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Bearing type and size

Bearing type and size

Assuming I have a plate as below with a hole in which I install a bearing.
Then I insert a shaft and in top of the shaft a weight. Let’s say is a round weight, can be a sphere or a disk.
- What type of the bearing should I use?
- From what material should be the bearing made?
- What size should be the bearing?
The purpose is to have the lowest possible friction using bearing that I can buy from market.
Plate: 30 x 30 cm
Weight: max. 20cm diameter and 1-2Kg
You may consider the plastic plate bigger if you consider is not stable, let's say 50X50cm and 5cm thick. It is not important. Just consider it a stable horizontal plate. It can be a table sheet which holds the bearing steady.
My focus is on bearing.
The plastic is just a steady support that never turns.
The purpose of the plastic sheet is only to keep the bearing in place. Imagine that the bearing is somehow embedded in the plastic sheet. The bearing is fixed there in the plastic with its outer shell. There is s hole in the plastic which allows the center of the bearing to move freely.
The shaft is fixed in the inner side of the bearing and at the end on the top is the weight.
The weight, the shaft and the inner side of the bearing move freely all together. The plastic sheet does not move, it just keeps the bearing in place. It is a simple construction, nothing fancy or sophisticated.
The shaft does not slide through the bearing. We can achieve that with various methods, for instance imagine the diameter of the shaft is increasing with few mm and then the shaft is stacked in the inner side of the bearing due to the weight/gravity.
The plate is always horizontal and the shat is always vertical.
At this stage, the above is just a theoretically experiment. I want to understand before I move to practical tests.
It is just a simple construction where a weight as a sphere or can be a thick disk is attached to a shaft and inserted in a bearing with only one purpose: to move freely, with lowest possible friction.

RE: Bearing type and size

A single, single row ball bearing does not tolerate "moment" loading well. Nor does it provide very good control/centering of the shaft.
For rotating machinery, or even moderately hard working mechanisms this means every shaft needs two bearings spaced at least a few shaft diameters apart.

Maybe a single bearing would be suitable for a nearly static display supporting a light weight globe or weather vane.

RE: Bearing type and size

Taking Tmoose's suggestions and the available technical info, I would look into a double-row ball bearing assembly. You can buy them grease filled with seals, and have generally low friction and some ability to handle moment loads.

RE: Bearing type and size

Why are you depicting a radial ball bearing in an example where it seems the major load is axial? Is there some reason for this?

Aidan McAllister
Metallurgical Engineer

RE: Bearing type and size

@ AidanMc
The radial bearing used in the picture is just an example. It may be any other type that you suggest.
I just made a picture setup with photos from internet. At this stage there is no physical setup nor CAD drawing. I just gather info and I need to understand what would suit better having in mind the major purpose: the lowest possible/Imaginable friction.

@ geesamand
I do not want to grease/lubricate the bearing unless will give me the lowest possible friction.
I am afraid that double-row ball bearing assembly will give me higher friction than single row, considering that I do not have high loads, neither high speed. What do you think?

@ Tmoose
Agree with your comments, but my load is small and the speed is not too high.
I would like to avoid the second bearing on the top, unless is really necessary.
If instead of the sphere will be a disk not very thick, then the entire construction will not be higher than 10cm. Under these conditions maybe I can live also with 1 row bearing.
Using 2 bearings, 2nd on the top, the friction will increase.
For my case, the friction is critical, it is the main thing.

RE: Bearing type and size

Few more details.
Before I asked the question here, I already searched the internet couple of days and I have read different info about the bearings. Then my head became bigger and the multitude and variation of info acquired started to be mixed up in my head resulting a salad. Nevertheless, I can still filter and split the question asked in clear more specific questions.
Here are some sources that I went through:
Then next:
and one of the best:

