Hey engineers - I need a mechanical elbow!
November 11, 2013 6:44 AM Subscribe
I'm working with some science-types on a project, and we're looking for a joint that gives a similar range of motion to a human elbow or shoulder. Is there a name for this, and is our current path to finding some a good one?
I'm helping out a friend with a project, and we've come upon a very specific need. We're both engineers by trial and failure alone (no formal training), and we need a joint that will give a good (180 degrees would be great) range of motion between two pieces.
I use the term "pieces", because depending on this path bearing fruit, those pieces may be anything from PVC tubing to aluminum extrusion. Weight is very much a factor, so ball joints for steering columns and the like is a bit too heavy for our use. The closest thing we've been able to find so far is this, which is fairly close to what we need, but ideally, the range of motion would be a wider cone.
If you can't see the link, it's essentially a three piece cup-and-ball system, where the cup is threaded on the outside, aligned with a top piece that completes the cup when threaded together. There is a hole in the top of the upper portion that allows the ball, with attached threaded bolt, to sit in the cup with its bolt sticking out. This allows the ball to move at about 120 degrees all the way around.
So here's the question: A) Is there an actual name for the kind of joint/part we're searching for (other than "ball joint", etc...) and B) Is there a better alternative in the industrial world than the projector mount linked above?
I'm helping out a friend with a project, and we've come upon a very specific need. We're both engineers by trial and failure alone (no formal training), and we need a joint that will give a good (180 degrees would be great) range of motion between two pieces.
I use the term "pieces", because depending on this path bearing fruit, those pieces may be anything from PVC tubing to aluminum extrusion. Weight is very much a factor, so ball joints for steering columns and the like is a bit too heavy for our use. The closest thing we've been able to find so far is this, which is fairly close to what we need, but ideally, the range of motion would be a wider cone.
If you can't see the link, it's essentially a three piece cup-and-ball system, where the cup is threaded on the outside, aligned with a top piece that completes the cup when threaded together. There is a hole in the top of the upper portion that allows the ball, with attached threaded bolt, to sit in the cup with its bolt sticking out. This allows the ball to move at about 120 degrees all the way around.
So here's the question: A) Is there an actual name for the kind of joint/part we're searching for (other than "ball joint", etc...) and B) Is there a better alternative in the industrial world than the projector mount linked above?
Best answer: My lab has a very specialized piece of equipment for positioning tools that allows for a very large range of motion and has at least two inline ball joints (which are very similar, if not exactly the same as the item in link you posted) connected by a elbow-like joint. Having two pivot joints in series will give you more range of motion than just one alone.
posted by scalespace at 7:18 AM on November 11, 2013 [1 favorite]
posted by scalespace at 7:18 AM on November 11, 2013 [1 favorite]
Ok I'm gonna pull rank here - an engineer "by trial and failure" doesn't exist, you're an engineer by training and certification. (Customer Service "Engineers", yes that means you too.) I didn't bust my lady-nuts in university for nothing. You're an experimenter or an inventor.
You're talking about degrees of freedom; human elbows have two (flex/extend and forearm rotation). Do you want it to only perform the flex-extend action of an elbow (but a full 180 deg extension) or do you want the full 6DOF (XYZ axis plus rotation)? I think you mean the latter, based on your ball-joint example. You could google "ball joint swivel" and try to find the right part. (Or google around for those celphone windshield mounts, they are made of plastic and have good range of motion & DOF, and then hack it.) Failing that, you may have to build a system of joints like scalespace said, or.... just make one. Do you have a milling machine or do you know someone with one? Just find a hobbyist on craigslist or kijiji and they could make you one.
posted by St. Peepsburg at 7:40 AM on November 11, 2013
You're talking about degrees of freedom; human elbows have two (flex/extend and forearm rotation). Do you want it to only perform the flex-extend action of an elbow (but a full 180 deg extension) or do you want the full 6DOF (XYZ axis plus rotation)? I think you mean the latter, based on your ball-joint example. You could google "ball joint swivel" and try to find the right part. (Or google around for those celphone windshield mounts, they are made of plastic and have good range of motion & DOF, and then hack it.) Failing that, you may have to build a system of joints like scalespace said, or.... just make one. Do you have a milling machine or do you know someone with one? Just find a hobbyist on craigslist or kijiji and they could make you one.
posted by St. Peepsburg at 7:40 AM on November 11, 2013
Ok I'm gonna pull rank here - an engineer "by trial and failure" doesn't exist, you're an engineer by training and certification. (Customer Service "Engineers", yes that means you too.) I didn't bust my lady-nuts in university for nothing. You're an experimenter or an inventor.
