© The Author(s), under exclusive license to APress Media, LLC, part of Springer Nature 2023
G. KochLearn Engineering with LEGOhttps://doi.org/10.1007/978-1-4842-9280-8_5

5. Mechanisms

Grady Koch1  
(1)
Yorktown, VA, USA
 

A mechanism is a system of mechanical parts working together in a machine to create a desired action. Such a desired action could be to lift, spin, push, nudge, drive, or shoot. This chapter presents four example mechanisms to build: a ratchet, cam, differential, and turntable. These mechanisms are commonly used in designing robots and vehicles, so an understanding of these mechanisms serves well in creating custom designs. A variety of parts will be used from the Robot Inventor to build these example mechanisms, including the building elements and gears of the previous two chapters. There are also several components in the Robot Inventor set that are meant specifically for use in particular mechanisms, including turntables, differential, and dart shooter. The final project in this chapter combines several mechanisms to build a steerable cannon.

Exercise: The Ratchet

A ratchet is a mechanism that allows an axle or wheel to rotate in one direction (say clockwise), but blocks rotation in the other direction (counterclockwise). In daily life, ratchets can be seen at turnstiles where people are allowed to move forward through a gate, but not backward, such as at a train station, amusement part, or places that scan a ticket for entry. Also, a ratchet wrench for tightening or loosening bolts is a common tool, which can be found in any hardware store. Figure 5-1 shows a LEGO version of a ratchet mechanism, which allows a spin handle to rotate in one direction, but not in the opposite direction. A gear is adapted here for the Ratchet, using the teeth of the gear to prevent rotation in one direction by catching against a connector. A rubber band, part of the Robot Inventor set, keeps the connector pressed up against the teeth of the gear. Building instructions follow Figure 5-1.

A 3-D model of a ratchet with a lift arm, an axle, and a gear. A rubber band with a mechanism prevents rotation in the other direction.

Figure 5-1

The Ratchet allows for a spin handle to rotate in one direction, but not the other direction

A 3-D model of a lift arm with an axle, gear, and axle connector assembled. Above it, are the 3-D models of each part separately.

A 3-D model of a lift arm with an axle, gear, axle connector, and an axle and tow ball attached. Above it, the 3-D models of an axle and tow ball are separated.

A 3-D model of a lift arm with an axle, gear, axle connector, an axle and tow ball, and a 3-hole axle connector attached. Above it is the 3-D model of the 3-hole axle connector separately.

A 3-D model of a lift arm with an axle, gear, axle connector, an axle and tow ball, a 3-hole axle connector, and another axle attached. Above it is the 3-D model of an axle separately.

A 3-D model of a lift arm with an axle, gear, axle connector, an axle and tow ball, a 3-hole axle connector, another axle, and an axle connector attached. Above it is the 3-D model of the axle connector separately.

A 3-D model of a lift arm with axles, gear, axle connectors, tow ball, a 3-hole axle connector, and another connector attached. Above it is the 3-D model of the connector separately.

A 3-D model of a lift arm with axles, gear, axle connectors, a tow ball, a 3-hole axle connector, a wheel, and a rubber band stretched over the tow ball. Above it, are the 3-D models of the wheel and rubber band separately.

Spinning the handle of the Ratchet should show that it can rotate in only one direction. In the allowed direction, the Ratchet makes a satisfying clicking sound, like a cricket, if the spin handle is turned quickly. A ratchet mechanism can be useful in many LEGO inventions, such as for preventing a wheeled robot or vehicle from rolling backward. Or for a LEGO model crane that lifts heavy objects, a ratchet can ensure the load doesn’t fall as it’s being lifted. When building a LEGO catapult, a stretched rubber band could be used to propel the catapult’s arm. In such a design, a ratchet could allow the rubber band to be stretched until ready to shoot by unlocking the catch that grabs the gear’s teeth.

Exercise: The Cam

A cam is a mechanism that provides a nudging or light lifting motion from a rotating power source, like a motor or engine. In cars that have gasoline or diesel engines, cams are hard at work opening valves to allow fuel into the engine and let exhaust out of it. In robotics, a cam is useful for an action that mimics a human finger pushing a button, which will be used later in this chapter to build the Mechanized Cannon.

But cams can be hard to see at work in a machine, since they’re often buried deep in the machinery. So the exercise shown Figure 5-2 is of a cam to explore how they work.

