SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
LESSON FOUR FOUNDATIONS OF
MACHINE DESIGN (PART l)
Bearings With Rolling Contact
In bearings with rolling contact,the shaft is
directly or indirectly supported by rolling elements,
such as balls,cylindrical or conical rollers,or
needles (i.e.,cylindrical rollers with a high L/D ratio).
The occasionally encountered name " antifriction
bearings" suggests that this type has little or no
friction,This is erroneous,since the friction is
merely of another nature than in journal bearings,In
bearings with rolling contact,friction losses are
caused by the elastic deformation of the surfaces in
rolling contact,sliding friction of rolling elements
with cages,retaining rings and seals,or with one
another,and also by some shear of lubricant.
conical roller
圆锥滚柱
needle 滚针
cylindrical
roller
圆柱滚柱
antifriction
bearing
减摩轴承
cage 保持架
retaining ring
档圈
bearing 轴承
journal bearing
颈轴承
滑动轴承
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Characteristics
l,Ball bearings have rolling elements in the form of balls,
which in all but the most inexpensive types are held in
cages,separators,or retainers,and inner and outer grooved
races.
2,Roller bearings have mainly cylindrical,conical,or
barrel-shaped rollers instead of balls,but are otherwise
quite similar to ball bearings.
3,Needle bearings usually have neither an inner race nor a
cage,The needles are retained by integral flanges on the
outer race,Transitional types between roller and needle
bearings are found in many catalogs.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Bearings with rolling contact have no
slipstick effect,low starting torque and
running friction,and unlike as in journal
bearings,the coefficient of friction varies little
with load or speed,Low starting torque is of
great advantage in railroad cars,and the
railroad industry has given the main impetus
for the development of mass-produced roller
bearings in the past,mainly for this reason.
These bearings may take both radial and
axial loads (depending upon the type),and
need less space axially but more radially
(except needle bearings),and less lubrication
and maintenance,than journal bearings,They
are also noisier and more expensive,and
cannot be repaired easily.
slipstick
滑动面粘附现象
impetus
刺激，促进
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since rolling elements and raceways are theoretically in
point or line contact and thus highly stressed in the contact area
by typically cyclical loads,all bearings with rolling contact will
eventually fail by fatigue when operated at their rated load.
Metal failure is typically by shear,just below the surface of
raceways or rolling elements--a result of the three principal
stresses in the x,y and z axis,all compressive,The
phenomenon is popularly called spalling and is characterized by
the presence of metal flakes in the grease,Even a small bearing
load will induce contact stresses above the yield point (up to
500,000 psi; called Hertz stresses after the mathematician who
analyzed them),As a result,the contact surfaces are under
residual compressive stresses,balanced by tensile stresses
under the surface,Friction between rolling elements and races
causes tensile stresses in the surface,believed to cause pitting.
The pits,in turn,are the starting point of fatigue failure crack.
剥
落薄
片 润
滑
脂
,
黄
油
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Life Expectancy
The life expectancy of ball bearings is not
uniform and is based upon statistics,Usually,
rated load values in catalogs give the minimum
life (revolutions or hours at a given rpm) for 90%
of a group of bearings,the so-called B-10 life.
Mean or average life is about three times rated
life,while median or 50% survival life is about
five times rated life.
The considerable spread of the life expectancy
curve is due to the many possible forms of
favorable or unfavorable interaction between the
different bearing elements on the basis of
dimensional tolerances,surface finish and varying
structural properties,as well as conditions of
service.
life expectancy
（概率,预计）
寿命
rated load
额定载荷
rated life
额定寿命
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For plain ball bearings,radial load ratings in pounds (as
given in catalogs) are based upon a given rpm and a given
number of hours of life with 90% survival,The product rpm
× 60 × hr life is then equal to 106.
Other manufacturers give rated loads on the basis of 106
revolutions instead of hours of life (again,90% survival),In
that case,the number of hours of life can be found by
dividing 106 by rpm × 60,It will be clear from the
foregoing that,for a given load,changes in rpm (within the
design limits as given by the manufacturer) affect life in
hours inversely; thus,
n1/n2 =H2/H1 6.5
where n=rpm and H=life (hr.),Changes in load,on the
other hand,affect life in revolutions exponentially; thus,
Round per minute
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(F1/F2)k = B2/B1 6.6
where F= load (lb) and B= life (revolutions),Values of k
range between 3 and 4 (3 for ball bearings; 3.3 for most
roller and needle bearings),Note that for k=3,doubling
the load means reducing the life in revolutions by a factor
of 8.
