Slider Crank Dead Spot Challenge

That's the best question this month so far, Mildred. Well done.

Electronic control of the motor and flywheel mass.

Not sure how you see this as links and sliders only?

"The rotation is going to be executed by a motor at the red dot"

Also, this is a start and stop and reverse and stop mechanism, not a constant motion situation.

I stand corrected, after being misled by the YouTube example.

Both systems, the original one and your alternate (neat concept, btw), suffer from a 'dead spot' where at the crossover point all the force acting on the vertical shaft is horizontal; there is no vertical component helping to lift or lower the platform.

I think if the vertical shaft was tilted, say 10 degrees, it would allow at least a small component of the force to resolve parallel to the shaft, at least partially minimizing the problem of the deadspot.

Actually, I realize that it is difficult to see, but my solution does apply force throughout, including the middle position because the shorter drive shaft link is applying force at the middle of the long link pushing the vertical slider up (and down). If you take a bit more time to think it through, you should see what I am talking about.

If there was a dead spot in my linkage it would be obvious when acticulating the model in SolidWorks, but the opposite is true. Articulating the model in SW, it becomes obvious that there is no dead spot.

Dead Spot ^ .

If you raised the (secondary) horizontal cam and slider, so that all 3 cams (or cranks?) were not parallel at this point, this would minimize the dead spot.

But I still think tilting the vertical shaft would be better.

As I look at this more, I wonder: What prevents both of the first two cranks from simply spinning in a circle when power is applied? Since the second crank is free to pivot at both points, even a small amount of friction at the first pivot points means it will simply follow the first crank. How does the first slider know it's supposed to go up?

The pin that goes through the slotted rail fixes the long link and the inner short link together as if they are a single link. Therefore the horizontal rail keeps the links from spinning in a circle at this point.

...Last post, then I think I'm done. Got other stuff to do.

I think I'd use a crank system that doesn't go through a dead spot.

To get the distance you need, have the crank drive a scissors mechanism.

I don't have to time to sketch it all out, but I'm sure you can see what I mean from these two parts. As the 1st crank goes around, it drives the second crank, which in turn drives the scissors assembly. The scissors assembly will expand and contact a lot, easily doubling the motion of the 1st crank.

Another good approach.

Make the link on one side at the top of the slider and on the other the bottom of the slider...

Can you illustrate how that would work, other than just adding another parallel dead spot at the same point of rotation?

Well you have to curve one of the arms on each assy , one one way the other the other way...

Even though your idea is going to require specific curvature cams to parallel the movement of the links and slider, I do like this idea and would probably go this route for simplicity in a more sophisticated manufacturing environment. The only downside is that the initial poster didn't want to have to make any special shapes that could not be made easily with basic drill and saw tools. Not that this goal was shared initially or that your approach is any less valid. I like it.

Thank you. Since the OP also mentioned the platform would move vertically I thought an elevator-style counterbalance system would be appropriate but did not include that suggestion since it went well outside the stated limitations. In its simplest form the slotted cam would have a perfectly straight slot since it would be partially assisted by the links in turn. There would be no need for curvature since the delivered torque would increase as the link torque decreased.

The only way the cam could be straight is if the contact point was going out to both sides to span a total distance equal to that of the entire linear travel, which would make it very similar to my mechanism. Otherwise the speed difference between linkage system and cam system would produce lockup. You would have to show me by visualization to convince me otherwise.

So why the fixation on trying to use a design that has an inherent flaw when there are other systems that are far more compact robust and easy to build and work with that can easily turn smooth rotational motion into smooth linear motion with less total components?

To me this is like saying you want to build a really good smooth riding vehicle with no suspension systems while using octagonal wheels rather than round ones.

Smooth as in amphibious?

Maybe instill a slight offset at the center of rotation?

can the motor be mounted on the vertical slide instead of fixed? If so, then put the slider for the platform higher up and use two arms attached to opposite side of the pulley, one attached to the platform and one to the base both free to pivot at each end.

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Another solution that does not require remounting the motor is to change the height of the slider and length of the arm such that less 180° is required for full range of motion and the upper pivot point either stays well above or stays well below the crank end pivot.

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The spot you are referring to as the dead spot is problematic because the position of the slider could easily vary widely in position with very little change in the crank end position. Weight will probably keep in at the lowest position. The motor has effectively the tallest gearing relative to the slider in that area, so position control will be numb. It may have a tendency to jam the slider.

