Hydrodynamic Bearings are named so because of the lubricating principle involved in the bearings. Apart from the construction that acts as the bearing, the lubrication plays a very vital role in the construction to act as a bearing.
Before we proceed to look into what a hydrodynamic bearing is, let us know about Hydrodynamic lubrication.
Hydrodynamic Lubrication is formed basically due to the dynamic motion or action of the moving parts. When sufficient quantity of lubricant is present between two surfaces in which at least one surface tends to move, the relative velocity of the moving surfaces tends to pump the lubricant between the two surfaces separating the two surfaces by a dynamic film of the lubricant.
Typical Construction Features of a Hydrodynamic Bearing:
A typical hydrodynamic bearing consists of the following:
1. The rotating member – Shaft
2. The supporting bearing – Outer Sleeve
Working Principle of the Hydrodynamic Bearing:
1. Before the shaft starts to rotate, both the shaft and the outer sleeve are in contact due to the self weight as shown below in the figure.
2. As the shaft tends to rotate the lubricant in between the shaft and the sleeve tends to get squeezed and ultimately forms a boundary lubrication between the shaft and the sleeve but still a slight amount of metal to metal contact remains as shown in the figure.
3. As the shaft tends to rotate rapidly, the fluid lubricant is pumped by the shaft. The lubricant surface near to the shaft has a velocity which differs from the surface velocity of the oil surface near the sleeve. This causes a pumping action in the lubricant. The pumped lubricant ensures that the shaft and the sleeve remain out of contact. This phenomenon of the lubricant pumping itself to keeps two mating surfaces out of contact is termed as “Hydrodynamic Lubrication" and hence the name “Hydrodynamic Bearing". See Figure below.
This principle ensures that the shaft remains out of contact when required i.e. during running. But a problem exists in this, the oil in between the sleeve and shaft tends to flow towards you or away from you when you look at. Hence a copious-continuous flow of the lubrication has to be ensured for the bearing to work perfectly compensating the lubricant loss.
Applications of Hydrodynamic Bearings:
Though all of us may not have seen the hydrodynamic bearing, they tend to work in some of the harsh and well know environments to us.
· The crankshaft and camshaft bearings in an automobile engine is a very good example for Hydrodynamic bearing
· All hand operated lube oil pumps do posses hydrodynamic bearings.
Design and Selection Aspects:
This is a type of bearing where all parameters are to be selected by the designer for designing the bearing. This is more of a bearing made for a application rather than the one which is taken from off the shelf.
· An important note for the design of hydrodynamic bearings, the hydrodynamic bearings are good in handling radial loads but will not be able to carry any axial load. Hence necessary precaution is to be taken during design of hydrodynamic bearings for accommodating the axial loads generated.