A 3-D model of a cam with a lift arm, axle, and gear.

Figure 5-2

The axle end of the Cam creates a pushing motion as the spin handle is rotated

The Cam uses a gear, but not for a typical gear function; instead, it acts as a circle with an off-center axle attachment point. When rotated on an axle, one end of the circle is at a farther distance from the axle than the other end of the circle. So, as the circle spins on the axle, one end of the circle reaches out farther than the other end of the circle. In the Robot Inventor set, there are two building elements that can serve as a cam: the 36-tooth gear (used in this exercise) or the round connector block.

A 3-D model of a lift arm with an axle, an axle connector, and a gear attached. Above it, are the 3-D models of each part separately.

A 3-D model of a lift arm with an axle, an axle connector, a gear, and 3 other axle connectors attached. Above it is the 3-D model of the axle connector separately.

A 3-D model of a lift arm with an axle, an axle connector, a gear, and 3 other axle connectors, another axle, and a connector attached. Above it, are the 3-D models of the axle and connector separately.

A 3-D model of a lift arm with an axle, axle connectors, a gear, another axle, a connector, another lift arm, and a cross block attached. Above it, are the 3-D models of the cross block and lift arm separately.

A 3-D model of a lift arm with an axle, various axle connectors, a gear, another axle, another lift arm, a cross block, a wheel, and a connector attached. Above it, are the 3-D models of the wheel and connector separately.

A 3-D model of a lift arm with an axle, various axle connectors, a gear, another axle, another lift arm, a wheel, and a rubber band stretched around a cross block. Above it, are the 3-D models of the wheel and connector separately.

Turning the Cam’s spin handle makes the axle move in and out. The rubber band continuously pulls the axle back so that the axle is always pressed up against the off-center rotating circle. The cam is similar to the eccentric built in Chapter 3, the difference being that the cam needs a spring (in the form of a rubber band) to work. Also, an eccentric can be made to push with more force than a cam. While the Cam used a gear for an off-center circle, LEGO also makes a piece specifically for a cam described in Figure 5-3, but it’s not included in the Robot Inventor set.

LEGO makes a part specifically for a cam, shown in Figure 5-3, but it’s not included in the Robot Inventor set. It can be found as part number 6575 on aftermarket parts dealers such as bricklink.​com. The advantage of the #6575 cam over using a circular element is that the #6575 cam has three possible off-center axle attachment points to give flexibility in choosing the length of the pushing action that the cam can apply, whereas a circular element only has one axle attachment point.

A 3-D model of a circular element.

Figure 5-3

It’s not part of the Robot Inventor, but the #6575 cam can be a good alternative to using a circular element

Exercise: The Differential

A differential is a mechanism for use with wheeled vehicles that solves a common problem with controlling a vehicle in a turn. If a vehicle uses a pair of wheels, or more than one pair, a problem arises when the vehicle is in a turn, as diagrammed in Figure 5-4. In a turn, one wheel is called on to turn slower than the other wheel due to the curve involved, because one wheel travels a shorter distance. But the wheels, being driven by the same motor source in the example of Figure 5-4, can only move at the same speed. The vehicle then has a problem in the turn, likely with a skid sideways and jerky motion. The differential solves this problem, by allowing one wheel to slow down when needed, such as in a curve. When the curving turn is over, the differential automatically switches back to both wheels being run at the same speed.

A 3-D diagram presents the movement of a pair of wheels before and during a turn. Before the turn, the wheels spin at the same speed. During the right turn, the left wheel spins faster.

Figure 5-4

During a turn, one wheel of a pair has to travel a farther distance than the other wheel

The differential works by halving an axle into two pieces, with each axle having a wheel on one end and a gear on the end inside the differential. Gears inside the differential split up the torque coming into the differential in such a way that one axle can be given less speed than the other axle.

Assembling the Differential

A differential is a bit complicated to build from scratch with gears and building elements, so LEGO provides a premade differential in the Robot Inventor set, as shown in Figure 5-5. The LEGO differential comes in pieces, though, requiring assembly as per the instructions that follow the figure.

A photograph of a differential with all its components assembled.

Figure 5-5

The assembled differential

A 3-D model of a differential gear connector with a gear attached. Above it, are the 3-D models of the gear connector and gear separately.

