Liftarms

The building of structures and devices using the Robot Inventor or SPIKE Prime set is based on liftarms, shown in their several varieties in Figure 3-1. Some liftarms are straight, some are bent, some are shaped like letters of the alphabet, and some are rectangles. Liftarms are meant to be attached to each other with pins, described in a following section. This connection arrangement is what distinguishes liftarms from LEGO beams—a beam, shown in Figures 3-2a and 3-2b, attached to a liftarm, has studs on top so that they can be stacked on top of each other. Beams and liftarms represent two different styles of building: studded (using beams) and studless (using liftarms). Some LEGO builders prefer to build in one of these two styles. And some LEGO fans tend to combine the two styles. The Robot Inventor and SPIKE Prime sets use the studless style, building with liftarms, so the studless style is used throughout this book. However, Figure 3-2 provides ideas for connecting liftarms to beams, for possibly combining Robot Inventor or SPIKE Prime parts with other LEGO bricks.

A 3 D model presents the building elements, such as I-shaped, rectangular, straight, and bent like lift arms.

Designs using Robot Inventor and SPIKE Prime use the studless style of LEGO building. But if the use of studded pieces, like LEGO System bricks or Technic beams, is desired, there are several ways to make this connection shown in Figure 3-2. One way, shown in Figure 3-2a, is to simply press a liftarm onto a beam. However, this connection is not very strong. So an alternate is to join a liftarm and beam side by side using pins, as in Figure 3-2b. Still another option is to use special bricks that combine stud and studless connections—one such special brick comes in the Robot Inventor set, shown in Figure 3-2c, that has a pin on one side and studs on the other side.

Three models a, b, and c exhibit 3 D lift arms joined one above the other, joined side by side using pins, and a special brick that has a pin on one side and studs on another side.

Straight Liftarms

Liftarms come in several shapes and sizes, with the most common being straight line liftarms—Figure 3-3 shows these straight liftarms that come in the Robot Inventor set. Different lengths are available, ranging from 2 to 15 holes long. Aside from the 2-hole-long liftarm, straight liftarms come in odd numbers of lengths. So there is no such thing as an 8-, 10-, or 12-hole-long straight liftarm.

A 3 D model exhibits eight straight lift arms available in different lengths of 1, 3, 5, 7, 9, 11, 13, and 15 long holes.

Bent Liftarms

A design may require to build along different directions, as opposed to attaching things all in a line. For such multidirectional applications, liftarms also come in the bent and letter-shaped varieties shown in Figure 3-4, all of which are in the Robot Inventor set. The longest bent liftarm turns through a right angle, which is useful for building along lines that are perpendicular to each other. The other two bent liftarms turn through an angle of 53.13 degrees, which is useful for building triangular structures, as will be seen later in this chapter. Aside from these bent pieces, letter-shaped pieces also come in useful for building structures. There are L-shaped liftarms, in two sizes, as well as H-shaped and T-shaped liftarms. These letter-shaped pieces will be used in several designs of this book.

A 3 D model presents three bent lift arms, two L-shaped lift arms in two sizes, one H-shaped lift arm, and one T-shaped lift arm.

Rectangular Liftarms

Another type of liftarm is rectangular designs, as pictured in Figure 3-5. The larger rectangles, which measure 11 × 15 and 7 × 11 holes long, are useful for the base of a structure or the chassis of a vehicle. The small rectangle, 5 × 7, comes in handy to hold a sensor or motor.

A 3 D model illustrates three rectangular lift arms available in measures of 11 cross 15, 7 cross 11, and 5 cross 7 holes long.

Connecting Liftarms

The holes in liftarms allow them to be joined with other pieces. Some of these holes are round, allowing insertion of a pin. Others are cross-shaped to accommodate an axle. The main difference between pin and axle connections is that pins allow rotation of the connected parts, whereas an axle will prevent rotation. As pins are more common, most of the openings found in liftarms are round. Rectangular and letter-shaped liftarms have no axle holes at all, and bent liftarms have axle openings at their ends only. Among straight liftarms, only the two-hole-long liftarm has an axle hole.

Pins

Pins come in several varieties, as shown in Figure 3-6. An important feature of pins is if they may have friction ridges, bumps along the surface of the pin. Pins come in varieties with (#2, 4, 6, 8, 9, 10, 11, and 12 in Figure 3-6) and without (#1, 3, 5, and 7 in Figure 3-6) friction ridges, meant for use in situations to control how easy it is to rotate a pin that has been inserted into a liftarm.

A 3 D model exhibits pins that come in 12 varieties of different sizes and shapes. Some pins have friction ridges and bumps along their surfaces.

