Cherry-picking conveyor components to optimize energy efficiency

And knowing best applications for component investment

By David Kaunitz

April 12, 2011

Conveyors are big energy consumers in manufacturing plants, warehouses, and distribution centers.

Conveyors are large energy consumers in manufacturing plants, warehouses, and distribution centers. In fact, an average distribution center may have more than 5 miles of conveyors.

Although many different applications and different types of conveyors are used to move product—and it's important to understand all these differences when selecting the best drive train style and component for each—obviously the system's efficiency is an important selection criteria.

Efficiency is the ratio of power output to power input:

Efficiency = Power Output
                   Power Input

Each component in the system has an impact on efficiency, so it is important to consider these common components individually to create the most efficient system (see Figure 1).

Induction Motors

Simply put, electric motors are devices that convert electricity into mechanical work. Therefore, the more interference with the conversion can be minimized, the more efficiency can be gained.

Electric motor efficiency is affected by many factors:

1. Ferrous Steel. Eddy currents can be reduced by designing a rotor with thin laminations made of high-quality, low-loss electrical steel. The thin laminations should be coated with a high-grade insulator and pressed together tightly to achieve the maximum efficiency and electromagnetic force. The additional steel of a longer core reduces the flux density and lowers eddy current losses.

2. Stator I²R Losses. Increased copper content and large conductors increase the cross-sectional area of the stator windings. This lowers the resistance of the windings and reduces losses from current flow.

3. Rotor I²R Losses. Large rotor conductor bars increase the cross-sectional area and therefore lower the conductor resistance and losses from current flow.

4. Friction and Windage Losses. Displacing the air from the motor fan (fan-cooled motors only) and overcoming the friction in the bearings create additional losses.

5. Stray Load Losses. Pulsations of flux in the rotor, stator, and air gap are the main cause of stray load losses.

Most motors applied in the unit handling industry are C-face footless mount motors. This is significant because the Energy Independence and Security Act of 2007 (EISA), which went into effect January 2010, requires that motors with this mounting configuration (140T frame and above) must meet a minimum efficiency level defined by the National Electrical Manufacturers Association (NEMA) Table 12-11. A more efficient option is the NEMA Premium® motor, which requires minimum efficiencies listed in NEMA MG-1 Table 12-12 (see Figure 2).

Gear Reducers

A gear reducer is a device used to reduce speed and multiply torque by using single or multiple gear sets (see Figure 3). Many different styles of gearing are used in unit handling applications; however, the most common gear types are worm, helical, and helical-bevel.

Worm Gearing. A worm gear set comprises a high-speed worm and a mating bronze gear (see Figure 4). Torque is transmitted primarily as the worm and a gear slide, but also as they roll relatively to each other. The interaction between the two gears allows for a right-angle change in direction.

Helical Gearing. A helical gear set is a pair of cylindrical gears mounted on parallel shafts on which the gear teeth are angled and unparallel to the rotational axis (see Figure 5). Typically, both gears are made of the same material, and torque is transmitted by rolling action, with a small amount of sliding.

Bevel Gearing. A bevel gear set is made up of a pair of bevel gears, typically mounted on shafts that are 90 degrees to each other (see Figure 3).

They are normally used in conjunction with at least one stage of helical gearing. Torque is transmitted through rolling action along with some sliding.

Best Applications for Efficiency Components

A common misunderstanding in the marketplace is that worm gearing is always an inefficient power transmission device (tool) across the board. Advancements in materials, lubrication, and design have led to improved energy efficiency levels. Furthermore, at low ratios, worm gear efficiency levels approach helical gear efficiency levels. As ratios increase, worm gear efficiencies decrease (see Figure 6).

At low ratios, the efficiency gains from a helical or helical-bevel gear reducer is marginal compared to the worm gear. However, when higher ratios are required, helical or helical-bevel gear reducers are more efficient.

Therefore, understanding the application requirements is critical for gearing selection, because generally the higher-priced helical and helical-bevel gear reducers may not produce the expected efficiencies and return on investment (ROI) in low-ratio applications.

Power Transmission Components

Several different methods can be used to connect parallel shafts in different planes. Three common connection components are sheaves, timing belt sprockets, and roller chain sprockets. All three transmit torque and change speed proportional to the ratio between the driver and driven sheaves or sprockets.

