Why do we need bearings in a car/truck/firetruck/torque lever/driveshaft/pulley/robot/mechanical anything…? Why are there so many different kinds of bearing for each application, why can’t there just be one design? Why have ball bearings when you can just have the two surfaces slide together. And why doesn’t a crankshaft use ball bearings. Finally, do all bearings need to be lubricated because of the friction or just ball bearings?
Start with the Wikipedia article:
Two surfaces in contact will wear due to friction if they are moving relative to each other (either one or both surfaces in motion). Lubrication is needed to keep them from wearing rapidly and producing excessive heat. Any kind of bearing needs lubrication unless it’s made of low friction material, and low friction materials are not very strong. Ball bearings minimize the contact area between moving surfaces and minimize friction so they don’t require constant lubrication. Crankshafts are constantly lubricated by crankcase oil which is constantly filtered to keep it clean and free of small particles that would increase wear so ball bearings aren’t necessary. In some two stroke engines where the crankshaft is not bathed in crankcase oil roller bearings may be used to extend life of the crankshaft. Lubricant added to fuel in those engines may help lubricate the crankshaft but it is insufficient for the load placed on them.
As a simple example think about socks. If you don’t wear them your shoes will rub against your feet and ankles when you walk or run. The socks are acting as a lubricant does to keep you from getting blisters.
Try rubbing your hands together really fast. Feel the heat?
Now try rubbing your knuckles together at the same speed. Less heat, right? Less contact area, less friction.
Rolling friction is a great deal less than sliding friction, even with grease.
That’s true but ball bearings don’t result in only rolling friction.
My three year old has a little shopping trolley he likes to push around. The wheels are attached by little bolts that fit inside steel sleeves that sit inside holes drilled in the wheels (ie. no bearings). I have to reattach those stupid little things on a weekly basis.
because otherwise the forces on those joints would cause the metal parts to grind against each other and wear out rapidly.
because different machines have different requirements. Engines with pressurized lubrication use plain bearings for things like the crankshaft, rods, and camshaft(s.) oil needs to pass through the crankshaft journals into passages to the rest of the engine, and the flow of oil both helps maintain the oil film, flush any contaminants out, and keep the journals cool. Also, the bearings are made of a different (softer) material like Babbitt metal so any particles that do get in there will embed into the bearing instead of being held there to score the journal.
ball/roller/needle bearings are used where you either have no lubrication supply (e.g. the bearing is packed with grease and sealed) or simple splash lubrication.
see above. Most engines don’t use ball bearings because oil is pumped through the plain bearing, through passages drilled into the crankshaft, and on to other parts of the engine. a ball/roller bearing would just let the oil pour out of the sides of the bearing back down into the sump.
The exceptions are 1) small two strokes where you mix oil into the gas, and 2) some motorcycle engines like Harley-Davidson’s. 1) is because the fuel-oil-air mixture goes through the crankcase and lubricates the bearings; no need for pressurized lubrication. 2) I don’t know exactly how or why they designed it that way.
ball/roller bearings actually work with less lubrication than plain bearings.
Because as mentioned, 2 pieces of anything just sliding around on each other will tear each other up. Some good easy example given already
because not all designs are physically suited to all purposes.
Ball Bearings are great and all, but when it is something with a lot of pressure or weight the ball concentrates it all in one spot.
If you have a wide shell bearing like in a larger engine, the load can be spread across the entire load bearing surface (journals)
Also putting a ball bearing on a crank that requires more than 2 outer bearing means the crank has to be made in multiple pieces and loses integrity, or the ball rages have to be made in multiple pieces.
Be cause a ball has such a small contact point, it will find any joint and beat it to pieces.
Ball bearing also do not seem terribly well suited to any appreciable side loading.
Ball and Roller bearing also require more space for the bearing to exist.
Some do, smaller engines with 2 outer crank bearing, see above
ALL bearing need lubricated
Some bearing are fed grease or oil, some bearing are embedded with lube semi permanently, and some bearing sacrificially lube them selves like graphite bearings.
All bearings need lubed or they wont be bearings for long.
Just on crankshafts, and ball-bearings.
With most engine crankshafts, which use plain bearings, the journals of the crank actually ride on the film of oil between the journal and the bearing. That makes the friction surprisingly low.
This is needed because the forces involved are very large, and non-linear - consider that the power stroke of an engine creates a lot more force on the bearing that say the exhaust stroke, and the oil film helps adsorb impact.
