SPUR GEARS PPT Flashcards
In precision machines, in which a definite velocity ratio is of importance (as in watch mechanism), the only positive drive is by
gears or toothed wheels
A __________ is also provided, when the distance between the driver and the follower is very small.
gear drive
According to the position of axes of the shafts.
* The axes of the two shafts between which the motion is to be transmitted, may be
(a) Parallel,
(b) Intersecting, and
(c) Non-intersecting and non-parallel.
These gears are called ___________ and the arrangement is
known as bevel gearing.
bevel gears
The bevel gears, like spur gears may also have their teeth
inclined to the face of the bevel, in which case they are known
as
helical bevel gears
The gears, according to the peripheral velocity of the gears, may be classified as :
(a) Low velocity,
(b) Medium velocity, and
(c) High velocity.
The gears having velocity less than 3 m/s are termed as
low velocity gears
Gears having velocity between 3 and 15 m/s are known as
medium velocity gears
If the velocity of gears is more than 15 m/s, then these are called
high speed gears
The gears, according to the type of gearing, may be classified as:
(a) External gearing,
(b) Internal gearing, and
(c) Rack and pinion
In ____________, the gears of the two shafts mesh externally with each other
external gearing
The larger of these two wheels is called __________ and the smaller wheel is called pinion.
spur wheel or gear
The larger of these two wheels is called spur wheel or gear and the smaller wheel is called ________.
pinion
In _____________, the gears of the two shafts mesh internally with each other as shown.
internal gearing
The larger of these two wheels is called ____________ and
the smaller wheel is called pinion. In an internal gearing, the motion of the wheels is always the same.
annular wheel
The straight line gear is called _______ and the circular wheel is
called _______.
rack and pinion
The teeth on the gear surface may be
(a) Straight,
(b) Inclined, and
(c) Curved.
are toothed wheels whose teeth are straight and parallel to the shaft axis. They are used to transmit motion and power between parallel shafts
Spur Gears
It is an imaginary circle which by pure rolling action, would give the same motion as the actual gear.
Pitch circle
It is the diameter of the pitch circle. The size of the gear is usually specified by the ____________. It is also called as pitch diameter.
Pitch circle diameter
It is a common point of contact between two pitch circles.
Pitch point
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°.
Pressure angle or angle of obliquity
It is the radial distance of a tooth from the pitch circle to the top of the tooth.
Addendum
It is the radial distance of a tooth from the pitch circle to the bottom of the tooth.
Dedendum
It is the circle drawn through the top of the teeth and is
concentric with the pitch circle.
Addendum circle
It is the circle drawn through the bottom of the teeth. It is
also called root circle.
Dedendum circle
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.
Circular pitch
It is the ratio of number of teeth to the pitch circle diameter. It is denoted by Pd.
Diametral pitch
It is the ratio of the pitch circle diameter to the number of teeth. It is usually denoted by m. Mathematically, Module, m = D / T
The recommended series of modules are 1, 1.25, 1.5, 2, 2.5,
3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40 and 50.
Module
It is the radial distance equal to the difference between the dedendum and addendum, in a meshing gear.
Clearance
It is the radial distance between the addendum and the dedendum circle of a gear. It is equal to the sum of the addendum and dedendum.
Whole depth or Total Depth
It is radial distance from the addendum circle to the clearance circle. It is equal to the sum of the addendum of the two meshing gears.
Working depth
It is the width of the tooth measured along the pitch circle.
Tooth thickness
It is the width of space between the two adjacent teeth measured along the pitch circle.
Tooth space
It is the difference between the tooth space and the tooth thickness, as measured on the pitch circle.
Backlash
It is surface of the tooth above the pitch surface.
Face of the tooth
It is the surface of the top of the tooth.
Top land
It is the surface of the tooth below the pitch surface.
Flank of the tooth
It is the width of the gear tooth measured parallel to its axis.
Face width
It is the curve formed by the face and flank of the tooth.
Profile
It is the radius that connects the root circle to the profile of the
tooth.
Fillet radius
It is the path traced by the point of contact of two teeth from the beginning to the end of engagement.
