Driving Question: How can we create a toy that uses a cams and cranks to change a rotary motion into a linear motion?

Challenge Overview: This project is designed to give students an introduction to basic design, engineering, mechanics, and construction procedures. Students will research, design and engineer, a toy that uses cams and cranks to change linear motion to rotary motion or visa versa. Students will then construct a toy following their design plans. Students will learn safe woodworking procedures, including the use of basic hand and power tools used in woodworking. Even though each student will produce his or her own toy, teamwork skills will be practised throughout this project.

Occupations: Communication – Design: Architect, Engineer Technology – Construction and Mechanics: Carpenter, Cabinetmaker, Mechanic trades

Cams A cam is a rotating or sliding piece in a mechanical linkage used in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path.

Cranks Engines that make their power with pistons usually need a way of converting back-and-forth (reciprocating) motion into round-and-round (rotational) motion. This drives the wheels. Most engines use cranks to do this. A crank is simply an off-center connection that provides energy to (or takes energy from) a rotating wheel. As the crank pushes back and forth, the wheel rotates (or vice-versa).

In this example, as the red wheel rotates, the green crank pushes the black and blue connecting rods back and forth, converting the wheel's rotary motion into reciprocal motion. So the red wheel moves round and round, but the blue rod moves back and forth. The same mechanism could be used the opposite way to drive the wheel from a piston. Steam train engine wheels are driven like this.

Car and boat engines have multiple cylinders that turn a single drive shaft, called the crankshaft. Each cylinder fires at a slightly different time so, at any given moment, there's always at least one cylinder adding power and driving the vehicle along. The cylinders are attached to the crankshaft by rods that connect to the piston rods inside the cylinders. Video

Cams can turn circular motion into a linear motion Cams can turn circular motion into a linear motion. This is referred to as reciprocating movement.   The cam-follower is connected to, and part off, a shaft known as the Push-Rod. The push-rod controls the direction of motion and transfers the cam's movement.

The amount of up and down movement is called the throw of a crank, and is measured by the size of the circle it scribes when turning which  will be twice the diameter.

An concentric cam is a disc with its centre of rotation positioned 'off centre'. This means as the cam rotates the flat follower rises and falls at a constant rate. This type of cam is the easiest to make and yet it is one of the most useful.  As it rotates it pushes the flat follower upwards and then allows it to drop downwards. The movement is smooth and at a constant speed.

Cams turn on a shaft and so need to be offset to create movement. If you have a circle with the shaft running through the center then nothing happens. However, if you offset it you can create a mechanism that can lift. The more you offset the cam, the greater the amount of lift you produce.

The concentric cam, is a circle with an offset center The concentric cam, is a circle with an offset center. By offsetting the center you produce the lift. The further you move away from the center point the greater the amount of lift you will produce. To calculate the amount of lift take the measurement from the center of the drive shaft to the highest point of the cam (15) and subtract this from the measurement to the lowest point (5). This calculation will give the amount of lift the cam will produce. (15-5=10)

To eliminate the turning affect you can either build stops to prevent turning or put guides either side of the cam.

This cam produces a smooth uplift which suddenly drops down This cam produces a smooth uplift which suddenly drops down. It is often referred to as a snail cam because of its shape or contour. This cam can only work in one direction. If you turn it the other way the cam-follower would jam. You need to bear this in mind when you are designing cams.  To ensure the rotation is smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of the slide. This cam produces several short up and down movements from one revolution.

From the basic round cam you can increase the diameter across one axis, to produce an egg-shaped, or Lobed cam.

Lobed Cam If you raise part of the circumference, you produce a lobe. This will lift the cam follower by the maximum height from the tip of the lobe to the circumference of the circle. When the cam follower returns to the circle it will pause and this is referred to as the dwell angle. You can produce a pause or dwell angle on top of the lobe if you design it properly.

Dropped Cam If you dip below the circumference of the circle then the cam follower drops, thus the term drop cam. You can calculate the drop of the cam by measuring from the lowest point of the drop to the circumference. A common form of drop cam is called the snail cam. This has a sudden drop that slowly rises to the next drop point. This cam is a blend of both drop and lobe cam.

The Pull-along Toy

Trouble Shooting

Cam mechanisms work well if they are made accurately Cam mechanisms work well if they are made accurately. Sloppy work can lead to the mechanism 'jamming' during rotation or the movement of the follower being less than smooth when rotating. 

A thin cam shaft may jam. To avoid this a circular cam-follower known as a Plate should be used. Because of it's large, flat contact area, it is less likely to jam. This type of follower works best with concentric and some lobed cams. It will not work on cams with complicated shapes.

Aligning the follower with the cam

Materials You May Use Spruce 2”x6” x 12” (38 x 140 x 300) or (1 ½” x 5 ½” x 12”) Spruce 1”x6” x 12” 19 x 140 x 300 Plywood, ¼”, ½”, and 3/4”. 1 ¼” concentric wooden cams. Wooden wheels, 2” dia. (3/8” axle). Axle holes should be drilled 7/16”

Materials 3/8” dowels. Carpenters glue (PVA or polyvinyl acetate glue) Nails, various sizes and types.

Selected Tools Mallet and wood chisels Back saw and bench hooks Drill press Twist drill bits Files and Rasps Coping Saw Try-square Rules Bandsaw

Safety ****Keep hands BEHIND chisel blades. Use a vise to hold the workpiece. Make sure the saw blades never touch metal parts on the vise. Bandsaw test must be taken before the bandsaw can be used. 100% must be achieved.

First: BRAINSTORM Design You will need a full size pattern to make your toy. First: BRAINSTORM

Brainstorm Draw 4 thumbnail sketches of three different designs. Choose your favourite design. Choose one within your capabilities and make sure that it uses both a cam and crank.

Research and Design You will need full size patterns to make your toy. Draw a two boxes that are exactly 200 x 300. One box will be the side view, the other the front view.

Your toy must sit inside the two boxes Your toy must sit inside the two boxes. Draw either the side or front view of your toy in the first box. Drilled holes must be at least 10 mm from the edge of the wood to the bottom of the hole. Models may be useful.

Design Toys that have moving parts should be modelled on cardboard. Use paper clips or similar items to connect moving pieces.

Building Aspects of the project will be demonstrated in the lab. Make sure you keep good notes on the procedures. You will have to show your notes when you hand in your project. Projects may be painted. Bandsaw test must be passed (100%) to use the bandsaw or scroll saw.

Marks Based On: Design Creativeness Construction Quality Work Habits, Safety, and Cleanup See Your Rubrics for specifics