A 3-D model of a differential gear connector with a gear and 3 other gears attached. Above it is a 3-D model of gear separately.

A 3-D model of a differential gear connector with 4 gears and 1 more gear that lies flat to the 3 gears attached. Above it is a 3-D model of gear separately.

A 3-D model of a differential gear connector with inner gears and a big outer gear attached. Above it is a 3-D model of the outer gear separately.

The Differential Demonstrator

Now that the differential has been assembled, its use can be explored by building the Differential Demonstrator pictured in Figure 5-6. A motor spins a pair of wheels in the Differential Demonstrator, allowing experimentation with the idea of how one wheel can be slowed down.

A 3-D model of a set-up to demonstrate the mechanism of a differential. It consists of a technic brick sheet with all components assembled on it.

Figure 5-6

The Differential Demonstrator uses a motor to spin the differential

A 3-D model of a technic brick sheet with 4 connectors attached. Above it, are the 3-D models of a brick sheet and connector separately.

A 3-D model of a technic brick sheet with 4 connectors, and 2 open center bricks attached. Above it is the 3-D model of an open center brick separately.

A 3-D model of a technic brick sheet with 4 connectors, 2 open center bricks, and 8 other connectors attached. Above it is the 3-D model of a connector separately.

A 3-D model of a technic brick sheet with connectors, 2 open center bricks, and another open center brick and 2 other bricks attached. Above it, are the 3-D models of brick and open center brick separately.

A 3-D model of a technic brick sheet with connectors, 3 open center bricks, 2 other bricks, 2 axles, and a differential attached. Above it, are the 3-D models of the differential and axle separately.

A 3-D model of a technic brick sheet with connectors, 3 open center bricks, 2 other bricks, 3 axles, a differential, and a gear attached. Above it, are the 3-D models of the differential and axle, and gear separately.

A 3-D model of a technic brick sheet with connectors, 3 open center bricks, 2 other bricks, 3 axles, a differential, a gear, and 3 more connectors attached. Above it, are the 3-D models of a connector separately.

A 3-D model of a technic brick sheet with connectors, 3 open center bricks, 2 other bricks, 3 axles, differential, gear, and 2 wheels attached. On the right and left of it, are the 3-D models of the tire and rim, respectively.

A 3-D model of an assembled piece of a motor. A 3-D model of a motor with connecting cable, and 4 connectors attached along with 3-D models of each part separately.

A 3-D model of a motor with connecting cable, 4 connectors, and 2 bricks attached. Above it is a 3-D model of a brick separately.

A 3-D model of a motor with connecting cable, 4 connectors, 2 bricks, and 4 other connectors attached. Above it is a 3-D model of a connector separately.

A 3-D model of a motor with connecting cable, connectors, 2 bricks, and 2 lift arms attached. Above it is a 3-D model of a separate lift arm.

A 3-D model of a motor with connecting cable, connectors, 2 bricks, 2 lift arms, and 4 pins attached. Above it is a 3-D model of a pin separately.

A 3-D model of a differential demonstrator with a differential and a motor attached. First, slide the motor onto the axle and then press the pins into the base. Above it is a 3-D model of a motor.

A 3-D model of a differential demonstrator with a differential, a motor, and 4 connector pins attached. Above it is a 3-D model of a pin separately.

A 3-D model of a differential demonstrator with a differential, a motor, 4 connector pins, and a hub attached. Connect the cable end to port B of the hub. Above it is a 3-D model of a hub separately.

The Differential Demonstrator can be activated to set the two wheels in motion by turning on the motor using the Hub. Details on running a motor with the Hub can be found in Chapter 1. With the wheels spinning, pressing with a finger on one wheel results in the wheel slowing down. And the harder this wheel is pressed, the more it will slow. But while pressing on this one wheel, the other wheel keeps on spinning at the same speed. Releasing the pressed wheel will have it go back to its prior speed. This shows that the differential acts as a speed reducer for one wheel of the pair when one wheel is called on to slow down. When used on a wheeled vehicle or robot, such a one-wheel speed reduction is needed anytime a turn is made. So driving is made much smoother using a differential.