Path of contact
It is the length of the common normal cut-off by the addendum circles of the wheel and pinion
Length of the path of contact
It is the path traced by a point on the pitch circle from the beginning to the end of engagement of a given pair of teeth. The ____________ consists of two parts, i.e.
Arc of contact
It is the portion of the path of contact from the beginning of
the engagement to the pitch point.
Arc of approach
It is the portion of the path of contact from the pitch point to the end of the engagement of a pair of teeth.
Arc of recess
It is the ratio of the length of arc of contact to the circular pitch i.e. number of pairs of teeth in contact.
Contact ratio
the common normal at the point of contact between a pair of teeth must always pass through the pitch point.
This is fundamental condition which must be satisfied while
designing the profiles for the teeth of gear wheels.
law of gearing
is the curve traced by a point on the circumference of a circle which rolls without slipping on a fixed straight line.
cycloid
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
epicycloid
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
hypocycloid
was one of the first regular profiles used for gear teeth.
cycloidal form
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
Involute Teeth
Used for general purpose gears. It is stronger but has no
interchangeability. The tooth profile of this system has cycloidal curves at the top and bottom and involute curve at the middle portion.
14 1/2° Composite system
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 ____________ and occurs when the teeth are being cut.
interference
The material used for the manufacture of gears depends upon the strength and service conditions like wear, noise etc.
Gear Materials
The gears may be manufactured from metallic or non-metallic materials. The metallic gears with cut teeth are commercially obtainable in
cast iron, steel and bronze
The __________ materials like wood, rawhide, compressed paper and synthetic resins like nylon are used for gears, especially for reducing noise.
nonmetallic
The _______ is widely used for the manufacture of gears due to its good wearing properties, excellent machinability and ease of producing complicated shapes by casting method.
The ________ gears with cut teeth may be employed, where smooth action is not important.
cast iron
The ______ is used for high strength gears and ______ may be plain carbon steel or alloy steel.
The ________ gears are usually heat treated in order to combine properly the toughness and tooth hardness.
steel
The _____________ is widely used for worm gears in order to reduce wear of the worms which will be excessive with cast iron or steel.
phosphor bronze
______________ assumed that as the load is being transmitted from one gear to another, it is all given and taken by one tooth.
Lewis Equation
The permissible working stress (Sw) in the Lewis equation depends upon the material for which an allowable static stress (So) may be determined.
Permissible Working Stress for Gear Teeth in the Lewis Equation
The ___________________ is the stress at the elastic limit of the material. It is also called the basic stress
allowable static stress
Every gear tooth acts as a cantilever. If the total repetitive dynamic load acting on the gear tooth is greater than the beam strength of the gear tooth, then the gear tooth will fail in bending, i.e. the gear tooth with break. In order to avoid such failure, the module and face width of the gear is adjusted so that the beam strength is greater than the dynamic load.
Bending failure
It is the surface fatigue failure which occurs due to many repetition of Hertz contact stresses. The failure occurs when the surface contact stresses are higher than the endurance limit of the material. The failure starts with the formation of pits which continue to grow resulting in the rupture of the tooth surface.
In order to avoid the ________, the dynamic load between the gear tooth should be less than the wear strength of the gear tooth.
Pitting
The excessive heat is generated when there is an excessive surface pressure, high speed or supply of lubricant fails. It is a stick-slip phenomenon in which alternate shearing and welding takes place rapidly at high spots.
This type of failure can be avoided by properly designing the parameters such as speed, pressure and proper flow of the lubricant, so that the temperature at the rubbing faces is within the permissible limits.
Scoring
The foreign particles in the lubricants such as dirt, dust or burr enter between the tooth and damage the form of tooth.
This type of failure can be avoided by providing filters for the lubricating oil or by using high viscosity lubricant oil which enables the formation of thicker oil film and hence permits easy passage of such particles without damaging the gear surface.
Abrasive wear
The corrosion of the tooth surfaces is mainly caused due to the presence of corrosive elements such as additives present in the lubricating oils.
In order to avoid this type of wear, proper anti-corrosive additives should be used.
Corrosive wear
