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.
Understanding Automotive Technology: How Engine Bearings Work
Most engine crankshaft bearings are plain bearings, not ball bearings. They use two thin metal shells that sit inside a precisely machined housing. Main bearings support the crankshaft in the engine block.
Connecting rod bearings sit between each rod and its crankpin. The shells are made in layers. A steel backing gives strength.
Softer metal layers provide a surface that can tolerate tiny dirt particles and brief contact during difficult conditions. Small locating tabs stop the shells from turning during assembly, but the tight fit in the housing does most of the holding.
Oil reaches these bearings through a planned path inside the engine. The oil pump draws oil from the sump through a pickup screen. It sends the oil through a filter, then into drilled passages called oil galleries.
Holes in the block feed the main bearings. Holes inside the crankshaft carry oil onward to the rod bearings.
This route means a problem at the pump, pickup, filter, or gallery can affect many moving parts at once. Oil pressure readings give useful information, though pressure alone cannot prove that every bearing has enough oil flow.
The hardest moments for a bearing often occur during starting and stopping. Before the engine begins turning, the full protective oil layer is not established. A thin oil coating remains on the surfaces, but some tiny high spots may touch.
This is called boundary or mixed lubrication. Modern engine oil contains additives that protect metal during these moments. Cold weather makes oil thicker, so it moves more slowly at first.
Very hot oil becomes thinner and may provide less separation under heavy loads. The correct oil grade helps the engine work across its expected temperature range.
Bearing damage usually leaves clues before total failure. A deep knocking sound that follows engine speed can point to excessive clearance at a rod bearing. Low oil pressure, glittering metal particles in drained oil, or copper colored material in the oil filter are serious warning signs.
Overheating can discolor a bearing shell. Dirt can scratch it. Fuel mixed into the oil can reduce its ability to carry load.
Lack of oil can make the shell overheat and seize to the crankshaft journal. The bearing may then spin in its housing, damaging the connecting rod or engine block.
When learning this system, focus on the link between motion, heat, oil flow, and clearance. Faster crankshaft speed increases sliding speed at the bearing surface. Heavy acceleration increases the load from combustion and inertia.
Both conditions make good lubrication more important. Understand that engine oil does several jobs. It reduces friction, carries heat away, transports contaminants to the filter, and protects surfaces from corrosion.
During service work, cleanliness matters greatly. A particle that seems tiny to the eye can be large enough to score a bearing surface or block a narrow oil passage.
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.