Sizing a motor to spin a box which weighs about 600lbs

[Jason mayak]

Not sure if this is better suited for the electrical engineers or mechanical but I figured id post it up here to start. I am looking to size a motor that will spin a box which weighs about 600lbs. The motor needs to be in line with the boxes rotational axis. It needs to be able to rotate 360 degrees in atleast 1 second. It would be parallel to the Earth and the box will be about 6 feet long and 4 feet high and 4 ft wide.

What formulas do i need to know in order to calculate necessary motor parameters?

 

[anorlunda]

The answer depends mostly on friction. None of the numbers you gave us relate to friction.

How is the weight supported? Perhaps with a roller bearing or a ball bearing.

 

[jrmichler]

What formulas do i need to know in order to calculate necessary motor parameters?

Wrong question. Right question: What information do i need to know in order to calculate necessary motor parameters? You need to know:
Is the load balanced around the rotational axis? If not, what is the maximum unbalance torque?
What is the rotational moment of inertia?
What is the motion profile?
Accelerate from zero velocity and keep rotating?
Start at zero velocity, end at zero velocity? If so, will you use a constant acceleration / triangular velocity motion profile?
What type of bearings? Even ball bearings have measurable friction torque.

Then you calculate the frictional torque, the unbalance torque, the peak acceleration torque, the peak deceleration torque, and the peak RPM. Last, you add up the torques to get the peak torque.

You start looking at motor parameters only after finding the necessary torque and RPM. Don't forget to include the motor inertia, and the effect of gear ratio, in your calculation. Hint: You will need a gear reducer. Another hint: This application will need a servo motor. Further hint: This application will need a servo class gear reducer. Penultimate hint: Reflected inertia is proportional to the square of the gear ratio. Last hint: Reduction drives have friction and inertia that must be included.

 

[CWatters]

Start by making a diagram with dimensions and details of how the mass is distributed. The moment of inertia will be different for a solid or hollow box. Is there any contents free to move about (eg loose rocks) inside?

You say it must rotate 360 degree in 1 second. Is that from a stationary? If you just want it to do 60rpm constantly how fast must it accelerate from stationary to 60rpm?

 

[Jason mayak]

Wrong question. Right question: What information do i need to know in order to calculate necessary motor parameters? You need to know:
Is the load balanced around the rotational axis? If not, what is the maximum unbalance torque?
What is the rotational moment of inertia?
What is the motion profile?
Accelerate from zero velocity and keep rotating?
Start at zero velocity, end at zero velocity? If so, will you use a constant acceleration / triangular velocity motion profile?
What type of bearings? Even ball bearings have measurable friction torque.

Then you calculate the frictional torque, the unbalance torque, the peak acceleration torque, the peak deceleration torque, and the peak RPM. Last, you add up the torques to get the peak torque.

You start looking at motor parameters only after finding the necessary torque and RPM. Don't forget to include the motor inertia, and the effect of gear ratio, in your calculation. Hint: You will need a gear reducer. Another hint: This application will need a servo motor. Further hint: This application will need a servo class gear reducer. Penultimate hint: Reflected inertia is proportional to the square of the gear ratio. Last hint: Reduction drives have friction and inertia that must be included.

Wow. Okay so I have been a little naive here thinking my problem was much simpler than what it is. Thank you for your reply. Rather than me asking a bunch more questions on how to solve this stuff is there any book you might recommend that would help guide me in solving the answers to some of these questions? I'll start searching the internet but if you know of a good book which might help me out I'd appreciate that.

Thanks,

 

[berkeman]

Rather than me asking a bunch more questions on how to solve this stuff is there any book you might recommend that would help guide me in solving the answers to some of these questions?

No worries, Jason. From your New Member Introduction post:

Hello all,
My name is Jason. I'm an EET student working on a final project. Looking for some help on solving a mechanical issue is what brought me to this forum. Hopefully I can contribute in some way to helping others out as well.

Thanks

it does seem like you have some of the background for this. If you could answer a few of the questions raised above, that would be the biggest help. If you can UPLOAD a sketch (PDF or JPEG) of how you picture this going together, that will help to answer our bearing loading and friction questions. And if you can answer the questions about the dynamics (how long to spin up the box, how long to spin it down, etc.), that will give us all a starting point.

