Kinematic design and linkage synthesis from scratch 3

[ves39]

Our team was recently tasked with a new project at work. We have to design a mechanism to move an object from point A to point B.

Projects of a similar scope were designed long ago by trial and error and ended up costing the company a lot of time and money. They would go through countless iterations and prototypes. We're hoping to change that. Unfortunately, our team is completely green when it comes to mechanism design.

We would like to start sketching stuff up on our CAD system, but we're not sure what type of mechanism would achieve the type of motion that we're looking for as we are not very familiar with this area of mechanical design.

I have an old kinematics textbook from long, long ago that I will have to brush the dust off. I'm hoping that will be a good start.

Would you guys be able to recommend any resources on kinematic synthesis and designing mechanisms from scratch? Are there any good resources out there that we can browse to see if there is something that already exists that might be accomplish the motion we're looking for?

 

[moon161]

https://www.powells.com/book/mechanism-design-9780132677820

 

[moon161]

https://www.innova-systems.co.uk/using-blocks-to-create-mechanisms-in-solidworks/

 

[moon161]

But that's development and implementation. First I would clarify (the problem) Ideate (come up with some ideas)
Select some ones you like.

Develop them in CAD as outlines of mechanisms and prescribed paths.

You have a new design problem: design & build your developed idea.

Note on brainstorming: Group of 3-7 read up on proper brainstorming, warm up on an inconsequential topic before you begin brainstorming on the consequential topic. Aim for quantity of ideas when brainstorming too.

 

[Compositepro]

"Projects of a similar scope were designed long ago by trial and error and ended up costing the company a lot of time and money. They would go through countless iterations and prototypes. We're hoping to change that. Unfortunately, our team is completely green when it comes to mechanism design."

Sounds to me that you are in exactly the same situation "they" were. The only things that make any difference is the individuals involved, and their talents and experience.

 

[3DDave]

See

https://blog.rectorsquid.com/linkage-mechanism-designer-and-simulator/

who has a free linkage design tool that is fast to use and has plenty of examples under

https://www.youtube.com/user/rectorsquid

The faster the turnaround the quicker you will get a solution.

Another resource is

https://www.youtube.com/user/thang010146/videos?shelf_index=0&view=0&sort=dd

See

https://www.youtube.com/user/thang010146/about

He has a zipped listing of his mechanisms:

http://www.mediafire.com/file/muio0i3hyvnhtj6/3200AMMe.zip/file

The list consists of 4 parts:
Part 1: Transmission of continuous rotation
Part 2: Other kinds of motion transmission
Part 3: Mechanisms of specific purposes
Part 4: Mechanisms for various industries

Autodesk Inventor is used to create all videos in this channel.
Author Information:
Name: Nguyen Duc Thang
Birth year: 1946 in Hue City, Vietnam
Mechanical engineer, 1969, Hanoi University of Technology
Doctor of Engineering, 1984, Kosice University of Technology, Slovakia
Job history:
- Designer of small mechanical engineering enterprises in Hanoi.
- Retirement in 2002.

 

[Jboggs]

"trial and error and ended up costing the company a lot of time and money"
I agree with CompositePro. Sounds like they haven't learned from their first mistake.

Unless you can learn from someone else's experience, you're likely to make the very same mistakes. Duh!

That being said, I find one of the most common mistakes of inexperienced designers is that they think they have to design everything from the ground up. They haven't been around long enough to have a good working knowledge of the wide variety of mechanical devices that are available on the market, either as standard or customized assemblies.

Can you get outside help? The good equipment vendors have very knowledgeable and helpful applications engineering capabilities, and they exist solely to help you succeed.

Can you give us a general summary of the task?

 

[CWB1]

Not sure what CAD package you have, but this is where parametric design shines. I would start off by making a basic model of your constraints, what needs to move and anything on the other end of the system that has to move it. Model the necessary motion or at least the initial and final points of that motion, then pull in some COTS parts from McMaster or otherwise to achieve that motion.

 

[Jake S.]

Planar 1 DOF mechanism design is truly one of my favorite topics, so please forgive the long winded post, but it is well worth the read.

Consider this a "crash course" in mechanisms, which by itself could be an entire 4 year degree, but I will try to be brief.

There are two ways that you could go about the synthesis of this mechanism (there are actually more ways, but these are the two that you SHOULD consider)

1. Utilize complex loop closure equations to develop a mathematical model for finite position synthesis (by complex, I mean complex numbers, not computationally complex)
2. Use GCP techniques (I'll come back to this later) in CAD software to build a virtual model of the mechanism's motion

Number two is very very fast if the user is familiar with mechanisms and the CAD software, however it offers no information as to what the link velocities, or accelerations may be, meaning that you will not be able to determine the loads that the mechanism is under during motion. Option one can be relatively slow (that is unless you already have a code/calculation that you can modify), but it gives every bit of information required about the mechanism.

Since you would like to start sketching mechanisms immediately, it seems that option two is going to be your best bet. The acronym "GCP" refers to "graphical constraint programming." And, if you are familiar with loop closure equations and what kinematic constraints actually are, it is easy to understand what is going on in the background of the software. To describe GCP completely in this post would be quite difficult, so I will leave it up to you and google, but ASME has some excellent articles about this topic.

