In addition to the purchase price, economic factors such as low service requirements, low life cycle costs as well as a long product service life are becoming increasingly important and often play a determining role. As a specialist in driveline and chassis technology for rail vehicles, ZF has developed solutions to these issues and improved the bearing concept with its single-stage cylindrical gear axle drives.
One detail innovation, many advantages
A closer look at the technical details reveals why metro operators in general, and Moscow Metro operators in particular, will benefit from ZF's cylindrical gear axle drive. In the case of conventional helical cut transmissions, forces are not only applied in the circumferential direction but also in the axial direction - with negative consequences: The forces put load on the bearings and reduce efficiency. To prevent this from happening, ZF has installed two innovative thrust collars for this gearbox.
Further development of the bearing concept
The axial forces generated as the teeth mesh are absorbed by the thrust collars, which leads to a reduction in the bearing forces. As a result, more affordable and smaller cylindrical roller bearings can be used instead of tapered roller bearings. In high-speed gearboxes in particular, it is possible to obtain considerably higher efficiency with the thrust collar bearing.
In simplified terms, these are rings that are attached to the left and right sides of the drive pinion and slide on an oil film at the output gear. This lateral guidance guarantees that no axial forces from the gearing take effect on the bearing and the housing – the basis of many other improvements for metro gearboxes.
On this basis, the ZF engineers designed input and output shafts with cylindrical roller bearings instead of conventional tapered roller bearings. The former require no special setups and make installation especially easy for service personnel, for instance in case of general inspections. In addition, this design is largely resistant to varying outside temperatures. For this reason, it works perfectly even in regions where temperatures drop way below zero. The temperature also remains cool inside the ZF Metro gearbox. This is because the bearings are mounted free of tension and without axial load, so that their temperature is only slightly above that of the oil sump. This relieves overall pressure on the bearings and results in a longer product service life. In commercial vehicles, this technology is well established: ZF has been using it successfully in this field for about 20 years. The cylindrical gear axle drive is encased in a one-piece housing that is extremely torsion-resistant and equipped with an oil sump cover. Thanks to wear-free ZF labyrinth seals, its leak tightness is very reliable in all driving situations.
ZF has also designed the cylindrical gear axle drive for the Moscow Metro to include a special feature: a hollow shaft drive. Rail vehicle manufacturers can thus attach the gearbox in a very easy way to the wheelset axle. Furthermore, it can be transported without the heavy wheelset axle which significantly reduces the logistics efforts and the transport costs for all parties involved. This helps not only during initial installation, but also in subsequent vehicle inspections and service work.