Slim bearings from Kaydon, part of the SKF Group, are in use in the design and construction of the CVT transmission of an innovative bicycle with automatic gear shift mechanism. In order to meet the application requirements, it was particularly important that both bearings should take up very little installation space.
by Elena Magistretti
Extremely compact and lightweight thin section bearings from Kaydon have proven their value in a bicycle…. We are not talking about a traditional model, but about an innovative bike which is currently being developed by students at the HTL Rennweg Higher Technical Education Institute in Vienna, Austria. Their objective was to design and build a CVT gear shift mechanism with the required size and transmission ratio. With a CVT transmission, the transmission ratio can be changed steplessly.
The operating principle of the CVT gear shift mechanism
The CVT mechanism consists of four conical pulleys between which a push belt is routed, or in this case a V-belt. When the distance between the first pair of pulleys becomes smaller, the distance on the other side becomes larger. In this way, the transmission ratio can be varied infinitely.
The rider can choose between manual and automatic mode with a switch on the handlebar, and in manual mode the gear ratio can be set by hand. In automatic mode, the gear is shifted automatically; the rider does not have to worry about it anymore. This electrically powered solution uses parameters such as pedalling frequency, force application, wheel speed and inclination to determine the appropriate gear ratio. An integrated rechargeable battery stores energy generated by the mileage travelled, but can also be charged via an electrical socket. The gear shift mechanism itself is mounted in the region of the water bottle holder, so complicated changes to the design of the bicycle are unnecessary. To install the device, the chain is taken off, the gear shift mechanism is installed and two new chains are fitted. This simple process makes it easy to retrofit the stepless automatic gear shift mechanism, which is usable on practically all bicycle types.
In search of bearings with highly compact construction
In all, two ball bearings are needed in the CVT gear shift: One bearing is fitted in the moving element. There, its function is to ensure that the rotating pulley can be shifted by the stationary transmission servodrive. A wide V-belt is installed in the gear shift mechanism to transmit torque. A compression spring ensures “non-slip” transmission. The servodrive must push the pulley against the pressure of the spring, which causes the spring pressure to bear on the thin section bearing. The purpose of the second bearing is to stabilise the trapezoidal spindle which is connected to the servodrive. A nut provided on the spindle is designed to shift the pulley.
It was particularly important that both bearings should take up very little installation space. The larger of the two bearings must be able to absorb axial forces in the order of 800 N and radial forces to a lesser degree at relatively low rotating speeds of not more than 120 rpm. At the same time, a very small installation width is important, which is why a combination of radial and axial bearings is not possible. It is true that the smaller bearing only has to absorb axial forces, but this installation site also offers only very limited installation space. The thin section bearings from Kaydon and their highly compact construction represented the ideal solution. The HTL students ultimately chose Kaydon Reali-Slim thin section bearing types KA 025 AR0 and KAA 10 AGO. These angular contact ball bearings without seals and with ball pocket separators have a cross section of just 6.350 mm and 4.7625 mm respectively.
Small and lightweight with no loss of performance
Bearings with a contact angle of 30° were used for the application, featuring a deep race on the inner ring and an outer ring which is reduced on one side. The outer ring can be pushed over the inner ring which is filled with balls and the separator with a slight pressing force. This snap-fit installation makes it possible to use a ball pocket separator with more balls.
Each series has a uniform cross section for all sizes. This means that the cross section remains constant even as the bore diameter increases. This ingenious design feature means that solid shafts can be replaced with hollow shafts. In many cases, a Reali-Slim thin section bearing constructed as a four-point bearing can even replace two ball bearings. This enables a more compact structure and simplifies installation of the ball bearing. With more than 250 different thin section bearing types up to hybrid variants with ceramic balls, Kaydon has the optimal solution for every design engineering problem.