Hydrodynamic journal bearing - a complete guide

Hydrodynamic Journal Bearing – A Complete Guide | Features & Usage

This article will go over the advantages and disadvantages of using a hydrodynamic journal bearing, as well as the situations and uses where it is most suitable.

Journal Bearing

As their name suggests, journal bearings are the objects that allow journals (shafts) to move. Journal bearings are made of phenolic (this type is used for power generation applications) or non-metallic materials such as brass or bronze.

Journal bearings can be classified as either hydrostatic or hydrodynamic bearings. This article will focus on the hydrodynamic type of journal bearing.

Hydrodynamic Journal Bearing

Hydrodynamic journal bearing - features and usage

Hydrodynamic bearings have two axial grooves for lubrication and a cylindrical bore in most cases. This bearing’s straightforward design is bi-rotational, compact, and has a high load capacity.

It is also simple to manufacture. It was discovered that this bearing had restrictions that necessitated modifications as the design speeds of the machines increased.

Hydrodynamic bearings can be found in industrial applications including steam turbines, electric motors, cooling pumps, and rock crushers.

They are also commonly found on ships in the clutch, blowers, pumps, and auxiliary machinery.

1. Lubrication

For hydrodynamic bearings to function safely, a suitable lubricant must always be used. To eliminate the heat produced by the oil shear, the lubricant must be cooled, and it also needs to be warm enough to flow freely. Filtering is necessary to reduce the average particle size of the lubricant to below the necessary minimum film thickness.

2. Speed Limits

Hydrodynamic bearings only encounter viscous friction because a fluid film layer is sheared throughout the bearing’s motion. When moving quickly, the land may experience hydrodynamic effects if it is too wide, which could generate a lot of heat. The approximate top speed is one million DN. The DN number indicates the top speed in rpm (N) and bearing diameter in mm (D).

3. Accuracy

The components’ accuracy dictates how accurate these bearing types will be. Submicron or meter accuracy has been achieved in the design of hydraulic linear motion bearings.

4. Deflection and Stiffness

In contrast to sliding or rolling contact bearings that are preloaded against one another, hydrodynamic bearings do not experience the same problems with loss of contact. In terms of stiffness, they can easily fall within the Newton per nanometer range.

These bearings have superior damping properties in both normal and tangential bearing directions because of the thin oil film in the bearing gap. When air is used, the low-viscosity air film in the bearing gap gives bearings moderate to low damping capabilities in the normal and tangential bearing directions, respectively.

5. Thermal Efficiency

Hydrodynamic bearings get their energy primarily from flow under pressure. A drip pan receives the oil that exits the bearing. Since the flow rate and pressure of the fluid are practically zero in the pan, all the energy that the initial flow and pressure represented is used to shear the viscous fluid as it leaks out of the bearings. Heat is created as a result of this power. Depending on how much heat is conducted by the machine, the oil will warm up.

6. Dimension and Weight

The space requirements for hydrodynamic bearings could be substantial, despite how little space they take up. Taking into account only the size and weight of the pump, the oil collection/distribution system, and the oil temperature control system, they have very high performance-to-weight ratios.

7. Lifespan and Maintenance

The cleanliness and level of the oil must be checked, and the pump’s filter must be changed frequently. It is important to keep an eye on the quality of the oil to make sure that its pH level stays within the desired range and that it is not contaminated. Hydrodynamic bearings have essentially infinite lives because they are non-contact mechanical components.

The Hydrodynamic Bearing’s Limitations

Most failure modes for hydrodynamic bearings involve a breakdown of the lubricant or oil film (when the oil film thickness is less than the roughness of the bearing or shaft journal). Machines that use hydrodynamically lubricated bearings do not endure high loads during startup. 

Final Thoughts

To reduce friction when rotation is started in hydrodynamic bearings used in rotary applications, external pressure may need to be applied to one of the bearing pads or a secondary bearing. Radial or thrust loads can be supported by hydrodynamic bearing designs.

Personally, I believe that the pivoted shoe design is the most efficient when it comes to hydrodynamic journal bearings. Because the pivot of the shoe allows for wedge formation and rotation, a spherical pivot surface allows for rolling in any direction. Hydrodynamically moving pivoting shoe bearings have many benefits. Compared to other bearing designs, they are more effective, less expensive, require less maintenance, and last longer. 

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