The relation between B (life in revolutions) and H (life
in hours),is as follows:
B = H × rpm × 60 6.7
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
On the other hand,life can be extended
considerably by providing superior lubrication.
Also,for the same size and materials,the
manufacturer can produce a bearing with
greater life expectancy by increasing the
accuracy of manufacture i.e,by reducing
manufacturing tolerances.
Other Factors Affecting Bearing Life
The rated life of any bearing with rolling
contact is based upon proper application
conditions,such as adequate lubrication,good
alignment,and adherence to recommended
interference fit values for the races,if applicable.
Any negative variance from these conditions
will affect fatigue life unfavorably.
alignment
调整
adherence
附着，坚持
interference fit
过盈配合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Static And Dynamic Load
In tables giving permissible load data for ball
bearings,a distinction is often made between
static and dynamic loads,i.e.,with the bearing
stationary or rotating,The permissible static load
is smaller than the permissible dynamic load due
to the possibility of permanent indentation
(flattening of the contact areas when the bearing
is at rest),This phenomenon is called Brinelling.
Some permanent indentation is almost
unavoidable due to the high contact stresses,
however,it has been found that permanent
deformations smaller than 10-4 times the rolling
element diameter usually do not unfavorably
affect the bearing's operation.
indentation
压痕
Brinell
布里涅耳
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Rotation Factor
When the outer race rotates and the inner race is
stationary (the reversal of common practice),the rated load
may be increased by a factor of l.2,since a ball rolls further
per revolution on the outer race when the outer race rotates
(the ball rotates in the same sense as the race) than on the
inner race (where ball rotation is opposite to that of the race),
and life is limited by ball revolutions.
Equivalent Dynamic Load
A plain ball bearing is designed to take radial loads.
However,due to the curvature of the ball tracks in an axial
plane of both inner and outer races,some axial load may be
imposed on a radial ball bearing as well,Note that an axial
load is taken by all balls,at least in theory,while a radial
load is carried by less than half their number.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Manufacturers' catalogs give simple conversion methods
to compute an equivalent radial load in cases where both
radial and axial loads occur simultaneously in a rotating ball
bearing,One manufacturer uses a load conversion factor FC
which,when multiplied with the
radial component R,gives the
equivalent radial load P,which
is then used to select a properly
sized bearing, These load
conversion factors depend upon
the ratio of the thrust load and
the radial load (T/R) and vary
for different types of bearings.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A friction drive consists of two cylinders
rolling together under some pressure,When
no slipping occurs,the tangential velocity V at
the line of contact of the two cylinders is,of
course,the same for each,From physics we
know that V= ω × r,in which V is the
tangential velocity (in./sec),ω is the rotational
velocity (rad/sec),and r is the radius of
rotation,Since V= ω1r1 and also V= ω2r2,we
can equate both equations,Thus,
ω1 × r1= ω2× r2 6.8
Dividing both sides by ω2× r1,we get
ω1/ω2 = r2/ r1 6.9
where ω1/ω2 is the speed ratio (mω).
Gearing
gear 齿轮
tangential
正切的，切线的
radius 半径
speed ratio 速比
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When ω is divided by 2π and multiplied by 60,we obtain
rpm (n),Therefore,
mω = n1/n2 = r2/r1 = d2/d1 6.10
That is,the speed ratio of two cylinders in rolling
contact is equal to the inverse ratio of their radii or
diameters,in consistent units.
The drawback inherent to a friction drive,such as that
described above,is that it is liable to slip when power of
any consequence is transmitted,It can be used only for
very small torque applications,such as phonograph
turntable drives and the like.
diameter 直径
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The positive prevention of slippage in the
transmission of larger quantities of power
requires the use of teeth,penetrating into the
surface of each cylinder of the friction drive.
Mating cylinders provided with teeth are called
gears,The smaller of two mating gears is called
the pinion,the larger one the gear,The diameter
of each of the original rolling cylinders is called
the pitch diameter,and the cylinder's sectional
outline is called the pitch circle,The curved
shape of the tooth outline must be such that no
change in speed ratio occurs during the passing
contact of each tooth with its mating tooth on the
other gear,This is a basic requirement for all
gearing,Curves that satisfy this requirement are
called conjugate curves.
pinion 小齿轮
pitch diameter
节圆直径
pitch circle
节圆
conjugate
共轭的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In spur gears,the simplest of
all gears,the teeth are straight
lengthwise and parallel to the axis
of rotation,The spur gear
nomenclature,which follows,is
mostly applicable to all other
types of gears as well.
