6dof torque v payload v output arm

Hi all,

I'm still researching motor/gearbox combinations for my 6dof flight simulator build.

Can somebody tell me the minimum torque requirements I should look for when sourcing components and how they relate to payload on a 6dof?

If I take 1/6th of the payload and apply it to one arm at the required distance from gearbox centre I get the minimum torque required to lift the payload. What % over that figure should I aim for? Is there a greater torque requirement for movement on the other axis?

My AUW on the moving platform will be less than 120Kg. To raise the platform on an arm 10cm long is 19.6Nm but this only accounts for movement where all six motors are acting together and is why I need help.

Am I safe at 25% higher?

I asked pretty much the same questions the other day for mine although it's only a 2Dof with traction loss.
From what I can see after the gearbox you'd ideally want 20Nm+ for most simulators. I based that on what other people have built. My thinking is that when it comes to the real world torque required then it's best to overspec a bit to give you the option of having speed and power should you be running an aggressive profile. I suspect 25% would be fine.

Someone with more experience will be able to give you more accurate specs but I think it will depend heavily on your center of gravity and distance between pivot points on your rig.

I just posted much the same question for a 3DoF stewart platform in the newbie faq forum.

I'm starting to lean toward maximum radial torque output = maximum payload per motor, but I have to check with a mechanical engineer. That bears the caveat that your platform is balanced, and also assumes that you don't have to derate the shaft rating because of a combined axial and radial load(depends on manufacturer, but seems to be about 20-30%). I have doubts that would ever be the case, but I'm having problems visualizing the force directions on a mobile platform, and I don't have the skills necessary to model it in CAD and simulate the physics.

Personally I'm thinking of sourcing the gearboxes and motors for a safety factor of 50%. This gives you options of greater speed or torque if necessary, and a real world safety factor so that a shaft is less likely to break under load if you overload the platform. And +50% safety rating =/= 50% increase in cost, so its all increasing returns.

The problem for me is I priced the next size up on the unit I was originally going to purchase and discovered a 10% increase in torque resulted in an 80% increase in cost. When you factor the cost of six it makes a big difference.

The unit I originally speced was rated at 40Nm of torque.

I'm doing calculations based on what you gave me for torque and weight of platform. Give me a few hours to get home from work and I'll post a breakdown that other more qualified people than me can pick apart.

Thanks @DMUSER

Overhung Load - A type of eccentric loading in which a force is applied to the shaft perpendicular to the shaft's axis beyond a support point. Overhung load is also called side loading.

http://www.electricmotorwarehouse.com/leeson_3phs_rt_angle.htm

I did all kinds of math for axial torque, radial torque, and was looking on how to calculate maximum shaft loading, then I found the above information, and threw everything out. Smarter people than me have already done this.

I believe that Overhung Load would apply directly in this circumstance:


at 49 seconds there is a good shot of the arm attached to the gearbox shaft almost flush with the end, and the rotation part where it meets the secondary support arm. The secondary support arm is outside of the shaft length, so overhung load comes into play.

This is greatly reduced, or possibly even removed entirely by the fact that they used a double output shaft with a central meeting point center of the gearbox. That means the motor supports way more weight this way than it ever would with a single output shaft and an overhung load.

Based on the load specs on these motors, it looks like a NEMA rated 3/4hp 3phase motor is going to be pretty good overkill even with 3 of them instead of 6. I'm willing to bet you could get away with 1/2 HP NEMA rated motors with 6 of them.

@cthiggin sent me this link:
http://www.automationdirect.com/adc..._100HP)/Marathon_Inverter_Duty_Motors_(by_HP)

Hopefully this helps you out in some way.

Have you guys looked at the principle of the inertia transfer on direction change, with out 12v sims we have found they double there amp draw on direction change, using 40nm 200W motors that draw up to 45-48amps on a sharp direction change, my opinion this is the max weight change the motors will experience and will be greater than just supporting the load.
But I must remember the 12v systems react so much quicker 180rpm against 26-30rpm so the need torque to change direction at high speeds is more than double the motors ratings. And yes they get a little warm doing this too.
For a 6DOF seen a couple running wiper motors and we know how weak they are, but they work.

Thanks for the help guys. I've ordered two units for testing. I went with my origional spec (40Nm 30 size). The more powerful option was just too expensive. I needed to bite the bullet or I'd be procrastinating forever.

If they don't work out I'll automate my lawnmower or something

A direction change would be a CW rotation, a braking operation, a CCW rotation. That's assuming you have electric braking, and it doesn't just directly reverse. This would be equivalent to locked rotor current, or starting current, which is generally 6 times the full load rated current in the case of an AC motor. I actually wouldn't be surprised to see the current spike up as high as 100A for a 16A FLA motor for a very brief interval if you hooked up an in-line ammeter with digital logging. It likely wouldn't even be long enough to blow a single element fuse unless it was really really tiny, certainly not as large as 40A.

How many RPM do you normally run your motors at? 200 RPM seems like a ton of jam to me for the application, that's over 3 rotations in a second. Do you need the excess speed to simulate rough road conditions or something I'm maybe not considering?

How much duty life do you get out of a motor you are running like that? The duty cycle I've seen on most industrial AC gearmotors is 1000-1500 operations per hour. Which is 16-25 per minute, that's a full start and a stop every 2-3 seconds maximum before you have to start derating the operating lifespan of the motor and gearbox. Do you compensate for this in some way with regular bearing replacement, cooling fans, or lower speeds?

Definition: Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute. When work is done by a source of torque (T) to produce (M) rotations about an axis, the work done is:
radius * 2pi * rpm * lb (or 2pi *TM)
When rotation is at the rate N rpm, the HP delivered is:
HP= (radius x 2pi x rpm x lb) / 33000
is equivalent to:
HP = TN/5250

So for example, solving for lb when HP is known:
(3/4HP * 33000) / ( 1 ft *2pi * 40rpm)
lb = 24750 / 251.33
lb = 98.4lbs

To me that is the lifting capacity of a 3/4hp motor running at 40rpm. So direct lifting capacity without a gearbox, just with an AC drive running the motor at 40rpm.

Someone can check my math for me.

http://www.reliance.com/mtr/flaclcmn.htm#kvahp