31. A Mechanical Cookbook

31.1 Connectors

31.1.1 Bearings

• The interface between moving parts that should minimize friction and wear. - Plain Bearings

• Generally used in low speed machines.

• The main bearing action comes from the lubricant. - Solid Bearings

• This looks like a section of tube that is placed in a hole, and the shaft rotates inside.

• Typical materials are,


sintered bronze (with graphite)

cast iron

• Made for slowly rotating equipment

• lubrication is required, and problems will arise when not properly maintained.

• Available in standard sizes. - Split Bearings

• Used on large machines at low speeds.

• The two halves of the bearings are adjusted in position using shims.

• Typical materials include,


bronze with babbitt

babbitt lined metal

• Oil grooves are used for lubrication. - Thrust Bearings

• Opposes axial thrusts of rotating shafts.

• Uses shoes of a variety of shapes,


kidney shaped

• An oil wedge approach is used to support the bearing. - Rolling Bearings

• Advantages,

low friction at all times


high accuracy

low wear

come in standard sizes - Ball Bearings

• Low friction, high speeds, low loads.

• ball bearings are packed between two rotating rings.

• The grooves that contain the ball bearings is given different shapes for different loading conditions. - Roller Bearings

• For heavy loads at medium or high speeds.

• The various roller bearings are designed for loads (radial and axial) and packing space. - Thrust Bearings

• A set of rollers or balls are held between two washers.

• Designed mainly for lower speed axial loads and occasionally light radial loads.


31.1.2 Threads

• One of the classic forms of mechanical connector. Also used to magnify motion and force, and to convert rotation to linear motion.

• The basic terminology is,


• Right hand threads are turned clockwise to tighten, left hand threads are turned the other way.

• Threads Per Inch (TPI) are the number of turns of the thread per inch of length.

• There are a number of standard threads, as outlined in the following subsections. - Metric - American National Standard - British Standard Whitworth (BSW) - The Unified Thread - American National ACME Thread - Brown and Sharpe Worm Thread - Square Thread


31.1.3 Tapers

31.2 Motion and Force Transmissions

31.2.1 Gears

• Gears are generally round or linear sets of teeth for transmitting forces or motions.

• Different combinations of gears will allow conversions of forces, motions and directions.

• Different types of gears are,








rack and pinion - Spur Gears

• transmit power between parallel shafts

• have straight teeth parallel to axis of rotation

• used for slow/moderate speeds

• when two of these gears are meshed, the larger is the gear, the smaller is the pinion - Internal Gears

• transmission between parallel shafts

• require less space

• better meshing/more contact between gears. - Helical Gears

• Gears with teeth on an angle.

• can convert rotary motion to rotary motion on a non-parallel shaft.

• a tooth does not suddenly engage/disengage fully, so noise and vibration are reduced.

• also, more than one tooth is typically in contact, so the strength is increased.

• these gears often generate longitudinal forces that require thrust bearings. - Herringbone Gears

• Looks like a helical gear, but it looks as if a second helical gear with a reverse helix has been attached.

• Similar to helical gears, except that thrust bearings are not required. - Bevel Gears

• Transmit rotations to another axis perpendicular to the first.

• These gears look like spur gears, but with a taper.

• When the two gears are the same size they are miter gears.

• for non-90° intersections, the gears are called angular bevel gears.

• the rotational axes of these gears intersect. - Hypoid Gears

• Like bevel gears, but with helical teeth

• the gears are often offset also (the axes of rotation do not intersect)

• these gears are commonly used in the auto industry. - Worm Gears

• The worm is a helical gear with one or more threads.

• The worm gear is typically a straight tooth gear that is turned by the worm.

• This combination is used to convert rotation to a perpendicular rotation, and reduce the speed. - Rack and Pinion

• A rotating spur gear drives a linear rack of teeth.

• this combination converts a rotation to a linear motion.


31.3 Power Transmission

31.3.1 Hydraulics

• Incompressible fluids are used to transmit volume and pressure changes throughout a system.

• Pascal’s law basically describes these systems,


• Hydrostatic force/motion multiplier,


• The Hydrodynamic Effect: when fluid is moving quickly, it has high levels of kinetic energy. If the fluid impacts a surface, it transmits a high quantity of energy in a short period of time.

• Hydraulic Circuits typically contain,

1. Hydraulic Fluid

2. An Oil Reservoir

3. A Pump to Move Oil, and Apply Pressure

4. Pressure Lines

5. Control Valves: to regulate fluid flow

6. Piston and Cylinder: to actuate external mechanisms

• Oil Reservoirs


• Pump types used include,

Reciprocating pumps: have intermittent pressures with a single piston

• Valves