Screw thread measurements and Gear measurement

8/20/2014 1Hareesha N Gowda, DSCE, Blore-78

Screw thread- definition
a screw thread is the helical ridge produced by forming a
continuous helical groove of uniform section on the
external or internal surface of a cylinder or a cone.
A screw thread formed on a cylinder is known as straight or
parallel screw thread, while the one formed on a cone is
known as tapered threads.

Types of thread
External thread: a thread formed on outside of a
work piece is known as external thread. Example:
on bolts or studs etc.
Internal thread: a thread formed on inside of a
work piece is known as internal thread. Example:
on a nut or female screw gauge.

Screw thread - use
Screw threads are used:
• To hold parts together-act as fastners (ex: V-threads)
• To transmit motion & power (Square, Acme threads)

Screw Thread terminology
Pitch
Crest
Root
Flank
Thread
Angle
Pitch line
Axis of thread
Axial thickness
Addendum
Dedendum
Flank
angle
Major dia Pitch dia Minor dia
EXTERNAL THREAD TERMINOLOGY

 Pitch: The distance from a point on a screw thread to a
corresponding point on the next thread measured parallel to the
axis.
 Lead The distance a screw thread advances in one turn. For a
single start threads, lead=pitch,
For double start, lead=2xpitch, & so on.
 Thread Form: The cross section of thread cut by a plane
containing the axis.
 Major Diameter: This is the diameter of an imaginary cylinder, co-
axial with the screw, which just touches the crests of an external
thread or roots of an internal threads. It is also called as ‘Nominal
diameter’

 Minor diameter: This is the diameter of an imaginary
cylinder, co-axial with the screw which just touches the roots
of an external thread or the crest of an internal thread. This
is also referred to as ‘root’ or ‘core diameter’.
 Effective diameter or Pitch diameter: It is the diameter of
an imaginary cylinder coaxial with the axis of the thread and
intersects the flanks of the thread such that width of the
threads & width of spaces between threads are equal.
 Flank: It is the Thread surface that connects crest with root.
• Depth of thread: It is the distance between crest and root
measured perpendicular to axis of screw.

 Angle of thread: Included angle between sides of thread
measured in axial plane.
 Helix angle: Angle that the thread makes with plane
perpendicular to thread axis.
 Flank angle: It is half the included angle of the thread.
 Addendum: It is the distance between the crest and the
pitch line measured perpendicular to axis of the screw.
 Dedendum: It is the distance between the pitch line &
the root measured perpendicular to axis of the screw.

MEASUREMENT OF VARIOUS ELEMENTS OF
THREAD
To find out the accuracy of a screw thread it will
be necessary to measure the following:
1) Major diameter.
2) Minor diameter.
3) Effective or Pitch diameter.
4) Pitch
5) Thread angle and form

Measurement of major diameter
The instruments which are used to find the major
diameter are by
 Bench micrometer
Bench micrometer

Ordinary micrometer:
 The ordinary micrometer is quite suitable for measuring the
external major diameter.
 It is first adjusted for appropriate cylindrical size (S) having
the same diameter (approximately).This process is known as ‘
gauge setting’ .
After taking this reading ‘ R the micrometer is set on the
major diameter of the thread, and the new reading is ‘R2

Measurement by Bench micrometer:
Clamp
Fiducial
Indicator
Measuring
Anvils Holding centres
Micrometer head
Supports
BENCH MICROMETER

For getting the greater accuracy the bench micrometer is used for
measuring the major diameter.
In this process the variation in measuring Pressure, pitch errors are being
neglected.
 The fiducial indicator is used to ensure all the measurements are made at
same pressure.
The instrument has a micrometer head with a vernier scale to read the
accuracy of 0.002mm. Calibrated setting cylinder having the same diameter
as the major diameter of the thread to be measured is used as setting
standard.
After setting the standard, the setting cylinder is held between the anvils
and the reading is taken

Then the cylinder is replaced by the threaded work piece and the new
reading is taken

Holding centre
Measuring anvil
Holding centre
Measuring anvil
StandardCylinder
ScrewThread
Measurement of Major diameter

Measurement of the major diameter of an
Internal thread:
An indirect approach of measuring internal dia is obtained by
obtaining the cast of the Thread. The main art thus lies in
obtaining a perfect cast.

