bearing - and not just about the radial type, but also about the linear type. A guide on

different linear slides was one of the most requested topics, and instead of jamming everything

into a single guide, i decided to break them out and cover each option in depth.

So, let’s start out with # radial ball bearings, because they work in exactly the same way

as linear ones. The basic idea behind them is that, instead of supporting a motion by

having two surfaces slide on each other, you’re adding a rolling element between them, in

this case, balls, but other bearings use needle pins or cylinders. This reduces the friction

and increases the life span of the bearing. Radial ball bearings are usually use for belt

idlers, in stepper motors and everywhere else where you need to support a rotating element.

Now, radial bearings can support forces perpendicular to the shaft or, in a more limited way, also

along the shaft. They can not support torsional moments, for example when you mount a pulley

on a shaft and support it like this. Which is why stepper motors have two ball bearings,

one on each end, to support these kind of moments.

Radial ball bearings are available in a huge variety of sizes, the most common ones are

the 62x series or the lighter 6 #0 x ones, particularly the cheapest bearing of all,

the 608 skateboard bearing with an 8mm inner diameter and the 623 and 624 bearings with

3 and 4mm inner diameters. Now, each of these bearings is available in different configurations,

and the ones that are most often used are ZZ and RS types. ZZ means that the bearing

will have metal dust caps on each side, guarding its balls from larger foreign objects, while

RS types have an actual rubber seal on each side, which both seal the bearings lubricant

inside the bearing, but also keeps pretty much everything, including liquids, from entering

the bearing. Both types are about the same price, so i’d recommend using sealed RS

types wherever possible. And staying with the subject of price, brand-name radial bearings

are a good bit more expensive than no-name, Chinese bearings, but since our 3D printers

usually don’t put enormous forces on the bearings, which would shorten their usable

service life, it doesn’t really make a difference which ones you use, since even the cheapest

bearings are still pretty decent. Now, these bearings typically are mounted

in our 3D printers by simply using a metric screw, and that does hold them in place, but

won’t make for a super-snug fit since male metric threads are always a bit smaller than

the actual diameter they’re named after - a male M8 thread is only about 7.8mm in

diameter on the outside. Just something you should keep in mind when using or designing

for this kind of mount. Now, the other type of ball bearing you’re

typically seeing in 3D printers is the linear type, which lets you guide things along a

round shaft. Now, because the balls of those bearings directly ride on the shaft instead

of a separate inner race, the quality of the shaft itself will # hugely influence the quality

of the linear motion you’ll get. First of all, you will want at least a hardened shaft,

preferably even a hardened # and chrome plated one, instead of a mild steel or plain stainless

steel one. Aluminum rods or tubes can absolutely not be used with linear ball bearings. On

shafts that are too soft, what will happen is that the balls will gradually form grooves

in the shaft’s surface, which cause extra backlash in the bearing. Often that backlash

is the cause for things like irregular layers or z-wobble, if the shafts of your z-axis

are worn and have too much backlash. The other reason why you’d want proper linear shafts

is that those are made to tighter tolerances than other rods that # aren’t specifically

made for linear motion. Again, this would lead to backlash and a poorer, less regular

quality on your prints. Rods with an g6 or h6 tolerance rating are the ones that work

best with the LM UU style bearings. You can check how tight the bearing is on the shaft

by rotating it, a good fit is when you can feel some resistance when rotating the bearing

around the shaft. If it’s loose and wobbly at all, you already have too much backlash,

if it’s too tight, to a point where it’s even running rough along the axis, the bearing

might fail and jam prematurely. So the typical type used is the LM8UU bearing,

made for an 8mm shaft. But the bearings are available in sizes from the 4mm LM4UU to things

like a crazy large LM100UU, but typically, only the 8, 10 and 12mm sizes are used in

3D printers. When using a larger bearing and shaft, you increase the rigidity of that axis,

which is especially important for the rods of the Z-axis, which usually have to deal

with the acceleration forces from the X-axis. Because these bearings are completely open

on the inside, lubrication is somewhat important. I’d recommend smearing some heavy grease

inside the bearing before mounting it, and then occasionally lubricating the rods they

ride on to keep the bearings running smoothly and to keep them from randomly jamming. They

do have a rubber seal on each end, but that usually isn’t tight enough to permanently

keep all the grease in and dirt out, so also keep your linear shafts clean and dust-free

at all times. Linear ball bearings are also pretty sensitive

to misalignment, which is why most carriages only use three bearings instead of four. If

you mount two bearings in a way where they are not aligned or have no way of aligning

themselves, one of the bearings will inevitably run noisier or even jam. So either use precise

mounts or provide a way for the bearings to align, for example by using only one zip tie

per bearing. And just like radial bearings, linear bearings

are also available in a couple of different varieties. For example, for the standard LM8UU

size, which is the short type, you can also get an LM8 #L UU, which is same bearing, but

twice as long. They are also available pre-fitted into aluminum blocks as the # SC 8UU, or as

a flange type as the LM # F 8UU. There’s a huge number of other variations available,

but typically, you’ll only be using the standard or long type.

Now, just like radial bearings, you can buy linear ball bearings from high-end brands

like SKF or from your typical Chinese ebay vendor, and even though the Chinese linear

bearings aren’t as quite good as their radial ones, i’d still say go for the cheaper option

here. You might get a couple of bearings that are a bit too loose or too tight, so buy some

spares, but considering that brand-name bearings cost about ten times as much as no-name ones,

it’s still cheaper to throw out a couple duds than paying so much more up front.

So to sum things up about the linear bearings, use precision shafts and keep them clean and

lubricated and you should have a very precise and reliable motion system.

I put a couple of ebay links in the description, just to give you a starting point what you

should look for when going shopping for affordable bearings.

As always, thank you for watching. Please share and like this video if you found it

helpful and subscribe if you want to stay updated when i upload new videos.

See you next week.