LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils
1. Purpose and Function of the Lubrication System
1.4. Bearing Clearance and the Hydrodynamic Wedge
LO 10-02: Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils. |
Bearing Clearance and the Hydrodynamic Wedge
It is important to remember a few points about the hydrodynamic wedge and bearing clearances on plain bearings. First, the faster the shaft rotates as it is supplied oil, the greater the pressure separating the surfaces Figure 9. Next, the thickness of the oil film matters. Thicker, more viscous oil forms stronger hydrodynamic wedges than thinner oil, but thicker, more viscous oil requires more space to fit between a bearing and a shaft to form a strong wedge compared to thinner oils. There is also an optimal bearing oil clearance that matches the viscosity of an oil. Thicker, more viscous oil requires a larger oil clearance than thinner, less viscous oil. Too large a bearing clearance combined with oil that is too thin will not form adequate pressure for an oil wedge to support a heavy load. The result can be shaft-to-bearing contact. Last, if bearing-to-shaft clearances are made larger, a greater volume of oil passes through the bearing clearance and eventually leaks off at the edges of the bearing Figure 10. If more oil flows through a larger clearance, more energy is required to push that oil through a lubricating system to maintain oil pressure Figure 11. Note too that ball and roller bearings operate much the same way, except that they do not have as much bearing surface area to form as large a wedge as a crankshaft or camshaft plain bearing.
FIGURE 10-9: Thinner, less viscous oils require a smaller clearance between the bearing and the rotating shaft to provide adequate load support. An optimal bearing oil clearance exists for each viscosity of engine oil. |
FIGURE 10-10: As the bearing clearance increases for this viscosity of oil, more oil volume is required to maintain pressure due to oil leak-off. |
FIGURE 10-11: Thicker oils require larger bearing clearances and consume more engine power to pump. Power loss is due to higher oil pump volume and the effect of shearing chains of oil molecules at the bearing surface. |
| Technicians Tip | ||
| Understanding how the hydrodynamic wedge separates a shaft journal surface from a plain bearing explains why some engines last longer before wearing out. Higher constant shaft speeds will generate more hydrodynamic pressure, separating a bearing and journal surface. This means an engine operated in slow-speed, stop-and-start urban driving conditions will not only have less oil pressure in its hydrodynamic wedge but also more frequent but brief shaft-to-bearing contact. An engine operated at steadier and higher speeds and loads driven over a highway will have much less wear for the same travel distance. |
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