"slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
"slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
(OP)
Hi, I am trying to design a lifting mechanism for a quite large (in area not weight) platform that only uses rotation for its actuation. I have a design that uses a mechanism similar to the slider-crank and I want to maximize the total height the platform can travel.
This is a sketch of my design:

The rotation is going to be executed by a motor at the red dot and my dilemma is about the behavior of the two arms when they reach the position at the middle of above image. I think this is called a singularity or dead point.
If the platform is going down, the way I have suggested in the sketch, at that point I see a weak mechanical position when the platform will tend to fall more than it should because of its weight. This is where I would loose precision in movement and I need a solution to overcome this. Is this true? What are the weak points of this design? Just a note: there is going to be direction changes in rotation at arbitrary positions.
A preloaded spring was suggested to me but I don't exactly know how should that be setup, to which direction is better? Is there any other (simple/cheap) solution employed by other such mechanisms/robot arms?
I am also thinking about having gears at the green dot ends of both arms but that would introduce backlash when the rotation would be reversed.
PS: I don't want to change the slider-crank-based design to a leadscrew or something else, dut to other factors it has to be rotational.
Thank you for your suggestions.
This is a sketch of my design:

The rotation is going to be executed by a motor at the red dot and my dilemma is about the behavior of the two arms when they reach the position at the middle of above image. I think this is called a singularity or dead point.
If the platform is going down, the way I have suggested in the sketch, at that point I see a weak mechanical position when the platform will tend to fall more than it should because of its weight. This is where I would loose precision in movement and I need a solution to overcome this. Is this true? What are the weak points of this design? Just a note: there is going to be direction changes in rotation at arbitrary positions.
A preloaded spring was suggested to me but I don't exactly know how should that be setup, to which direction is better? Is there any other (simple/cheap) solution employed by other such mechanisms/robot arms?
I am also thinking about having gears at the green dot ends of both arms but that would introduce backlash when the rotation would be reversed.
PS: I don't want to change the slider-crank-based design to a leadscrew or something else, dut to other factors it has to be rotational.
Thank you for your suggestions.





RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
If you add a fixed sprocket at the drive end and a sprocket at the pivot of the driven lever with a chain around them then the second sprocket will be driven to orbit the first and have moment continuity to continuously carry the load. A similar mechanism is used in orreries to drive moons around the planets. http://ecg.mit.edu/george/lego/orrery.html
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
https://video.search.yahoo.com/video/play;_ylt=A2K...
... sorry about the long link.
The mechanism comprises a fixed sprocket of size 2N, about the center of which a primary arm is driven by obvious means, and a secondary arm free to rotate at the tip of the primary arm, and driven in rotation by a sprocket of size N. The two arms are of the same length L center to center, and the total stroke of the mechanism is 4L. The tip of the secondary arm is constrained to move in a straight line. There is no dead spot.
I think it's a derivative of work by James Watt, who built it with gears, not chain and sprockets, to get around a slider-crank patent. I think you can see how it would work with gears of size 2N and N and an intermediate idler gear of not quite arbitrary size.
Mike Halloran
Pembroke Pines, FL, USA
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
1st: @Jboggs: I think your suggestion goes in line with 3DDave's and MikeHalloran's. I wonder how come I didn't find that video during my research, that would have saved you this thread. Nevertheless I am glad because I am sure I'll have many more things to ask.
I have seen the "How it's made" (Robot Arm on How it's Made ) for robotic arms but it didn't click to me that one of the many belts going from shoulder to elbow does exactly this.
@tbuelna: I can't have clearance in the joints or flexible arms/etc because this platform needs to go up and down with a certain precision, with no deviations from the vertical.
One more thought: if the initial position of the platform was up and it only descended, wouldn't the weight of the platform "help" it decide to go down at the singularity? This is what I was imagining when I started thinking about the application. The only mechanical problem would then be when I raise the platform all the way up, at that middle point it wouldn't choose to go above the center by itself, more likely it will go back down because of this weight bias.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
The other thing I don't like about the linkage approach is the wide variations in speed and torque as the constant rotary speed is converted to linear motion. You haven't mentioned a specific load or speed but normally that would be an issue.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
What about the weight of the platform helping overcoming the singularity indecision of the linkage? I am pretty interested in your opinions about this.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
I have sketched another design after I have realized funny things will happen at the center but I hoped there would be a solution to overcome that issue gracefully. Here's my sketch:
This will give me about 80% of the diameter the driving pin describes.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
TTFN
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RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
I am thinking also about having 2 such pairs, with one pair having a slightly longer arm that would put it in a position other than the singularity and which will dictate the direction when the other pair is in the weak position. But I am still thinking about such a solution feasibility.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
You can buy specialized roller chain with some links modified to accept plates to make a conveyor and instead bolt the plates to a beam, but for vertical travel it may be unnecessary.
In your case, imagine pillow blocks and a lineshaft with sprockets along its length, all placed near the top of your apparatus, and lengths of roller chain supporting your platform. Tie the unterminated ends of the roller chain to a dangling length of pipe or bar just to keep some tension in the chain.
Mike Halloran
Pembroke Pines, FL, USA
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
http://www.mcmaster.com/#racks-and-pinions/=z5hip1
unless you think that's still too expensive
TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
The difference is that instead of going the easy way, I set myself goals that would prevent me down the road to choose comfort. So I apologize for insisting but I feel I am quite close to a solution. If I need to lower my expectations I'd rather do that. @MikeHalloran's video suggestion proves it's possible, as well as his work if you believe him.
Back to my idea, having a platform to lift in a horizontal position you have to define the plane by at least 3 points in space. These points can be the upper ends of the rotating arm. Each arm can only rotate in a single plane, that's why the three will constrain a vertical movement.
Now we are left with the singularity issue. I know that pistons in a gasoline motor fire up in a sequential order to cover all 360 degrees of push -or at least that's how I came with the idea of having at least one of the slider-crank assemblies phase-shifted with 10-15degrees. This way it will still be in full torque when the other cranks will get in the weak position. What do you think would this work? Unfortunately I can't think of something even simpler that would stick to the goal of having rotary-only actuation.
Sorry for my long post, I expected at some point to be suggested some better alternatives that would go against my set goals.
PS: in my opinion, leadscrews/ballscrews/roller screws/rolling ring actuators are the best option for linear motion.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
We've had lots of problems with resonances in long lead screw systems
TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
I've built slider crank mechanisms to move some biggish stuff and it is disappointing when you see what you haven't considered up close.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
@dvd: I can't have neither the crank nor the rod too long due to size constrains (the lifting platform will end up having a too high lowest point) yet I want to maximize stroke.
I used the formulas at http://ocw.metu.edu.tr/pluginfile.php/3961/mod_resource/content/6/ch7/7-2.htm and for an crank=100, rod=150, eccentricity=20 the stroke gets me around 203, which is acceptable.
It doesn't compare though to the stroke of the mechanism linked by MikeHalloran.
I will start putting together an excel with what info I have so far. I also need to calculate how much stroke travel for each stepper motor step, so that I can tune the dimensions/gear ratios for the desired resolution.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
From this statement it sounds like you aren't planning on using any guide features to constrain the platform to only allow vertical motion. You will likely end up with twisting/destruction of your mechanism without something holding it in the proper orrientation.
Yet another option you could consider looking into for vertical motion of a platform is a scissor lift.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
It remains for a prototype to be built I guess... and improved from there. I am sure it's possible.
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
The first paragraph discussed a different arrangement, where the roller chain is actually kept straight, and bolted or welded (!) to a straight bar to make a light-duty rack. As mentioned, it needs to be de-rated because of the zero wrap angle, or it needs to be fastened only at the ends and equipped with wrap idlers so it looks a little like a Rolamite. (Remember the Rolamite? Did anyone ever find a real use for it?)
I should also mention that working with stepping motors can provide a large number of surprises.
One of the first you will encounter is that every single step is a complete motion of everything, so the first problem in analysis is that you have to know the inertance of everything, and if it's too great, the stepper will buzz, but not move.
The second surprise is that the stepper's performance is integrally linked to the performance of the circuit that drives each coil in turn, so any comparisons you do have to include a particular motor and a particular drive circuit.
There exist many books on application of steppers (search on Al Leenhouts in particular), and you will need to digest several such books before you are properly equipped to apply a stepping motor to a particular problem.
Mike Halloran
Pembroke Pines, FL, USA
RE: "slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?
Regarding your explanation, it would be very helpful for me to have some pictures to look at. Maybe you have some. Or maybe something similar... I have a hard time following especially as english is not my native language.
Thank you.