Need help with motor/gear selection for robotic arm

[imaginatorgtr]

Hello, I am building a 4 DOF robotic arm for a university project. As this is my first experience with robots, I need a bit of help with the selection of hardware.

The requirement for the joint at the base is as follows:

Speed: 3.33 rpm (this is the given requirement)
Torque: 133 Nm (calculated with simple statics)
Power output: 46.37 W (calculated with P = T x speed, please correct me if I am wrong)

I have found the following geared motor from Xajong, a Taiwan company:

Operating speed: 3200 rpm
Torque output: 0.21 Nm
Power output: 40W

My questions are as follows:
1. Am I right that I will need a gearing system with a ratio of approximately 1000 to step down the speed to 3.33 rpm?
2. By stepping down the speed with a gear, do I also step up the torque output by a factor of 1000?
3. Therefore, including efficiency of gear, am I getting 1000 x 0.21 x 0.9 = 189 Nm of torque output?
4. Since 189 Nm > 133 Nm, which is the amount if required, this motor should be good enough. However the power output is less than the amount required, and it contradicts the conclusion based on torque. This is when I get confused.
5. Lastly, any recommendation of a suitable motor for this case? Suggestions will be greatly appreciated. Motor should be as light as possible.

Helps will be greatly appreciated, since I am rather lost with what I should do as this is my first ever experience with building a robot.

Thank you!!

[Soeren]

Hi,

1 - Yes.
2 - Yes, minus the gearing loss caused by friction.
3 - You don't know the loss in the gearing until you design and build it. Precision and material selection will affect this in a very big way.
4 - According to the formula: P=T*2*Pi*RPM/60, the Xajong motor is 70.4W   (You still have to factor in friction losses in your gear box though).
5 - Hey, you have to do a little of your work yourself

[imaginatorgtr]

Hello, thanks for the reply, but I wish to clarify something regarding question 4.

I thought with that formula that you mentioned, we are calculating the power that the motor has to produce for this particular task. So since the motor specification says that its power output is only 40W, does that mean that the motor is not suitable?

Thank you.

[colorclocks]

Note that torque is not constant, but varies with speed.  See, for example, http://lancet.mit.edu/motors/motors3.html#torqueunits.  You won't get any useful information by multiplying the max speed by the max torque.  The motor will have zero torque at its max speed.  (That's why they also call it the no-load speed.)  Most likely the manufacturer is quoting the max torque, which the motor will deliver when it's turning at roughly half of its no-load speed.

[Admin]

You'd do best by buying a motor with built in gearing. A bit expensive however . . .

Search the sites that sell motors for battle bots, plenty with your specs.