Automated guided vehicles - AGVs for short - are proving to be extremely economical and productive when it comes to distributing picked goods in large warehouses and shipping halls, for example. These vehicles, often referred to as automatic guided vehicles (AGVs), are also increasingly being used to transport individual parts and prefabricated components to other assembly and processing stations in assembly and production halls. These systems usually operate autonomously and navigate their surroundings with the help of sophisticated sensors.

AGVs are particularly productive and economical when they can cover long distances and travel times without repeatedly recharging the energy storage units they carry. This places special demands on the design and execution. Especially in the increasingly smaller shuttle vehicles for distribution systems in warehouses, there is little space available for the drives. The usual drive units with axially aligned electric motors and multi-stage spur and planetary gearboxes are difficult to install due to their length. With this in mind, gearbox manufacturer Framo Morat and motor manufacturer Dunkermotoren have joined forces to create a particularly compact DC drive system.

Due to their compactness and high efficiency, drive systems made up of DC motors and bevel gearboxes and hub gearboxes are ideal for mobile applications.

© Dunkermotors

A key component of the new solution is the hub gear designed by Framo Morat. According to Wolfgang Sühling, who heads up the development of customer-specific drives at the Eisenbach-based gearbox manufacturer, this gearbox design is being used for the first time in a travel drive for AGVs. In addition to the compact design, this gearbox design has other decisive advantages, particularly for the wheel and toothed belt drives frequently installed in shuttle vehicles.

In conventionally designed wheel or belt drives, in which a planetary gear or spur gear unit is used, large radial loads act on the bearing due to the load introduction into the cantilevered shaft. The rolling bearings installed in the gearbox must be able to withstand this. However, according to Sühling, the usual shaft bearings with deep groove ball bearings or preloaded tapered roller bearings have a number of decisive disadvantages: With deep groove ball bearings arranged close together, radial loads acting on the projecting shaft end lead to high bearing loads and, due to the bearing clearance in conjunction with the short bearing distance due to the design, also to a skewed position of the output shaft and individual gear components, for example the planet carriers. As a result, the gearboxes wear quickly and do not guarantee the required service life and reliability. The skewed position also causes increased noise emissions. Although tapered roller bearings preloaded in pairs can easily support larger radial loads, they have greater bearing losses than deep groove ball bearings, especially in the partial load range. They are also larger and heavier than deep groove ball bearings.

Large radial forces no problem

In the hub gearbox that has now been realized, standard deep groove ball bearings support the high radial loads. "The name of this gearbox type already indicates the special design features. The entire gearbox can be accommodated in a narrow wheel hub," reports Sühling. The input and output shafts are arranged coaxially in this gearbox design. The radial forces are transmitted via the hollow shaft almost centrally between the two ball bearings. The small distance between the ball bearings also has a positive effect on the bending stiffness of the hollow output shaft. In other words, the shaft hardly bends at all.

© Framo Morat

A planetary gearbox transmits the torques between the input and output shafts. It ensures the required speed ratios between the input and output sides. In the attached, electronically controlled DC motors from Dunkermotoren, these are usually between i = 20 and i = 30. The gearbox runs very quietly due to the insignificant inclination of the planet carrier.

To make the entire drive package particularly compact, a bevel gear connects the DC motor to the input shaft of the hub gearbox. "With this combination, we achieve the smallest installation space and the highest energy density," emphasizes Stefan Tröndle, Product Manager at Dunkermotoren in Bonndorf, adding: "From the combination of our brushless, electronically controlled DC motors with over 90 % efficiency and the smooth-running, low-loss bevel gearboxes and planetary gearboxes, we achieve optimum efficiency and thus high energy efficiency for the entire drive system."