The Heavy load carrying rollers von Walther Flender are used as skid conveyor elements for the transport of vehicles from the body shop to the paint shop to the vehicle assembly and must withstand high loads. Installed in it Tolerance sleeves from Dr. Tretter, which frictionally connect the pulleys for the drive with the tubes on which later the transport carriages for the vehicles run.

Walther Flender GmbH in Dusseldorf is one of the leading manufacturers of timing belt drives and has been listed as a manufacturer of heavy duty idlers in the automotive industry for over 30 years. "The competition does not sleep and the automobile market is subject to constant change," sums up Andreas Schmitz, Group Leader Technology at Walther Flender GmbH in Düsseldorf. The mechanical engineering company has been around for 85 years and employs about 300 employees at seven production sites in Europe and Asia. "Our core business is the development and production of timing belt drives, drive groups, geared motors, clutches or drive components such as our heavy-duty idlers," says Andreas Schmitz. Among other things, the group leader sees challenges in the development of electromobility, as this influences the weight of the vehicles and thus the loads on the idlers. 

Test bench in the test laboratory

"In order to be able to assert ourselves in this very tough competitive environment, we have to work extremely economically," says the group leader, pointing to a test bench in the test laboratory. "Here we simulate the actual use of our idlers." A sledge, also called skid, moves from left to right and back again in continuous operation on the runway. The load on the rollers can be adjusted according to the customer requirements. Walther Flender has already designed several thousands of carrying roller variants.

The "M-Flex" series, which has been used successfully in numerous applications, consists at its core, for example, of the polyamide PA66 and is overmoulded with a bandage made of thermoplastic polyurethane. If the requirements in terms of transport load are increased, the transport weight can also amount to several tons. In these cases, the core is made of steel. In the case of the classic tube variant, the running body and toothed disc components are installed as a fixed unit on the support tube and then pushed together onto an axle.

Wear-free spindle nut made of plastic for heavy loads

"Generally, we try to use standard components," says the group leader. "Thanks to the standardized construction method, we are able to shorten the delivery time considerably." The Dusseldorfer always produces shafts and pipes to customer specifications. Of course, the standards are customized by reworking. Not infrequently, however, the geometry deviations are so great that they would not be covered by the normal standard. But even in this case, Walther Flender supports its customers and, if necessary, develops tailor-made individual solutions.

Material or form fit decide on risk of breakage

“The worst thing that can happen to our idlers in use is to break them,” says Andreas Schmitz. Then operations come to a standstill, which can be expensive for the user. This probability is highest at the joint between the toothed lock washer and the pipe. “Because here, among other things, the permanent rotation, the weight of the slide or the tensions that the Timing belts causes the highest alternating bending loads, ”explains the designer Dennis Fundke. "Each type of connection has its justification, but it must be carefully selected for the respective application."

The engineers were aware of the tolerance sleeves of Dr. Ing. Petter decided. These components are placed in the toothed disks and thus connect the two components positively with each other. You can transfer the peripheral force of the toothed belt to the pulley and from there to the pipe. "When welding, on the other hand, the structure of the materials changes. As a result, the fatigue strength suffers and there is an increased risk of breakage due to the high bending cycle moments at the connection point. This danger goes with the tolerance sleeves to zero, "says Fundke. In addition, the tube must be sized larger when welding. In addition, suffer from the distortion, the concentricity properties.

Small components with great effect

Tolerance sleeves are slotted sheet metal sleeves. Corrugations like wave crests, which look like small ones, are embossed into these over the entire circumference compression springs Act. The wave crests are elastically deformed by a certain oversize of the component. This creates a frictional connection between the bearing and the shaft. “With this type of connection, unlike welding, we can easily detach the connection again - without it being destroyed as is the case with welding,” explains Mr. Fundke. The assembly speed is also much higher. The fitter only needs a small pressing tool, and he can dismantle it with a small puller.

If the cohesive welded joint is broken, there is no second chance for the assembly. The tolerance sleeve on the other hand - should they slip through - then resumes after work. In addition to welding, clamping sets could also be used. "That would be much more expensive than the use of tolerance sleeves and requires significantly more assembly work," says Andreas Schmitz. Due to the extreme interference fit the fatigue strength also suffers. In contrast, the interference fit of the tolerance sleeve is low, because the spring steel nestles relatively gently over the shaft.

Available in different designs

Tolerance sleeves in various designs are available. In the unitary shaft (AN type) system, the wave profile projects inward. The ends of the tolerance sleeve are open. It is fixed in a groove in the bore and pressed onto a unit shaft (h9). The gap narrows when pressed with the shaft. In the unit bore system (BN type), the wave profile protrudes outwards. Overlap the ends of the tolerance sleeve. It is fixed in a groove on the shaft and pressed into a unit bore (H9). "For the toothed pulleys, we prefer to press the AN tolerance sleeve onto the support tubes," says Schmitz. In some cases, the hubs of the rollers are connected to the pipe or shaft by means of a BN tolerance sleeve. "Here on the test bench, we can swap them easily if the component fails or after the trials are over."

The components are installed centered. The AN sleeve is inserted into a groove in the bore and the BN sleeve in a groove in the shaft. The groove width is given to guarantee an ideal connection. Even when mounting this installation is more advantageous because the inserted into the groove sleeve can be supported on the side surfaces. Deformation due to tilting can thus be avoided. The spring effect automatically results in a certain self-centering and a compensation of center offsets. Furthermore, the spring properties reduce vibrations and vibrations and thus contribute to noise reduction.

In terms of economy unsurpassed

Walther Flender standardized the tolerance sleeves in the heavy-duty idlers. "They are predestined to transmit small and medium torques in this application," says the group leader. Many customers are so convinced of the use of tolerance sleeves at Walther Flender GmbH that they have defined this connection solution as the standard that marketers of the machine manufacturer must adhere to.

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