Aside from clipping wool from sheep, many of us have only a vague notion what the word shearing means. Shearing refers to any activity that involves cutting or scraping the surface of an object. Where combustion engines are concerned, this term refers to the distortion metal parts in motors and machines undergo when they scrape and rub under intense heat and pressure. Mechanics speak of engine parts shearing and then freezing, or locking into place, when the oil in the engine runs dry. Wing shear on an airplane's wings refers to the distortion of the the plane's wings when they are under the tremendous pressure of flying. The heavy metal plates that from the plane's wings and must be reshaped or replaced so that they can be firmly bolted in place, or the plane would be unsafe to fly.
The mechanical shearing of the parts in a combustion engine of a car or a lawn mower, refers to the scraping of metal parts under heat and pressure until they wear down or break.
Oil manufacturers spend millions of dollars evaluating the use of polymers in engine oils to prevent the shearing of engine parts, which is the natural consequence of running a motor. Without oil that can remain fluid at high temperatures and is capable of lubricating and protecting the metal surfaces under extreme conditions, an engine could be destroyed after running just a few minutes.
Many of us have had the experience of running an old car that had an oil leak. In 1982, my husband and I owned such a car. Every time we filled the tank with gas, we had to add a quart of oil. My husband tried dumping those cans of sludge into the oil tank that are supposed to stop oil leaks, but, of course, the leak did not go away. I am sure you can guess what happened. In the midst of a long trip, the engine froze. The shearing action of the metal parts scraping against each other under intense heat and pressure caused them to fuse. The cost of taking the engine apart piece by piece, sanding all the valves and cylinders so they could fit together smoothly and move again was more than the car was worth--especially after the mechanic informed us that even after all that labor, he could not guarantee that the engine would function. If there was too much play between the cylinders and the sleeves they fit into, the engine would lack power.
Combustion engines are under stress. The metal parts wear down under the stress of normal use. Keeping an engine--whether in a lawn mower or a car--well lubricated will help it survive longer so that you may get years of good use from it.
The company that manufactured your car and your mechanic know of engine oils that would be most appropriate for the make and model of your car.
