BrickEngineer: LEGO Design

LEGO Engineering for LEGO NXT and Robot Enthusiasts

Smooth LEGO Clutch with Differential


In a previous post, I describe the design of a smooth LEGO clutch.

Here I introduce a version where I have added a differential to the top of the clutch. By rotating the main differential gear in one direction, the clutch will engage and rotate one axle. By rotating it the other direction, the clutch will engage and rotate the opposite axle.

LEGO clutch with differential

It would be straightforward to attach a motor to this system.

Here are the building instructions.

Enjoy,
Kevin

10+1 LEGO Design and Engineering Tips

1. Use only parts that are necessary…no less and no more.
In the course of trying to fortify a LEGO construction, it is extremely easy to get carried away and add too many parts. If you are like me, you have a limited supply of parts.  Using too many parts rapidly depletes your supply and can add significant weight to your creation.  If you are building a robot, this extra weight can really slow it down.

2. Build strong connections
We all know that when stacking bricks to make a wall, you need to stagger the bricks so that the next layer of bricks holds the bricks in the lower layer together by covering up the cracks.  This is one way to build strong connections.  You can do even better by bracing the wall with a beam by pinning it to technic bricks embedded in the wall.  By adding a few carefully chosen pieces, it is possible to significantly strengthen a structure.  Just be careful not to go crazy and violate Tip #1.

3. Be aware of design constraints
Every time you add a LEGO part to your creation, you limit the possibilities of what you can make.  When you have nothing you can make anything.  When you grab a brick, you can now only make things that have that brick in it.  Each part constrains the creation. Be aware of parts or constructs or mechanisms that place too strong of a constraint on your design.  You dont have to get rid of them, but just be aware of the role they play in constraining what you do next.

4. Dont become TOO attached to your creation.
Sometimes we find that we really really like a certain aspect of the construction or a set of parts, but nothing else works. The tendency is to get rid of everything else and then build up around those parts we really like. However, these parts are placing strong constraints on the design… often too strong, which is why nothing else works. The solution is to get rid of the problem. In this case, you have to get rid of the set of parts you like because it is over-constraining the rest of the design. You dont have to destroy it (see Tip #9)… just set it aside, but be sure to remove it from your creation.

5. Be open to new ideas.
There is always more than one way to solve a problem. Be open to new, and seemingly crazy ideas. Sometimes these lead to ingenious solutions. If you suspect that you are having problems similar to those described in Tip #4, take some time out to brainstorm and see if you can come up with a new idea!

6. Build in stages.
Designing a complex structure or mechanism in one step is almost impossible. Break the construction up into stages, and consider each stage separately. Sometimes a given stage will still be too complex. If so, break that construction up into stages as well. Just be aware of the dangers in Tip #4 above. Your solution for one stage might be awesome, but if it doesn’t work well with your solution as a whole… it has to go. Again, if you have enough parts, keep your creations. Otherwise, at the very least preserve their memory by building them in a LEGO Computer Aided Design (CAD) system.

7. Watch for opportunities.
Sometimes we get lucky and we find that a part or a set of parts can serve two or more functions. This is an excellent situation as it saves you both parts, size and weight. Watching out for these opportunities, and taking advantage of them when you can, can really help make an elegant and efficient design. Just be careful not to become too attached to the idea, as you could end up in trouble as described in Tip #4.

8. Study your design.
When you are all finished, take some time to study and test your design. Consider both form and function. When considering form, you are concerned mainly with aesthetics (beauty). What do you like about the design? What dont you like? Is it too big? Is it bulky? Can it be made smaller, sleeker, more elegant? When considering function, you are concerned mainly with its operation and effficiency. Does it do what it is supposed to? Do the parts go together well? Does it vibrate? Do the gears mesh properly? Does it get stuck? Is there too much friction in the system? Could it be smoother? Is it safeguarded against parts breaking in high torque situations? Then consider the big picture. What did you do right? What did you do wrong? If you could do it over again, what would you change?

9. Keep your designs
I have mentioned this tip several times above. If you have enough parts, and enough room, keep your creations. Otherwise, at the very least preserve their memory by building them in a LEGO Computer Aided Design (CAD) system and generate building instructions. That way you can keep a good idea. Who knows if it will come in handy later?

10. Do it over again!
When building mechanisms that require careful consideration of either form or function, you should plan to make several prototypes (a prototype is a first design). Don’t destroy what you just constructed. You may need to refer to it by copying a part of the design that worked well. You may also need to see if you have improved the form or function by comparing it to your first attempt. I personally plan to make at least three prototypes until settling down with a final design.

11. Color coordinate your creation
I dont apply this rule to my first prototypes, but as I settle in on a final design, I work to choose the brick colors carefully. Of course, we do not always have enough parts to do this, but it is worth the extra effort. Colors scattered all over a design leads the eye to seeing it as haphazard rather than elegant. A careful choice of colors can really enhance the form of your creation. You can also use colors to enhance the function by color-coding functional segments of your design. This is maybe better for illustrative purposes (as in a LEGO CAD design), but usually I choose the latter and aim for an elegant coloration.

Kevin Knuth
Albany NY

Smooth LEGO Clutch

At times one may want to control the transmission of rotational motion by engaging or disengaging an axle from a motor.  Such a mechanism is called a clutch.

LEGO has special parts that can be used to make a clutch.  However, these are relatively hard to find, and take some force to engage or disengage.

Many clutch designs simply move gears back and forth across one another, but this causes the gears to grind and eventually wears down the parts.  In these systems, the clutch does not engage smoothly.

Smooth Clutch Mechanism

I was aiming to design a clutch that can engage or disengage with very little force, while also being made of common parts.  Below is my most recent design which relies on the easy meshing between two pin with towball  pieces on the 24 tooth gear and the Technic connector with axlehole on the central drive.  The only parts that are less common are the two Technic disks with axlehole.  These can probably be replaced with other parts.  Perhaps axles extending from the lever would work.

Here are the instructions in pdf format, as well as a zip file containing the LDraw files.

The following animations demonstrate the basic mechanism:

Clutch Animation

This one shows the coupling mechanism in detail from below:

Clutch Animation

I will post a description of how I made the animations.
Note that the engaging lever even pauses until the coupling mechanism meshes with the towball pins.

This mechanism can be powered with a motor coupled to a differential. This enables one motor to control both the drive and the switching. Rotating the motor in one direction will switch to one side and drive it, while rotating the motor in the other direction will switch to drive the other side. I will post a detailed description of how to do this as well.

Geneva Mechanism

The Geneva Mechanism takes smooth rotary motion and converts it to intermittent rotary motion. One can think of it in electronics terms as changing the duty cycle of the oscillation.  This is a mechanical version.

Geneva mechanisms were invented in Switzerland for use in clockwork so that the hands of a clock would snap rapidly to their new positions rather than move smoothly across the face of the clock. They are also used to advance movie film in film projectors and are responsible for that clicking noise that film projectors make.

LEGO Geneva Mechanism

Here is a rendering of a Geneva Mechanism designed from LEGO parts. This rendering was made using MLCad in the LDraw package in conjunction with L3PAO and POV Ray.  Below the mechanism in operation.  This process will be described in detail in a future post.

LEGO Geneva Mechanism

The building instructions are straightfoward and can be downloaded in this zip file.

I am looking into using such a mechanism in a LEGO laser scanner, which can be used in instrument or robotics applications.

A smaller LEGO Geneva Mechanism created by Leo Dorst of the Intelligent Systems Laboratory in Amsterdam can be found here, although I have not been able to get it to work myself.

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