Gears Flashcards
defines the average number of teeth in contact at any one time
contact ratio
A minimum contact ratio for smooth operation
1.2
Most spur gearsets will have contact ratios between
1.4 and 2.
A disadvantage of unequal-addendum tooth forms is an
increase in sliding velocity at the tooth tip.
Interference (and undercutting caused by manufacturing tools) can be prevented simply by
avoiding gears with too few teeth
As the number of teeth is reduced for a fixed diameter gear,
the teeth must become larger.
Severe undercutting will promote
promote early tooth failure.
arc length along the pitch circle circumference measured from a point on one tooth to the same point on the next.
circular pitch
defines the tooth size.
circular pitch
tooth is measured along the axis of the gear.
face width
tooth thickness is measured at the
pitch circle,
tooth space width is slightly larger
tooth thickness.
Tooth space width - tooth thickness
backlash
dedendum is slightly larger
addendum
tip of one mating tooth
(addendum circle)
bottom of the tooth space
(dedendum circle).
tooth height is defined by the
addendum
tooth height is defined by the
addendum and dedendum
(AGMA)
American Gear Manufacturers Association
They define a spectrum of quality numbers and tolerances ranging from
lowest (3) to the highest precision (16).
backlash can cause potentially
“hunting”
back-to-back on the same shaft that can be rotated slightly at assembly with respect to one another,
antibacklash gears
Another factor affected by changing center distance is
backlash.
clearance between mating teeth measured at the pitch circle
Backlash
As the center distance increases
pressure angle and vice versa
The common normal is
still tangent to the two base circles and still goes through the pitch point.
The velocity ratio is unchanged despite the shift in center distance.
Unchanged
center distance errors ____ the velocity ratio.
Do not affect
the gear tooth form is not an involute, then an error in center distance will violate the fundamental law, and there will be variation, or
“ripple,”
There will also be an ideal center distance ( CD ) that will give us the nominal pitch diameters for which the gears were designed. U
center distance
defined as the angle between the axis of transmission or line of action ( common normal)
pressure angle
direction of velocity at the pitch point
pressure angle
distance along the pitch circle within the mesh
arc of action
subtended by these points and the line of centers
subtended by these points and the line of centers
The arc of action on both pinion and gear pitch circles
The arc of action on both pinion and gear pitch circles
The distance along the line of action between these points within the mesh i
length of action,
common normal of the tooth profiles, at all contact points within the mesh
pitch point
The ratio of the driving gear radius to the driven gear radius
remains constant as the teeth move into and out of mesh.
curves at the contact point,
common tangent
perpendicular to the common tangent.
common normal
common normal, which is also the
axis of transmission,
The amount of tooth that sticks out
addendum,
The cylinders from which the strings are unwrapp
base circles
base circles are necessarily
necessarily smaller than the pitch circles,
The string is always
angent to the cylinder.
The center of curvature of the involute is always at the
the point of tangency of the string with the cylinder.
The contact point between the cylinders lies on the line of center
pitch point.
reciprocal of the angular velocity ratio (mV)
torque ratio (mT)
equal to the ratio of the radius of the input gear to that of the output gear.
angular velocity ratio (mV)
equal to the ratio of the radius of the input gear to that of the output gear.
pinion and gear
are ones in which the teeth are parallel to the axis of the gear. This
SPUR GEARS
are ones in which the teeth are at a helix angle, Ψ, with respect to the axis of the gear
HELICAL GEARS
They run quieter than spur gears because of the smoother and more gradual contact between their angled surfaces as the teeth come into mesh.
HELICAL GEARS
are formed by joining two helical gears of identical pitch and diameter but of opposite hand on the same shaft. These
HERRINGBONE GEARS
which has only one tooth wrapped continuously around its circumference a number of times, analogous to a screw thread.
Worm
means that the worm gear teeth are wrapped around the worm.
Single enveloping
resulting in an hourglass-shaped worm.
Double enveloping
based on the rolling centrodes of a Grashof double-crank fourbar linkage.
Noncircular Gears
If the teeth are angled with respect to the axis,
SPIRAL BEVEL GEARS
If the teeth are angled with respect to the axis,
a spiral bevel gear
bevel gears are based on
rolling cones
rotary to linear motion conversion or vice versa. It
Rack and Pinion
