Chapter 4 Mechanisms

Introduction Mechanism: moving mechanical components that are supported by a rigid structure. Linkage: rigid mechanical components that are linked together for the purpose of transmitting force and controlling motion.

Linkages Linkage systems: basic elements (rockers, cranks, connecting bars, and sliders) joined together via movable pin joints. Four-bar linkage: consists of three movable bars, two fixed pivot joints, and two pin joints. The fourth bar is a fixed structure. Parallel linkage: four-bar linkage system that follows the geometry of a parallelogram.

Figure 4-3: Four-bar linkage. Linkages Figure 4-3: Four-bar linkage. © Cengage Learning 2012

Linkages Scissor linkage: used in devices that are designed to amplify linear motion. Slider-crank: converts reciprocating linear motion into continuous rotary motion. Crank: bar that has one fixed pivot point. Slider: takes the form of a piston.

The Cam and Follower Cam and follower: mechanism that changes continuous rotary motion into intermittent linear motion. Cam is the rotating component; makes contact with the follower.

Figure 4-10: Cam and follower. The Cam and Follower Figure 4-10: Cam and follower. © Cengage Learning 2012

The Cam and Follower Cams and followers are used within the piston-type internal-combustion engine. Cams are used in the design of clamping mechanisms.

Bearings Bearing: used to reduce friction where two surfaces meet and slide against each other as a result of linear or rotary motion. Linear-motion bearings: one component must slide along another in one direction. Plain bearings: no moving parts and are shaped like cylindrical tubes.

Bearings Ball-bearing slides: tiny metal spheres that are free to spin 360°. Rotary motion bearings: interface with rotating shafts. Radial load bearings: used when a rotating shaft must be supported by a fixed structure.

Gears In mechanical systems that involve precisely controlled rotary motion, gears: –Control rotational direction –Control rotational speed –Control torque

Gears Gear: toothed wheel used to transmit rotary motion and torque from one shaft to another. Pitch circle: imaginary circle around which the teeth on a gear are evenly spaced. Diametral pitch: size of a gear tooth.

Figure 4-27: Gears are toothed wheels. © Cengage Learning 2012

Gears Gears are classified based on the orientations of their gear shaft: –Parallel shaft gears –Nonparallel and intersecting shaft gears –Nonparallel and nonintersecting shaft gears

Gears Helical gear: teeth wrap around the gear hub in the form of a helix. Worm and wheel: worm is a screw thread that shares similar geometric features as the gear to which it meshes. Bevel gears: conical form; teeth are the same shape as on spur gears.

Gears Rack and pinion: spur gear (pinion) that interfaces with a flat bar into which straight gear teeth have been cut (rack). –This gear is used in automobile steering systems. Gear train: two or more gears in mesh.

Figure 4-39a: Rack and pinion gear. Gears Figure 4-39a: Rack and pinion gear. © Cengage Learning 2012

Gears Simple gear train: two or more gears in mesh that have only one gear per shaft. Idler gear: bridges the gap between the input and output gears. Compound gear train: carries two gears of different sizes locked to the same idler gear shaft.

Gears Gear ratio: ratio between the number of revolutions of the input gear and output gear in a gear train. Rotational speed: measure of how fast an object rotates about an axis.

Gears Speed ratio (SR): ratio between the rotational speed of the input gear and output gear in a gear train. Torque: rotational equivalent of the concept of force.

Sprockets and Chain Drives Sprocket: toothed wheel used in conjunction with a continuous chain. Chain: series of links that are uniform in length; fitted together to form a continuous band. Roller chains: two repeating components, pin link and roller link.

Sprockets and Chain Drives Figure 4-48: Power chain system. © Cengage Learning 2012

Sprockets and Chain Drives Chain pitch: distance between centers on one chain link. Chain drive ratio (CDR): ratio between the rotational speeds of the input (driver) and output (driver) sprockets.

Summary A mechanism is comprised of two or more moving mechanical components. Most common linkage systems: four-bar, parallel, scissor, and slider-crank mechanism. The cam and follower is found in piston engines and pumps.

Summary Bearings: linear motion and rotary motion. Gear: toothed wheel. Two or more gears working together form a gear train. Gear train gear ratio: ratio between the number of revolutions of the input gear and the output gear.

Summary Most common types of chain drive mechanism is the roller chain and sprocket. Chain drive mechanisms: used for transferring motion and manipulating rotational speed between rotation shafts.