While several of such carves
exist,the one almost universally
used at this time is the involute.
This curve is that described by a
point on a string as it is being
unwound from a cylinder.
involute 渐开线
spur gear
直齿圆柱齿轮
nomenclature 术语
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Nomenclature And Definitions
The pitch point P is the point of tangency of
the pitch circles,The pitch diameter Dp,is the
diameter of the pitch circle,and is equal to
twice the pitch radius,The addendum (i.e.,that
which is added) is the radial distance from the
pitch circle to the top of the tooth (the crest).
The dedendum (i.e.,that which is deducted) is
the radial distance from the pitch circle to the
bottom of the groove between adjacent teeth
(the root),Clearance is the difference between
addendum and dedendum in mating gears.
Clearance prevents binding caused by any
possible eccentricity.
addendum
齿顶高
dedendum
齿根高
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The circular pitch Pc is the distance
between corresponding sides of neighboring
teeth,measured along the pitch circle,The
diametral pitch Pd is the number of teeth of a
gear for each inch of pitch diameter,(Do not
confuse Pd with Dp),The circular pitch and
the diametral pitch are related as follows:
Pd × Pc = π 6.11
The line of centers is a line passing
through the centers of two mating gears,The
center distance C (measured along the line of
centers) equals the sum of the pitch radii of
pinion and gear (Rpp+Rpg) or half the sum of
the pitch diameters:
C = (Dpp + Dpg)/2 6.12
circular pitch
周节
diametral pitch
径节
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Tooth width is the width of the tooth,measured
along the pitch circle,Tooth width is also referred
to as tooth thickness,Space width is the distance
between facing sides of adjacent teeth,measured
along the pitch circle,Tooth width plus space width
equals the circular pitch,Face width measures
tooth width in an axial direction,The circle from
which the involute is generated is called the base
circle,Backlash is the space width minus the tooth
width,It is necessitated by the tolerance of the
manufacturing processes used.
The face of the tooth is the active surface of the
tooth outside the pitch cylinder,The flank of the
tooth is the active surface inside the pitch cylinder.
The fillet is the rounded corner at the base of the
tooth.
base circle
基圆
backlash
侧隙
flank 齿根面
fillet
圆角，倒角
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The working depth is the sum of the addendum of a gear
and the addendum of its mating gear (i.e.,twice the
addendum in standard gears).
The base pitch is similar to the circular pitch but is
measured along the base circle instead of along the pitch circle.
It can easily be seen that the base radius equals the pitch radius
times the cosine of the pressure angle,Since,for a given angle,
the ratio between any subtended arc and its radius is constant,
it is also true that the base pitch equals the circular pitch times
the cosine of the pressure angle φ,The pressure angle φ is the
angle between the common tangent to the base circles,and the
common tangent to the pitch circles at the pitch point,At
present,the preferred pressure angle for spur gears is 20o,In
newer designs this angle replaces the value of 14.5oformerly
used.
压
力
角
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The AGMA (American Gear Manufacturers Association)
has established the following proportions for 20opressure
angle standard spur gears:
Addendum = l / Pd = Pc / π (in.) 6.13
Dedendum = 1.157 / Pd (in.) 6.14
Minimum clearance = dedendum - addendum
=,157/ Pd (in.) 6.15
In order to mate properly,gears running together must
have,(l) the same pitch,(2) the same pressure angle,and (3)
the same addendum and dedendum,The last requirement is
valid for standard gears only.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since the number of teeth of each of two mating gears
is proportional to its respective pitch diameter (prove),it
is easier and thus customary to express the speed ratio in
tooth numbers rather than pitch diameters,thus:
mω = np / ng = Ng / Np = (Dpg / Dpp) 6.16
where subscript p = pinion,and g = gear,Dpg = pitch
diameter of gear,and Dpp = pitch diameter of pinion; n =
rpm,and N = tooth number.
Recall that the involute is the curve almost
universally used at present for shaping the outline of gear
teeth,Let us now analyze the action of a pair of mating
involute teeth.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
1.the pressure casting
Key words,tendency,ferrous metal,cast iron
2.the welding of steel bars
Key words:welding deformations,mechanical properties,
carrying capacity(normal and be heated)
3.the numerically-controlled machine tool
Key words:compose,application,tendency,etc.
