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Tolerance Compensation of NORGLIDE
A mark of quality in any product is the ability to perform consistently from part to part. This is how a company can build a brand by having a consistent and smooth feel when parts are actuated. Often, this can lead to a difficult decision: consistent performance can be guaranteed through precision manufacturing but this comes at a significantly increased cost.
With NORGLIDE® bearings, Saint-Gobain offers a solution to this dilemma due to the forgiving configuration of the low friction PTFE layer. This layer is able to absorb tolerance stack ups and perform consistently for a wide range of customer part tolerances, resulting in less total cost for our customers without the need to compromise on performance.
(See Misalignment Compensation and Tolerance Compensation contents below for more details.)
Two major considerations of performance throughout a tolerance range are assembly force and torque. If the parts are made with too much interference due to a tolerance stack-up, then both the assembly force of the shaft into the housing-mounted bearing and the torque to turn the shaft within the bearing can become too high. Not only this but the force and torque will be quite different compared to that of the same system but in the opposite tolerance extreme, i.e. very low interference or even clearance.
Figure 1 shows the assembly, or press in, forces of a shaft into a housing-mounted bearing for five different sizes of pin, simulating a tolerance range of 200 µm. As can be seen the NORGLIDE® solutions are significantly superior in maintaining a consistent assembly force for the different pin sizes. Two of the competing PTFE-based solutions failed as the pin (shaft) diameter was increased as the assembly forces became too high.
The torque was also tested for different pin sizes. The torque is generated purely from the radial surface of the bearing. Torque can also be generated by applying an axial force to the flange. The results can be seen in Figure 2. A similar result as the assembly test is obtained, as expected. This shows a consistent performance of the NORGLIDE® bearings throughout the 200 µm range whilst the alternative PTFE-based solutions increased significantly. Obviously if the pin couldn’t be assembled in the press-in force test for high interference, then they could not be tested for torque either.
failed after 0.10 mm and 0.15 mm interference
Figure 1– The press in forces of pins of varying diameters into the bearing mounted in a housing. NORGLIDE® bearings perform consistently throughout the pin dimension range whereas the competing solutions press in forces increase significantly for higher diameter pins. Competing solutions 1 and 3 both fractured at 10.10 mm and 10.15 mm respectively. Test conditions: housing and pin are dummy components made out of hardened steel; the assembly speed is 1 mm/s.failed after 0.10 mm and 0.15 mm interference
Figure 2– The torque generated by the radial surface of the bearings for a range of pin diameters. The NORGLIDE® solutions offer consistently low torque when compared to the competing solutions. Competing solutions 1 and 3 could not be tested at higher pin diameters because they fractured during assembly. The housing and the pin are dummy components made out of hardened steel. Test conditions: housing and pin are dummy components made out of hardened steel; sliding speed = 45°/s; axial load = 0 N; test stroke = 90°; cycles = 5.
Some systems are controlled by electric motors which require software to run. In these applications, consistent performance will decrease the complexity of the software. One input parameter for the software will be the speed of actuation. If the bearing’s torque changes too much for different speeds, then calculating the right amount of input power to the motor can become a difficult task. Therefore, it gives confidence to our customers to know that we test our bearings with such detail that sliding speed is considered, especially as PTFE performance is sensitive to speed and pressure. Figure 3 presents the change in torque for three different angular speeds, 45°/s, 90°/s and 135°/s. The stability of torque over the three speeds is significantly better than all but one of the competing solutions. This allows confidence for our customers that not only will their systems have a consistent feel but also that the software doesn’t have to become needlessly complex.
Figure 3 – left: torque generated by the bearings for three different speeds (45°/s, 90°/s and 135°/s). This test highlights the choice in torque level our customers have by choice of bearing. Right: the same test as the left graph but plotted as a change in torque between the 3 data points per material (Δ torque) to highlight the consistency of the bearings when the speed changes. NORGLIDE® bearings perform consistently when compared with all but one of the alternative PTFE-based bearings. Test conditions: housing and pin are dummy components made out of hardened steel; pin size = 10.05 mm; axial load = 0 N; test stroke = 90°; cycles = 5.
The impressive tolerance compensation qualities exhibited by NORGLIDE® bearings are achieved due to the unique composition of the products. The PTFE layer seen in Figure 4 has the ability to absorb tolerance stack-ups of mating components. The low friction layer also has the benefit of reducing the impact of wear on the performance throughout the lifetime of the system.
Figure 4– NORGLIDE® T (left) and a sintered bronze and PTFE bearing (right). The PTFE layer on the NORGLIDE® solutions has a unique structure which allows greater compensation of tolerance and more consistent performance over time than sintered bronze and PTFE bearings.
Misalignment of an assembly can cause issues for manufacturers such as, increased assembly forces, excessive torque, and higher wear rates. These issues can diminish the perceived quality of a system.
An example of misalignment is found in the seat track mounting in the automotive industry. The tier 1 seat manufacturer could design a perfectly aligned seat, however the mounting holes in the car body in white can be misaligned which can distort the shape of the seat. Figure 5 shows a mechanism in which the distortion of the seat can produce misalignment between two housings that hold the seat cross-tube. Misalignment in this case can cause excessive torque and wear when the seat is adjusted.
Figure 5: left - the diagram above shows a mechanism of misalignment where two housings with a shaft running between them are misaligned. Right - when the shaft is rotated within the bearing, torque is produced. Misalignment between the two housings can result in higher torque values. Saint-Gobain engineers replicated the test as shown.
Saint-Gobain engineers replicated the test as shown in Figure 5. One of the housings was misaligned by varying amounts and the torque to turn the shaft was measured. This was performed on different bearing solutions. The resulting torques are shown in Figure 6. The plastic POM bushing showed a significant increase in torque, even at low misalignment values. The NORGLIDE® bearings show a great improvement in torque consistency throughout the misalignment range. Particularly, the NORGLIDE® MP material shows excellent misalignment compensation properties.
Where plastic bushings have a hard and unforgiving surface, our unique NORGLIDE® bearings maintain their low friction properties of the PTFE, as shown previously, which helps to minimise issues caused by misalignment in assemblies.
Figure 6: The graphs above show the resulting torque under different values of misalignment between two housings. POM plastic bushings show a significant increase in torque, even at low value of misalignment. NORGLIDE® bearings show far greater compensation for misalignment with the NORGLIDE® MP material performing best.