How to estimate friction on the bearings to select motor?

[Oleg]

TL;DR

Friction at bearings depends on the properties of the bearings themselves, but how can I estimate the "extra friction" under the load? Is there any formula or rule of thumb?

I'm willing to build a motorized gimbal head for my camera (something like this: https://www.amazon.com/dp/B01JN5D40A/?tag=pfamazon01-20).
If my understanding is correct, given that camera will be well balanced on the platform, the motors would not need to hold the load and only should be strong enough to overcome the static friction of the bearings. Obviously, it will depend on the properties of the bearings themselves, but how can I estimate the "extra friction" under the load? Is there any formula or rule of thumb?

 

[Baluncore]

You must select the bearings for the task, then evaluate the friction from manufacturers data.
Will you use ball bearings alone or in pairs, or sleeve bearings.
In either case, to minimise friction, use the minimum diameter shaft or bearing.

 

[Borek]

the motors would not need to hold the load and only should be strong enough to overcome the static friction of the bearings.

No, they need enough power to deal with the moment of inertia of the camera.

Which probably makes the bearing friction question a moot.

 

[Oleg]

No, they need enough power to deal with the moment of inertia of the camera.

Which probably makes the bearing friction question a moot.

I see.
Do you have an idea, how to estimate the required torque?

 

[Baluncore]

The torque required to move will depend on how fast you want to accelerate the camera.
It will also depend on how carefully you can counterbalance the load and structure.

 

[Oleg]

The torque required to move will depend on how fast you want to accelerate the camera.
It will also depend on how carefully you can counterbalance the load and structure.

Acceleration is a simple part. Something around ~3-5 degrees/sec^2 would be good enough.
For the balance part, it is not so straightforward. What is a measure of "carefulness"? I will have the possibility to adjust the position of the load and I'll do my best, but we all understand that it never will be ideal.

 

[Baluncore]

For the balance part, it is not so straightforward. What is a measure of "carefulness"? I will have the possibility to adjust the position of the load and I'll do my best, but we all understand that it never will be ideal.

Without the gears on the motors engaged, adjust the position and counter-balance weights to make it possible for the camera to remain in any position you place it. Then you know that the unbalance torque is less than the bearing torque. If you cannot achieve that balance, then bearing torque is irrelevant and you must design for unbalance torque.

 

[jrmichler]

If you build the gimbal without the motor, and turn and tilt it by hand:

1) If it moves smoothly and easily, measure the torque. A motor drive sized for that torque will move it smoothly and easily.

2) If it moves smoothly with resistance, measure the torque. A motor drive sized for that torque will move it smoothly and easily.

3) If it moves rough and jerky, start over. A motor drive would still be rough and jerky.