Eh, not really sure what rank you think you're pulling... but while I can respect the fact that I'm sure you worked very hard on an engineering degree in college and I'm sure you're also a very good engineer in practice, the two are not required and many times in my experience not related. Engineering is simply the application of scientific, economic, social, and practical knowledge in order to design, build, and maintain structures, machines, devices, systems, materials and processes (thanks, Wikipedia!). I know several mediocre-bordering-on-terrible engineers that have engineering degrees and I know some excellent engineers who are engineers by experience only. I'm an engineer by certification, but not by undergraduate education. Was I not an engineer for all those years between beginning work in engineering and my certification, when I was working circles around peers that had engineering degrees?
Sorry for swerving off topic, OP. It sounds like you understand the type of joint you're seeking, but rather are looking for one with greater range of motion than you've found. I'm not familiar with a single piece allowing for your desired specs, but you could certainly achieve this fairly easily with a system of joints as scalespace mentioned.
posted by Vonnegut27 at 9:44 AM on November 11, 2013
Eh, not really sure what rank you think you're pulling... but while I can respect the fact that I'm sure you worked very hard on an engineering degree in college and I'm sure you're also a very good engineer in practice, the two are not required and many times in my experience not related. Engineering is simply the application of scientific, economic, social, and practical knowledge in order to design, build, and maintain structures, machines, devices, systems, materials and processes (thanks, Wikipedia!). I know several mediocre-bordering-on-terrible engineers that have engineering degrees and I know some excellent engineers who are engineers by experience only. I'm an engineer by certification, but not by undergraduate education. Was I not an engineer for all those years between beginning work in engineering and my certification, when I was working circles around peers that had engineering degrees?
Sorry for swerving off topic, OP. It sounds like you understand the type of joint you're seeking, but rather are looking for one with greater range of motion than you've found. I'm not familiar with a single piece allowing for your desired specs, but you could certainly achieve this fairly easily with a system of joints as scalespace mentioned.
posted by Vonnegut27 at 9:44 AM on November 11, 2013
The physical volume in which you can position a point on your linkage defines something called the 'configuration space'. There are usually many approaches to getting a linkage designed that will get you there. St. Peepsburg used the appropriate term 'degrees of freedom' and it's a concept you should get familiar with. It comes in handy.
A lot (gazillions) of old-timey linkages and mechanisms are out there for achieving specific goals, and I am not sure what to recommend because your description doesn't give me a clear enough definition of your problem.
There are very few spaces you can't design a linkage to navigate, so it doesn't seem like too much of a problem to me. If you haven't got the imagination, tools, experience to come up with it, maybe you need to assume it and hire the job out while you deal with other parts of the problem that are closer to your main competencies. I do this a lot, historically. You can't be really good at everything.
posted by FauxScot at 1:45 PM on November 11, 2013 [1 favorite]
A lot (gazillions) of old-timey linkages and mechanisms are out there for achieving specific goals, and I am not sure what to recommend because your description doesn't give me a clear enough definition of your problem.
There are very few spaces you can't design a linkage to navigate, so it doesn't seem like too much of a problem to me. If you haven't got the imagination, tools, experience to come up with it, maybe you need to assume it and hire the job out while you deal with other parts of the problem that are closer to your main competencies. I do this a lot, historically. You can't be really good at everything.
posted by FauxScot at 1:45 PM on November 11, 2013 [1 favorite]
Response by poster: Thanks to everyone! For whatever myopic reason, I hadn't considered an assemblage of a series of joints to build the system I needed. I think that, coupled with the suggestions of specific parts, might get us there. Thanks again
posted by ASoze at 8:15 AM on November 14, 2013
posted by ASoze at 8:15 AM on November 14, 2013
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I would look for U-joints, like this one meant for ratchet sets.
posted by backseatpilot at 7:02 AM on November 11, 2013 [1 favorite]