4.others
LESSON FOUR FOUNDATIONS OF
MACHINE DESIGN (PART l)
Bearings With Rolling Contact
In bearings with rolling contact,the shaft is
directly or indirectly supported by rolling elements,
such as balls,cylindrical or conical rollers,or
needles (i.e.,cylindrical rollers with a high L/D ratio).
The occasionally encountered name " antifriction
bearings" suggests that this type has little or no
friction,This is erroneous,since the friction is
merely of another nature than in journal bearings,In
bearings with rolling contact,friction losses are
caused by the elastic deformation of the surfaces in
rolling contact,sliding friction of rolling elements
with cages,retaining rings and seals,or with one
another,and also by some shear of lubricant.
conical roller
圆锥滚柱
needle 滚针
cylindrical
roller
圆柱滚柱
antifriction
bearing
减摩轴承
cage 保持架
retaining ring
档圈
bearing 轴承
journal bearing
颈轴承
滑动轴承
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Characteristics
l,Ball bearings have rolling elements in the form of balls,
which in all but the most inexpensive types are held in
cages,separators,or retainers,and inner and outer grooved
races.
2,Roller bearings have mainly cylindrical,conical,or
barrel-shaped rollers instead of balls,but are otherwise
quite similar to ball bearings.
3,Needle bearings usually have neither an inner race nor a
cage,The needles are retained by integral flanges on the
outer race,Transitional types between roller and needle
bearings are found in many catalogs.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Bearings with rolling contact have no
slipstick effect,low starting torque and
running friction,and unlike as in journal
bearings,the coefficient of friction varies little
with load or speed,Low starting torque is of
great advantage in railroad cars,and the
railroad industry has given the main impetus
for the development of mass-produced roller
bearings in the past,mainly for this reason.
These bearings may take both radial and
axial loads (depending upon the type),and
need less space axially but more radially
(except needle bearings),and less lubrication
and maintenance,than journal bearings,They
are also noisier and more expensive,and
cannot be repaired easily.
slipstick
滑动面粘附现象
impetus
刺激，促进
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since rolling elements and raceways are theoretically in
point or line contact and thus highly stressed in the contact area
by typically cyclical loads,all bearings with rolling contact will
eventually fail by fatigue when operated at their rated load.
Metal failure is typically by shear,just below the surface of
raceways or rolling elements--a result of the three principal
stresses in the x,y and z axis,all compressive,The
phenomenon is popularly called spalling and is characterized by
the presence of metal flakes in the grease,Even a small bearing
load will induce contact stresses above the yield point (up to
500,000 psi; called Hertz stresses after the mathematician who
analyzed them),As a result,the contact surfaces are under
residual compressive stresses,balanced by tensile stresses
under the surface,Friction between rolling elements and races
causes tensile stresses in the surface,believed to cause pitting.
The pits,in turn,are the starting point of fatigue failure crack.
剥
落薄
片 润
滑
脂
,
黄
油
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Life Expectancy
The life expectancy of ball bearings is not
uniform and is based upon statistics,Usually,
rated load values in catalogs give the minimum
life (revolutions or hours at a given rpm) for 90%
of a group of bearings,the so-called B-10 life.
Mean or average life is about three times rated
life,while median or 50% survival life is about
five times rated life.
The considerable spread of the life expectancy
curve is due to the many possible forms of
favorable or unfavorable interaction between the
different bearing elements on the basis of
dimensional tolerances,surface finish and varying
structural properties,as well as conditions of
service.
life expectancy
（概率,预计）
寿命
rated load
额定载荷
rated life
额定寿命
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
For plain ball bearings,radial load ratings in pounds (as
given in catalogs) are based upon a given rpm and a given
number of hours of life with 90% survival,The product rpm
× 60 × hr life is then equal to 106.
Other manufacturers give rated loads on the basis of 106
revolutions instead of hours of life (again,90% survival),In
that case,the number of hours of life can be found by
dividing 106 by rpm × 60,It will be clear from the
foregoing that,for a given load,changes in rpm (within the
design limits as given by the manufacturer) affect life in
hours inversely; thus,
n1/n2 =H2/H1 6.5
where n=rpm and H=life (hr.),Changes in load,on the
other hand,affect life in revolutions exponentially; thus,
Round per minute
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
(F1/F2)k = B2/B1 6.6
where F= load (lb) and B= life (revolutions),Values of k
range between 3 and 4 (3 for ball bearings; 3.3 for most
roller and needle bearings),Note that for k=3,doubling
the load means reducing the life in revolutions by a factor
of 8.
