Drum motors’ benefits can further market penetration
Contained design helps promote food safety measures
The drum motor belt drive design entered the conveyor industry in the early ’50s as a unique drive for belt conveyor applications. What sets the drum motor apart from a conventional conveyor drive is that the drum motor has all the drive components, such as the electric motor, gear reducer and all bearings, housed inside the drum.
Without exposed drive components and with no other drive parts rotating outside the conveyor frame, the drum motor features a unique belt drive design. The drum motor also maximizes space utilization and worker safety. When a drum motor is used, external electric motor, gear reducer, bearings, chain and chain guards mounted outside the conveyor frame are not needed.
Most standard conventional conveyor drives use either a 90-degree gear reducer mounted direct on the drive shaft or sprockets and chains to drive the head conveyor drive drum. Using a 90-degree gear reducer impedes on the efficiency of the drive, resulting in mechanical losses. In the drum motor conveyor drive, the motor is mechanically connected in-line with the gear reducer. This increases mechanical efficiency by 20 to 40 percent, depending on the type of gear reducer with which it is being compared.
Because the drum motor drive uses no external drive components, it offers a more streamlined conveyor design.
Because the drum motor drive uses no external drive components, it offers a more streamlined conveyor design. (Image courtesy of Van der Graaf)
When looking at the drum motor, one notices a cylinder with two square shafts protruding on either side, and an electrical connection box on one side of the drum motor that houses the electrical connections. The shafts are square and do not rotate. They are fixed and mounted on the conveyor frame, eliminating the need for pillow block bearings. The electric motor that is housed inside the cylinder (drive drum) is an AC squirrel cage design motor. The stator does not rotate, therefore there is no need for rotating brushes or slip rings delivering power to the stator windings.
The rotor shaft is the input pinion driving either a two- or three-stage gear reducer. The last stage of the gear reducer is driving a gear ring that is bolted direct to the end-flange, and the end-flange is bolted directly to the drive drum, rotating the drum motor.
All internal components, motor, gears, and bearings are working in an oil bath. The drum motor is hermetically sealed and filled one-third of the way with oil. The oil inside the drum motor is used as a lubricant, and also provides cooling. When the drum motor is running, the oil transfers the heat that is generated from the electric motor and gear reducer to the rotating drum and then is dissipated to the conveyor belt. As the temperature inside the drum motor rises, the internal pressure can rise up to 1 atmosphere or 14.6 psi. Because of the internal pressure, it is necessary for the drum motor to be hermetically sealed to prevent oil leakage.
Depending on the manufacturer, drum motors are available in different drum diameters, belt speeds and horsepowers (HP). The diameter of the drum motor is dictated by the required HP and the HP is therefore geometrically restricted because all mechanical and electrical components have to fit inside the cylinder of the drum motor. In order to achieve the required belt speed, drum motors offer a range of fixed belt speeds. When a different belt speed is required, this can be accomplished by changing the ratio of the gear reducer or using a frequency inverter.
Because of the self-contained design of the drum motor, it is able to withstand high-pressure wash-down procedures, promoting sanitation and hygiene, representing a unique and attractive option for food processing applications.
Drum motors also are more efficient than external mounted belt drives, resulting in lower energy consumption without any sacrifice in torque and performance. Because drum motors contribute to a more streamlined conveyor design, they promotes workplace safety as chain and sprockets are not required.
The amount of time required to install a drum motor into the conveyor frame is much less than the time required to install an external conventional drive.
The drum motor does not require any maintenance. However, in the event of electric motor or gear reducer failure, the drum motor has to be removed from the conveyor frame, resulting in downtime to get back to production. Using the conventional external motor/gearbox drive system, repairs can be completed in less time because all drive components are located outside the conveyor frame and more accessible to maintenance personnel.
In North America, the drum motor represents approximately 7 percent of the overall conveyor drive applications market. With all the benefits that drum motors offer, it would be reasonable to ask why the drum motor has such a low market penetration.
