Boundary Lubrication

Boundary lubrication in marine diesel engine bearings occurs during start up and stopping, relatively slow speeds, high contact pressures, and with less than perfectly smooth surfaces. As running conditions become more severe such as with rough surfaces, and high contact pressures, wear becomes a severe problem to the system.

With mineral oil,  it is possible to create a lubricant that forms a surface film over the surfaces, strongly adhering to the surface. These films are often only one or two molecules thick but they can provide enough of a protection to prevent metal to metal contact.

Boundary lubricating conditions occur when the lubricant film is insufficient to prevent surface contact.  This results in bearing wear and a relatively high friction value.

Hydrodynamic Lubrication

Fluid film or hydrodynamic lubrication is the term given when a shaft rotating in a bearing is supported by a layer or wedge of oil so that the shaft is not in contact with the bearing material.

The principle which allows large loads to be supported by this film of oil is similar to that which causes a car to aquaplane on a wet road surface. Aquaplaning occurs when water on the road accumulates in front of your vehicle's tyres faster that the weight of your vehicle and the pumping action of the tyre tread can push it out of the way. The water pressure can cause your car to rise up and slide on top of a thin layer of water between your tyres and the road. Just like a car has to be travelling at a certain speed before aquaplaning will occur (this varies according to tyre condition and road surface but is generally about 55mph), a shaft must be rotating at a certain speed before hydrodynamic lubrication takes place.

Hydrodynamic lubrication was first researched by Osborne Reynolds (1842-1912).When a lubricant was applied to a shaft and bearing, Reynolds found that the rotating shaft pulled a converging wedge of lubricant between the shaft and the bearing. He also noted that as the shaft gained velocity, the liquid flowed between the two surfaces at a greater rate. This, because the lubricant is viscous, produces a liquid pressure in the lubricant wedge that is sufficient to keep the two surfaces separated. Under ideal conditions, Reynolds showed that this liquid pressure was great enough to keep the two bodies from having any contact and that the only friction is the system was the viscous resistance of the lubricant.

The operation of hydrodynamic lubrication in journal bearings is illustrated below.   Before the rotation commences the shaft rests on the bearing surface. When the rotation commences the shaft moves up the bore until an equilibrium condition is reached when the shaft is supported on a wedge of lubricant.  The moving surfaces are then held apart by the pressure generated within the fluid film.   Journal bearings are designed such that at normal operating conditions the continuously generated fluid pressure supports the load with no contact between the bearing surfaces.   This operating condition is known as thick film lubrication and results in a very low operating friction and extremely low bearing load

The rotating shaft drags a wedge of oil beneath it that develops a pressure great enough to support the shaft and eliminate contact friction between the shaft and bearing

Viscosity of the lubricant is an important feature. The higher the viscosity, the higher the friction between oil and shaft, but the thicker the hydrodynamic film. However friction generates heat, which will reduce the viscosity, the thickness of the film and may result in metal to metal contact. Using an oil with a low initial  viscosity will also result in a reduced oil film thickness. We have to be very careful that the distance between the two surfaces is greater than the largest surface defect. The distance between the two surfaces decreases with higher loads on the bearing, less viscous fluids, and lower speeds. Hydrodynamic lubrication is an excellent method of lubrication since it is possible to achieve coefficients of friction as low as 0.001 (m=0.001), and there is no wear between the moving parts. However because the lubricant is heated by the frictional force and since viscosity is temperature dependent, additives to decrease the viscosity's temperature dependence are used. The oil of course is cooled before it is pumped back through the engine.

Thrust Bearing

In a thrust bearing, the surfaces have to be such that a converging wedge of fluid can develop between the surfaces, allowing the hydrodynamic pressure of the lubricant to support the load of the moving surface.   This is obtained in a number of ways, a common design is the tilted pad bearing, where a tilted pad skims over a sheet of fluid. This was developed by Australian engineer George Michell in 1905. Albert Kingsbury, an American, simultaneously and independently invented a bearing operating on the same principle.   It should be noted that the movement is relative. On some bearings the pads are stationary and the thrust collar rotates (used on most marine diesel applications), whist in some applications the pads rotate against a fixed collar   This principle is the one used on main propulsion thrust bearings

For some smaller thrust bearings, the wedges are machined into the bearing as shown. This photo illustrates the thrust bearing from a MAN B&W turbocharger.

The following related articles are available in the Members Section

Thickwall Bearings

Main Bearing Design

Bottom End Bearing design

Crosshead Bearing Design

The Piston pin Bearing

 The Thrust Bearing

Bearing Faults