1. Fill in the blanks for the following (20%)

A. Ratchets give stepped motions B. For ratchets, the second pawl (detent) stops the wheel from slipping back C. Drives and Gearing provide rotary-to-rotary connections D. In friction drives, a flange is often employed to prevent slip E. An idler, jockey wheel or tensioner is a tensioning mechanism is used to take up slack in a belt F. In timing belts, the belt has teeth to engage the notches in the pulley wheels G. In chain and drives, links can be added or removed H. Spur have the same number of teeth for regulated motion I. In a rack and drive, the geared wheel meshes with a toothed rack J. In a worm drive, the shaft has a thread that meshes with a toothed wheel K. In bevel gears, the 2 wheels mesh at 45 degrees L. The pin wheel is simpler form of the and is easier to fabricate M. In bevel gears, the plane of rotation changes from horizontal to vertical (and vice-versa)

2. Answer the following (10%)

A. Sketch a friction belt drive where the driving and driven wheels counter-rotate

In the above diagram, the belts are twisted (once). This reverses the direction of motion of the driven wheel. The arrows represent the motion of the wheels and belt.

B. Sketch a friction belt drive where the driving and driven wheels are rotating in different rotational planes

The above mechanism is the same as the counter rotate mechanism but here the plane of rotation of one of wheel, is rotated 90°. The twist in the belt while providing a counter rotation effect also allows the belt to use itself for support for transmitting motion between the two wheels. If the twist is not present, the belt most likely will slip during motion and will not be a stable way to transmit or motion.

3. Define (1 to 2 sentences), describe (1 to 3 paragraphs) and provide sketches to support you definitions and descriptions for the following (10%)

A. Worm drive Definition: A worm drive is a combination of a worm (a gear in the form a screw on a shaft) and the worm

Description: The worm gear is a wheel/gear with teeth on it that is similar to a spur/helical gear. The pitch and profile of the tooth meshes well with the screw pitch and profile. This provides a reliable method of changing the plane of rotation 90°. Since both the components mesh, there is no slip and the of motion/torque is reliable and accurate. Such mechanisms are used to generate motion or torque where slight changes are necessary. The only downside of this mechanism is that it can be only used for relatively slow speed applications. One of the major advantages is the transmission of motion/torque is unidirectional. i.e. the worm drives the gear. This cannot be reversed due to friction between the components.

B. Ratchets used to lift heavy loads

Definition: A ratchet along with at least one pawl is a mechanical device that provides stepped motion on in one direction. The profile of the notches on the ratchet prevent the mechanism from rotating the opposite direction

Description:

The above diagram shows a basic layout of the ratchet mechanism used to lift heavy loads.

From the diagram it can easily be inferred:

1. Rotation of the ratchet clockwise, winds the wire that pulls up the load. 2. The pawl, unless released manually prevents, the ratchet from turning counter clockwise due to the load.

This mechanism provides the distinct advantage that the lifting mechanism doesn’t have to utilize energy to keep the load in place or from falling down. It also allows the load to be lifted in steps (depending on the notches) and manually releasing the pawl can lower the load.

4. Sketch and describe the 4 cycles of an 8-notch ratchet-crank mechanism (10%)

Shown above are the 8 steps of the ratchet crank mechanism.

The operation of the mechanism is described below.

1. As the rotates is moved forward, it pushes against the profile of the ratchet. 2. As the crank is pushed more, it pushes the notch of the ratchet I pushed forward turning the ratchet. Due to profile of the notch near the pawl, the pawl if pushed down against the spring. 3. The rotation of the ratchet continues as the crank is pushed towards its maximum displacement. Similarly, the pawl also is displaced as the ratchet rotates. 4. Once the crank has reached its max displacement, it is unable to push the notch any further. This displacement/ position of the crank can be adjust to let one complete notch to be shifted on the ratchet. Note that the pawl has also position itself at the new notch face. This will prevent any reverse turn of the ratchet. 5. As the mechanism on the crank continues to rotate, the crank is pulled back. 6. As the crank further continues to be drawn back, due to the profile of the notch, the crank slides back and the pawl prevents any reverse rotation. 7. Finally, in the eight step as the crank returns to its initial position, it lodges itself against the face of the new notch. Any further turn will push the ratchet forward.