Exercise: Turntables

A turntable is a mechanism that provides a platform for rotation. Old-fashioned record players use a turntable to spin vinyl records. Another example of a turntable, sometimes called a Lazy Susan, is in a kitchen cabinet for more convenient use of space. And giant turntables are used in train terminals to reverse the direction of locomotives. In the realm of LEGO, a turntable can be useful for aiming a sensor or turning the head of a robot. The Robot Inventor set includes two types of turntables, shown in Figure 5-7, with one large turntable and two small turntables. Each turntable comes in two pieces in the set, as shown in Figure 5-8, that are pressed together to look like Figure 5-7.

Three 3-D models of a big and 2 small turntables.

Figure 5-7

The Robot Inventor set includes three turntables: one large and two small

Two sets of 3-D models of gears and rims of big and small turntables. Both gear and rim are assembled to form a piece of a turntable.

Figure 5-8

Each of the turntables comes in two parts that are pressed together to assemble

The turntables have teeth around their rims that can mesh with a gear, so gears are a good way to get a turntable moving. A gear can be meshed with a turntable either along a straight line or at a right angle. Both of these meshing techniques are used in the Turntable Playground of Figure 5-9. Three turntables are put into action in the Turntable Playground, used to give minifigures a fun ride. One turntable spins continuously, and an interesting twist is used for the other two turntables to have them spin in intermittent motion using a gear mounted off center. Intermittent means that the motion is on and off, in contrast to continuous movement.

A 3-D model of a turntable playground with a motor, brick base, other bricks, and turntables attached.

Figure 5-9

The Turntable Playground gives a ride to minifigures

A 3-D model of a brick base with 8 and 2 different connectors attached. Above it, are the 3-D models of each part separately.

A 3-D model of a brick base with connectors, 2 lift arms, a brick piece, and a motor attached. Above it, are the 3-D models of the lift arm, brick, and motor separately.

A 3-D model of a brick base with connectors, 2 lift arms, a brick piece, a motor, 4 other connectors, a brick, and a pin attached. Above it, are the 3-D models of the connector, brick, and pin separately.

A 3-D model of a brick base with connectors, 2 lift arms, bricks, a motor, a pin, an open center brick, a gear, and 2 more connectors attached. Above it, are the 3-D models of the open center brick, gear, and connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, bricks, a motor, an open center brick, a gear, and 4 more connectors and a brick attached. Above it, are the 3-D models of the connector and brick separately.

A 3-D model of a brick base with connectors, 2 lift arms, bricks, a motor, an open center brick, a gear, and 2 more open center bricks attached. Above it, are the 3-D models of the open center brick separately.

A 3-D model of a motor with 2 open center bricks, gear, 2 other bricks, and 4 connectors attached. Below is a 3-D model of an open-center brick with 4 connectors attached along with models of each part separately

A 3-D model of an open center brick with 4 connectors and 1 more open center brick attached. Above it is a 3-D model of an open center brick separately.

A 3-D model of 2 open center bricks with 4 connectors and 4 more connectors attached. Above it is a 3-D model of a connector separately.

A 3-D model of 2 open center bricks with connectors, a gear, a rim, and a motor attached. Above it, are 3-D models of the gear, rim, and motor separately.

A 3-D model of 2 open center bricks with connectors, a gear, a rim, a motor, and an axle attached. Above it is a 3-D model of the axle separately.

A 3-D model of 2 open center bricks with connectors, a gear, a rim, a motor, an axle, and 1 more gear attached. Above it is a 3-D model of the gear separately.

A 3-D model of 2 open center bricks with connectors, gears, a rim, a motor, an axle, and 2 more axles and a brick attached. Above it, are 3-D models of the axle and brick separately.

A 3-D model of 2 open center bricks with connectors, gears, a rim, a motor, axles, a brick, and 2 more connectors attached. Above it is a 3-D model of the connector separately.

A 3-D model of 2 open center bricks with connectors, gears, a rim, a motor, axles, a brick, and 1 more brick attached. Above it is a 3-D model of the brick separately.

A 3-D model of a turntable playground assembly with a motor set and 1 more motor piece set attached. Above it is a 3-D model of the motor piece assembly.

A 3-D model of a turntable piece set with bricks and gear attached. Below is a 3-D model of assembled gear, 2 axles, 2 connector bricks, and a rim along with 3-D models of each part separately.

A 3-D model of assembled gear, axles, connector bricks, rim, 2 more axles, and 2 more connector bricks. Above it, are 3-D models of axle and connector brick separately.

