Robotmaker, first, I fully understand what the term OZ-IN.... "MEENZ". I got the formula from Society of Robots article. It's not mine. Second, I understand that this is the stall weight. So the rated OZ-IN will have to be higher or the weight lifted lower for it to work. As you saw in the post I was only using thumbnail picture to show that in order to lift the smallest weight at a robots arm length you will need a very high torque servo. Lastly, in your "tutorial" you neglected any mention of the weight of the arm as well. If I am in error, I will gladly admit it.

total weight depends on the torque of the servo,you need to add weight of the total length of the arm plus servo's you are using plus weight of the item you want to lift and how high you want it to lift plus another more then 25 % for over weight,and the servo has to be rated at 75% of the rated torque

and the article by society of robots is for basic arms with motors NOT SERVO'S its a little different then using servo's

• So in the beginning you just repeated what I said. The weight of the arm includes the servos and the max payload. Why reword it to sound like you made that up? The reason you add all that up and compute using 1/2 of the arm length is due to the center of balance, but even that is a thumbnail figure of where it might be. Its possible to design one where the center of ballance is 3/4 of the total length. If that were the case, then you need to rework your figures. The best way is to take the COMPLETE arm with the max payload you want to carry attached to the gripper and balance it on the back of a chair. Of course your robots elbow needs to be straight as well. Now measure that balance point to the pivot point. From there on the math is simple. If you want to make lifting easier you could add weight on the other side of the pivot point and will help. The greater the weight and the greater the distance from the pivot point (other side) the more it brings the center of balance to the pivot point thereby reducing the torque required. Most of our toy robots dont have room for that though.
• "and how high you want to lift", this statement is in error. Gravity is not going to vary. The numbers are rated at horizontal from pivot point. It is actually easier to raise the closer you get to verticle. Test it on your expensive machines.
• Adding 25% I can see just to be sure it breaks the stall point. "Servo rated at 75% of rated torque" is just increasing your margin of error with no basis on why. You could do one or the other. I havent done the math on which gives the greater margin of error but lets keep things simple for people. Remember there are a lot of people just starting out.
• Lastly, the article is about physics of torque. Not about any special device. It can be gas, electric or heck even nuclear machine. The physics remain the same.

only thing difference is that you ,that when building a arm you need to add the ounce per inch for servo,s on motors you dont need that ,society of robots article is about the physics of torque (yes you are right about that )

there is a very simple test to check to see how much a servo will lift take any servo attach 1 inch arm to it and a piece of string add a weight and see how much it will lift shound have a ampmeter inline with the power supply for stall current then try a 2 inch arm same weight you will see it half and so on. so if using a 400 oz servo and the arm is 24 inches long ,stall torque is 16.6 oz the above test can comfirm this

ALSO when you start building a hand design using servo's also start at the hand part and what max weight you want to pick up ,like all like the (beer fetching robot) so the grip area is first,then rotate ,need to add up all design weights ,like beer ,servos's that was picked for the right torque,plus all parts,then get total weight of the first arm part for elbow lift and see what torque is needed ,need to measure from beer can to center point of the servo motor in elbow, example 14 inches total then times total weight of that arm part,that will tell how much torque for that part then same for elbow servo the society of robots is fairly close to it ,but need to add total length of complete arm and divide by the torque of the servo,that will tell how much it can lift

"when building an arm you need to add the once per inch" Where would you add them? "on motors you don't need that". Are you saying that motors don't produce torque? I disagree. The servo already has a rating that shows how much it will lift from the center of balance of the total weight of the arm to the pivot point of the servo. It is divided down to ounce per inch. So yes, as the other formula has been trying to show, divide your sum total weight of the arm and payload by the distance of the "center of BALANCE" of that arm to the pivot point of the servo. total of arm weight=400 center of balance=24 inches from servo stall torque=16.6

Imagine an imaginary arm that has NO weight. This servo will stall at 400 ounces from one inch.

Now imagine that arm weighs 400 ounces but it is TWO inches in length. Its center of balance is still at the ONE inch mark and will stall at the SAME number.

Now imagine an arm that is FOUR inches in length but most of its weight is near the servo making is center of balance at the one inch mark. The servo will still stall at 400 ounces EVEN THOUGH ITS FOUR INCHES LONG.

Again this is stall torque rating given my the manufacturer. If you don't get the same results as described by them you may have a fake servo from china. There are been many reports to this effect and also in false mAh ratings on rechargeable batteries.

As Robotmaker says, figure in a margin of error. If you don't you may be disappointed after you made a hefty investment.

I hope that its more clear now that center of balance plays a part in arm design.

motors do produce torque ,but dont have a gear box like a servo ,so its not the same ask servocity techs they will tell you much more on selecting a servo for a arm design

try the test it works and proves the point,buy not trying a lot of rework so you lose much money arm designs are the hardest to design,took me awhile with many tests and losing some money and rework building arm,still i used part of the society of robot article and understanding OZ-INCH and running a lot of tests you need about 2000 oz servo to lift a beer with added weight ,plastics,gears, and all other parts if using a 400 oz servo it will lift a about a pound,check some og the robot arm designs the sell using a 400 oz servo max lift on a arm is under a pound beer weights about a pound for small can,bigger can 1.5 lb about and glass much more,so you need at least 4 lbs lift

my design at shoulder is 2000 servo (special) and 400 oz servo at elbow and 133 oz at wrist rotate ,it can lift little over 10 lbs big reason to use a higher torque servo is use half the current and stay under stall torque ,stall torque on mine is near 19 lbs lift so with lifting a beer it use much less current oin each servo main thing i look at in designing robots is using low current sensors,servo's ,motors and hardware even the led on the ezb ad ups too ,since it inside and not needed for anything,so use less weight on a extra battery or a short run time

Is that 2000 at the elbow or 2000 at the shoulder?

To lighten the weight of an arm, one could use "tendons" that attach to motors in the torso. I never tried it, but heard it could work.