How’s it going, everyone?
I’m Tom and today i’m going to talk a
bit about ball 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.
