ARALING PANLIPUNA 4 Q3 WEEK 1.pptx

1. Gears
Theory of Machines

2. Gears
Gears are used to transmit
motion from one shaft to
another or between a shaft and a
slide. This is accomplished by
successively engaging teeth.
Gears use no intermediate link
or connector and transmit the
motion by direct contact.
Gears 2

3. Gears
If power transmitted between two shafts is
small, motion between them can be
obtained by using two plain cylinders or
discs.
If there is no slip, a definite motion can be
transmitted. Such wheels are termed as
friction wheels.
However, as power transmission
increases, slip occurs between the discs
and the motion no longer remains definite.
Gears 3

4. Gears
To transmit a definite motion and to prevent slip,
projections and recesses on the two discs can be
made which can mesh with each other. This leads
to the formation of teeth on the discs and the
motion changes from rolling to sliding. These discs
with teeth are known as gears or gear wheels.
Gears
4

5. Classification of Gears
Gears
5
Based on relative position of the shafts, gears can be classified as:
1. Parallel Shafts
(a) Spur Gears
(b) Spur Rack and Pinion
(c) Helical Gears
(d) Double Helical and Herringbone Gears
2. Intersecting Shafts
(a) Bevel Gears
3. Skew Shafts
(a) Cross Helical Gears
(b) Worm Gears
(c) Hypoid Gears

6. Spur Gears
Gears
6
They have straight teeth parallel to the axes and thus are not subjected to axial thrust due to
tooth load.
At the time of engagement, the contact extends across the entire width on a line parallel to the
axes of rotation. This results in sudden application of the load, high impact stresses and
excessive noise at high speeds.
Gears having external teeth
rotate in the opposite
direction.
Gears having internal teeth on an annulus
ring will mesh with an external pinion (smaller
gear) and rotate in the same direction.

7. Spur Rack and Pinion
Gears
7
Spur rack is a special case of a spur gear where it is made of infinite diameter.
The spur rack and pinion combination converts rotary motion into translatory motion, or vice-
versa.
It is used in a lathe in which the rack transmits motion to the saddle.

8. Helical Gears or Helical Spur Gears
Gears
8
In helical gears, the teeth are curved, each being helical in shape. Two mating gears have the
same helix angle, but have teeth of opposite hands.
At the beginning of engagement, contact occurs only at the point of leading edge of the curved
teeth. As the gears rotate, the contact extends along a diagonal line across the teeth.
Thus the load application is gradual which
results in low impact stresses and reduction
in noise.
Therefore, the helical gears can be used at
higher velocities and have greater load
carrying capacity.
The disadvantage is of having end thrust as
there is a force component along the gear
axis. The bearings and the assemblies
mounting the helical gears must be able to
withstand thrust loads.

9. Double Helical and Herringbone
Gears
Gears
9
A double helical gear is equivalent to a pair of helical gears secured together, one having a
right-hand helix and the other a left-hand helix.
The teeth of the two rows are separated by a groove used for tool run out.
Axial thrust is eliminated as the axial thrusts of the two rows cancel each other out. They can
be run at high speeds with less noise and vibrations.
If the left and the right inclinations meet at a common apex and there is no groove in between,
the gear is known as Herringbone Gear.

10. Bevel Gears
Gears
10
Kinematically, the motion of two intersecting shafts is equivalent to the rolling of two cones,
assuming no slipping. The gears, in general are known as Bevel Gears.
When teeth formed on the cones are straight, the
gears are known as Straight Bevel Gears.
The teeth are straight, radial to the point of
intersection of the shaft axes and vary in cross
section throughout their length.
Usually used to connect shafts at right angles
which run at low speeds.
Gears of the same size and connecting two shafts
at right angles to each other are known as mitre
gears.

11. Bevel Gears
Gears
11
When the teeth of a bevel gear are inclined at an angle to the face of the bevel, they are known
as Spiral Bevels or Helical Bevels.
They are smoother in action and quieter than
straight tooth bevels as there is gradual load
application and low impact stresses.
There exists an axial thrust calling for stronger
bearings and supporting assemblies.
These are used for the drive to the differential of an
automobile.
Spiral bevel gears with curved teeth but with zero
degree spiral angle are known as Zerol Bevel Gears.
Their tooth action and end thrust are same as
straight bevels but they are quieter in action due to
the curved teeth.

12. Crossed Helical Gears
Gears
12
In case of skew (non-parallel, non-intersecting) shafts, uniform rotary motion is not possible
by pure rolling contact.
In case of skew shafts, crossed helical gears can be
used for motion transmission.
The use is limited to light loads.
By a suitable choice of helix angle for the mating
gears, the two shafts can be set at any angle.