Peristalsis in the opposite direction of rotation
Soda described 2 mechanisms of bearing-ring creep opposite to the bearing's direction of rotation. The easier of the 2 mechanisms to understand is caused by clearance between the bearing ring and its mating shaft or housing (Figure 1). When a rotating load acts on the bearing, the changing load direction causes relative movement between the ring and the mating housing surface. If the clearance between the mating surface and ring is c, the ring lags by πc for each bearing revolution, producing creep opposite to the bearing's direction of rotation.
Figure 1 Peristalsis caused by clearance
Another mechanism that produces creep is elastic deformation and slippage of the ferrule when there is no gap. Imai proved the existence of this mechanism through experiments, while Soda explained the peristalsis phenomenon through examples, as shown in Figure 2. The tea cylinder is covered with a rubber belt, and the slight sliding between the rubber belt and the tea cylinder at the loading position moves forward cumulatively as the tea cylinder rolls. Initially, the rubber belt deforms and stretches in the circumferential direction due to the load of the tea cylinder.
The deformation is uniform and the sliding is consistent at both left and right ends. From this, it can be concluded that movement in one direction cannot produce creep. However, when the tea cylinder rotates, the deformation caused by the previous sliding is still maintained, causing asymmetry, because the increase in sliding in one direction causes peristalsis, and the peristalsis direction is opposite to the rolling direction of the tea cylinder.
Figure 2 Peristalsis caused by sliding rubber belt
Creeping in the same direction as rotation
Ten et al. explained that "the force that causes the outer ring to rotate in the circumferential direction is caused by the strain generated each time the rolling elements pass by, which pushes the outer ring forward a little bit, eventually producing creep." The principle is shown in Figure 3. This mechanism explains that peristalsis occurs when the accumulated strain moves forward on the circumference, which just confirms Soda's explanation and can be considered a reflection of the actual phenomenon.
Figure 3 Strain-induced creep
traveling wave peristalsis
Tsuyoshi assumes that traveling waves are generated on the surface of the ferrule, and the traveling waves move the ferrule and produce creep. In order to better explain the mechanism. It is introduced using the transmission mechanism of ultrasonic motors. In a disk ultrasonic motor, the traveling wave is excited on the surface of the elastomer by the piezoelectric element, which will produce an elliptical motion on the rotor in the opposite direction to the direction of the traveling wave on the surface of the elastomer. The transmission mechanism of ultrasonic motor is shown in Figure4.
Figure 4 Ultrasonic motor mechanism
According to Sakajiri et al., when the load from the rolling elements acts on the surface of the ring, the contact point between the rolling elements and the ring is immediately squeezed, and traveling waves are generated on the bearing. Surface fluctuations can reach several micrometers due to differences in load. When the bearing rotates, the rolling elements move simultaneously, and these surface waves become traveling waves.
As shown in Figure 5, the ferrule itself generates traveling waves, which causes the ferrule to squirm. According to the direction of creep, the ring acts on the bearing seat in the direction opposite to the direction of travel of the rolling elements. However, because the bearing seat is fixed, the rings are pushed back to travel in the same direction as the rolling elements, that is, they rotate in the same direction as the bearing rotates.
Figure5 Traveling wave creep of bearing rings
The traveling waves generated on the ferrule surface exhibit a peristaltic-like motion in the circumferential and radial direction of the entire loading area, which is characterized by a rotational motion in the opposite direction to the traveling waves at the mating surface located directly beneath the rolling elements. The phenomenon that the ferrule is transferred by this mechanism is called traveling wave peristalsis.
References:
[1]Soda: Bearings, Iwanami Shoten (1964), 196.
[2]Imai: Lubrication, 4 (1959) 307.
[3]Ten, Sakajiri, T akemura, Yukawa: NSK Technical Journal No. 680 (2006) 13.
[4]Sakajiri, Takemura: Proceeding of Japan Tribology Conference (2003-5) 103.
[5]Tsuyoshi NIWA.A Creep Mechanism of Rolling Bearings.2017.
(Source: Bearing Magazine)