To answer your question about books, any standard intro ME book should cover a lot of this, and your basic physics classes covered friction (like bearing friction) and how the Moment of Inertia affects angular acceleration, right? Please tell us a bit more about the classes you have taken so far in your EET education that apply to your project. Thanks.

 

[CWatters]

There are some articles on the web with titles such as "Motor sizing made simple/easy" but don't get too excited. You always need to know and understand the nature of the load in some detail. Pretty sure you will also need a gearbox.

 

[Jason mayak]

Here is quick sketch of what I'm getting at. As far as my background I am going through an electronics engineering technologies course with Lincoln tech. Its an accelerated program for electronics technicians. So far my course have been basic and specific math in regards to electronics and theory in the same manner. So we will receive a course on physics in a later term but I haven't gotten there yet. I don't believe they will cover it in the way a traditional school will either.

I will have to think more about what I am doing in order to answer the previous questions. I work fulltime on top of school. I'm an industrial mechanic so some of this is not necessarily foreign to me but I have no mechanical or mechatronics schooling only on the job training. I have a lot to learn obviously.

My final project for school is going to be taking data from an accelerometer via a radio signal and sending it to a motor controller to rotate this box. I have about 3 weeks until my prpjects abstract is due which is why I'm looking into all of this now.

 

[berkeman]

I am looking to size a motor that will spin a box which weighs about 600lbs. The motor needs to be in line with the boxes rotational axis. It needs to be able to rotate 360 degrees in atleast 1 second. It would be parallel to the Earth and the box will be about 6 feet long and 4 feet high and 4 ft wide.

Thanks for the sketch, it's a big help.

Can the horizontal axle go all the way through the box, or will it only contact the two sides as shown? If it only contacts the two sides, the loading on those bearings will be quite high and hard to manage, IMO.

And when you say you need to be able to do one full horizontal rotation per second, is that after a spin-up period of, say, 30 seconds, or do you need to be able to go from a full stop all the way through one turn in one second. If the latter, that is going to take a very powerful motor...

 

[jrmichler]

You can simplify your design by using one of these gear reducers to support the box: https://alpha.wittenstein-us.com/pr...273.511492982.1540936180-789768976.1540936180. They are designed to support overhung loads, so you can support the box without needing an outboard bearing. Just size the reducer to handle both the peak drive torque and the overhung load.

You really need the moment of inertia of that box in order to have any idea of the torque required to rotate it. The easiest way to get that is to find somebody to model the box in SolidWorks, then have SolidWorks calculate the moment of inertia (Tools > Evaluate > Mass Properties). The model does not need to be detailed as long as the outside dimensions and total mass are reasonably close. It's a five minute job for somebody familiar with SolidWorks.

Note that half the items I listed do not require getting a book. Those items need to be determined before you start looking for a book.

 

[Jason mayak]

Thanks for the sketch, it's a big help.

Can the horizontal axle go all the way through the box, or will it only contact the two sides as shown? If it only contacts the two sides, the loading on those bearings will be quite high and hard to manage, IMO.

And when you say you need to be able to do one full horizontal rotation per second, is that after a spin-up period of, say, 30 seconds, or do you need to be able to go from a full stop all the way through one turn in one second. If the latter, that is going to take a very powerful motor...

The axle cannot go through the box. And the rotation would need to be from a dead stop so yes a very powerful motor is what I am anticipating...and exspensive I am sure.

 

[Jason mayak]

You can simplify your design by using one of these gear reducers to support the box: https://alpha.wittenstein-us.com/pr...273.511492982.1540936180-789768976.1540936180. They are designed to support overhung loads, so you can support the box without needing an outboard bearing. Just size the reducer to handle both the peak drive torque and the overhung load.

You really need the moment of inertia of that box in order to have any idea of the torque required to rotate it. The easiest way to get that is to find somebody to model the box in SolidWorks, then have SolidWorks calculate the moment of inertia (Tools > Evaluate > Mass Properties). The model does not need to be detailed as long as the outside dimensions and total mass are reasonably close. It's a five minute job for somebody familiar with SolidWorks.

Note that half the items I listed do not require getting a book. Those items need to be determined before you start looking for a book.

