Versatile wheel blocks help engineers build better transfer cars.
By Dennis Clark
Area Manager Mannesmann Dematic Corp.,
EDITED BY AMY HIGGINS
In many automation systems, material moves horizontally from onearea to another by way of conveyors or travel systems supported on wheels. Wheels, used for scrap transfer, bulk handling, mold transfers, or general moving applications, are typically found on transfer cars, shuttle conveyors, scissors-lift tables, and turntables.
When designing a wheel system for transfer cars, traditionally, the first step is to determine the wheel size, then design or select the proper axle. Engineers must then establish the correct pillowblock bearings for support based on the axle diameter, distance, load, and speed, and design the car's undercarriage to support the entire package. Finally, they must determine how to get power to the wheel. Typically, this is accomplished with a chain and sprocket connection to a gearbox and motor.
Many companies still use this traditional approach, but improvements in wheel design may make for safer and more cost-effective production. One new design, for instance, the Demag Travel Wheel System from Mannesmann Dematic Corp., Cleveland, consists of what is called the "DRS Wheel Block," a preassembled wheel, bearings, and support in a compact, adaptable housing, along with a gearmotor and splined output shaft to directly drive the wheel. Amazingly, this custom-engineered travel wheel system supports as much as 88,000 lb/wheel. Here's a closer look at each component.
The DRS wheel block wheel comes in three materials: spheroidal graphite cast iron, polyamide, and polyurethane.
The original design uses GGG65 spheroidal graphite cast iron, a cast-iron alloy impregnated with graphite in a spheroidal form. This combination improves the cast iron's damping characteristics, giving it properties similar to steel without the high costs. Also, the graphite is self-lubricating, so friction between the wheel flanges and the track is reduced which, in turn, reduces wear on the wheel and rail. The latest design, however, uses a new alloy cast iron (GGG70) with better vibration-damping and self-lubrication properties than the original. For example, load capacity per wheel for the DRS125 spheroidal graphite cast iron is 11,000 lb (4.9-in. wheel diameter).
Polyamide wheels, made from cast polyamide, PA6, are used when quiet running and low rail wear is necessary. The polyamide wheel combats abrasion and has a higher traction resistance than spheroidal graphite, but significantly lower load capacity. For example, each DRS125 polyamide wheel handles loads to 2,200 lb/wheel (4.9-in. wheel diameter).
Another material called hydropur, a polyurethane elastomer, offers higher traction resistance than the spheroidal cast iron, making speeding up or slowing down easier. Additional benefits include less running noise and good damping characteristics. The material's soft exterior makes these wheels particularly suitable for use on concrete, wood, or plastic-coated running surfaces. However, as with the polyamide wheel, consideration must be given to the hydropur wheels' reduced capacity, which, for the DRS125 polyurethane wheel is 1,740 lb/wheel (4.9-in. wheel diameter).
The travel wheels come in seven profiles to fit concrete, steel, or wood surfaces above or below the floor. Shapes include double flanged, single flanged, and flangeless, as well as concave wheels for vertical and horizontal guide arrangements, hydropur wheels without flange for increasing speeds, and large diameter wheels for difficult ground clearance situations. Wheel sizes range from 4.4 to 19.7-in. diameter with loads to 88,000 lb/wheel.
Bearings are carefully selected and designed into the housing, and capacity is usually based on the bearings' diameter. The DRS Wheel Block supports the bearings inner race in the wheel block housing rather than on the axle, this allows for larger-diameter bearings with more capacity. The standard bearings used for smaller wheels are typically shielded ball bearings while larger wheel sizes use sealed tapered roller bearings. All are lubricated for life for maintenance-free wheel blocks.
The two-part wheel block housing is made from either corrosion-resistant diecast aluminum or cast iron, depending on its size, and powder coated. Prepared mounting surfaces let wheels be top or side mounted, or pinned inside a fabrication.
DRIVING THE WHEELS
With a variety of motor types, the travel wheel system lets users customize their power source. Motor options include cylindrical rotor travel motors, indrive variablespeed motors, conical rotor travel motors, and the company's microspeed drive. The cylindrical rotor travel motor, with or without a brake, and the conical rotor travel brake motor include high-slip windings and an inertia wheel built into the motor's rotor. This combination provides smooth acceleration and deceleration for cross the line starts, which is critical when driving wheels because it keeps the wheels from slipping. This boosts wheel and rail life.
Variable-frequency controllers and inverter-duty motors are also available. Pneumatic or hydraulic motors may also be used for specific requirements.
As far as gearboxes go, as many as nine different offset and 10 right-angle gearboxes can be used with the travel wheel system, with gear ratio's ranging from 5:1 to 485:1. Hardened and ground gearing is either machined into the shaft or connected by interference fit with special adhesives.
Further, system maintenance is quite manageable. A flange wear indicator lets operators see when it's time to replace the wheel. Once the wheels have been aligned, any maintenance or replacement is handled simply by removing the side plate — without disturbing the alignment.
The Demag Travel Wheel System is also available with a variety of options including guide rollers, bumpers, several torque brackets, and shaft protectors.