Measurement of the major diameter of an
Internal thread:

Measurement of Minor diameter
The minor diameter is measured by a comparative
method by using floating carriage diameter measuring
machine and small ‘ V pieces which make contact with
the root of the thread.
These V pieces are made in several sizes, having
suitable radii at the edges.
 V pieces are made of hardened steel.
The floating carriage diameter-measuring machine is
a bench micrometer mounted on a carriage.

The threaded work piece is mounted between the centres of the
instrument and the V pieces are placed on each side of the work
piece and then the reading is noted.
After taking this reading the work piece is then replaced by a
standard reference cylindrical setting gauge.
Measurement of Minor diameter

Measurement of Minor diameter of
Internal threads:
The Minor diameter of Internal threads are
measured by
1. Using taper parallels
2. Using Rollers.

Internal threads:
1. Using taper parallels:
 For diameters less than 200mm the use of Taper parallels and
micrometer is very common.
The taper parallels are pairs of wedges having reduced and
parallel outer edges.
The diameter across their outer edges can be changed by sliding
them over each other.

Internal threads:
Using rollers:
 For more than 200mm diameter this method is used.
Precision rollers are inserted inside the thread and proper
slip gauge is inserted between the rollers.
The minor diameter is then the length of slip gauges plus
twice the diameter of roller.

Pitch measurement
The most commonly used methods for
measuring the pitch are
1. Pitch measuring machine
2. Tool makers microscope
3. Screw pitch gauge

Tool makers microscope:
Lamp
Hollow base
Collimator lens
Base
Column
Eye piece
Optical head
Mirror
work table
with carriage

Tool makers microscope:
1. Worktable is placed on the base of the instrument.
2. The optical head is mounted on a vertical column it can be moved up
and down.
3. Work piece is mounted on a glass plate.
4. A light source provides horizontal beam of light which is reflected
from a mirror by 90 degree upwards towards the table.
5. Image of the outline of contour of the work piece passes through the
objective of the optical head.
6. The image is projected by a system of three prisms to a ground glass
screen.
7. The measurements are made by means of cross lines engraved on the
ground glass screen.
8. The screen can be rotated through 3 60°.
9. Different types of graduated screens and eyepieces are used

Pitch measuring machine
When the pointer is accurately placed in position, the micrometer reading is
noted. The stylus is then moved along into the next thread space, by rotation of
the micrometer, and a second reading taken.
The difference between the two readings is the pitch of the thread. Readings are
taken in this manner until the whole length of the screw thread has been
covered.
Spring loaded head permits the
stylus to move up the flank of the
thread and down into the next space
as it is moved along.
Accurate positioning of the stylus
between the two flanks is obtained
by ensuring that the pointer T is
always opposite to its index mark
when readings are taken.

Measurement of screw thread angle
(Flank angle)

Measurement of effective diameter
Effective diameter measurement is carried
out by following methods.
1 two wires method
3. three wires method.
4. Micrometer method.

Two wire method:
 The effective diameter can not be measured directly but
can be calculated from the measurements made.
 Wires of exactly known diameters are chosen such that
they contact the flanks at their straight portions.
 If the size of the wire is such it contacts the flanks at the
pitch line, it is called the ‘best size’ of wire which can be
determined by geometry of screw thread.
 The screw thread is mounted between the centers &
wires are placed in the grooves and reading M is taken.
 Then the effective diameter E =T+P
where T =M-2d, & P is a value which depends on diameter
of wire, pitch & angle of the screw thread.

Two wire method:
M
M-Dimension over the wire

Three Wire method
The three-wire method is the accurate method.
 In this method three wires of equal and precise
diameter are placed in the groves at opposite sides of
the screw.
In this one wire on one side and two on the other side
are used. The wires either may held in hand or hung
from a stand.
 This method ensures the alignment of micrometer
anvil faces parallel to the thread axis.