I will be looking into these gearboxes tonight. Thank you. I don't know anyone with solid works, I imagine its exspensive software. I will look locally for someone with it to possibly get a model simulated. Thanks again.

 

[berkeman]

You could just start by assuming the mass of the box is evenly distributed throughout the volume (so look up the MOI of a rectangular solid). For the angular acceleration you would start with 2π radians/s^2, I think. Given that MOI and acceleration α, what do you get for the torque (before any losses or gearbox)? What ballpark motor size does that imply?

 

[berkeman]

You could just start by assuming the mass of the box is evenly distributed throughout the volume (so look up the MOI of a rectangular solid). For the angular acceleration you would start with 2π radians/s^2, I think. Given that MOI and acceleration α, what do you get for the torque (before any losses or gearbox)? What ballpark motor size does that imply?

So using the assumptions I mentioned (and using some online units conversion calculators because I'm not good at going back and forth between metric and imperial units), I get that the torque required is about what you get from a Ford F150 truck engine, about halfway between their standard motor and their top-of-the-line biggest engine.

Are you sure you want to go there? Is this just an academic exercise?

 

[Jason mayak]

So using the assumptions I mentioned (and using some online units conversion calculators because I'm not good at going back and forth between metric and imperial units), I get that the torque required is about what you get from a Ford F150 truck engine, about halfway between their standard motor and their top-of-the-line biggest engine.

Are you sure you want to go there? Is this just an academic exercise?

Lol yeah I'm sure that I don't want to go there. So at this point I'm just going to continue to solve my problem for fun sake and choose a different direction or approach for what I was trying to do. Appreciate the help and guidance. You and the other members have been extremely helpful. I'll post my conclusions as I obtain them so you can see my progress and comment on any mistakes or oversights on my part.

 

[CWatters]

Is there anything going in the box? If not why so heavy?

 

[Jason mayak]

Is there anything going in the box? If not why so heavy?

It was an assumption of weight. What would be inside the box is about 150lbs of gear a person with max weight of 250 and about 200lbs of aluminum tubing to make the "box". I wanted to create a cockpit that would respond to an accelerometer from an rc plane. These aren't measured weights or anything so far this has just been an idea in my mind. The sketch i uploaded i drew yesterday during class and was the first time i even attempted to put anything on paper. I've seen a similar design by a chinese company on youtube which is where I got this thought from.

 

[jrmichler]

Since you are an EET, not a senior in ME, we can do some calculations for you.

Assume 600 lbs rotating mass that is, on average, 2 feet from the center of rotation. The moment of inertia (sorry about the English units) is 600 / 32.2 * 2^2 * 12 = 895 in-lb-sec^2.
Assuming start from dead stop, rotate one revolution to a dead stop in one second: Acceleration = 2 * PI / 0.5^2 = 25.1 rad/sec^2.
Maximum velocity = 25.1 rad/sec^2 X 0.5 sec = 12.6 rad/sec = 120 RPM.
Torque = 895 in-lb-sec^2 X 25.1 rad/sec^2 = 11,250 in-lbs = 9400 ft-lbs.
Peak power = 9400 X 120 / 5250 = 22 hp = 16 kW.
Assume a 20:1 ratio gear reducer, then the required motor torque is 11,250 / 20 = 563 in-lbs, and peak motor RPM is 20 X 120 = 2400 RPM.
Tentative motor choice is Allen-Bradley MPL-B560F, rated 600 in-lbs peak torque and 3000 RPM peak speed.

If you want to see what's available for servomotors, check this document on page 79: https://literature.rockwellautomation.com/idc/groups/literature/documents/td/knx-td001_-en-p.pdf. The MPL-B560F is a middle size motor in their middle size servo motor line. If you want to drive big loads, their big HPK series servo motors are on page 139.
Siemens has a similar line of servo motors.

 

[Jason mayak]

Thanks. You've done a good amount of work for me and i appreciate it. That motor isn't as exspensive as I was imagining it would be either.

 

[Baluncore]

Seeing the two powered perpendicular axes in the diagram immediately raises the need to assess the gyroscopic effects on the motors, bearings and structure. You may be able to ignore those effects if it is used as a slow positioning system, but not if it is a continuous rotation system such as a rotary molding machine.