Before you begin with option two, you must first decide the mechanism "task", that is, what type of motion are you trying to accomplish with the mechanism? In this case, it would seem that you have two options:
1. Motion generation - in which both the path and the orientation of the body matters
2. Path generation - in which only the path of the object matters, and the orientation is of no concern
There are other tasks, but I don't think you will be requiring them for this specific application.

Many situations call for option 1, but you will have to make that choice yourself.

Once you have the task defined, you must determine the design positions - remember, there is a finite number of positions that we can design for the mechanism motion (in the case of motion generation, the theoretical maximum number of design positions is five, which is due to the number of free choices that one can make regarding the ground positions, drive link angles, etc. being zero, but I digress.) That is not to say that you cannot calculate what the path will be at discrete points using what is called position analysis, but we can only DESIGN for a specific number of locations.

In your case, you state "from A to B", which might initially lead you to think that you only need two design positions. However, I would warn you that in some cases, this may not be a good choice because your mechanism could behave erratically between your design positions, and the mechanism may be more likely to experience circuit or branch defects. In this case, I would aim for 3 position motion generation, it is usually a good place to start. This will limit some of your "free choices" but will have a higher probability of providing a robust design.

Now that you have an understanding of what task your mechanism needs to perform, it is time to select the appropriate linkage type, and inversion. This will depend on how complex your motion is, and a handful of other requirements such as packaging, required input torque, etc. However, in my experience, most problems can be solved with a standard 4-bar linkage (as opposed to six bar or eight bar linkages), which is quite easy to synthesize using GCP techniques. If you do choose a four-bar, you will have to determine if you want your drive link to be fully rotatable (for instance - if you wanted to drive it with an electric motor), or if you want it to have a limited travel range, in which case you would most likely need to drive it with an actuator (brief note: if the actuator is the linear type, then you would be creating a Watt II 6 bar chain!). Some good terms to search for are: "rocker-crank", "crank-rocker", "double-crank", "double-rocker", and Grashof condition.

Here is a good example of a 4-bar mechanism that I designed (in MATLAB) that performs 3 position motion generation, with a fully rotatable input link:

https://www.youtube.com/watch?v=AfUJhBKs-4U

Hopefully that helps to give a little bit of context of what was previously discussed. The red circles are the design positions, and the link passing through them is called the coupler link, which is the link that provides the most complex motion in a four bar mechanism, highly non-linear as you can see.

There will be some other items of concern, such as circuit defects, branch defects, toggle positions, poor transmission angles, the list goes on, but what I have written should at least point you in the right direction. Some good names to google are: Arthur Erdman, George Sandor, Thomas Chase, John Mirth, and Larry Powell (compliant mechanisms)

Should you need additional assistance, please don't hesitate to let me know. I hope this was helpful, and wow, what a first post!

On a side note, should you start having to design a lot of mechanisms, building a mathematical model is the superior method, as it will allow you to determine every bit of knowledge that you want about the mechanism. Position, velocity, acceleration (which becomes very very important in fast moving mechanisms), input torque, mechanical advantage etc. You can even start getting fancy and OPTIMIZING these mechanisms!

 

[CWB1]

Number two is very very fast if the user is familiar with mechanisms and the CAD software, however it offers no information as to what the link velocities, or accelerations may be, meaning that you will not be able to determine the loads that the mechanism is under during motion.

Every modern CAD package has motion study capabilities in the base license that will measure every potential velocity and acceleration throughout the mechanism given an input or range or inputs. These are typically used as inputs for FEA/modal to size components and optimize the system.

 

[ves39]

Thank you all for the responses to my question. A lot of good information above.

3DDave and moon161 thank you for the information and links to some great resources.

Jake S. I would like to especially thank you for the extremely detailed response. You have provided us with an awesome starting point for this project.

Our project will actually be task number two, path generation. We're trying to move an object out of the way (180 degrees around a corner to be specific - almost similar to a side swinging aircraft door) to make room for another operation to be performed. The orientation of the object is not critical, only that is clear for when operation two takes place.

Once again, your response is greatly appreciated and gives us a concrete starting point. There was a lot of new terminology introduced and it points us right in the direction we need to go. I'm excited to start researching some of these new topics and gathering the information for our design.

Our team is green, but we're eager to learn, quick to adapt, and I believe we can arrive at a solution to the problem we're trying to solve much more quickly, efficiently, and cheaply.

 

[rb1957]

does it have to translate, or could it rotate out of the way (like a house door) ?

don't copy airplane door mechanism. They have a great deal of complexity as they need to react pressure when closed.

what are your space constraints … the volumes you have to stay out of, the volumes you can use for your mechanism.
don't post … just understand the restrictions placed on your design.

what are the time restrictions ? you say "only that it is clear when operation 2 takes place" …
can you inter-link with "operation 2" to ensure no conflict (but probably greatly increasing the complexity) ?
what controls the sequence of operations ?
does the door have to close again ?
could you use the start of "operation 2" to start opening the door (like a sled hitting the door, pushing the door open or activating the mechanism) ?

is this on a plane ?? (noting you're "aerospace")

another day in paradise, or is paradise one day closer ?