BrickEngineer: LEGO Design

LEGO Engineering for LEGO NXT and Robot Enthusiasts

Center of Mass of LEGO NXT Motors

I am working on designing a walking machine, but I needed to know the Center of Mass of the LEGO NXT Motors.  When using Newton’s Laws to compute the forces on the system, we can treat the motor as if all of its mass is located at a single point.  The Center of Mass is the location of this point.

Hanging an NXT motor to find its center of mass.

There are several ways to find the Center of Mass of the motor.  The most straightforward way is to hang the motor from an axle placed in one of the holes.  The motor will orient itself so that the Center of Mass lies directly below the axle.  By hanging a mass on a string from the axle, the Center of Mass must lie somewhere along the line defined by the string. 

The Center of Mass Lies along the line defined by the vertical string

After performing this experiment, I placed a small piece of Scotch tape over the string so that I can keep track of where that line is.  I then cut the string off of the axle.

A piece of Scotch tape holds the string in place

Now to find the precise point, we simply perform the experiment again, but place the axle through a different hole.  This gives us a second line.  Since the Center of Mass must be on both the first line and the second line, it is located at the intersection of these two lines.

The intersection of the two strings indicates the position of the Center of Mass

The Center of Mass is very close to being aligned with the holes on the motor.  Below is an MLCAD image of the NXT motor (from Philo).  I have overlayed a Cartesian coordinate system that corresponds to that used to define the 3-D image file.  The origin of this system is at the center of the axle hole on the motors drive axis.  This is perfect for me since I will be rotating the motor and trying to compute the position of the Center of Mass after the motor has rotated through some arbitrary angle.

The dimesions of the LEGO NXT Motor

This image not only helps with identifying the Center of Mass of the NXT motor, but also in understanding the dimensions of the NXT motor overall.

Kevin Knuth
Albany NY

Matlab Package for LEGO Mindstorms

I recently received a comment on my post on controlling NXT robots with Matlab that pointed me to the RWTH – Mindstorms NXT Toolbox for MATLAB®, which is a public domain Matlab package that enables one to interface with and control LEGO mindstorms.

The RWTH – Mindstorms NXT Toolbox for MATLAB® was developed as a student project in the Institute of Imaging and Computer Vision at RWTH Aachen University in Aachen Germany. It provides a Matlab interface with the NXT brick that includes Bluetooth communication, sensor interface and motor interface. It requires a working Matlab license, of course.

The package is very easy to set up. It took me less than ten minutes to successfully test the example programs over Bluetooth.

There are some very nice motor features, such as motor synchronization and speed ramp-up and ramp-down.

I have yet to explore how easy it is to modify or extend the code, but it ought to be a straightforward matter.

The package can be downloaded from

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.

Storing your LEGO Collection

It can be very difficult to figure out exactly how to store one’s LEGO parts. This depends greatly on the extent of your collection, and how mobile it needs to be. Below I offer a wide array of suggestions ranging from the size of one NXT set through tens of NXT sets with thousands of dollars of extra parts. Here I will review the solutions that I have found useful, and at the end I will provide a detailed description of my particular storage strategy.

Utility Boxes with Compartments
Clear plastic utility boxes with adjustable compartments are excellent for storing LEGO parts, keeping them separated, and making sure that they are accessible. I have found the products manufactured by Plano Molding Company to be especially useful. The Plano Prolatch with Adjustable Dividers
is secure and allows one to carry the box around without fear of accidental opening. These utility boxes are also included in the larger Plano Tackle Boxes, which provides an excellent means of both storage and transport.The new Custom Divider Systems (CDS) give you a great many ways to divide each box into bins with the greatest versatility. The other divider systems work well, but this is a nice option.

Small Box
Plano No. 2-3500-20
9-1/8″ Wide. x 1-1/4″ High x 5″ Deep
Up to 9 adjustable compartments
Order from
These fit into the Plano 1354 Tackle Box
Medium Box
Plano No. 2-3650-20

11″ Wide x 1-3/4″ High x 7-1/4″ Deep
Up to 18 adjustable compartments
These fit into the Plano 1364 Tackle Box below
Large Box
Plano No. 2-3750-20

14″ Wide x 2″ High x 9-1/4″ Deep
Up to 20 adjustable compartments
These fit into the Plano 1374 Tackle Box below

Tackle Boxes
Plano tackle boxes are designed to hold multiple (typically four) utility boxes above. This provides for a readily mobile storage solution. Your LEGO collection can be carried from your home, to the lab, a friend’s house, or into the field at a moment’s notice. Here is a photo of my Plano 1364 Tackle Box that I use for most of my construction needs.

