This article is part of a 6-part series on engine lubrication systems. Part 3 explains how the hydrodynamic wedge lifts a rotating shaft off its bearings, the three basic ways oil is delivered to engine parts, and how the oil pan and sump are designed to keep the pickup tube submerged. Seeing how oil gets from the pan to the farthest overhead camshaft helps explain why a plugged pickup screen or low oil level can destroy an engine in seconds.
Hydrodynamic Lubrication and Wedging Action
As a shaft begins to rotate within a friction bearing, it pulls oil into the high-pressure zone. The rotation wedges the oil between the bearing surface and the journal. Under rotation, the pressurized oil film physically lifts the shaft off the bearing surface. At operational speeds, the shaft is pulled toward the center of the film, ensuring no metal-to-metal contact occurs. Resistance to movement is highest at the film-to-metal interface and lowest at the center of the oil film. Most internal friction occurs near the center of the oil film itself.
Bearing Clearances
Precision clearances are required to maintain the hydraulic wedge without excessive pounding or oil starvation. The standard automotive tolerance is .002 inch or 0.05 mm. Clearances must be loose enough to allow oil entry and film formation, but tight enough to support the mechanical load and resist the reciprocating forces of the engine.
Full Pressure System
The full pressure system uses an oil pump to force lubricant through drilled and cast passages known as oil galleries. Pressurized oil is forced directly to crankshaft main bearings, connecting rod journals, and camshaft lobes. Timing gears, chains, lifters, and rocker arms are lubricated via spray or direct pressurized feed. Spurt holes are machined into the connecting rod to lubricate cylinder walls and camshaft lobes via throw-off. Some connecting rods are drilled the full length, called rifle drilling, to provide pressurized oil to the wrist pin bushings.
Splash System
The splash system relies on dippers attached to the bottom of the connecting rods. Dippers dip into shallow oil troughs or the sump, splashing oil over the internal engine components as the crankshaft rotates. This system is generally restricted to small, low-load, one-cylinder engines.
Combination System
The combination system utilizes a pressurized pump to supply main bearings and critical galleries while relying on splash or dippers for auxiliary lubrication.
Startup Dynamics
When an engine is shut down, the oil film is squeezed out, and the shaft rests directly on the bearing. Only a microscopic residual film remains. Upon startup, there is a measurable delay before the oil pump builds full system pressure. Internal components are under maximum stress during these first few seconds. To prevent premature wear, engines should be operated at idle in neutral or park until the oil pressure gauge or light indicates the system is fully pressurized.
The Oil Pan Sump
The oil pan acts as the primary storage vessel. A lowered section, the sump, serves as a collection point for the oil pickup screen. Precision-engineered metal plates, called baffles, are used within the pan to prevent oil sloshing. Stabilizing the oil prevents the lubricant from moving away from the pickup tube during cornering or braking, and ensures the crankshaft does not strike the oil surface, which would cause aeration or foaming. The drain plug is strategically located at the absolute lowest point of the sump to ensure heavy contaminants and metallic particles are gravity-flushed during service.
Combination Pressure-Splash Delivery
The primary pressure path works as follows. An oil pump draws lubricant through a pickup screen. Oil is forced under pressure through discharge pipes to the main bearings and crankshaft journals. Pressurized oil is routed to the cylinder head to lubricate the camshaft, rocker arms, and valve stems. The secondary splash path includes nozzle injection, where specific discharge nozzles spray oil directly into troughs or dippers on the connecting rod. Centrifugal distribution occurs as the crankshaft rotates, oil from these dippers is thrown onto the cylinder walls, piston pins, and timing gears. Spurt holes are machined passages in the connecting rods that use internal pressure to spurt oil onto high-friction areas not reached by primary galleries.
The Lubrication Circuit
The relationship between the oil pump capacity and gallery diameter determines the systems operating pressure. After oil passes through a pressurized bearing, it is thrown off by the rotating component, creating an oil spray or mist. This mist is critical for lubricating secondary components like valve springs and pushrod ends before it returns via gravity to the sump. Pushrods are often used as hollow conduits to allow pressurized oil to reach the rocker arm assembly, where it then drains back through the cylinder head return holes to the oil pan.
System Versatility
The combination pressure-splash system is the standard for most modern internal combustion engines due to its ability to provide localized high-pressure lubrication to bearings while using residual throw-off for broad-area cylinder wall cooling and lubrication.
Without a proper oil film clearance measured in thousandths of an inch, bearings fail from pounding and heat. Proceed to Part 4.
Local Shop Note:
You know, I heard a great story from a mechanic over on S Main St in Perry, N.Y. about a job that looked routine but turned into a real diagnostic challenge. He was at a TST seminar, and he was telling me about a sedan that came in with a complaint that the oil pressure light would flicker at hot idle, but the pressure was fine at speed. The customer had already replaced the oil pump and the oil pressure switch. Still flickered.
He hooked up a mechanical gauge and found the pressure was 12 psi at cold idle, but at hot idle it dropped to 4 psi. The switch’s threshold was 5 psi, so the light was flickering on and off. He checked the oil level — full. Checked the oil viscosity — correct weight. Then he dropped the oil pan and found the pickup screen was partially plugged with sludge and debris. The pump was fine, but the screen was restricting flow at low RPM. At speed, the pump was pulling hard enough to overcome the restriction, but at idle, it couldn’t draw enough oil to maintain pressure.
He cleaned the pan and pickup screen, flushed the engine, and refilled with the correct oil. The pressure came back to 8 psi at hot idle with no flickering.
Years later, I still remember that one because it showed me that oil pressure is a function of flow and restriction. If the pickup screen is plugged, the pump can’t pull oil, and the pressure drops. A new pump won’t fix a blocked screen. Always drop the pan and inspect the pickup when you’ve got an oil pressure problem — sometimes the pump is fine and the screen is just full of junk.