Sheaves. A pair of sheaves have grooves machined in their ODs to match the profile of a mating V-belt (see Figure 7). The V-belt is wedged in the groove through tension in the system, and the friction between the two faces transmits the torque. The belt will slip in the groove if the load exceeds the frictional force.

The benefit of this arrangement is that it isolates the connected equipment from shock loads and vibration. However, because there is no positive engagement between the two sheaves and belts, the system's efficiency is lower than other alternatives'.

Timing Belt. A timing belt drive consists of two timing belt sprockets and a belt. A synchronous drive does not slip and therefore transmits shock loads within the system (see Figure 8). Because of this positive engagement, a timing belt drive achieves higher efficiency than a sheave and V-belt drive.

Roller Chain Sprockets. Roller chain sprockets comprise a pair of metal sprockets (see Figure 9) and a roller chain to connect the sprockets. The chain rolls over the sprocket teeth, transmitting the torque in the system. Benefits to applying roller chain systems include low installation tension and high efficiency.

It is important to understand how each individual component's efficiency affects the total system efficiency (see Figure 10). Each component efficiency is multiplied to reach a total. For example, in a drive train with three components, the efficiency of the total drive train would be calculated as follows:

Total Drive Train Efficiency = Component 1 Efficiency × Component 2 Efficiency × Component 3 Efficiency

Because no component is 100 percent efficient, removing an unnecessary component in the system can improve the system efficiency. For example, if the drive system of a conveyor that runs continuously at fixed speed is being powered by a variable-frequency drive, the company is actually using more energy than if it were run on sine wave power.

Conveyor Types

Pick. When an order is released, the products are picked, labeled, and put on a pick conveyor to be processed through the system. Because a person is physically loading the order, this conveyor typically runs slowly. Quite often the conveyor is long, which increases the power requirement.

Because of its high horsepower and slow speed, a pick conveyor is suitable for a NEMA Premium efficient motor, helical gear reducer, and a synchronous drive (depending on the drive design).

Accumulation/Transport. With this conveyor type, products are moved through the system and are accumulated. As packages or products accumulate, spaces and gaps are removed to form a slug. Typically, accumulation conveyors run relatively fast.

In general, because the gear ratios typically are low and the power required is low, this application is suitable for Premium efficient motors and worm gear reducers.

Meter Belt. As a package moves through the meter belt system, it must be scanned to track its location for proper sorting. A meter belt consists of two conveyors attached by sprockets that move at two different speeds. Packages are fed onto the infeed belt at a given speed and then are discharged at a faster speed. The difference in speed leaves a gap between the packages or product. Optical sensors, or photo eyes, detect the gap to define the beginning and end of each package.

Because this type of belt requires a high number of cycles, a Premium efficient motor may not be an optimal choice. Upgrading to helical gearing may offer marginal efficiency gains over worm gearing.

Sorters. Packages are diverted to the appropriate shipping lane by the sorter. It is common to run the sorter at a constant speed continuously throughout the day. In most systems, the motor driving the sorter is the largest motor in the system.

There are many different styles of sorters that run at different speeds. In the past the most common drive train for sorters was a motor that drove a shaft mount reducer through a set of sheaves. Although the sheaves offer good shock load capability, they are not as efficient as other alternatives. By changing the sheave combination to timing belt sprockets at the same ratio, it is possible to increase the efficiency. Replacing the drive with a shaft mount reducer may also increase efficiency. The use of Premium efficient motors and helical reducers is also recommended for this application.

Trailer Loaders. As packages are diverted to the shipping lane, they move through the take-away onto a trailer loader and are loaded on a truck. This can be accomplished by a trailer loader belt conveyor.

A typical drive train design comprises a motor, a reducer driven by a roller chain or timing belt, and a driven pulley connected to the reducer. The reducer ratios typically are low, and the roller chain or timing belt drive fine-tunes the speed. A Premium efficient motor and worm gear with a synchronous drive can provide an efficient drive train cost-effectively in this application.

Note that differences in the number of hours the conveyors are operated, the price of electricity, and the drive installed may change the analysis.