MACHINE DESIGN TERMS

Thin wall pressure vessel

is one in which the ratio of the wall thickness to the inside diameter is less than 0.07

Thick wall pressure vessel

is one in which the ratio of the wall thickness to the inside diameter is more than 0.07

Shaft

a rotating member transmitting power

Axle

a stationary member carrying rotating wheels, pulley, etc

Spindle

a short shaft or axle on machines

Machine shaft

a shaft which is an integral part of the machine

Transmission shaft

shaft which is used to transmit power between the source and the machine absorbing the power

Lineshaft or mainshaft

transmission shaft driven by the prime mover

Countershaft, jackshaft, headshaft, short shaft

transmission shaft intermediate between the line shaft and the driven machine

Keys

a machine member employed at the interface of a pair of mating male and female circular cross sectional members to prevent relative angular motion between this mating members

Keyway

a groove in the shaft and mating member to which the key fits

Splines

permanent keys made integral with a shaft and fitting into keyways broached into the mating hub

Coupling

a mechanical device which is used to connect length of shafting permanently

Rigid couplings

couplings that do not allow angular axial or rotational flexibility and used with collinear shaft

Flange coupling

type of rigid coupling which consists of two halves of flanges connected to each other by bolts

Sleeve or collar coupling

rigid coupling which is a cylindrical collar pressed over the ends of two collinear shaft

Flexible couplings

couplings which allow angularity to take care of misalignment of the shaft

Flywheel

a rotating energy reservoir which absorbs energy from a power source during a portion of the operating cycle and delivers that stored energy as useful work during the other portion of the cycle

Bolts and screws

are threaded fasteners which are used to hold together machine members which require easy dismantling

Unified national course

UNC; for general use except where other types are recommended

Unified national fine

UNF; frequently used in automotive compressor pumps and aircraft work where a fine adjustment is required

Unified national extra fine

UNEF; use in aeronautical equipment and where very fine adjustment is required

Lead

is the actual distance a thread advances in one revolution

Major diameter

is the outside diameter of threads and is the nominal diameter

Minor or root diameter

is the smallest diameter of the threads

Pitch diameter

is the main diameter of the major and minor diameters

Stress area

is the area of an imaginary circle whose diameter is the mean of the pitch and minor diameters

Screw

is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread and of being tightened or released by torquing a nut

Power screws

are used to move weights and machine parts and uses square acme or buttress threads

Pitch

the distance between adjacent threads, inches

Spring

stores energy when it is extended or compressed.

flat belt

used with flat pulleys and allows long distance between shaft

v belt

used with shaves or groove police and provide stronger grip at short distance between shaft

toothed belt

paired with toothed pulleys and used as timing belt or speed ratio must be maintained

addendum

height of tooth above pitch circle or the distance between the pitch circle and the top of the tooth

Addendum circle

the circle that bounds the outer ends of the teeth

arc of action

arc of the pitch circle through which a tooth travels from the first point of contact with the mating tool to the point where the contact ceases

arc of approach

arc of the circle through which a tooth travels from the point of contact with the meeting tooth to the pitch point

arc of recess

arc of the pitch circle through which a tooth travels from its contact with the meeting tooth at the pitch point to the point where the contact ceases

axial plane

in a pair of gears it is the plane that contains the two axes in a single gear it may be any plane containing the axis and the given point

backlash

the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles

base circle

the circle from which an involute tooth is generated or developed

base helix angle

the angle, at the base cylinder of an involute gear, that the tooth makes with the gear axis

base pitch

in an involute gear it is the pitch on the base circle or along the line of action

normal base pitch

is the base pitch in the normal plane

axial base pitch

is a base pitch in the axial plane

center distance

the distance between the parallel axis of spur gears and parallel helical gears or between the crossed axis of helical gears and worm gears also it is the distance between the centers of the pitch circles

central plane

in a worm gear this is the plane perpendicular to the gear axis and contains the common perpendicular of the gear and the worm axis

chordial addendum

the height from the top of the tooth to the chord subtending the circular thickness arc

cordial thickness

length of the chord subtended by the circular thickness arc (the dimension obtained when a gear tooth caliper is used to measure the thickness at the pitch circle)

circular pitch

length of the arc of the pitch circle between the centers or other corresponding points of adjacent teeth

normal circular pitch

is a circular pitch in the normal plane

circular thickness

the length of the arc between the two sides of a gear tooth on the pitch circles unless otherwise specified

normal circular thickness

is the circular thickness in the normal plane

clearance

the amount by which the dedendum exceeds the addendum of the meeting tooth. it is also the radial distance between the top of a tooth and the bottoms of the mating tooth

contact ratio

the smallest diameter on a gear tooth with which the meeting gear makes contact

cycloid

the curve formed by the path of a point on a circle as it rolls along a straight line. when this circle rolls along the outer side of another circle the curve is called an epicycloid.

