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

Terminology of screw threads

Screw thread- definitiona 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

PitchCrest

Root

Flank

ThreadAngle

Pitch line

Axis of thread

Axial thickness

Addendum

Dedendum

Flankangle

Major dia Pitch dia Minor dia

EXTERNAL THREAD TERMINOLOGY

Screw Thread terminology

Pitch: The distance from a point on a screw thread to acorresponding point on the next thread measured parallel to theaxis.

Lead The distance a screw thread advances in one turn. For asingle start threads, lead=pitch,

For double start, lead=2xpitch, & so on.

Thread Form: The cross section of thread cut by a planecontaining the axis.

Major Diameter: This is the diameter of an imaginary cylinder, co-axial with the screw, which just touches the crests of an externalthread or roots of an internal threads. It is also called as ‘Nominaldiameter’

Screw Thread terminology Minor diameter: This is the diameter of an imaginary

cylinder, co-axial with the screw which just touches the rootsof an external thread or the crest of an internal thread. Thisis also referred to as ‘root’ or ‘core diameter’.

Effective diameter or Pitch diameter: It is the diameter ofan imaginary cylinder coaxial with the axis of the thread andintersects the flanks of the thread such that width of thethreads & 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.

Screw Thread terminology

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) Pitch5) 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

FiducialIndicator

MeasuringAnvils Holding centres

Micrometer head

Supports

BENCH MICROMETER

Measurement by 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

Measurement by Bench micrometer:

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

Measurement by Bench micrometer:

Measurement by Bench micrometer:

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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 diameterThe 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.

Measurement of Minor diameter

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 by1. Using taper parallels2. Using Rollers.

Measurement of Minor diameter of 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.

Measurement of Minor diameter of 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 machine2. Tool makers microscope3. Screw pitch gauge

Tool makers microscope:

Tool makers microscope:

Lamp

Hollow base

Collimator lens

Base

Column

Eye pieceOptical head

Mirror

work tablewith carriage

Tool makers microscope:

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.

Screw pitch gauge

Measurement of screw thread angle(Flank angle)

Measurement of effective diameter

Effective diameter measurement is carried out by following methods.

1 two wires method3. three wires method.4. Micrometer method.

Two wire method:

The effective diameter can not be measured directly butcan be calculated from the measurements made.

Wires of exactly known diameters are chosen such thatthey contact the flanks at their straight portions.

If the size of the wire is such it contacts the flanks at thepitch line, it is called the ‘best size’ of wire which can bedetermined 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 diameterof wire, pitch & angle of the screw thread.

Two wire method:

M

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Two wire method:

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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

Three Wire method

This method is more accurate than two wire methodas it ensures alignment of micrometer faces parallelto the thread axis.

Here, three wires of exactly known diameters areused, 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 abovethe pitch line, r=radius of wire, H=depth of thread,D=major diameter of the thread.

Three Wire method

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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

Helical Gear…

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

WORM AND WORM GEAR

Forms of Teeth

• In actual practice following are the two types of teeth commonlyused

1. Cycloidal teeth ; and 2. Involute teeth.

Cycloidal Teeth

• A cycloid is the curve traced by a point on the circumference of acircle 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 knownas epi-cycloid.

• On the other hand, if a circle rolls without slipping on the inside of afixed circle, then the curve traced by a point on the circumference ofa 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.

NOMENCLATURE OF SPUR GEARS

• 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 cycloidalgears, due to the following advantages :

Advantages of involute gears

• The most important advantage of the involute gears is that the centre distance for apair 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 bemaintained.

• In involute gears, the pressure angle, from the start of the engagement of teeth to theend of the engagement, remains constant. It is necessary for smooth running and lesswear of gears. But in cycloidal gears, the pressure angle is maximum at the beginning ofengagement, reduces to zero at pitch point, starts decreasing and again becomesmaximum 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 incycloidal gears, double curves (i.e. epi-cycloid and hypo-cycloid) are required for the faceand flank respectively. Thus the involute teeth are easy to manufacture than cycloidalteeth. In involute system, the basic rack has straight teeth and the same can be cut withsimple tools.

• Note : The only disadvantage of the involute teeth is that the interference occurs withpinions having smaller number of teeth. This may be avoided by altering the heights ofaddendum 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 rootand remove part of the involute profile of tooth on the wheel. This effect is knownas interference, and occurs when the teeth are being cut.

• In brief, the phenomenon when the tip of tooth undercuts the root on its matinggear 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 chordaladdendum.

W=AB=2AD

WKT, θ=360/4N,

Where N= number of teeth.