Low market penetration of drum motors is because conveyor manufacturers and end users implement the drum motor into their conveyor systems only when there are space constraints. In addition, market feedback showed issues with oil leakage, premature bearing, and gear and electric motor failure.
When drum motors first appeared in the market, the overall selection of belt speeds, HP, drum diameters, and drum lengths was very limited, with belt speed selection the larger issue. For this reason, the majority of conveyor drives contained chains and sprockets to power the head roller.
Changing belt speeds on conventional drives can be achieved by simply changing the drive sprockets. Changing the belt speed of the drum motor required removing the drum motor from the conveyor frame and changing the gear ratio of the gear reducer. Today, this problem has been largely eliminated with the introduction of variable frequency inverters. Adjusting the belt speed with a frequency inverter contributed to a relatively small increase in market penetration of the drum motor.
All of these issues were mainly an inconvenience to end users but not a major detriment at the time because belt conveyor engineers over designed and were liberal in selecting motor HP. More often, the specified HP of a drive would have been twice or more than the required HP needed to drive the belt conveyor. Because of this over design, conveyor drives including drum motors were not subjected to continuous full load conditions.
Design issues and solutions
As an exclusive drum motor manufacturer with facilities in the United States, Canada and the Netherlands, Van der Graaf has invested extensively into research and development. In 2012, the company dedicated its resources to solving drum motor design issues. After two years of testing and analysis, results revealed that lack of heat dissipation was impacting the overall product reliability.
Currently, the majority of electric motors used as conveyor drives are fan cooled. Because all drive components of the drum motor are housed internally, fan cooling is not possible. The drum motor uses oil to transmit the heat from the electric motor to the drum shell and dissipates to the belt. Cooling of the drum motor through the oil submersion is not as effective as the method of fan cooling of a standard conveyor motor.
In electro mechanical devices such as drum motors, two sources are responsible for generating heat: the electric motor and the gearbox. The gear reducer accounts for about 15 percent of the heat generation while the majority of the heat is produced by the electric motor. In addition, the electric motor has two sources of heat generation as well. The two sources are current density of the stator winding and the magnetic density of the laminated core. By increasing these densities, the temperature generated by the electric motor will increase, and by reducing them, the temperature will decrease. However, reducing the current and magnetic densities in the electric motor to achieve lower temperature also will reduce the amount of work the motor will produce. This results in a reduction of torque and HP. That means the standard method of calculating electric motors no longer is valid and cannot be used to design electric motors for drum motor applications.
Energy conservation is more important today than any other time in history. Therefore, conveyor equipment needs to operate longer hours, at full load and close to the rated drive performance. Equipment and drum motor manufacturers alike have been facing the same challenges.
Additional testing revealed the negative impact on heat dissipation of the drum motor when rubber lagging is applied for belt traction. Stainless steel drum motors for sanitary and food processing applications are particularly sensitive to temperature elevations because stainless steel has poor heat-transfer properties. Without sufficient heat dissipation, oil temperature rises, oil viscosity decreases and might not provide adequate lubrication, resulting in premature mechanical component failure. Elevated oil temperature causes higher internal pressure, which can affect the sealing system of the drum motor, and a potential for oil leaks.
As such, the dedicated research and development of the drum motor manufacturer resulted in a mandate for a new motor design. This new manufacturer-produced design included a new method of calculating to correct the drum motor temperature issue. With this, the heat problem has been addressed by using different lamination sizes and materials with different metallurgic composition. Today, the new drum motor design offers an unmatched option for powering belt conveyors. Although, as with most mechanical products, price, quality and reliability vary among those in the drum motor market.
Ongoing investments in research and development by the drum motor manufacturer has produced a new intelligent drive. The intelligent design drum motor is capable of communicating with central plant control or other plant equipment. It is available in different gear configurations, from high to low belt speeds, 3.1 inch diameter to 42 inch diameter, and horsepower ratings from 0.25 HP to 500 HP. BI