Disadvantages of Hydrodynamic Bearings or journal bearings

Disadvantages of Hydrodynamic Bearings

• One major disadvantage of hydrodynamic bearings is that a certain minimum speed is required to generate a full fluid film that completely separates the sliding surfaces. Below that speed, there is mixed or boundary lubrication, with direct contact between the asperities of the rubbing surfaces. 

• For this reason, even if the bearing is well designed and successfully operating at the high rated speed of the machine, it can be subjected to excessive friction and wear at low speed, such as during starting and stopping of journal rotation.

• In particular, hydrodynamic bearings undergo severe wear during start-up, when they accelerate from zero speed, because static friction is higher than dynamic friction.

• A second important disadvantage is that hydrodynamic bearings are completely dependent on a continuous supply of lubricant. If the oil supply is interrupted, even for a short time for some unexpected reason, it can cause overheating and sudden bearing failure. 

• It is well known that motor vehicle engines do not last a long time if run without oil. In that case, the hydrodynamic bearings fail first due to the melting of the white metal lining on the bearing.

• This risk of failure is the reason why hydrodynamic bearings are never used in critical applications where there are safety concerns, such as in aircraft engines. 

• Failure of a motor vehicle engine, although it is highly undesirable, does not involve risk of loss of life; therefore, hydrodynamic bearings are commonly used in motor vehicle engines for their superior performance and particularly for their relatively long operation life.

• A third important disadvantage is that the hydrodynamic journal bearing has a low stiffness to radial displacement of the journal (low resistance to radial run-out), particularly when the eccentricity is low.

•  This characteristic rules out the application of hydrodynamic bearings in precision machines, e.g., machine tools. 

• Under dynamic loads, the low stiffness of the bearings can result in dynamic instability, particularly with lightly loaded high-speed journals.

• The low stiffness causes an additional serious problem of bearing whirl at high journal speeds. 

• The bearing whirl phenomenon results from instability in the oil film, which often results in bearing failure.

Further discussions of the disadvantages of journal bearing and methods to overcome these drawbacks are included in the following POSTS.