Plano 1354 Tackle Box (3500 size)
I have found that the 1354 Tackle Box is a bit on the small side for even the smallest collections. You will be surprised at how many distinct kinds of Lego parts there are, when you find that they will never fit into the 9×4 compartments. However, this tackle box can hold 4 utility boxes, and you often can put an additional box in the top compartment.
Order at
Plano 1364 Tackle Box (3650 size)
This is a really nice size for a tackle box. I can easily store at least two NXT sets in this tackle box as well as many extra parts. Four Plano ProLatch 3650 stowaway boxes
fit in the tray resulting in 4×18 = 72 compartments. I also manage to keep an additional 3650 box in the top of the tackle box. I can often work with just this subset of my collection. In addition, it makes travel easy.
Plano 1364 Tackle Box at
Plano 1374 Tackle Box (3750 size)
This tackle box can hold a large number of parts. The top bulk storage compartment is sufficiently large and deep to actually hold smaller robots! The 3750 size utility boxes are very spacious, and this tackle box comes with one of the newer Custom Divider System (CDS) utility boxes, and one flip-sider, which is good for fishing tackle, but not so useful for LEGO parts. For this reason, you may need to buy an extra 3750 box to replace the flip-sider. And you may want an extra 3750 to fit in the top compartment.
Plano 1374 Tackle Box at

Compartmentalized Storage
For a more extensive LEGO collection, one may require more storage space for small parts, such as pins, axles, etc. Akro-Mills makes some very nice organizers with a large number of small drawers.

Akro-Mills 10724 24 Drawer Storage

This organizer can hold a healthy supply of small LEGO parts. The organizer comes with partitions that can be inserted into the drawers allowing one to keep more parts.Overall Size: W=20″ H=15.81″ D=6.38″ (inches)
Drawer Size: 4.25″ W x 5.25″ D x 2.0″ HI have three of these. One holds all of my pins and connectors. The second holds all of my axles and axle connectors, and the third holds all of my gears.
Akro-Mills 10124-2 44 Drawer Storage
This organizer can hold a healthy supply of small LEGO parts. The organizer comes with partitions that can be inserted into the drawers allowing one to keep more parts.Overall Size: W=20″ H=15.81″ D=6.38″ (inches)
Large Drawers: 4.38″ W x 5.25″ D x 2.0″ H
Small Drawers: 2.13″ W x 5.25″ D x 1.50″ H
Akro-Mils 10764 64-Drawer Storage

This organizer can hold many small LEGO parts. The organizer comes with partitions that can be inserted into the drawers allowing one to keep even more parts.Overall Size: W=20″ H=15.81″ D=6.38″ (inches)
Small Drawers: 2.13″ W x 5.25″ D x 1.50″ H

Large Storage Bins with Drawers
The larger collections require more extensive storage. Storage bins with drawers are excellent for storing large numbers of bricks of many colors and types. Stackable bins with drawers facilitate organization and access, although they require a good bit of space… or rather, your collection does!

Three Drawer Organizer
I have found the Sterilite ClearView™ Wide Three Drawer Organizer 2093 to work very well. Its dimensions are suffciently large to hold a large number of brick. Or one could make partitions (they do not come with the drawer) to store a wide array of parts in a single drawer. At this point, I use 20 of these to store my bricks, plates, liftarms, wheels, and specialized NXT parts. The clear drawers make it easy to see what is inside, and one can easily afix large easy-to-read labels.The dimensions of the three drawer organizer is:
14 5/8″ L x 14 1/2″ W x 10 5/8″ H.

Large Mobile Storage
My lab requires that a large proportion of my parts be easily transported. The best solution that I have found for this is the mobile toolbox together with a healthy set of Plano Utility Boxes (above). I was surprised to find that there is not the diversity in toolbox designs that I expected. I finally settled on the
Stanley Consumer Storage 033023R Pro Mobile Tool Chest.

This tool chest when packed full holds:
4 large Plano 3750 compartment boxes
3 medium Plano 3650 compartment boxes
6 small Plano 3500 compartment boxesThis is in addition to the large top tray that can hold Mindstorm NXT bricks, a great deal of cabling, and other tools that you might need.The tool chest has a handle and wheels, which makes for easy transportation.
Order from here.

See the more recent posts: COLORFUL LEGO STORAGE IDEAS and New LEGO STORAGE OPPORTUNITIES for more ideas.

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