hypocycloid

when it rolls along the inner side of another circle it is called a hypocycloid

dedendum

the depth of tooth space below the pitch circle or the radial dimension between the pitch circle and the bottoms of the tooth space

diametral pitch

the ratio of the number of teeth to the number of millimeters of pitch diameter

normal diametral pitch

is the diametral pitch as calculated in the normal plane and is equal to the diametral pitch divided by the cosine of helix angle

effective face width

that portion of the face width that actually comes into contact with meeting teeth as occasionally one member of a pair of gears may have a greater face width then the other

efficiency

the actual torque ratio of a gear set divided by its gear ratio

external gear

a gear with teeth on the outer cylindrical surface

face of tooth

that surface of the tooth which is between the page circle to the top of the tooth

face width

the length of the teeth in axial plane

fillet curve

the concave portion of the tooth profile where it joins the bottom of the tooth space the approximate radius of this curve is called the fillet radius

flank of tooth

that surface which is between the circle pitch and the bottom land the flank includes the fillet

helix overlap

the effective phase width of a helical gear divided by the gear axial pitch also called the face overlap

helix angle

the angle that a helical gear tooth makes the gear axis

Highest point of single tooth contact

the largest diameter on a spurgear at which a single tooth is in contact with the mating gear

internal diameter

the diameter of a circle coinciding with the tops of the teeth on an internal gear

internal gear

a gear with teeth on the inner cylindrical surface

involute

the curved formed by the path of a point on a straight line called the generatrix as it rolls along a convex base curve this curve is generally used as the profile of gear teeth

land

the top land is the top surface of the tooth and bottom land is the surface of the gear between the fillets of adjacent teeth

length of action

the distance on an involute line of action through which the point of contact moves during the action of the tooth profile

line of action

the path of contact in involute gears it is a straight line passing through the pitch point and the tangent to the base circle

lowest point of single tooth contact

the smallest diameter on a spurgear at which a single tooth of one gear is in contact with its mating gear

module

the ratio of pitch diameter in millimeters to the number of teeth english module is the ratio of pitch diameter in inches to the number of teeth

normal plane

a plane normal to the tooth surface at a point of contact and perpendicular to the pitch plane

outside diameter

the diameter of the outside circle

pitch circle

a circle the radius of which is equal to the distance from the gear axis to the pitch point

pitch plane

in a pair of gears it is the plane perpendicular to the axial plane and tangent to the pitch surface

pitch point

this is the point of tangency of two pitch circles and is on the line of center

pitch surface

the surface of the rolling cylinder that the gear may be considered to replace

plane rotation

any plane perpendicular to a gear axis

Pressure angle

the angle between the tooth profile and a radical line at its pitch point. in involuted, the angle between the line of action and the line tangent to the pitch circle

principal reference plane

these are a pitch plane actual plane and transverse plane all intersecting at a point and mutually perpendicular

ratio of gearing

ratio of the numbers of teeth on meeting gears

roll angle

the angle subtended at the center of a base circle from the origin of an involute to the point of tangency of the generatrix from any point on the same involute

root circle

a circle coinciding width or tangent to the bottoms of the tooth spaces

root diameter

diameter of the root circle

tangent plane

a plane tangent to the tooth surfaces at a point or line of contact of material is removed near the tip of the gear tooth

tip relief

an arbitrary modification of toothprofile whereby a small amount of material is removed near the tip of the gear tooth

tooth thickness

the width of tooth measured along the pitch circle

tooth space or space width

the space between the teeth measured along the pitch circle

total face width

the actual width dimension of gear blank

transverse plane

a plane perpendicular to the actual plane and to the pitch plane

trochoid

the curve formed by the path of a point on the extension of a circle as it rolls along the curve or line it is also the curve formed by the path of a point on a perpendicular to a straight line as a straight line rolls along the convex side of a base circle

true involute form diameter

the smallest diameter on the tooth at which the involute exits usually this is the point of tangency of the involute tooth profile and the fillet curve this usually referred to as a tip diameter