Kaydon case studies

Kaydon Reali-Slim® bearings save space on bicycle gear mechanism

Reali-Slim® thin section bearings from Kaydon, an SKF Group brand are extremely compact and lightweight. These space-saving components have proven their value, especially in the semiconductor, medical engineering, and robotics industries. But they also have value in less obvious applications.

Recently, Kaydon Reali-Slim bearings were selected for use in an innovative bicycle automatic gear shift mechanism. The gear shift is currently being developed by students at the HTL Rennweg, the Higher Technical College of Vienna in Austria.

"As part of our final examination project for graduating from HTL Rennweg, my fellow students Stefan Neunteufl, Philipp Kraftl and I decided to develop an automatic gear shift mechanism for a bicycle," says Thomas Wendl, project manager for the CVT (continuously variable transmission) bike. "Our objective is to design and build a CVT gear shift mechanism with the required size and transmission ratio."

An innovative CVT (continuously variable transmission) bicycle being developed by students in Vienna, Austria employs Kaydon Reali-Slim® thin section bearings
(Photo courtesy of Rodriguez GmbH)

With a CVT transmission, the transmission ratio can be changed steplessly. The transmission consists of four conical pulleys connected by 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," explains Stefan Neunteufl. "In manual mode, the rider sets the gear ratio by hand. In automatic mode, the gear is shifted automatically; the rider does not have to worry about it anymore."

A CVT gear shift mechanism enables the transmission ratio to be changed steplessly—when the distance between the first pair of pulleys becomes smaller, it increases on the other side
(Photo courtesy of Rodriguez GmbH)

The 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, and 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.

The CVT gear shift mechanism's mounting design renders complicated structural changes to the bicycle unnecessary
(Photo courtesy of Rodriguez GmbH)

In search of: bearings with highly compact construction

The students discovered Kaydon Reali-Slim bearings on the internet, and sent an enquiry to Kaydon's German distribution partner, Rodriguez. Two ball bearings are needed in the CVT gear shift. One bearing is fitted in the moving element 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 second bearing stabilizes the trapezoidal spindle that is connected to the servodrive. A nut provided on the spindle is designed to shift the pulley.

In all, two ball bearings are needed in the CVT gear shift (one bearing in the moving element and one stabilizing the trapezoidal spindle)
(Photo courtesy of Rodriguez GmbH)

It was particularly important that both bearings take up very little installation space. Additionally, the larger of the two bearings must 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. The thin section bearings from Kaydon and their highly compact construction represented the ideal solution. The HTL Rennweg students ultimately chose Kaydon Reali-Slim thin section bearing types KA025AR0 and KAA10AG0. These angular contact ball bearings feature ball pocket separators and no seals, and have a cross section of just 0.25" and 0.1875" respectively.

Despite miniaturization, Kaydon thin section bearings exhibit performance and durability capabilities comparable to regular roller bearings
(Photo courtesy of Rodriguez GmbH)

Small and lightweight—with no loss of performance

"These separable bearings have a contact angle of 30°, a deep race on the inner ring, and an outer ring which is reduced on one side," explains Ulrich Schroth, division manager for value added products at Rodriguez. 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. "These bearings are capable of withstanding both radial and unidirectional axial forces despite their compact construction, so they are perfectly suited for use in the CVT bike," says Schroth.

Angular contact ball bearings withstand both radial and unidirectional axial forces, making them ideal for the CVT bike
(Photo courtesy of Rodriguez GmbH)

Each Kaydon bearing 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 four-point Reali-Slim thin section 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, including hybrid variants with ceramic balls, Kaydon has the optimal solution for every design engineering problem.

The students received the Kaydon thin section bearings from Rodriguez for cost. "We are delighted to promote innovative ideas," says Ulrich Schroth. "After all, today's talented students are tomorrow's expert technicians." He notes that Rodriguez has supported the Formula CAT-Racing Student Team at Coburg University for years.

The HTL Renwegg students' CVT bike may not reach Formula 1 speeds—but it will get top marks for riding comfort.