Three Wire method
 This method is more accurate than two wire method
as it ensures alignment of micrometer faces parallel
to the thread axis.
 Here, three wires of exactly known diameters are
used, one on one side & the two on the other side.
The wires may be held in hand or hung from a stand.
 From the fig, M=diameter over the wires
E= effective diameter (to be found)
d= diameter of wires, h=height of wire center above
the pitch line, r=radius of wire, H=depth of thread,
D=major diameter of the thread.

Three Wire method
E M
H
A
B C
D

P
h
E
M
Dia 'd'

E

Three Wire method
2
cot
22
cosec1dEMOr
2
cot
22
cosec122
2
cot
42
rcosec2EMi.e.
2r2hEMwires,over theDistance
2
cot
42
cosec
2
)(
2
cot
42
H
CDand
2
cot
22
cot
2
cosec
22
cosecABAD,ABDetriangl





P
P
rEr
P
Pd
CDADhFurther
PP
DEH
d
theFrom








































Three Wire method
out.foundbecanEknown,isdasM,ofluecorrect vathefindingAfter
above.derivedformulaeusingvaluesaltheoreticwith thecompare
then&ypracticallMofvaluethemeasurecanWe
P5155.1d3DM
732.1
2
cot,2
2
eccos,60,P6495.0DE
0.6495PthreadofDepththreads,MetricFor
thread.theofdiametermajortheisDwhereP605.1d1657.3DM
921.1
2
cotand,1657.2
2
cosec0.64P,-DE
0.64Pthreadofdepth,55thread,WhitworthFor
o
o















BEST WIRE SIZE
P



A
P/2
P/4
Pitch line
BEST SIZE OF WIRE
B

BEST WIRE SIZE
sec
P
sec
P
2D
thread.theofpitchtheisPwhere
4
P
ABline,pitchon theliesABsinceAlso.
2
secAB2OB2Di.e.
)(Dwiresizebestofdia
2
1
wireofradiusOBBut
.
2
secAB
2
cos
AB
2
-90sin
AB
OB
OB
AB
2
-90sinor,
OB
AB
AOBSinOAB,triangleIn the
line.pitchat thethreadtheofportionflanklar toperpendicuis
OBfiginshownaswords,otherInthread.screwtheofdiametereffectiveor
linepitchat thecontactmakeswhichonetheiswiresizebestThe
b
b
b

















 






 





 

GEAR…..
• Power transmission is the movement of energy
from its place of generation to a location where
it is applied to performing useful work
• A gear is a component within a transmission
device that transmits rotational force to another
gear or device

TYPES OF GEARS
1. According to the position of axes of the
shafts.
a. Parallel
1.Spur Gear
2.Helical Gear
3.Rack and Pinion
b. Intersecting
Bevel Gear
c. Non-intersecting and Non-parallel
worm and worm gears

SPUR GEAR
• Teeth is parallel to axis
of rotation
• Transmit power from
one shaft to another
parallel shaft
• Used in Electric
screwdriver, oscillating
sprinkler, windup alarm
clock, washing machine
and clothes dryer

External and Internal spur Gear…

Helical Gear
• The teeth on helical gears are cut at an angle
to the face of the gear
• This gradual engagement makes helical gears
operate much more smoothly and quietly than
spur gears
• One interesting thing about helical gears is
that if the angles of the gear teeth are correct,
they can be mounted on perpendicular shafts,
adjusting the rotation angle by 90 degrees

Rack and pinion
• Rack and pinion gears
are used to convert
rotation (From the
pinion) into linear
motion (of the rack)
• A perfect example of this
is the steering system on
many cars

Straight and Spiral Bevel Gears

Forms of Teeth
• In actual practice following are the two types of teeth commonly
used
1. Cycloidal teeth ; and 2. Involute teeth.
Cycloidal Teeth
• A cycloid is the curve traced by a point on the circumference of a
circle which rolls without slipping on a fixed straight line.
• When a circle rolls without slipping on the outside of a fixed circle,
the curve traced by a point on the circumference of a circle is known
as epi-cycloid.
• On the other hand, if a circle rolls without slipping on the inside of a
fixed circle, then the curve traced by a point on the circumference of
a circle is called hypo-cycloid.