It would be better to keep the motion such that the connection between the slider and crank end is more aligned with the load, and never perpendicular.

Yes, putting the motor at either end of the second link would fix the problem and is another good option in situations where it is ok to have the motor moving.

The second thing suggested in my comment it actually better and doesn't involve moving the motor.

For a while I was thinking that I should keep my linear linkage a secret. However, I have been racking my brain now for several days for different applications that would make my "Lucas Linkage" an exclusive best mechanism, but alas I am not so sure that there are any applications that just can't live without it. In short, it is now public domain: https://www.youtube.com/watch?v=6iLu__COq4w

I believe that this would be a good way to get rid of the sliders for this particular application. Even though the lift doesn't pass through the input axis (as stipulated), it is nice in that it allows infinite input rotation, unlike its geared cousin.

Enjoy...

That seems to be fairly devoid of simplicity. Will the working be readily visible? Is there something unmentioned in the criteria? Maybe like a requirement to impress/dazzle others with unusual mechanisms?

It might be an indicator worth considering if a mechanism is far easier to find as a generated image than any actual pictures/video. If it were a good idea, it would be in use, unless it is so novel it is just in the early stages. That seems unlikely for the task at hand.

I will admit that my linear linkage is really a second discussion, but as my correspondence with the original poster continued, the goal evolved into a linear linkage without gears. However, the reason I posted this as a discussion as opposed to a question was to stimulate creative dialogue as opposed to a fight to "win". This group troubles me a bit with how much it can stifle creativity for the purpose of "winning" best answer. I hope not everyone thinks every worthwhile mechanism has already been invented.

I get it now.

I enjoy examining different solutions to interesting non-pressing problems.

I misunderstood this to be a continuation of a discussion about something that required a functional reliable solution to be ready soon....which is not the time to be considering esoteric linkages that have failed to be adopted. My words were an attempt to steer you away from a path that would not likely result in a developed solution any time soon.

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I did not intend to take any wind out of any sails. I hate that when I see someone else do it.... there is a real investment required to foster calaboration, whether for creativity or team performance or attaining consensus, yet tearing it all down requires almost nothing at times, certainly no admirabe skill or clever insight.

True, and much appreciated.

Then there are those that submit an observation, without the desire for a "best answer" who are provided with terse remarks from the OP.

"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 lose precision in movement and I need a solution to overcome this."

Is it possible to attach counterweight, weighing more than the platform plus the payload? Such that the possible fall while crossing the center position will not arises due to the constant upward pull.

Replace the crank with a drum winch, fit a pulley at the top and a cable to the platform and you will have as much travel as you want with as much control as needed without any of the problems that you are creating for yourself here.

For some reason I cannot get CR4 to accept a drawing. I have tried .jpg, .png & .gif to no avail so I am going to try and describe a solution. Are you sitting comfortably? Then I'll begin. Note unless you have a good imagination and a pencil and paper the solution may prove harder than the problem.

Imagine the assembly so that the lower linkage is in the 12 o'clock position. Add an extension to the lower linkage (+1/3 the existing length) below the drive pivot towards 6 o'clock and mount a cam pivot at the end. Add a second extension (same length) from the drive pivot towards 3 o'clock and drill a hole at the end to fix one end of an extension spring. Add a cam follower wheel to the top of the upper linkage. Now imagine a boomerang shaped cam with a rounded external bend. Draw the cam with the bend at 9 o'clock to the drive pivot and the two boomerang legs pointing towards 2 o,clock and 4 o,clock. Half way down the lower leg of the cam add a pivot hole that coincides with the cam pivot pin on the modified lower linkage. At the bottom of the lower of the cam leg drill a hole to fix the other end of the spring. The length of the spring (un-extended) should hold the cam in the position drawn.

As the lower linkage rotates anti clockwise the cam follower roller lowers and the upper arm of the cam arks round so the the cam engages on the roller between 10 and 11 o'clock. Further rotation causes the follower to roll along the cam towards the bend, displacing the cam and extending (storing energy in) the spring. When the lower linkage reaches 9 o'clock the cam is above the cam follower and pushing down through the dead spot with the force of the stretched spring. When the lower linkage is at 3 o'clock the cam is below the follower and pushing it up through the dead spot. Beyond the dead spot the follower rolls back along the cam until it disengages.