Gears For Parallel Shafts

1.
Gears for Parallel Shafts:

The motion between parallel shafts is same as to the rolling of two cylinders. Gears
under this category are the following:
1.1 Spur Gears:

Straight Spur gears are the simplest form of gears having teeth parallel to the gear
axis. The contact of two teeth takes place over the entire width along a line parallel
to the axes of rotation. As gear rotate , the line of contact goes on shifting parallel
to the shaft.
1.2 Helical Gears:
In helical gear teeth are part of helix instead of straight across the gear parallel to the
axis. The mating gears will have same helix angle but in opposite direction for proper
mating. As the gear rotates, the contact shifts along the line of contact in in volute
helicoid across the teeth.
1.3. Herringbone Gears:
Herringbone gears are also known as Double Helical Gears. Herringbone gears are
made of two helical gears with opposite helix angles, which can be up to 45 degrees.
1.4. Rack and Pinion:
In these gears the spur rack can be considered to be spur gear of infinite pitch radius
with its axis of rotation placed at infinity parallel to that of pinion. The pinion rotates
while the rack translates.
2. Gears for Intersecting Shafts:
The motion between two intersecting shafts is equivalent to the rolling of two cones. The
gears used for intersecting shafts are called bevel gears. Gears under this category are
following:
2.1 Straight Bevel Gears:
Straight bevel gears are provided with straight teeth, radial to the point of intersection of
the shaft axes and vary in cross section through the length inside generator of the cone.
Straight Bevel Gears can be seen as modified version of straight spur gears in which teeth
are made in conical direction instead of parallel to axis.
2.2 Spiral Bevel Gears:
Bevel gears are made with their teeth are inclined at an angle to face of the bevel.
Spiral gears are also known as helical bevels.
3. Gears for Skew Shafts:
The following gears are used to join two non-parallel and non-intersecting shafts.
3.1 Hypoid Gears:

The Hypoid Gears are made of the frusta of hyperboloids of revolution. Two matching
hypoid gears are made by revolving the same line of contact, these gears are not
interchangeable.
3.2 Worm Gears:
The Worm Gears are used to connect skewed shafts, but not necessarily at right
angles. Teeth on worm gear are cut continuously like the threads on a screw. The
gear meshing with the worm gear is known as worm wheel and combination is
known as worm and worm wheel.
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,
Interference in involute gears.
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.
The number of teeth on the pinion (TP) in order to avoid interference may be
obtained from the following relation :
Design Considerations for a Gear Drive
In the design of a gear drive, the following data is usually given :
1. The power to be transmitted.
2. The speed of the driving gear,
3. The speed of the driven gear or the velocity ratio, and
4. The centre distance.
The following requirements must be met in the design of a gear drive :
(a) The gear teeth should have sufficient strength so that they will not fail under static
loading
or dynamic loading during normal running conditions.
(b) The gear teeth should have wear characteristics so that their life is satisfactory.
(c) The use of space and material should be economical.
(d) The alignment of the gears and deflections of the shafts must be considered because they
effect on the performance of the gears.
(e) The lubrication of the gears must be satisfactory.
Beam Strength of Gear Teeth – Lewis Equation
Tooth of a gear
Permissible Working Stress for Gear Teeth in the Lewis Equation
Dynamic Tooth Load
Static Tooth Load
The static tooth load (also called beam strength or endurance strength of the
tooth) is obtained by Lewis formula by substituting flexural endurance limit or
elastic limit stress (σe) in place of permissible working stress (σw).
∴ Static tooth load or beam strength of the tooth,
Wear Tooth Load
The maximum load that gear teeth can carry, without premature wear, depends upon
the radii of curvature of the tooth profiles and on the elasticity and surface fatigue
limits of the materials. The maximum or the limiting load for satisfactory wear of gear
teeth, is obtained by using the following Buckingham equation, i.e.
Design Procedure for Spur Gears
1. First of all, the design tangential tooth load is obtained from the power transmitted and
the pitch line velocity by using the following relation :
The following particulars of a single reduction spur gear are given :
Gear ratio = 10 : 1; Distance between centers = 660 mm approximately; Pinion transmits
500 kW at 1800 r.p.m.; Involute teeth of standard proportions (addendum = m) with
pressure angle of 22.5°; Permissible normal pressure between teeth = 175 N per mm of
width. Find :
1. The nearest standard module if no interference is to occur;
2. The number of teeth on each wheel;
3. The necessary width of the pinion; and
4. The load on the bearings of the wheels due to power transmitted.
1. Nearest standard module if no interference is to occur

Gears For Parallel Shafts

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1.

Gears for Parallel Shafts:

1.1 Spur Gears:

3.1 Hypoid Gears:

1. Nearest standard module if no interference is to occur

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