The relation between B (life in revolutions) and H (life
in hours),is as follows:
B = H × rpm × 60 6.7
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
On the other hand,life can be extended
considerably by providing superior lubrication.
Also,for the same size and materials,the
manufacturer can produce a bearing with
greater life expectancy by increasing the
accuracy of manufacture i.e,by reducing
manufacturing tolerances.
Other Factors Affecting Bearing Life
The rated life of any bearing with rolling
contact is based upon proper application
conditions,such as adequate lubrication,good
alignment,and adherence to recommended
interference fit values for the races,if applicable.
Any negative variance from these conditions
will affect fatigue life unfavorably.
alignment
调整
adherence
附着，坚持
interference fit
过盈配合
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Static And Dynamic Load
In tables giving permissible load data for ball
bearings,a distinction is often made between
static and dynamic loads,i.e.,with the bearing
stationary or rotating,The permissible static load
is smaller than the permissible dynamic load due
to the possibility of permanent indentation
(flattening of the contact areas when the bearing
is at rest),This phenomenon is called Brinelling.
Some permanent indentation is almost
unavoidable due to the high contact stresses,
however,it has been found that permanent
deformations smaller than 10-4 times the rolling
element diameter usually do not unfavorably
affect the bearing's operation.
indentation
压痕
Brinell
布里涅耳
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Rotation Factor
When the outer race rotates and the inner race is
stationary (the reversal of common practice),the rated load
may be increased by a factor of l.2,since a ball rolls further
per revolution on the outer race when the outer race rotates
(the ball rotates in the same sense as the race) than on the
inner race (where ball rotation is opposite to that of the race),
and life is limited by ball revolutions.
Equivalent Dynamic Load
A plain ball bearing is designed to take radial loads.
However,due to the curvature of the ball tracks in an axial
plane of both inner and outer races,some axial load may be
imposed on a radial ball bearing as well,Note that an axial
load is taken by all balls,at least in theory,while a radial
load is carried by less than half their number.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Manufacturers' catalogs give simple conversion methods
to compute an equivalent radial load in cases where both
radial and axial loads occur simultaneously in a rotating ball
bearing,One manufacturer uses a load conversion factor FC
which,when multiplied with the
radial component R,gives the
equivalent radial load P,which
is then used to select a properly
sized bearing, These load
conversion factors depend upon
the ratio of the thrust load and
the radial load (T/R) and vary
for different types of bearings.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
A friction drive consists of two cylinders
rolling together under some pressure,When
no slipping occurs,the tangential velocity V at
the line of contact of the two cylinders is,of
course,the same for each,From physics we
know that V= ω × r,in which V is the
tangential velocity (in./sec),ω is the rotational
velocity (rad/sec),and r is the radius of
rotation,Since V= ω1r1 and also V= ω2r2,we
can equate both equations,Thus,
ω1 × r1= ω2× r2 6.8
Dividing both sides by ω2× r1,we get
ω1/ω2 = r2/ r1 6.9
where ω1/ω2 is the speed ratio (mω).
Gearing
gear 齿轮
tangential
正切的，切线的
radius 半径
speed ratio 速比
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
When ω is divided by 2π and multiplied by 60,we obtain
rpm (n),Therefore,
mω = n1/n2 = r2/r1 = d2/d1 6.10
That is,the speed ratio of two cylinders in rolling
contact is equal to the inverse ratio of their radii or
diameters,in consistent units.
The drawback inherent to a friction drive,such as that
described above,is that it is liable to slip when power of
any consequence is transmitted,It can be used only for
very small torque applications,such as phonograph
turntable drives and the like.
diameter 直径
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The positive prevention of slippage in the
transmission of larger quantities of power
requires the use of teeth,penetrating into the
surface of each cylinder of the friction drive.
Mating cylinders provided with teeth are called
gears,The smaller of two mating gears is called
the pinion,the larger one the gear,The diameter
of each of the original rolling cylinders is called
the pitch diameter,and the cylinder's sectional
outline is called the pitch circle,The curved
shape of the tooth outline must be such that no
change in speed ratio occurs during the passing
contact of each tooth with its mating tooth on the
other gear,This is a basic requirement for all
gearing,Curves that satisfy this requirement are
called conjugate curves.
pinion 小齿轮
pitch diameter
节圆直径
pitch circle
节圆
conjugate
共轭的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
In spur gears,the simplest of
all gears,the teeth are straight
lengthwise and parallel to the axis
of rotation,The spur gear
nomenclature,which follows,is
mostly applicable to all other
types of gears as well.