Construction of cycloidal teeth for gear
• The cycloidal teeth of a gear may be constructed as shown in Fig. (b).
• The circle C is rolled without slipping on the outside of the pitch circle
and the point P on the circle C traces epi-cycloid PA, which represents
the face of the cycloidal tooth.
• The circle D is rolled on the inside of pitch circle and the point P on
the circle D traces hypo-cycloid PB, which represents the flank of the
tooth profile.
The profile BPA is one side of the
cycloidal tooth. The opposite side
of the tooth is traced as explained
above.

Involute Teeth
• An involute of a circle is a plane curve generated by a point on a tangent,
which rolls on the circle without slipping or by a point on a taut string which
in unwrapped from a reel as shown in Fig.
• In connection with toothed wheels, the circle is known as base circle. The
involute is traced as follows :
• A3, the tangent A3T to the involute is
perpendicular to P3A3 and P3A3 is the
normal to the involute.
• In other words, normal at any point of an
involute is a tangent to the circle.

• Pitch circle. It is an imaginary circle which by pure rolling action would give
the same motion as the actual gear.
• Pitch circle diameter. It is the diameter of the pitch circle. The size of the
gear is usually specified by the pitch circle diameter. It is also known as pitch
diameter.
• Pitch point. It is a common point of contact between two pitch circles.
• Pitch surface. It is the surface of the rolling discs which the meshing gears
have replaced at the pitch circle.
• Pressure angle or angle of obliquity. It is the angle between the common
normal to two gear teeth at the point of contact and the common tangent
at the pitch point. It is usually denoted by φ. The standard pressure angles
are 14 1/2 ° and 20°.

• Addendum. It is the radial distance of a tooth from the pitch circle to
the top of the tooth.
• Dedendum. It is the radial distance of a tooth from the pitch circle to
the bottom of the tooth.
• Addendum circle. It is the circle drawn through the top of the teeth
and is concentric with the pitch circle.
• Dedendum circle. It is the circle drawn through the bottom of the
teeth. It is also called root circle.
Note : Root circle diameter = Pitch
circle diameter × cos φ , where φ is
the pressure angle.

Circular pitch. It is the distance measured on the circumference of the
pitch circle from a point of one tooth to the corresponding point on
the next tooth. It is usually denoted by Pc ,Mathematically,
• A little consideration will show that the two gears will mesh together
correctly, if the two wheels have the same circular pitch.
Note : If D1 and D2 are the diameters of the two meshing gears having
the teeth T1 and T2 respectively, then for them to mesh correctly,

Diametral pitch. It is the ratio of number of teeth to the pitch circle
diameter in millimetres. It is denoted by pd. Mathematically,
Module. It is the ratio of the pitch circle diameter in millimeters to the
number of teeth. It is usually denoted by m. Mathematically,
Clearance. It is the radial distance from the top of the tooth to the
bottom of the tooth, in a meshing gear. A circle passing through the
top of the meshing gear is known as clearance circle.
Total depth. It is the radial distance between the addendum and the
dedendum circles of a gear. It is equal to the sum of the addendum
and dedendum.

Face of tooth. It is the surface of the gear tooth above the pitch surface.
Flank of tooth. It is the surface of the gear tooth below the pitch surface.
Top land. It is the surface of the top of the tooth.
Face width. It is the width of the gear tooth measured parallel to its axis.
Profile. It is the curve formed by the face and flank of the tooth.

Comparison Between Involute and Cycloidal Gears
• In actual practice, the involute gears are more commonly used as compared to cycloidal
gears, due to the following advantages :
Advantages of involute gears
• The most important advantage of the involute gears is that the centre distance for a
pair of involute gears can be varied within limits without changing the velocity ratio.
This is not true for cycloidal gears which requires exact centre distance to be
maintained.
• In involute gears, the pressure angle, from the start of the engagement of teeth to the
end of the engagement, remains constant. It is necessary for smooth running and less
wear of gears. But in cycloidal gears, the pressure angle is maximum at the beginning of
engagement, reduces to zero at pitch point, starts decreasing and again becomes
maximum at the end of engagement. This results in less smooth running of gears.
• The face and flank of involute teeth are generated by a single curve where as in
cycloidal gears, double curves (i.e. epi-cycloid and hypo-cycloid) are required for the face
and flank respectively. Thus the involute teeth are easy to manufacture than cycloidal
teeth. In involute system, the basic rack has straight teeth and the same can be cut with
simple tools.
• Note : The only disadvantage of the involute teeth is that the interference occurs with
pinions having smaller number of teeth. This may be avoided by altering the heights of
addendum and dedendum of the mating teeth or the angle of obliquity of the teeth.