They also represent a fairly compact package considering the forces they’re expected to endure
Small 2-stroke engines and the like that use ball-bearings usually employ a very-large bearing relative to the size of the engine - that is so it can survive the shock loadings imposed by the engine cycles. To use similar bearings in a normal car engine, they would be HUGE and then you have a size/weight penalty that basically reduces any benefit you may get from (slightly) lower frictional forces.
cheers,
edit : removed extra word
Some bearings don’t need lubrication, but they’re designed for low speed, light loads, and short life, the main selling feature being low cost. See e.g. lisiate’s post re: his daughter’s toy shopping trolley.
Sometimes you’ll see small motors (e.g. box fans or bathroom ventilation fans) that have a steel shaft spinning inside what looks like ordinary bronze. Those are lubricated: that piece of bronze is actually oilite, a porous material that is deliberately saturated with machine oil during assembly. The oil provides the necessary lubrication to “float” the shaft in the bore. When the motor gets old, the oil may dry up and the shaft can start binding in the hole; wear accelerates, the hole gets larger, and the shaft can start whirling in its bore, causing a lot of noise and wear. This points to the necessity of lubrication.
Ball/roller bearing units need less lubrication than hydrodynamic bearings, but if operating at rated load they will destroy themselves in fairly short order if they aren’t lubricated at all. The surfaces of the balls and races are ground to a particular surface finish that’s pretty smooth, but not telescope-mirror smooth. They rely on the presence of oil to fill in the gaps and bear the load so that you never end up with metal-to-metal contact between the surface asperities of the balls/rollers and the races. The trick is that low-viscosity machine oil can develop extremely high viscosity (think asphalt) when exposed to the contact pressure between ball and race, forming a protective pad between the two surfaces. As viscous as that is, it will still eventually flow out of the contact area, which means you need to keep the bearing moving; ball/roller bearing catalogs often list the minimum speed necessary to bear the rated load without shortening its life.
I like the animation in the Wikipedia article.
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Consider two flat metal pieces with ball bearings between them They can slide around with little friction. Now bend the flat pieces around to form circles and you have the animation. Since the inside and outside diameters circumferences are no longer the same you will get some friction, but not a lot. If you took out the bearings and and the two metal pieces directly against each other there’d be a lot more friction.
Note that the size of the bearings affect the friction. The smaller they are, the closer approximation you get to two flat pieces rolling by each other. But then there are trade-offs: tiny bearings are easier to break or deform, secondary losses accumulate, etc.
You know how automobile engine and wheel bearings are cylinders rather than ball bearings ? This is so that at regular usage speed, they float, rather than roll or slide… They float on the liquid oil/grease ! Thats why they have to be different.
Meanwhile, something used for low speed is going to have to be a roller.
It all depends on cost ,effectiveness and requirement.
If you require it to work for a couple of hours, it can jsut be a slider.
If you need it to work for 1000 hours, it can be a ball bearing roller.
If its got to work for a million hours, more of a floater is required.
The problem with nuts coming off can be solved with suitable washers, AND/OR the nylon lock nuts… True story, the spare parts shed at the USA’s Afghanisan vehicle maintenance yard was filled with shelves for parts… but right out the front, obviously in high usage … A rack with all sorts of different nylon lock nuts … really they were just going to have to lock everything on so parts don’t go missing ,correct torque never working to hold anything in Afghanistant.
Forget about the wear on the parts-- You need bearings just to reduce the friction. Take one of those fidget-spinner toys that are so popular nowadays, and give it a twirl. See how long it spins for? Now imagine how long it’d spin if it were just a cylinder through a hole for the bearing.
Even a teflon bearing would not let that spinner move for very long. Just too much surface area in contact.
OK, I’m with you so far.
Wait…what? :dubious:
Are we still talking about ball/roller bearings? Can’t make sense of what you’re saying.
Journal bearings are commonplace in engines, but not in wheels; the latter are typically ball or roller bearing units.
I assume he’s talking about the geometry issue with ball bearings. The outer race and inner race surfaces have different diameters so the ball can’t simply roll on both surfaces.
Not just that, but for any form of motion control, e.g., a servomotor-actuated piston or servo-actuated anything. If you want fine-grained control over distance and/or force, reducing the friction is much more critical than any effects of wear. It’s not just that controlling the current becomes tricky, but even getting things moving can be tricky. Think of it like overcoming the shear in ketchup or wet cornstarch.
Although heat has been mentioned, I’ll point out that enough heat can cause the surfaces to weld together, e.g., friction welding, such as friction stir welding.
And of course customer satisfaction is a thing. Bearings make things quieter by eliminating friction.
Something’s getting lost in translation; I’m still not understanding the issue. Certainly the outer/inner races have different diameters, but why would the balls have difficulty rolling on both surfaces? Can you point to a picture or YouTube video or something?
if that was true, planetary gears wouldn’t be possible.