Insufficient Lubrication - MTD P90 series Shop Manual

P90 Series Vertical Shaft Engines

Insufficient lubrication

The bearing surfaces in an engine are not smooth. The machining processes used to make the engine parts,
leave little peaks and valleys that are only visible under a microscope. These peaks are called asperities. As the
engine breaks in, the asperities break off leaving plateaus that become the bearing surface. The valleys become res-
ervoirs for the lubricant.
The oil in an engine performs three functions:
• Cools - It carries heat away from the hotter parts to the cooler parts of the crankcase, allowing the heat to
dissipate
• Clean - The oil carries metal particles and contaminants away from the contact surfaces.
• Lubricates - It forms a cushion between moving parts, protecting them from contact with each other.
When an engine is properly lubricated, all of the moving parts glide on a thin film of oil. If that film breaks down or
carries enough grit to bridge the film, damage will occur.
1. When the parts are at rest, they push the lubricant or oil away resting on the bearing surfaces. As the parts
rotate, they climb over the oil, pulling the oil between the bearing and the part, riding on a film of oil.
The asperities are the first thing to make contact between two moving engine parts with an insufficient oil film
between them. This creates friction and causes a transfer of metal between the parts. The heat and friction fur-
ther breaks down the oil film, accelerating the process.
2. Insufficient lubrication failures include:
• Low oil level
• Wrong oil for the application
• Contaminated oil
• Degraded oil (heat, age, acids)
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