While several of such carves
exist,the one almost universally
used at this time is the involute.
This curve is that described by a
point on a string as it is being
unwound from a cylinder.
involute 渐开线
spur gear
直齿圆柱齿轮
nomenclature 术语
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Nomenclature And Definitions
The pitch point P is the point of tangency of
the pitch circles,The pitch diameter Dp,is the
diameter of the pitch circle,and is equal to
twice the pitch radius,The addendum (i.e.,that
which is added) is the radial distance from the
pitch circle to the top of the tooth (the crest).
The dedendum (i.e.,that which is deducted) is
the radial distance from the pitch circle to the
bottom of the groove between adjacent teeth
(the root),Clearance is the difference between
addendum and dedendum in mating gears.
Clearance prevents binding caused by any
possible eccentricity.
addendum
齿顶高
dedendum
齿根高
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The circular pitch Pc is the distance
between corresponding sides of neighboring
teeth,measured along the pitch circle,The
diametral pitch Pd is the number of teeth of a
gear for each inch of pitch diameter,(Do not
confuse Pd with Dp),The circular pitch and
the diametral pitch are related as follows:
Pd × Pc = π 6.11
The line of centers is a line passing
through the centers of two mating gears,The
center distance C (measured along the line of
centers) equals the sum of the pitch radii of
pinion and gear (Rpp+Rpg) or half the sum of
the pitch diameters:
C = (Dpp + Dpg)/2 6.12
circular pitch
周节
diametral pitch
径节
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Tooth width is the width of the tooth,measured
along the pitch circle,Tooth width is also referred
to as tooth thickness,Space width is the distance
between facing sides of adjacent teeth,measured
along the pitch circle,Tooth width plus space width
equals the circular pitch,Face width measures
tooth width in an axial direction,The circle from
which the involute is generated is called the base
circle,Backlash is the space width minus the tooth
width,It is necessitated by the tolerance of the
manufacturing processes used.
The face of the tooth is the active surface of the
tooth outside the pitch cylinder,The flank of the
tooth is the active surface inside the pitch cylinder.
The fillet is the rounded corner at the base of the
tooth.
base circle
基圆
backlash
侧隙
flank 齿根面
fillet
圆角，倒角
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The working depth is the sum of the addendum of a gear
and the addendum of its mating gear (i.e.,twice the
addendum in standard gears).
The base pitch is similar to the circular pitch but is
measured along the base circle instead of along the pitch circle.
It can easily be seen that the base radius equals the pitch radius
times the cosine of the pressure angle,Since,for a given angle,
the ratio between any subtended arc and its radius is constant,
it is also true that the base pitch equals the circular pitch times
the cosine of the pressure angle φ,The pressure angle φ is the
angle between the common tangent to the base circles,and the
common tangent to the pitch circles at the pitch point,At
present,the preferred pressure angle for spur gears is 20o,In
newer designs this angle replaces the value of 14.5oformerly
used.
压
力
角
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
The AGMA (American Gear Manufacturers Association)
has established the following proportions for 20opressure
angle standard spur gears:
Addendum = l / Pd = Pc / π (in.) 6.13
Dedendum = 1.157 / Pd (in.) 6.14
Minimum clearance = dedendum - addendum
=,157/ Pd (in.) 6.15
In order to mate properly,gears running together must
have,(l) the same pitch,(2) the same pressure angle,and (3)
the same addendum and dedendum,The last requirement is
valid for standard gears only.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
Since the number of teeth of each of two mating gears
is proportional to its respective pitch diameter (prove),it
is easier and thus customary to express the speed ratio in
tooth numbers rather than pitch diameters,thus:
mω = np / ng = Ng / Np = (Dpg / Dpp) 6.16
where subscript p = pinion,and g = gear,Dpg = pitch
diameter of gear,and Dpp = pitch diameter of pinion; n =
rpm,and N = tooth number.
Recall that the involute is the curve almost
universally used at present for shaping the outline of gear
teeth,Let us now analyze the action of a pair of mating
involute teeth.
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING
1.the pressure casting
Key words,tendency,ferrous metal,cast iron
2.the welding of steel bars
Key words:welding deformations,mechanical properties,
carrying capacity(normal and be heated)
3.the numerically-controlled machine tool
Key words:compose,application,tendency,etc.
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