The following is mainly for vacuum loading machine oil film dynamic pressure bearing, for air bearing part of the application.

1. First of all, a brief description of the principle of dynamic pressure sliding bearings:

 

In dynamic pressure sliding bearings, due to gravity, unbalance force, etc., journal (journal) relative to the bearing (housing) eccentric, resulting in convergence and divergence of the oil wedge, in the convergence of oil wedge pressure in the divergent oil The pressure in the wedge is reduced, resulting in bearing capacity.

 

2. Vacuum feeding machine cavitation phenomenon:

 

In the divergent wedge, if the continuous liquid model is calculated, the oil film pressure is often much lower than the ambient pressure. In practice, when the oil film pressure is slightly lower than the ambient pressure, the dissolved gas in the oil film becomes a bubble group, which becomes a two-phase flow in the divergent wedge, and the fluid pressure of the two phases is small, so that the oil film pressure is not further reduced , Basically stable at slightly below atmospheric pressure. Therefore, the oil film pressure is usually regarded as the ambient pressure in the diverging wedge. As shown below:

 

 

3. Why can not the dynamic pressure bearing run at low speed?

The bearing capacity of the dynamic pressure bearing is positively correlated with the rotational speed. When the bearing capacity is constant, the lower the rotational speed, the closer the journal is to the bearing pad, since the wedge requires a greater shrinkage to produce the bearing capacity; When the journal and the bearing close to the scratch, there will be scratched or even locked. So the dynamic pressure bearing running speed can not be too low.

 

4. What prevents high-speed operation of dynamic pressure bearings?

in short:

A. For low-speed heavy-duty vacuum feeders Large rotor bearings (gas turbines, medium and large compressors, etc.), the upper limit of speed is limited by bearing stability;

B. For high-speed light-load small-sized rotor bearings (such as vehicle turbochargers, etc.), the upper limit of the speed is mainly limited by the oil film temperature.

The following two sections are analyzed in detail.

 

5. Oil film bearing stability

 

The support force of the oil film bearing is not directed to the center of the bearing pad, that is, the direction of the force and the direction of the journal eccentricity is not on a line, there is an angle. And this angle increases with the reduction of the crest of the bearing! The angle increases, which means that the bearing support force along the tangential component of the proportion of increase, not rigorously said that the increase in tangential force will have a tendency to produce precession, when the crest centrifugal rate is small to a certain extent, They began to violent precession, the general term is called oil film vortex. The front mentioned that under the same bearing capacity (usually gravity), the higher the speed, the smaller the crest centrifugation, so more and more about instability, high speed to a certain extent, the oil film vortex, the rotor violent vibration, The rotor, which is terrible! Therefore, in general, low-speed heavy-duty bearing stability, stability is to limit the upper limit of large rotor bearing the most important factor.

 

The following figure is an example of my simulation, the black circle for the bearing gap space, blue line for the journal trajectory. I set the initial position of the journal and concentric with the bearing, and then release the journal, under the action of gravity began to move at low speed can finally stabilize in the equilibrium position, but at high speed, the sharp vortex occurred.

 

6. High-speed light load bearing & oil film temperature:

For high-speed light-load bearings, stability requirements are not suitable, because its working speed is much higher than the "critical stability speed", so the vortex is inevitable, but fortunately for small rotor, whirlwind battle is not to large rotor So scary, as long as the vortex amplitude is not too much, or can be used normally. But also through various methods to control the eddy amplitude, in the vehicle turbocharger, the bearing using floating bearing structure. The structure of a double layer of oil film, which is mainly to provide the outer layer of oil film damping, absorbing vibration energy, so that greatly reduced eddy amplitude. As shown below:

 

So the stability is not high-speed light load bearing to consider the problem, but because the speed is further increased, the oil film temperature will rise sharply in the vacuum feeder limit, the highest point of oil film temperature has reached 190 ℃, Good flash point of the oil is only 210 ℃, and then raise the temperature, the oil will be a lot of carbonization, blocking bearing clearance. Therefore, for small vacuum feeder high-speed light load rotor bearings, temperature is the limiting factor of speed.