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Ball bearing reduces friction to a large extent than sliding friction. Motors use a metal ring called bush which slides against the shaft when the shaft is rotating. If a force is exerted on the shaft in direction perpendicular to the direction of the shaft, the motor stops because of the so much friction present between the shaft and the bush. Here's a photo to be clear :

enter image description here

... Whereas, if a ball bearing is used instead, the force won't be that much effective in that case because of the balls present in the ball bearing as they have to roll and not slide. For those thinking what a bush is, here is a photo:

enter image description here
(Photo taken from Google, edited)

Why is the bush not replaced with a ball bearing?

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  • $\begingroup$ All the answers here are correct, but sometimes there are bearing accommodated within the housing, they're just not visible. Here i made a very comprehensive roller vs journal bearing maybe you find it useful: engineering.stackexchange.com/questions/28531/… $\endgroup$ Sep 16 '19 at 14:17
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The manufacturer chooses between a bush and a roller bearing based on the conditions that the motor will used in.

So, if the side loads are small or negligible and the speed within what they decide then they will use a bush.

If there are significant side loads and speed conditions then they will choose a roller bearing.

Other constraints are the available space for a bearing which is limited by the total size of the motor and the cost of parts and assembly...

There are some internal combustion engines that have roller bearings on the crankshaft but most engines rely on shell bearings with white metal, similar to a bush, but designed for forced lubrication.

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Ball bearings are much more expensive than oil-impregnated bushings so they are only included in the motor design when the load or service conditions explained by Solar Mike require them.

Note also that although ball bearings are not needed in internal combustion engine crankshaft supports, some engine manufacturers (like Suzuki) will nonetheless use them so the engine will continue to run even if the oil pump fails during a race. So this is an example of a service condition that is independent of a load condition, in which including ball bearings is prudent.

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In addition to what @ solar mike and @ niels nielsen said there are some properties of bushings that are better than rolling element bearings ( ball, roller,taper roller, pin, etc). A bushing can imbed some debris and keep working. They are normally chosen as thrust bearings over rolling element. And although generally cheaper than rolling element , they are chosen for application where cost is not a factor such as some large radial and axial compressors where solid silver bushings are used. You can get more info at GGB ( Glacier Garlock Bearing) and " tilting pad" type bearings.

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