Offset Load Torque Calc 2

[RichD80]

Hi,

I am really struggling to calculate the torque required to rotate a load. Can anyone help me?

Imagine a crane head holding a 600kg load (horizontal to the ground)and the load needing to be rotated. The load is a slab of material around 3m wide and 1.5m high.

The speed can be very slow (1-2 rpm) and it will never complete a full rotation anyway (maybe 200 degrees max) - it will however change direction at times so accerlation can't be too slow (imagine maneuvering the load into position with a joystick and fine movements). I assume I need to calculate the moment of inertia and also and angle of momentum for this however it becomes more complicated.

In some scenarios, it is possible that the load will be off set from centre by around 100kg rather than balanced. I have no idea how to calculate the torque I need my motor / gear solution to produce to turn the load (supported on an external bearing - not the gear shaft) at the given speed and agility.

Thanks for any assistance offered

Richard

 

[zeusfaber]

You've already identified the two main parts to this problem - the torque needed to rotate or to stop the load, and the torque needed to control the off-centredness of the load.

To calculate the first of those, you need to have decide how much angular acceleration you want (this will depend on how much fine control you'd like, how impatient your operators are and how much shaking around the whole lot will bear - but is essentially a requirements capture job). Make sure you understand this both for getting the load to start spinning and for stopping it too - the requirements may be different. Couple that with your moment of inertia and you know enough to apply the angular acceleration equation.

The second part is the static bit. Assume that a load with the worst possible offset is stopped with its centre of mass level with the pivot (any other orientation will be less challenging) and calculate the resulting moment.

Now simply add the magnitudes of the two moments together and you'll get the worst-case torque requirement.

Finally, bear in mind that with an offset load, both steady-state rotation and angular acceleration will also react linear loads into the pivot point - always good to know about that sort of thing when the whole lot is hanging off a crane.

A.

 

[MintJulep]

I am imagining a 600 kg gyroscope hanging on a crane with a rotation control system all design by someone who "has no idea" and I am terrified.

The amount of assistance that you need is far far more than can be provided by a web forum.

Stop.

Before you get someone killed.

Hire an engineer with an idea to do this for you.

 

[tygerdawg]

For symmetric system shown in picture the math is straightforward:

starting/stopping torque T = Jα
α ≅ Δω (angular velocity) / Δt (time)
J = mass moment of inertia of the rectangular plate (get this from Machinery's, CRC Math Tables, or CAD system)

Add extra torque for windage, friction, safety factor, whatever. Don't scrimp on torque. Size the gearmotor accordingly. An Old-Skool Detroit axiom is "steel is cheap, use plenty of it". Same applies for torque: go big. Your motors will only draw as much current as necessary to provide the torque demand.

For non-symmetric system, is similar but now static torque may be significant. You'll have to analyze, and possibly add that quantity to the total Torque demand.

Want to start / stop quickly? You need a lot of torque because Δt will be small and in the denominator. Will never be instantaneous, but if you spend enough money then it can be quick. That is, IF your bearings can survive the beatings they'll receive.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[RichD80]

Thanks for the feedback TygerDawg and zeusfaber.

MintJulep - I'm afraid you're assuming a lot about the application. I used the word crane head to show how the application is configured. The actual system is a simple lift that will lift the load no more than 0.5m off the ground.

The current solution is manual, is tried and tested for many years and works well - we're simply looking to replace the manual rotational effort with a simple motor / gear solution. The risk of the load falling or whatever else you imagine it will do does not increase by adding an external motor, I simply need to make sure the system is dynamic enough to maneuver the load effectively.