Let's Make Robots!

Heavy Duty Mechanical Components

Hey Guys,

I'm looking to make a semi-functional heavy duty robotic arm.  It's for an art project so it won't need absolute precision, it's more for aesthetics and entertainment.  The arm could potentially be 4-5 feet but it wouldn't need to lift anything except itself or a flashlight.

I have a handle on programming, metal fabrication, and electronics.  

What I have no clue about is where to look for joint components and related hardware.  I have been reading this forum and a few other websites and most of what I see is focused on smaller scale high precision robots, or the large scale precision joints all cost $500+.  I'm in the NY metro area in case anyone knows of a local spot.



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Don't try to make your motors do all the work. It's much easier to "spring load" your joints or use gas-filled cylinders to support the weight of the arm to a zero balance point. Then, all your motors have to lift is whatever your actual load is in the gripper.

Thanks for the info.  I'm actually less concerned about motors.  I read a thread where someone joked about using winch motors with accelelormeters to figure out angle.  I'm fine with something along those lines.

What I don't know is where I could purchase joints (physical joints) and gears, etc.  Let's say I bought a larger stepper motor, is there anywhere I could source mounting components that would let me easily construct the joint or do I need to design/fabricate myself.



I mentioned the PVC and weight for the same reason he said the arm couldn't weigh much of anything. We really need to know what you are attempting to have it look like rather than tell you how to make a joint. :)

got it.  thanks guys.  let me play with some modeling software a little then I can show you what I'm thinking

Please see attached this image

The dimensions are 4', 3' and 2'.  This is negotiable but I would like the robot to be on the larger side.  The black lines represent joint motion.  Bottom joint is rotational the top 2 are elbow like.

I do not need to cary much weight, just the structure itself and perhaps max 1lb of weight on the tip.

I have a decent (but not huge) budget so I can order professional joints and do various metal fabrication.  If you guys have any advice would be great.  So far ideas that have made sense are using springs to try to balance the weight, and also using brake line cables so that I do not need to mount the actual motors on the robotic arm (although that apparently has quite a bit of give).


274 Oz/In servos means that your 5 foot arm can't weigh any more than 4oz - which is possible but fiendishly hard.

If you need to lift something like a torch - lets assume a maglite - at that length, your looking at something like 960 Oz/in.

You either need to be thinking about gearing or a mechanical method which gives mechanical advantage, like block and tackle or hydrualics.

Lets say you use a nema 23 stepper stepper motor at about £100 each, you'll need a 3:1 stepdown per joint in terms of gears - which will also need around 3amps - 4amps per joint as well to hold the joint still when not in motion.


If your going to make it, I would use plywood, and cut out a load of webbing sections to strip as much weight as possible out. I would laminate 2 or 3 sheets of thin wood, rather than use one thick piece, and I would also use thin ( 2mm square ) stringers - it'll easily bear the torque your looking to apply, and be lighter than pretty much anything else. 

Joints etc depend on the material of the main arm lengths, their sectional shape and their size. If you post up some more details, we can offer more advice.



page, the best I could manage was http://www.dfrobot.com/index.php?route=product/product&path=47&product_id=445 .

Those are listed as 1/4 scale servos rated at 274 oz-in torque @ 4.8v. I would think that a pair at each joint would allow one to move a couple of pounds per arm segment. I looked and found that 2 1/2" PVC is said to weigh 1.07 lbs/ft. I am truly unsure if you can stack servos for more torque at a given joint, but, it is either that or you step up to stepper motors and stepper motor drivers.

The preceding is only my 2 cents, if that.