 # Calculate max mass of camera on a servo

I'm making a servo-controlled camera device controlled by arduino. I am using a HD1235MG Giant Servo. I'm using it at 7.4v so I should get 40 kg·cm of max torque.

I think I understand that this means it will lift a max of 40kg on a servo-arm of 1cm. But I am actually turing an object (camera) the centre of gravity of which is directly over the drive spindle. Is there any way to estimate the max size of an object it can rotate?

also I am helping support the weight using a flat needle bearing assemblage (see pic attached). I guess I may be over engineering this, my camera is only a couple of Kilos, but I want to understand the theory here too. Otherwise how can I learn? Maybe in future I can use it for heavier objects too. Cheers

Sorry here is a picture of the servo with bearings.

Servo Images

You need to look up Moment of Inertia

http://www.engineeringtoolbox.com/moment-inertia-torque-d_913.html

You probably will need to slow the servo. Look at the servo sweep example in the Arduino IDE.

...R

trendski: Sorry here is a picture of the servo with bearings.

Servo Images

If the axis is vertical there's no torque component at all from moving the mass of the camera, but you will have to overcome friction. needle bearings will help a lot.

Note that torque = force x off-axis-distance. If your load is on-axis or the axis is vertical then you only have friction to overcome. And MoI if you want to turn fast.

MarkT: If the axis is vertical there's no torque component at all from moving the mass of the camera,

Not true. The torque required depends on the MoI as calculated about that rotational axis. It's a rotational analogue for the mass in the case of a linear acceleration.

In the linear case we have F=ma, and the rotational analogue for that is tau = I alpha, tau being torque and alpha the angular acceleration, with I the moment of inertia.

I depends on the distribution in space, of the particles that make up the mass. You're saying that to rotate a mass depends on the position of the CoG wrt to that axis, so that if the CoG is on the axis the torque required is zero since there's no moment, which is not true. It actually depends on the sum of the squares of the distances of the particles: it's to do with their distribution, not their collective centre.

Once the rotating item as at speed though, the torque required is zero except for that required to overcome friction, as is the case in linear f=ma type motion.

What you say about torque being the force x distance is true though, and that's how you calculate the force required to produce the torque which you have to calculate from the MoI.

So for the camera, I'd say approximate its shape as a cuboid, its mass is known, hence MoI. Then decide what the angular acceleration is, and hence the torque required.

JimboZA: Not true. The torque required depends on the MoI as calculated about that rotational axis. It's a rotational analogue for the mass in the case of a linear acceleration.

I acknowledged that at the end of my post - I was pointing out you don't need torque to move the thing, I said nothing about how fast to move it or whether it would be accelerating or decelerating or just moving steadily. The earth turns without torque being applied.

The OP was imagining a massive torque was needed because the object had mass...