To feel the difference that friction ridges can make, Figure 3-7 shows an exercise to insert a pin with friction ridges and a pin without friction ridges side by side in a liftarm. Spinning each of these installed pin should show that the one without friction ridges spins freely, but the one with friction ridges will only turn with effort. So if a design calls for liftarms to be freely rotating when connected together, a pin without friction ridges will accomplish this.

A 3 D model presents how two pins with and without friction ridges are inserted in a lift arm. The lift arm has 9 long holes, in which the pin with the friction ridge is placed at the fourth hole and the pin without the friction ridge is placed at the sixth hole.

Pins come in a longer length, such as #6, 7, and 8 in Figure 3-6, for situations in which there’s a need to go through three liftarms. Certain pins are designed to adapt axle openings (like #4, 5, and 8 in Figure 3-6) or to a perpendicular pin (like #10 and 11). Others adapt to a stop bush (like #9), which is useful to grab hold of a pin for easy removal. Still others adapt to a tow ball (like #12), useful for connecting a rubber band to a structure or to block against rotation of a liftarm past a certain angle. Pins will be used in these ways throughout the projects in this book.

Axles and Bushes

Some of the pins in Figure 3-6 have axles on one end, meant to join pieces with a pin on one side and axle on the other side. But very often there’s a need to join the axle ends of two liftarms or connect to a motor or a gear. It may even be needed to connect devices that are separated by some distance, so axles come in several different lengths. Figure 3-8 summarizes the types of axles that come in the Robot Inventor set. Axles range in length from 2 to 12 long. Some axles are plain, but some have additional features built into them, such as a stop, like the axles labeled (#3, 5, 6, 8, 9, and 13 in Figure 3-8), which serves to keep the axle from sliding past a certain point. Some stops are on the end of an axle (#3, 6, 8, and 13 in Figure 3-8), and some stops are somewhere along the middle length of the axle (#5 and 9 in Figure 3-8). The 2-long axle (#1 in Figure 3-8) has notches in it that helps in removing the axle by giving a place to get a fingernail into the axle. Otherwise, these short axles can be difficult to remove. The colors of axles can help to quickly find the particular length called for in building instructions, but for added clarity the building instructions in this book will often have a number in a yellow square next to the axle to indicate length.

An illustration exhibits the 15 varieties of axles that come in the Robot Inventor set. Each axle has different sizes and structures. Some axels are plain, and some have special features.

A piece sometimes used with an axle is known as a bush, which slides onto an axle. Bushes can be used as a spacer for an axle to keep parts separated and from rubbing together. Or a bush placed on the end of an axle can serve as a lock to keep the axle from sliding. Figure 3-9 shows two kinds of bushes that come with the Robot Inventor set—a full-size bush and a half-size.

A 3 D model exhibits the types of buses named full-size bush and half-size bush in the Robot Inventor set.

Exercise: Triangular Structures

Liftarms, joined together by pins and axles, are the basis for LEGO structures. Figure 3-10 shows an example liftarm structure, called the Triangle Trick, that takes advantage of bent liftarms to incorporate triangular shapes. Triangles are important in building, because they offer the best performance in strength and handling a heavy load. A triangle is stronger than any other shape for sturdiness. But there’s a trick in getting a triangle to fit within the usual up-and-down, side-to-side layout of LEGO building. The secret to this trick is in the angle that bent liftarms are at, that is, 53.13 degrees. The Triangle Trick shows how to use this angle with the instructions following Figure 3-10.

A 3 D model presents a triangular trick lift arm structure. Two straight lift arms are connected at an angle of 90 degrees, and a bent lift arm is used to strengthen the structure. A toy is placed near the horizontal lift arm end.

A 3 D model demonstrates how to connect the pins and axles with a 13-hole long lift arm.

A 3 D model exhibit how to connect a 3-hole long lift arm and a 7-hole long straight lift arm with a 13-hole long lift arm using pins and axles.

A 3 D model presents how to connect a 3-hole long lift arm and a 7-hole long lift arm with a 13-hole long lift arm using pins and axles.

A 3 D model presents how to connect a 3-hole long lift arm, a 5-hole long lift arm, and a 7-hole long lift arm with a 13-hole long lift arm using pins and axles to form a triangular structure.

A pause in building is warranted to examine the triangle built after step 4. As diagrammed in Figure 3-11, this is a right triangle with dimensions of 3, 4, and 5 holes long on the sides. The angle opposite the 4-hole-long side is 53.13 degrees, which matches the angle of a bent liftarm. Since the triangle’s angle and the bent liftarm angle are the same, bent liftarms can now be added onto the triangle, and LEGO hole spacing will be maintained for the rest of the structure. Continuing on with the building steps following Figure 3-11 shows how this works.