Advantages of cycloidal gears
Following are the advantages of cycloidal gears :
• Since the cycloidal teeth have wider flanks, therefore the cycloidal gears are
stronger than the involute gears, for the same pitch. Due to this reason, the
cycloidal teeth are preferred specially for cast teeth.
• In cycloidal gears, the contact takes place between a convex flank and concave
surface, whereas in involute gears, the convex surfaces are in contact. This
condition results in less wear in cycloidal gears as compared to involute gears.
However the difference in wear is negligible.
• In cycloidal gears, the interference does not occur at all. Though there are
advantages of cycloidal gears but they are outweighed by the greater simplicity and
flexibility of the involute gears.

Interference in Involute Gears
• Fig. shows a pinion with centre O1, in mesh with wheel or gear with centre O2.
• MN is the common tangent to the base circles and KL is the path of contact
between the two mating teeth.
• The tip of tooth on the pinion will then undercut the tooth on the wheel at the root
and remove part of the involute profile of tooth on the wheel. This effect is known
as interference, and occurs when the teeth are being cut.
• In brief, the phenomenon when the tip of tooth undercuts the root on its mating
gear is known as interference.
•A little consideration will show,
that if the radius of the addendum
circle of pinion is increased to O 1
N, the point of contact L will move
from L to N.
•When this radius is further
increased, the point of contact L
will be on the inside of base circle of
wheel and not on the involute
profile of tooth on wheel.

• Similarly, if the radius of the addendum circle of the wheel increases beyond O2M,
then the tip of tooth on wheel will cause interference with the tooth on pinion. The
points M and N are called interference points.
• From the above discussion, we conclude that the interference may only be avoided,
if the point of contact between the two teeth is always on the involute profiles of
both the teeth. In other words, interference may only be prevented, if the
addendum circles of the two mating gears cut the common tangent to the base
circles between the points of tangency.
• When interference is just avoided, the maximum length of path of contact is MN
when the maximum addendum circles for pinion and wheel pass through the points
of tangency N and M respectively as shown in Fig.
Obviously, interference may be
avoided if the path of contact does
not extend beyond interference
points. The limiting value of the
radius of the addendum circle of the
pinion is O1N and of the wheel is
O2M.

• Measurement of tooth thickness
• The tooth thickness is generally measured at pitch
circle and is therefore, the pitch line thickness of
the tooth. Following method is used for
measuring the gear tooth thickness :
• Measurement of tooth thickness by gear tooth
vernier caliper.
• The gear tooth thickness can be conveniently
measured by a gear tooth vernier as shown in the
fig.
• Since the gear tooth thickness varies from the tip
to the base circle of the tooth, the instrument
must be capable of measuring the tooth thickness
at a specified position on the tooth.
• The gear tooth vernier has two vernier scales. The
vertical vernier scale is used to set the depth (d)
along the pitch circle from the top surface of the
tooth at which the width (w) has to be measured.
While the horizontal vernier scale is used to
measure the width (w) of the teeth.

• Considering one gear tooth, the theoretical
values of w and d can be found out which may
be verified by the instrument.
• As shown in the figure , w is a chord ADB, but
tooth thickness is specified as an arc distance
AEB. Also the depth d adjusted on the
instrument is slightly greater than the
addendum CE", width w is therefore called
chordal thickness and d is called the chordal
addendum.
W=AB=2AD
WKT, θ=360/4N,
Where N= number of teeth.

Screw thread measurements and Gear measurement

Screw thread measurements and Gear measurement

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (20)

Ähnlich wie Screw thread measurements and Gear measurement

Ähnlich wie Screw thread measurements and Gear measurement (20)

Mehr von Hareesha N Gowda, Dayananda Sagar College of Engg, Bangalore

Mehr von Hareesha N Gowda, Dayananda Sagar College of Engg, Bangalore (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Screw thread measurements and Gear measurement