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Why Engine Crankshafts Using Sliding Bearing, But Not Roller Bearing?

Time:2019.12.18  Source:Roller Bearings Suppliers


Nowadays, the rapid development of society and the continuous advancement of science and technology have shown that nothing can be better than the gradual popularization of automobiles.


As people's incoming gradually increase, many families have purchased cars for the convenience of traveling or working. There are more people driving and fewer people knowing cars and knowing that the core component of a car is the engine. The quality of an engine depends on how the components inside the engine are made. Why is the crankshaft designed as a sliding bearing instead of a roller bearing?


We should learn the difference between sliding bearing and roller bearing.


Sliding bearings are the bearings that work under sliding friction. Sliding bearings work smoothly, reliably, and no noise. Under the condition of liquid lubrication, the sliding surface is separated by the lubricating oil without direct contact, which can also greatly reduce friction loss and surface wear, and the oil film also has a certain ability to absorb vibration.


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Commonly used sliding bearing materials are bearing alloys (also known as pasteurized alloys or white alloys), wear-resistant cast iron, copper-based and aluminum-based alloys, powder metallurgy materials, plastics, rubber, hardwood and carbon-graphite, polytetrafluoroethylene (Teflon, PTFE), modified polyoxymethylene (POM), etc.


The sliding bearing absorbs and transmits the force between the relatively moving parts, maintaining the position and positioning accuracy of the two parts. In addition, the directional motion must be converted into rotary motion (such as a reciprocating piston engine).


The roller bearing is a precise mechanical element that changes the sliding friction between the running shaft and the shaft seat into roller friction, thereby reducing friction losses. Roller bearings are easy to use and maintain, work reliably, have excellent starting performance, and have a high load-carrying capacity at medium speeds. Compared with sliding bearings, roller bearings have larger radial dimensions, poorer vibration damping capacity, lower life at high speeds, and more significant noise.


Sliding bearings are highly reliable and will not lock up immediately when worn! And in the process of work is very stable and secure, and there is not much noise. Compared with the above sliding bearings, roller bearings only change sliding friction into roller friction, and the purpose is to reduce friction losses.


If the roller bearings fall apart, they will immediately lock up, so in most high-speed shafts, sliding bearings are used more than just engines, and like large compressors are used this kind of sliding bearings.


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If a roller bearing is installed on the crankshaft of the engine, there is no way to install it, because everyone must know that the ball track of a roller bearing cannot be sewn. If this phenomenon occurs, it will be easily damaged. This is the reason that roller bearings are large can’t be used in the engine.


Compared with roller bearings, sliding bearings have many advantages that roller bearings do not have. For example, sliding bearings, when the lubricating oil is sufficient, whether it is impact resistance, service life, precision, noise, small space, etc., roller bearings are not available, so a sliding bearing must be used under a comprehensive consideration.


Due to the large pressure bearing area of the sliding bearing, its bearing capacity is generally higher than that of rolling bearings, and the rolling bearing's ability to withstand impact loads is not high. Still, the fully liquid lubricated bearings can buffer and absorb shocks due to the lubricant film, which can withstand larger impact loads. When the speed is high, the centrifugal force of the rolling elements in the rolling bearing increases, and its bearing capacity should be reduced (prone to noise at high speed). For dynamic pressure sliding bearings, its bearing capacity increases as the speed increases.


When we understand the above information, we will know why the engine crankshaft uses sliding bearings instead of roller bearings.


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