A 3 D model exhibits a side view of a triangular structure formed by connecting a 3-hole long lift arm, a 5-hole long lift arm, and a 7-hole long lift arm with a 13-hole long lift arm using pins and axles.

A 3 D exhibits how a bent lift arm is connected to a triangular structure formed by connecting a 3-hole long lift arm, a 5-hole long lift arm, and a 7-hole long lift arm with a 13-hole long lift arm using pins and axles.

A 3 D model presents how a 1 x axle is connected to the free end hole of the bent lift arm that is connected with a triangular structure.

A 3 D model represents how a bent lift arm is connected with another bent lift arm that is already connected with a triangular structure.

A 3 D model exhibits how a bent lift arm is connected with another bent lift arm that is already connected with a triangular structure.

A D model presents how a straight lift arm is connected side by side with a bent lift arm and a straight lift arm to strengthen the vertical column.

Pushing on one of the long sides of the Triangle Trick will show that the structure is rather strong—it doesn’t bend much. This is because the triangles built into the design are effective at distributing a load. So when strength is needed in a LEGO design, a triangular load-bearing shape is a good idea. The 53.13 angle of bent liftarms allows incorporation of these triangles.

Connectors

This chapter has shown that many shapes and structures can be built out of liftarms, pins, and axles. But even more can be done with connectors that come in the Robot Inventor set, diagrammed in Figure 3-12. Connectors can allow structures to be smaller than building with only liftarms. Also, connectors can change the angle along which things are build, allowing large three-dimensional structures. Connectors can include both pin and axle openings.

A 3 D model reveals many types of connectors in the Robot Inventor set. Each connector has different shapes and sizes.

Exercise: Symmetric Objects

An interesting challenge in building with LEGO is to make three-dimensional shapes that are symmetrical. Symmetry means that if an imaginary line were drawn down the middle of a side, then the two halves of the side are mirror images of each other. Figure 3-13 shows two examples of structures that have three-dimensional symmetry. For each side viewed, there’s a mirror image at the line down the middle of the side. Building these two examples starts with building instructions following Figure 3-13 to first build the Symmetric Cube.

A 3 D model of two structures that have three-dimensional symmetry, a cube and a symmetric cross.

A 3 D demonstrates how to connect the two different 1 x connectors.

A 3 D model demonstrates how to connect the three pieces of 1 x connectors, two of which are the same type and one of which is a different type.

The Symmetric Cube shows the idea of symmetry, as diagrammed in Figure 3-14. If an imaginary line were drawn down the center of a side of the cube’s face, the left and right halves are mirror images of each other. This symmetry will be the case no matter what side of the Symmetric Cube is being viewed.

A 3 D model of the side view of a symmetric cube. An imaginary one decides the cube into two halves. The left and the right halves are mirror images of each other.

The Symmetric Cube is a simple design, but a more complex example can be studied with the Symmetric Cross as follows:

A 3 D model exhibits the interconnection of a 1 x connector with a 1 x axle.

A 3 D model exhibits the interconnection of 1 connector, 2 elbow-type connectors, and 1 axle.

A 3 D model demonstrates how to connect the 2 axles at the endpoints of elbow-type connectors.

A 3 D model demonstrates how to connect the 2 axles at the endpoints of elbow-type connectors. Other 2 connectors are attached to the axles.

A 3 D model demonstrates how the connectors and axles are interconnected to build a symmetric cross.

For an even more extensive study of symmetry, the Symmetric Cube and the Symmetric Cross can be combined as in Figure 3-15 to build the Symmetric Star. Directions to build the Symmetric Star follow Figure 3-15. The Symmetric Cube goes into the center of the Symmetric Cross, held in place with connectors and pins.

A 3 D model demonstrates how a symmetric cross and a symmetric cube are combined to build a symmetric star. The cube is placed at the center of the cross. A toy is placed near the star.

A 3 D model of a symmetric star. The symmetric cube is set loosely at the center of the symmetric cross.

A 3 D model demonstrates how two pins are connected at the two opposite ends of a symmetric star.

A 3 D model exhibits how a straight lift arm is connected with a symmetric star in the middle.

A 3 D model demonstrates how two straight lift arms are connected on both sides of the vertical bar, creating a symmetric star.

Project: Mechanical Linkages

This chapter has given a tour of the building elements that come with the Robot Inventor and SPIKE Prime sets: liftarms, pins, axles, bushes, and connectors. Exercises have built several structures to see how all these building elements can be used together. But aside from structures, parts that move can also be built, called linkages. A mechanical linkage is an assembly of building elements to control movement. Two example linkages are built as follows: a universal joint and an eccentric.

