Machinedesign 1470 Motorized Pulleys 0 0

Motorized pulleys bounce back

March 1, 2001
Motorized drive pulleys for belt conveyors offer an alternative to traditional chain and belt drive systems

Of all the components in a conveyor system, the drive mechanism is the most expensive. It can account for as much as two-thirds the cost of the entire system, as well as 80% of the maintenance and repair costs. It’s no surprise, then, that many engineers put the drive mechanism at the top of the list when spec’ing out a conveyor system.

External chain and V-belt drive mechanisms are the traditional choice, but motorized pulleys — an old idea reborn with new technology — are beginning to live up to earlier promises. With integrated electronic intelligence, today’s self-propelled pulleys are reliable, efficient, and virtually maintenance-free in many applications.

What is a motorized pulley?

A motorized pulley is simply a drum motor in which the outer shell rotates around a stationary shaft. The revolving member serves as a traction pulley, gripping and driving the conveyor belt. Electrical power leads pass through the stationary shaft to terminals within the pulley housing. Since the shaft end does not rotate, there is no need for slip rings to deliver electrical energy to the motor windings.

The entire internal mechanism — including motor, gears, and bearings — is completely sealed and protected by the outer metal drum. Oil inside the drum continuously lubricates the bearings and gears, and acts as a heat-transfer fluid by dissipating heat away from the motor windings. This keeps the motor cool, extends service life, and reduces maintenance. It also means designers can put pulleys in hot environments (up to 120°F) and in non-conventional applications such as sprocket drives and kick rollers.

By bringing the drive mechanism inside, a motorized pulley is much more compact than a conventional drive system with all its external components. This dramatically reduces space requirements and allows for the installation of more conveyor in less floor or overhead space. And since all moving parts and electrical connections are completely enclosed in steel, external safety hazards are virtually eliminated.

Typically the only service required on a motorized pulley is a simple oil change every 20,000 hours. To put this in perspective, that’s nearly 10 years of service (40 hour week) without maintenance.

Self-propelled pulleys are also quiet. Most designs — because gears and other moving parts are enclosed within a sealed, oilfilled chamber — operate in the 57 to 62 dB range, the sound intensity of normal conversation. At this level, there is no need for workers to wear ear protective devices, as specified by OSHA guidelines and requirements.

Another advantage for motorized pulleys is efficiency. They typically transfer about 95% of the input horsepower to the conveyor belt. In today’s cost-conscious plant, that can help a system pay for itself in short order.

Motorized pulleys are not without problems, however. One in particular has been oil escaping through the end seals. Earlier designs put end seals in contact with the shaft, causing both components to wear. The more the conveyor was used, the less time it would take the oil to find a leak path to the outside world.

New sealing technology corrects this problem by inserting a hardened, ground stainless steel sleeve between the main shaft and double-lip oil seal. The new design works so well that pulleys often perform reliably without leaking for many years — even after repeated washdowns with scalding water, caustic solutions, and steam at pressures to 1,200 psi.

Applications for motorized pulley conveyor belts

It would be an oversimplification to say that any conveyor system can be driven by a self-contained motorized pulley, although the statement is certainly true. There are, however, several conveyor applications that can realize significant benefits from self-contained motorized pulleys.

Chief among these are in the food processing and handling industry. Because foods for human consumption are involved, the FDA has stringent rules governing cleanliness. Conveyor systems and associated equipment must be washed down daily with high-pressure steam, hot water, and/or caustic chemical solutions. Sealed pulleys are impervious to these harsh cleanings and remain so for years.

Package handlers — from shippers, to package-forwarding organizations such as Airborne Express, to companies receiving hundreds of parcels daily — are also benefiting from the simplified design and space-saving features offered by motorized pulleys. This includes airlines that handle a variety of items inside terminals every day, and the truckmounted conveyors that carry parcels in and out of cargo hold on parked aircraft.

The foundry industry is also benefiting as sealed, enclosed drives reduce the potential for shutdowns caused by airborne dust and dirt fouling bearings, gears, and other exposed parts.

Selecting the right pulley for your application

Determining which motorized pulley will handle your application is not a difficult task. Most manufacturers provide computer software that takes basic application information — conveyor length and speed, load, belt weight, whether the system is a roller or slider-bed conveyor — and provides guidelines for inputting such variables as the coefficient of friction; whether or not the conveyor is inclined and, if so, the steepness of the incline; hours of operation per day; and the type of load that will be handled (uniform loading, moderate shock, or heavy shock). From all of this data, the program calculates the required pulley horsepower and recommends a model for the specific application.

Alternately, manufacturers may also offer mathematical formulas, either in print or on their websites, along with instructions on how to calculate required horsepower and specify a particular drive. Some manufacturers even go through the calculation process for you. Just fill out a form that requests all conveyor operating data, and usually within 24 hours, the manufacturer will fax back or e-mail an equipment recommendation list.

Timothy Wallace is an application specialist with Sparks Belting Co., Grand Rapids, Mich. He can be reached at (616) 949-2750.

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