And general info as:
and some more…

Strange that no one asked me so far, and I forgot to mention it: what is the speed of the shaft?
Probably you assumed that I spin it by hand.
It is around 600 RPM, easy to be handled by most of the bearings. The worse case would be 10 times higher around 6000 RPM, but for the moment we consider it 600 RPM.
I do not intend to use magnetic bearing, unless is somewhere with permanent magnets only, accessible price, but here is a big discussion and we should ignore it for the moment, unless you point to a direct manufacturer/product that I can check it. I do not intend to use hydrodynamic, aerostatic or hydrostatic bearings.
Basically I just want a simple mechanical bearing, probably with balls, unless a nice magnetic bearing with permanent magnets is indicated.
Then comes the force.
My initial drawing and explanations says about a weight (sphere or disk) of 1-2kg. That makes you think that only the gravity force perpendicular on the bearing is present and therefore a thrust ball bearing would be enough.
If the shaft rotates with 600RPM then must be something to rotate it.
Let’s consider a air blow pressure from the side, thinking we have a propeller attached to the shaft or if instead of the sphere/disk weight there is a rectangular metal weight (as propeller) and I blow the air onto it.
Then we have also radial force and the thrust bearing is not good anymore.

Let’s break the initial questions in more specific questions:
1) When I asked what size, I had in mind next:
a. Should it be 1mm bearing or 10cm bearing? Why one or another?
b. Is it any optimum size in-between? How do you know what size to choose?
c. There are sizes having the same inner diameter, but different outer diameter. Which one to choose? Keep in mind that I am flexible with what inner and outer diameter I can choose. I am not force by the setup to a specific diameter.
The only explanation that comes into my mind now to choose a specific size is related with next things:
- A smaller size my imply a lower friction – this is what I need.
- A too small size will not assure the stability of the load, the shaft with the weight when rotates with 600 RPM.
- And opposite: higher size gives stability but too much friction.
From here the question: how to choose proper the size knowing the size, the weight of the load and the construction setup?

2) When I asked about type, I had in mind next:
a. Axial, radial, thrust, ball, rolls…whatever mechanical constriction has the bearing.
Which one is better for me, knowing the size, the weight of the load and the construction setup, shaft position, speed of the shaft, forces on the shaft…, considering lowest possible friction?
I was thinking at rolls bearings which can support also radial force.
But then comes next ( angular contact, radial contact, 4 points contact or whatever else may be…?
What about the cage for balls? With or without or what type?
Logical sounds without cage less friction, but would that give enough stability to ky setup? I do not have another bearing on the top.
b. Material: should be stainless steel, or special plastics or ceramic?
I have read a lot about ceramic bearings having 10 times less friction than normal steel bearings. I do not know about plastics. Can you tell me something?
c. I do not want to lubricate the bearing, unless is really lower friction. I know that are some self-lubricating materials, but I do not think are used for such bearings. Any idea?

Maybe this construction without cage (
) and ceramic type will be lowest friction, but I need to buy something available on the market. And I am not so sure about such bearing without cage when are radial forces involved.
Speaking about ceramic bearings, I discussed with the next company ( ) and they recommended me ZrO2 as lowest friction.
They have a lot of types ( ).
The y asked me 100$ for one piece having 10mm inner diameter, 20-20mm outer diameter and width 5-10mm.
I found on German market 30-40€ for such bearing. On Ali-Express it is a lot cheaper, only that takes weeks.
Please give me more details about type and size as I described above.
I added 2 more pictures here:

RE: Bearing type and size

600 to 6000 rpm. As much as 2 kg ( 4.4 lbs. ) UNBALANCE !!! rotor dynamics !! Basic machine design.

How is this to be driven?
How many revolutions / hours of operation are required?

As far as "lowest possible friction", how would you rank a bicycle wheel with axle pointing vertical?

The size of the plate, and its effect on stability, pales in comparison to the jamming and binding the poor bearing is subjected to when forced to control that spinning

RE: Bearing type and size

Please consider only 600RPM and 1KG, continuously run 24h.
Imagine that plate is a stable heavy table and the bearing is well fixed in the top of the table.
My pictures are only to get an idea of what I want, but is more theoretically, not the actually design.

