Conical Rotor Motors
Raise Standard for Industrial Crane

Corrosive environments demand rugged hoists and gearmotors

--edited by Richard Mandel

In 1996, Invacare, a manufacturer of wheel chairs, crutches and other healthcare equipment, set out to modernize its Elyria, Ohio, facility. An operation such as Invacare's involves not only tube bending and assembly lines, but also in-house plating lines for easy-to-maintain finishes. Upgrades would include the replacement of its existing rack-and-pinion lifting device and a sprocket-to-cross-axle travel device originally built and installed by the company during the mid-60s.

Invacare required a reliable, cost-effective and efficient solution for raising and lowering materials into a series of chrome-plating tanks containing highly corrosive chemicals.

But hoist and motor parts replacement in the plating area presented some unique challenges. In particular, the in-house chrome plating line involves a process that requires highly corrosive chemicals. The production equipment used for raising and lowering materials into process tanks, as well as the travel drive equipment, are exposed to vapors from acidic chemicals, which can significantly impact maintenance time and costs as well as affect the equipment's life span. Invacare had originally used wheels and motors from Demag Cranes and Components, Cleveland, OH (formerly Mannesmann Dematic) for the system and, after evaluating the performance of those first lifting and travel devices, some of which has been in use for more than 30 years, Invacare decided to call on Demag to handle the upgrades.

Finding the solution

The transfer car for this line weighs about 1,000 pounds and handles several hundred pounds of cargo with each load. The original travel drive unit was replaced by a system consisting of a Demag DRS Wheel Block at each corner of the transfer car and a Demag KBF (self-braking) Conical Rotor Brake Motor, with a three-stage gearbox, driving two of the wheel-block units by a cross shaft. While one KBF conical rotor motor acts as a travel motor to move the overhead car from one tank to the next, another KBF unit performs as a lift motor, lowering and raising materials into and out of the tanks. The compact, ruggedly designed wheel blocks, which feature high-strength, modular cast-iron travel wheels for low wear and long life, are virtually maintenance free. The totally enclosed, fan-cooled motors drastically reduce the potential for internal damage to the motor from caustic fumes.

Demag Conical Rotor Brake Motor construction.

Ted Mucha, currently Invacare's safety specialist for Ohio operations and maintenance supervisor at the time of initial installation, said, "The Demag drives and wheels worked very well, even better than I had expected."

Conical Rotor Motors versus conventional cylindrical rotor motors

The Demag KBF Conical Rotor Brake Motor provides unprecedented reliability, efficiency and safety features. "The use of modern materials and insulators in conjunction with modern production methods make these compact motors highly reliable," said Dennis Clark, Demag regional sales manager.

In these squirrel-cage self-braking motors, the rotor and spring-loaded brake are located on a single shaft that slides in the stator housing. At rest, the motor is braked by spring pressure. When the motor is switched on, an axial force is created as a result of the electro-magnetic field and the characteristic air gap between the cone-shaped rotor and stator. This axial force overcomes the return force of the brake spring, moving the rotor shaft and brake disc in an axial direction. The brake is then released, allowing the motor to start up. After the motor is switched off or if the voltage fails, the magnetic force collapses and the motor mechanically brakes to a standstill by the return force of the brake spring. This classical principle of interlocking the functions of driving and braking has proved itself many times over, and makes components like rectifiers and additional magnetic coils obsolete.

A key advantage to using the KBF motor are its softer windings and higher inertia. This provides more slip between synchronous and full-load speed, producing an increased load sharing capability. The higher inertia also provides smoother acceleration and deceleration.

"From a maintenance standpoint, you get less wheel slippage during starting and stopping and reduced wear on the rail and on the wheels, which is a direct result of the smoother acceleration and deceleration," said Clark. Comparatively, other motors must use some kind of external control to accelerate and decelerate. Usually, this is accomplished with an external soft-start device, a variable frequency unit or the equivalent. The Demag motor accomplishes the same function mechanically without any added controls.

Clark also points out that the KBF motor has simple integral braking systems and a high torque-to-rotor-inertia ratio. The sliding conical rotor design produces simultaneous starting and brake release, thereby eliminating the heat produced by motor-brake overlap inherent in conventional brake motors. Heat produced by separately energized coils is also avoided by the sliding rotor design. High brake torque is an advantage provided by the conical brake profile. The KB motors are also specially designed for high cycling and/or close positioning applications.

(continued)