I am trying to spec out a motor's torque required to rotate a platform about an axis. 
The attached diagram makes it better to understand(it's like a rotisserie). Dimensions are in cm:
[![enter image description here][1]][1]
I have an arbitrary shaped object on the platform that I simplified to a cuboid to calculate the moment of inertia. 
The combined weight is about 25kg.
I use the moment of inertia formula for a cuboid along the length axis and use the parallel axis theorem to move the axis to where I have the rod, I get:

    I = (1/2) * m * (w^2 + h^2) + m*((0.5*h)^2)
      = (1/2) * 25kg * (0.4^2+ 0.5^2) + 25 * (0.2^2)
      = 6.125 kg-m^2

Assuming I want to reach 5 rpm in 5 seconds. I have 

      5rpm = (2 * pi * 5)/60  rad/s

      alpha = ((2 * pi * 5)/60 - 0)/(5-0)  rad/s^2

            = pi/30 rad/s^2

`T = I * alpha    = 6.125 * (pi/30)   = 0.64N-m`

Now I am not entirely sure if this calculation is correct. I had a 5N-m rated dc motor lying around and I fit it to the platform. The motor was able to rotate the platform about 45 degree clockwise but was not able to come back to zero degrees. Am I missing something in the above calculation? Gravity doesn't feature in my equations.
There could be other factors like friction, or the gearbox in the motor?


  [1]: https://i.stack.imgur.com/VdqdM.jpg