„As far as "lowest possible friction", how would you rank a bicycle wheel with axle pointing vertical?”
I would not consider it the lowest possible friction.
When I think at lowest possible friction I have in mind the mechanical bearing, probably with balls, maybe made of ceramics, which tends to come as close as possible to a magnetic bearing for instance. Of course will never ne as a magnetically bearing, but to be as close as possible.
But I am not so sure about geometry, where the balls should touch, cage or without cage, maybe another material (special plastics? ), then the proper size compared with the load and setup.

RE: Bearing type and size

Sounds like you're asking for a magnetic bearing in a vacuum or an air bearing.

Based on my experience with bicycle wheel hubs, ceramic bearings sound nice but the performance different (lower friction) is scarcely measurable. More practical factors such as misalignment, lubricants, and air resistance dominate the system.


RE: Bearing type and size

If it is of any help, I can consider the weight lower, down to 0.5Kg.
We can consider also the radial forces that turns the shaft equal distributed around 360°, so not only one force from one side.
We can imagine an AC squirrel motor where my shaft is the rotor, but only vertical position and max. 600RPM. Then you can imagine the electromagnetically forces distributed around the rotor.
Normally I need a 2nd bearing on the top, but only because I need the lowest friction I would like to use only one bearing at the bottom. We can imagine a flat rotor as a disk, where the diameter is longer than thickness.
The purpose of the lowest friction is to keep the rotor in motion with the lowest possible current in the coils of the stator. We may assume that I put the rotor in motion by hand only to break the initial torque.

RE: Bearing type and size

Hi vick12000 ,

How is this device going to be driven to 600 rpm ?

What assures the 1 kg object is going to have it's mass distributed precisely on the axis of rotation defined by the bearing. Like within 50 microns ?


Dan T

RE: Bearing type and size

Then let me ask you opposite: what do you consider max. acceptable speed in RPM for such system with only one bearing (what type?) when the mass is distributed as follows:
- the rotor is a 0.5Kg disk where the material is considered homogeneous and the cutting precision of the disk is 0.1mm.

RE: Bearing type and size

A single ball bearing running with the shaft tilted will have more friction and make more noise than if properly guided and supported.

NOTE - SKF won't even mention moment load and single row ball bearing in the same breath.

Or, ANY single bearing, really.
"Bearing arrangements consisting of a single bearing that can support radial, axial and moment loads, e.g. for an articulated joint, are not dealt with in this catalogue. If such arrangements are required it is advisable to contact the SKF application engineering service."

ANY offset of the CG from the mechanical rotating axis will create a "moment" load, and try to make the assembly lean to the side, like a marionette with a broken string.
With the bearing at the bottom of the shaft, The limit of lean will first be the clearance within the bearing.

Depending on how this device will be driven to 600 rpm, the unbalance as a result of the lean will be the entire disc/sphere being offset.
Standard machinery residual unbalance tolerance of G6.3 would permit maximum offset of about 0.005" for 600 rpm service.
Page 3 here -

Kind of like the left hand half of this device from 0 to 0:20.

RE: Bearing type and size

The document form here:
contains one of the best info that I missed, very valuable for me, I did not know about it, thank you for it.
Let's consider the shaft/rotor is electrical driven by electromagnetic forces.
These are distributed around. Does that mean will help to center the rotor?
Now, let's suppose the limits of displacement from document above are not respected due to the tolerances of rotor manufacture, is then the usage of the second bearing on the top a help to have lower friction for the rotor compared with the situation when only one bearing is used and the center of gravity vs weight vs RPM and is outside of the recommended limits and starts wobbling?
In other words I see these 2 options and I do not know which one is better for lower friction:
1) Using only one bearing and making high precision rotor
2) Using 2 bearings and lower tolerances for rotor

For me important is only to have the lowest possible friction, the rotor to move as easy as possible, driven with lower possible electromagnetic field, even if I start the rotor by hand initially, because the start moment involves always high inrush currents and more energy than when is in motion.

RE: Bearing type and size

viki2000 said "Does that mean will help to center the rotor?"

Certainly Others will know better than I, but I doubt it.

I am suspicious the inertia of the device would be a far greater impediment to starting than the friction from an innocent pair of little old ball bearings.

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