engineers to search for a new design.

The first "safe elevator" designed by Otis incorporated a mechanical device that would grab onto a large steel rail located in the hoistway, preventing a catastrophic fall if the rope, typically made of hemp, broke. Since the 19 th century heavy steel cables have been required by U.S. building codes to provide the public with even safer elevators. However, the rules restricted engineers to a design that would not bend the cable less than 40 times its diameter (ratio of 40:1). Otherwise, the high-stress cycling could cause it to fail.

"In a typical elevator the diameter of the traction sheave always had to be over 5 times larger due to the cable-bending radius restrictions, but now that we are using a belt that is more flexible we are free of this design constraint," says Ken Woronoff, building manager of the Otis Research Center, who has over 30 years experience as a field engineer. "We can therefore use a smaller sheave and motor because less torque is required to drive the elevator." A typical 1/2-inch diameter cable required a 20-inch diameter sheave for a motor operating at a rated torque of 520 N-m. Since the polyurethane belt's effective diameter is much less, engineers can achieve the same 40:1 ratio with a sheave that is 4 inches in diameter, requiring a motor torque of only 105 N-m.

The polyurethane-coated belts have a number of added advantages. "We needed to make sure the belts were capable of handling an annual duty cycle of 300,000 so we had to redesign the belt technology that was initially provided to us by Contitech, and in the process we were able to achieve a durability that is 2 to 3 times that of conventional heavy steel cables lasting up to 20 years," says Saxe. A consortium of companies participated in improving the belt design, including Contitech, Otis, United Technologies, and Pratt and Whitney. Contitech, an affiliate of Continental Tire, is located in Danenburg Germany, and is an original manufacturer of polyurethane flat-belts used in the transportation industry.

"Another benefit is that the belts do not need continual maintenance compared to the heavy cables that need regular oiling or will fail," says Saxe. "The system is also more efficient because the polyurethane has a higher coefficient of friction, reducing slippage and increasing traction."

The idea for a new belt sprang from a week-long brainstorming session that brought in a variety of experts from their subsidiaries and partners from all over the world. "Our goal was to create an elevator that could save building space by eliminating the need for a separate machine room-often on the roof" says Saxe. "Essentially, we wanted to put the drive system in the hoistway." Conventional elevators with much larger motors force architects to design a separate room, and sometimes a whole floor, to house the components. Now that the drive system is located in the hoistway, engineers and architects can design a building that does not need an external room housing the motor and its associated components, saving time and money while improving building aesthetics.

During the design process the engineering team had to decide whether to use an ac induction motor, or a dc surface or embedded permanent magnet motor. Although ac induction motors are less efficient than permanent magnet ones, they are typically used for elevator systems because, to produce high torque, large permanent magnet motors require large magnets that are cost prohibitive. Since 5 times less torque is needed to drive the Gen2 system, Kollmorgen custom designed a permanent magnet motor that is smaller and more cost effective for the Otis machine design. An elevator machine is composed of the motor, drive shaft, and break system. Although an embedded permanent magnet motor requires more sophisticated control algorithms it is generally more reliable than a surface magnet motor. "Safety and reliability mean everything in an elevator and all aspects of the design must reflect this requirement," says John Boyland, Otis Program Manager for Kollmorgen. "At the speed ranges we are operating, there has never been a case where an interior permanent magnet separated from the rotor, thus making the selection inherently more reliable."

With strict space requirement in mind, the control system engineers were able to reduce the cabinet cooling volume using E-PAC technology developed by Hewlett Packard. E-PAC allowed engineers to place the control and drive system in the hoistway while increasing efficiency and reliability. The material has a Styrofoam-type consistency so it was easily cut into sections and placed at key locations in the cabinet so that channels could be designed allowing fan-induced airflow to directly cool the hottest components. "This enabled us to reduce the cabinet to 20% of its original volume and significantly increase efficiency, because in previous designs the whole cabinet had to be cooled by convection," says Rudy Steiger, chief engineer for the control unit. "It also enabled us to design a unit that is more reliable because the material is excellent at absorbing mechanical vibrations," recalls Steiger.

Despite the space improvements provided by the Gen2 elevator, a safety issue arose requiring engineers to incorporate more sophisticated electronic systems. Since the motor is located in the hoistway, during an emergency it remains inaccessible to building technicians who may need to move the car to the nearest floor for passengers to escape. In conventional elevators, technicians are able to manually lower the car by using a mechanism attached to the motor. Otis engineers are convinced, however, that by engaging in risk analysis and incorporating the latest in sensor and control technology the product will be as safe as the conventional designs. For example, in the case of a fire, heat sensors will shut down the system and send the car to the nearest landing if there is extreme heat in the hoistway.

Gen2 will initially be available for small and medium sized elevators in Europe this year and in the U.S. by the end of 2001. The maximum height is 21 floors, however Otis engineers continue to investigate the innovative belt technology and plan to expand it to taller buildings in the near future. So far the system has been so well received that it is the fastest selling product in Otis history, with the projection of 2,000 units to be sold in Europe before the product release toward the end of this year. DN

Embedded Vs Surface Permanent-Magnet Motor
Embedded Magnet Motor Surface Magnet Motor

Smoothness

Possibility of smoother torque production

Somewhat higher torque ripple

Control

More complicated current control required

Less complicated current control

Cost

Lower cost

Higher cost

Magnet Retention

Does not require mechanism to hold

magnets in place

May require mechanism, typically a wire, to hold

the magnets in place to prevent catastrophic failure

Magnet Cost

Simple block magnet shape

More complicated arc or bread-loaf magnet shape

Machining Costs

Lower machining requirements

More exacting machining requirements


The Gen2 Engineering Design Process

Engineers decide the primary design goal of fitting the machine and control unit in the hoistway to dramatically save new building cost and construction time.

Brainstorming: Engineers from all over the world gather in Farmington, Connecticut to discuss cutting edge ideas in elevator technology. As a result, Otis engineers decide that incorporating belt technology is the only way to meet their design requirements.

With the knowledge that in the past the design of belt technology in elevator systems failed at competitor companies, the Otis team is confident that they can do the job with the help of Contitech

The use of belt technology has a significant advantage in that they can use a much smaller motor. Kollmorgen is contacted and suggests using an embedded permanent magnet motor with the advantage of providing a smooth, reliable ride.

The design team enters the next phase by engaging in two critical safety processes in parallel-ensuring the cable is strong and reliable to carry an elevator car of the same weight and speed, and risk analysis so that the new feedback-control system used for a smaller machine can be integrated into the system to ensure safety in the case of an emergency.

The result-70% reduction in machine volume while designing a more reliable and smoother ride.

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