As the crankshaft turns, it drags oil into a narrowing gap between the journal and the bearing shell. This creates hydrodynamic lubrication, where oil pressure inside the film supports the load much like water can support a fast moving boat hull. Bearing clearance must be small enough to build oil pressure but large enough to allow oil flow and thermal expansion.
If the oil film breaks down, metal contact can quickly damage the bearing and crankshaft.
Key Facts
- Friction force can be estimated by Ff = μN, where μ is the coefficient of friction and N is the normal force.
- Engine speed is often measured in revolutions per minute, rpm, and 3000 rpm means 3000 crankshaft rotations each minute.
- Bearing clearance is the small gap between the crankshaft journal and bearing shell, often about 0.02 mm to 0.08 mm in many engines.
- Hydrodynamic lubrication forms when a moving surface drags oil into a wedge shaped gap, building pressure in the oil film.
- Oil viscosity is a measure of resistance to flow, and it changes strongly with temperature.
- Power lost to friction can be calculated by P = Fv, where F is friction force and v is sliding speed.
Vocabulary
- Crankshaft journal
- A smooth round surface on the crankshaft that rotates inside a bearing.
- Bearing shell
- A curved replaceable metal liner that supports a rotating shaft while matching its shape.
- Oil film
- A thin layer of oil that separates moving metal surfaces and carries load.
- Hydrodynamic lubrication
- A lubrication condition where motion pulls oil into a gap and creates enough pressure to keep surfaces apart.
- Bearing clearance
- The small space between the shaft journal and the bearing surface where oil flows.
Common Mistakes to Avoid
- Thinking the crankshaft should touch the bearing shell during normal running is wrong because proper operation depends on an oil film separating the metal surfaces.
- Using oil that is much too thick or too thin is wrong because viscosity affects how well the oil flows, cools, and forms a load carrying film.
- Ignoring bearing clearance is wrong because a gap that is too small can restrict oil flow, while a gap that is too large can lower oil pressure and reduce support.
- Assuming bearings only reduce friction is wrong because they also position the crankshaft, carry combustion loads, conduct heat, and allow controlled oil flow.
Practice Questions
- 1 An engine runs at 2400 rpm. How many crankshaft rotations occur in 10 seconds?
- 2 A crankshaft journal has a diameter of 50 mm and the bearing inside diameter is 50.06 mm. What is the total diametral clearance, and what is the radial clearance on one side?
- 3 Explain why an engine bearing can be damaged quickly if oil pressure is lost, even though the bearing surfaces are made of metal.