The Universal Joint

The Universal Joint, shown in Figure 3-16, allows an axle to spin in an arrangement where the axle isn’t in a straight line. This can be useful for a situation in which one part of a shaft needs to flex, such as with cars and trucks that have wheels bounce up and down as the vehicle drives. A pivoting mechanism allows the entire length of the axle to spin, even though there is an angle between two sections of the axle. Connectors are used in the following LEGO example to create a pivoting action that accommodates the angle between the two axle sections. When one end of the axle spun, such as by the handle on the right side of the Universal Joint, the entire length of the axle will rotate. The angle between the two sections of the Universal Joint can be changed and still be able to rotate the axle.

A 3 D model demonstrates how two straight lift arms are connected on both sides of the vertical bar, creating a symmetric star.

A 3 D model demonstrates how the five pins, three on one side and two on the other are connected with a 15-hole long straight lift arm.

A 3 D model demonstrates how the two 7-hole long straight lift arms are connected side by side with a 15-hole long straight lift arm.

A 3 D model exhibits how the two 15-hole long straight lift arms are connected end to end.

A 3 D model demonstrates how the two pins are connected on the second 15-hole straight lift arm.

A 3 D model exhibits how the two 7-hole long straight lift arms are connected on the second 15-hole long straight lift arm with two pins.

A 3 D model demonstrates how the two connectors are inserted on both 15-hole long straight lift arms in the middle.

A 3 D model demonstrates how to construct a universal joint using different types of connectors.

A 3 D model demonstrates how to construct a universal joint using different types of connectors.

A 3 D model demonstrates how to construct a universal joint using different types of connectors along with an axle.

A 3 D model demonstrates how to join the universal joint with the help of axles on a platform built by joining 15-hole long straight connectors.

A 3 D demonstrates how to join the two connectors on the free end of the axles that are connected with the universal joint.

The Eccentric

One problem that often comes up when designing machinery is how to change rotary motion into linear motion. Rotary motion is spinning motion, commonly encountered from a motor, engine, or manual crank handle. In contrast, linear motion moves along a straight line that might be wanted to use to push something forward or in a back-and-forth action. A linkage known as an eccentric solves the problem of converting rotary motion to linear motion by attaching a part called a link to a rotating circle. As a result, the link converts the rotary movement created by turning the crank into the linear movement of the axle: the axle slides back and forth as the crank is turned. Figure 3-17 shows a LEGO implementation of an eccentric, with building instructions following the figure. Turning the crank handle results in the axle sliding back and forth along a line. A version of this eccentric will be used in a more advanced project in Chapter 9.

A 3 D exhibits an example of an eccentric. On rotating a crank handle, an axel moves back and forth linearly.

A 3 D model demonstrates how the two pins are connected with a 13-hole long straight lift arm.

A 3 D model demonstrates how two pins, an axle, and a connector are connected with a 13-hole long straight lift arm.

A 3 D model exhibits how two pins, an axle, a connector, and a 5-hole long straight connector are connected with a 13-hole long straight lift arm to build an eccentric.

A 3 D model demonstrates how a long axle is connected with a platform formed on a 13-hole long straight lift arm to build an eccentric.

A 3 D model demonstrates how a long axle is connected with a platform formed on a 13-hole long straight lift arm to build an eccentric. 2 pins are inserted at the left end.

A 3 D model demonstrates how a bent axle is connected with a platform formed on a 13-hole long straight lift arm to build an eccentric.

A 3 D model demonstrates how a pin is connected at the free end of the bent axle to build the crank handle of an eccentric.

A 3 D model exhibits how a pin and a connector are connected at the free end of the bent axle to build the crank handle of an eccentric.

Summary

This chapter gave a tour of the building elements of the Robot Inventor set including liftarms, pins, axles, bushes, and connectors. Liftarms are the fundamental building element in the Robot Inventor and SPIKE Prime sets and come in shapes of straight, bent, rectangular, and letters of the alphabet. Example projects showed how these various liftarm shapes can be used, such as the angle of bent liftarms allowing triangular supports to make structures strong. Liftarms include attachment holes for either pins or axles so that liftarms can be joined together. Pins are inserted into the circular openings of liftarms, while axles are inserted into cross-shaped openings. Some pins have an axle on one end for conversion from pin connection to axle connection. Pins can also be with or without friction ridges to allow choice in whether a connection between liftarms should be resistant to or freely allowed to rotate. The many varieties of connectors that come with the Robot Inventor set are another means to join liftarms, offering more design flexibility than the simpler approach of pins and axles. While building elements are often used to build nonmoving structures, building elements can also be used to build machines with moving parts, as demonstrated with the example linkages built in the chapter summary projects.