A 3-D model of assembled gear, axles, connector bricks, rim, and 2 connectors. Above it is a 3-D model of the connector separately.

A 3-D model of assembled gear, axles, connector bricks, rim, connectors, and 1 more brick. Above it is a 3-D model of the brick separately.

A 3-D model of a turntable playground assembly with 2 motor piece sets and a turntable piece assembly attached. Above it is a 3-D model of the turntable piece assembly separately.

A 3-D model of a turntable playground assembly with 2 motor piece sets, a turntable piece assembly, 1 gear, rim, and 4 connectors attached. Above it is a 3-D model of the rim, gear, and connector separately.

A 3-D model of a turntable playground assembly with 2 motor piece sets, turntable piece assemblies, a brick, and 4 connectors attached. Above it is a 3-D model of the brick and connector separately.

A 3-D model of a turntable playground assembly with 2 motor piece sets, turntable piece assemblies, brick, connectors, and 6 pins attached. Above it is a 3-D model of the pins separately.

A 3-D model of a turntable playground assembly with 2 motors, turntables, brick, connectors, pins, and 4 more connectors attached. Above it is a 3-D model of the connector separately.

A 3-D model of a turntable playground assembly with 2 motors, turntables, and a hub attached. Connect the ends of cables to ports A and E of the hub. Above it is a 3-D model of the hub separately.

The turntables can be set in motion by control from the Hub, as described in Chapter 1. This exercise is a little different in motor control in that two motors are involved, with the Hub’s controls applied to both motors.

Project: The Mechanized Cannon

The final project of this chapter shows that mechanisms can be combined together in a machine, in this case to build the Mechanized Cannon of Figure 5-10. The Mechanized Cannon can aim with a spin handle, then shoot off a dart by activating a motor. The mechanisms involved include a turntable to aim the cannon and a cam to press a button to trigger a dart shooter. The dart shooter is also a mechanism, though a fully contained one and not something to be built from scratch. The Robot Inventor set includes two dart shooters, along with the darts. The dart shooter has a button on the back to trigger shooting, which can be tried out by loading a dart into the front of the shooter and then pressing the button, but making sure that the dart is pointed away from the operator or anyone else. Instead of manual shooting, the Mechanized Cannon automates control of the dart shooter by triggering the shooter with a motor-driven cam mechanism. The cam mechanism is similar to the design built earlier in this chapter, except for the use of a round connector block in place of the gear in order to make the cam smaller. Like the gear, the round connector block has an off-center axle mounting hole, so it can be used to build a cam. The cam in this design doesn’t use a rubber band like the cam built in the exercise earlier in this chapter. The rubber band isn’t needed here, because there’s a spring inside the dart shooter that serves the same function as the rubber band: keeping the cam’s pusher up against the rotating gear. There’s a large rectangular liftarm at the back of the Mechanized Cannon that acts as a safety feature to prevent two accidents: aiming a dart at the operator and spinning the turntable so far that the motor’s cable gets pulled too far.

A 3-D model of a cannon assembly with a brick base, lift arms, open-center bricks, motors, and connectors attached.

Figure 5-10

The Mechanized Cannon aims with a spin handle to shoot a dart

A 3-D model of a brick base with 4 connectors attached. Above it, are the 3-D models of each part separately.

A 3-D model of a brick base with 4 connectors, and 2 lift arms attached. Above it is the 3-D model of the lift arm separately.

A 3-D model of a brick base with connectors, 2 lift arms, and 4 more connectors attached. Above it is the 3-D model of the connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, and an open center brick attached. Above it is the 3-D model of the open center brick separately.

A 3-D model of a brick base with connectors, 2 lift arms, an open center brick, and 4 more connectors attached. Above it is the 3-D model of the connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, an open center brick, and 2 more open center bricks attached. Above it is the 3-D model of the open center brick separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, an axle, and 2 pins attached. Above it, are the 3-D models of the axle and pin separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, an axle, pins, a gear, and a connector attached. Above it, are the 3-D models of the gear and connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, an axle, pins, a gear, and 1 more axle and a pin attached. Above it, are the 3-D models of the axle and pin separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, axles, pins, a gear, and 1 wheel attached. Above it, are the 3-D models of the wheel separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, axles, pins, a gear, a wheel, 1 more gear, rim, and 4 connectors attached. Above it, are the 3-D models of the gear, rim, and connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, axles, pins, gears, wheel, and 4 more connectors attached. Above it, are the 3-D models of the connector separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, axles, pins, gears, wheel, and 2 bricks attached. Above it, are the 3-D models of the brick separately.

