BrickEngineer: LEGO Design

LEGO Engineering for LEGO NXT and Robot Enthusiasts

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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