The hydrodynamic bearing has the following characteristics:
- Lubricant film thickness and pressure are maintained by lubricant which is drawn into a wedge shaped zone between the journal and the bearing, due to their relative velocity and the viscosity of the lubricant.
- Lubricant flow conditions are laminar.
- Lubricant film thickness is sufficient to prevent metal to metal contact under all operating conditions.
The behaviour of a hydrodynamic bearing is described by the Reynolds equation:
where:
h = film thickness
p = hydrodynamic pressure
µ = absolute viscosity
u = surface velocity
z = direction transverse to rotation
x = direction parallel to rotation
Two phenomena exist which, unless appropriate precautions are taken, will disrupt hydrodynamic action on a large journal bearing. These phenomena are:
- The large journal diameter means that a single long bearing would have an exceptionally long lubricant wedge with high side leakage, and highly critical tolerance requirements.
- Large journals deflect significantly from the theoretical cylindrical shape under operating conditions.
Each shoe is mounted on a spherical pivot at the center of hydrodynamic pressure, so that the bearing shoes freely pivot to maintain optimal shoe/journal alignment.
The design of a large hydrodynamic bearing involves many factors which have a relatively complex interrelationship. In this relationship several implicit feedback loops exist. For example, increased lubricant temperature results in reduced viscosity which in turn may cause increased bearing friction which may further raise the lubricant temperature.
Laminar lubricant conditions typically occur when the Reynolds number is less than 2,000. At higher Reynolds numbers, laminar behaviour can not be guaranteed as initially transition, and ultimately turbulent flow occurs. This turbulent flow will then cause increased side leakage, reduced film thickness, increased friction, and ultimately result in lubricant film collapse.
The multiple pivoted shoe journal bearing described, by adapting to minor journal deflections, makes design of a practical bearing feasible for the large journal diameters described. However, it is still necessary to ensure that the geometry of the oil film wedge is not compromised by dimensional changes in the journal or the bearing shoes.
In order to operate successfully, the bearing must attain thermal equilibrium at a relatively stable temperature for all operating conditions. For a typical machine this temperature is in the range of 40 – 50 degrees C.
The lubricant viscosity is a critical element in the thermal equation. The viscosity of typical lubricants is considerably reduced at elevated temperature.