A 3-D model of a brick base with connectors, 2 lift arms, 3 open center bricks, axles, pins, gears, wheel, bricks, and 4 more connectors attached. Above it, are the 3-D models of the connector separately.

A 3-D model of an assembly of a motor with gear, bricks, lift arm, axle, and connecting pins. Another 3-D model is of a motor with an axle and connecting pins along with 3-D models of each part separately.

A 3-D model of a motor with an axle, connecting pins, and a lift arm attached. Above it is a 3-D model of the lift arm separately.

A 3-D model of a motor with an axle, connecting pins, a lift arm, and 2 more pins attached. Above it, are the 3-D models of the 2 pins separately.

A 3-D model of a motor with an axle, connecting pins, a lift arm, and 2 more lift arms attached. Above it, are the 3-D models of the 2 lift arms separately.

A 3-D model of a motor with an axle, connecting pins, 3 lift arms, a connector, and an axle attached. Above it, are the 3-D models of the connector and axle separately.

A 3-D model of a motor with axles, connecting pins, 3 lift arms, a connector, and gear, and a brick attached. Above it, are the 3-D models of the gear and brick separately.

A 3-D model of a motor with axles, connecting pins, 3 lift arms, a connector, gear, a brick, and 1 more connector and an axle attached. Above it, are the 3-D models of the connector and axle separately.

A 3-D model of a motor with axles, connecting pins, 3 lift arms, connectors, gear, a brick, and 2 more lift arms attached. Above it, are the 3-D models of the 2 lift arms separately.

A 3-D model of a motor with axles, connecting pins, 5 lift arms, connectors, gear, a brick, and 2 more connecting pins attached. Above it, are the 3-D models of the 2 connecting pins separately.

A 3-D model of a motor with axles, connecting pins, 5 lift arms, connectors, gear, a brick, and 1 more lift arm attached. Above it is the 3-D model of the lift arm separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, and bricks. Above it is a 3-D model of the motor with assembled components separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, and 5 more connecting pins. Above it is a 3-D model of the connecting pin separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, connecting pins, and 1 more lift arm. Above it is a 3-D model of the lift arm separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, connecting pins, and an open center brick. Above it is a 3-D model of the open center brick separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, connecting pins, an open center brick, and 1 more pin. Above it is a 3-D model of the pin separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, connecting pins, an open center brick, and 4 more connectors. Above it is a 3-D model of the connector separately.

A 3-D model of a brick base with an assembly of motor, gears, lift arms, connectors, bricks, connecting pins, an open center brick, and a hub. Connect the end of the cable to port C of the hub. Above it is a 3-D model of the hub separately.

The Mechanized Cannon is used by placing it on a table with the dart pointing away from anyone. Powering up the Hub and pressing the center button will prepare the motor. The dart can then be aimed by turning the spin handle that drives the turntable. Once lined up with a target, a dart can be fired by pressing either the left or right button on the Hub. Now that a means has been built to activate the dart shooter with motorized control, an extension may be to have a computer or robot decide when to shoot a dart. This idea will be implemented in Chapter 8.

Summary

This chapter explored the design and use of mechanisms, systems of parts working together in a machine to create a specific action like lifting, spinning, pushing, nudging, driving, or shooting. Four example mechanisms were built of a ratchet, cam, differential, and turntable, which can be useful for building custom inventions. These mechanisms showed that

  • A ratchet allows rotation in only one direction.

  • A cam gives a light pushing motion from a rotational power source.

  • A differential allows one wheel of a pair to slow down when needed, such as in a turn.

  • A turntable provides a platform for rotation.

Some mechanisms, such as the example exercise of the Ratchet and Cam, can be built from scratch from parts in the Robot Inventor set. Other mechanisms, such as the differential and turntable, use parts made by LEGO specifically for use in a mechanism. The final project of this chapter combined several mechanisms to build a Mechanized Cannon. Mechanisms are often used in inventions, as will be seen in the projects of upcoming chapters.

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