31. A Mechanical Cookbook

31.1 Connectors

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

31.1.1.1 - Plain Bearings

• Generally used in low speed machines.

• The main bearing action comes from the lubricant.

31.1.1.1.1 - Solid Bearings

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

• Typical materials are,

bronze

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.

31.1.1.1.2 - 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

bronze with babbitt

babbitt lined metal

• Oil grooves are used for lubrication.

31.1.1.1.3 - Thrust Bearings

• Opposes axial thrusts of rotating shafts.

• Uses shoes of a variety of shapes,

flat

kidney shaped

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

31.1.1.2 - Rolling Bearings

• Advantages,

low friction at all times

compact

high accuracy

low wear

come in standard sizes

31.1.1.2.1 - 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.

 

31.1.1.2.2 - Roller Bearings

• For heavy loads at medium or high speeds.

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

 

31.1.1.2.3 - 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.

 

• 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.

31.1.2.1 - Metric

 

31.1.2.2 - American National Standard

 

31.1.2.3 - British Standard Whitworth (BSW)

 

31.1.2.4 - The Unified Thread

 

31.1.2.5 - American National ACME Thread

 

31.1.2.6 - Brown and Sharpe Worm Thread

 

31.1.2.7 - Square Thread

 

31.2 Motion and Force Transmissions

• 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,

spur

internal

helical

herringbone

bevel

hypoid

worm

rack and pinion

31.2.1.1 - 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

 

31.2.1.2 - Internal Gears

• transmission between parallel shafts

• require less space

• better meshing/more contact between gears.

 

31.2.1.3 - 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.

 

31.2.1.4 - 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.

 

31.2.1.5 - 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.

 

31.2.1.6 - 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.

 

31.2.1.7 - 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.

 

31.2.1.8 - 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

• 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