Roller bearings are an essential component in many rotating systems that require low friction. The choice of bearing mount method can have a significant effect on the roller bearing’s ability to perform. Press-fitting is a common method that introduces problems such as high assembly forces, loss of retention force of the mounted bearing at high temperatures and precision manufacturing costs.
RENCOL® Tolerance Rings can offer a choice of benefits as a bearing mount such as enabling the relaxation of machining tolerances, reduction of press fit forces compared to conventional interference fit into the housing or reduction of the noise level. It should be noted that not all the benefits can always be combined as there are cases when a noise reduction can’t be achieved in conjunction with relaxed tolerances as the dynamic stiffness has a high dependency on the clearance between the housings and the mounted bearings. Therefore, it is important to work closely with our customers to ensure the best solution based on our experience and expertise.
Figure 1: A motor assembly showing the roller bearing mounted with a RENCOL® Tolerance Ring. Right – A hard disk drive exploded assembly view showing the roller bearing mounted with a RENCOL® HDD solution.
Using a RENCOL® Tolerance Ring as a bearing mount solution allows the relaxation of tolerances and the precision requirements of the bearing housing. In an example case the tolerance on the housing was relaxed from IT6 to IT11. For a bearing housing with a 22 mm diameter, this equates to going from a 13 µm tolerance to 130 µm. By eliminating the precision turning process, a cost saving for the manufacturer for a batch size of 100,000, including the tolerance ring cost, is about $0.04 to $0.06 depending on the final design. Based on the batch size, this equates to a direct saving of $6,000 per batch. This study was calculated using a thermosetting plastic housing, the potential savings for metal housings will be greater.
Costs due to scrap can also be reduced when replacing an adhesive solution with a RENCOL® Tolerance Ring because failed assemblies can be reworked instead of scrapping the whole housing. Eliminating adhesive will also eliminate glue curing times saving assembly time as well as cost.
A mega-trend in automotive and other industries is the shift to lightweight materials such as aluminium. Using press-fits with different materials can lead to problems because of differential thermal expansion over the required operational temperature range. This is known to cause issues because Aluminium housings will expand at a higher rate than steel bearings reducing the interference fit. This can lead to a potential for components to become loose and lead to rattling noise.
RENCOL® Tolerance Rings work in the elastic range of the waves, providing higher retention forces of mounted bearings than a press fit solution at higher temperatures (Figure 2) and confidence to our customers where thermal expansion could otherwise cause issues.
Figure 2: A calculation of the push-out force of a steel roller bearing mounted in an aluminium housing, causing differential thermal expansion. The results show two mounting solutions, a RENCOL® Tolerance Ring and a press-fit. RENCOL® technology offers more consistent and higher retention forces at increased temperatures.
In cases where a system has an excessive noise level or other NVH issues, a RENCOL® Tolerance Ring can be used to help reduce the overall noise level. Our solutions can be tuned for dynamic stiffness, enabling us to provide a part that can isolate problem frequencies. Figure 3 shows that for an electric motor with the sound pressure level of the system can be reduced by 4 dB(A) for RENCOL® Tolerance Ring bearing mount design.
Figure 3: Sound pressure levels of an electric motor with and without a RENCOL® Tolerance Ring as the bearing mount solution. Each configuration was tested 5 times and an average was taken. The motors were tested at 1000 RPM. A 4 dB(A) reduction was achieved using the RENCOL® solution.
For a rotating system, one way that quality can be assessed is the consistency of torque through the full 360° rotation of the system. This can be adversely affected by deformation of the outer bearing race if uneven external pressures exist. This might manifest itself as a notchy steering feel for a steering column or a first order vibration mode in a motor driven system.
RENCOL® Tolerance Rings help avoid this by spreading out the uneven external forces onto the outer bearing race. Figure 4 shows finite element analysis (FEA) of the deformation of the bearing outer race. In this application the housing wall thickness is uneven around the circumference of the bearing. This means inconsistent stiffness which results in uneven pressure loading. The analysis shows the RENCOL® solution spreads out the loading onto the bearing, meaning it can provide more consistent torque as the system rotates.
Figure 4: Finite element analysis of the deformation of an outer bearing race due to uneven external pressures for a press fit bearing (left) and a RENCOL® Tolerance Ring solution (right). The RENCOL® solution results in a much more consistent pressure around the outer race of the bearing.
RENCOL® solutions also offer good concentricity as the tolerance ring tends to centre a bearing or other parts with each wave pushing towards the centre. With a free mounting arrangement eccentricity will usually be in the region of 2-3% of radial clearance.
A tolerance ring isn’t just a stand-alone component; it is an integral part of a larger system that means a detailed, knowledge-based approach is needed for tolerance ring design. That is why a RENCOL® Tolerance Ring is designed in collaboration with our customers, engineer to engineer. This process is made more powerful by using our FEA capabilities along with in-house testing to analyse the system, with mating components included and under key conditions. This means what we and our customers can have confidence in the solutions we provide.