Spur Gears
Spur gears
are the simplest form of Gears available. They are also the most
commonly used gears in the market. They are found generally in the form
of a cylinder or disk. Thes gears are applied for varying the force and
speed of a rotating axle. They have a straight teeth. They are usually
mounted on parallel shafts. These days mostly the tooth form is based
on the involute curve. For creating large gear reductions, many spur
gears are used together.
However, these gears can mesh correctly only if they get fitted to
parallel axles. That is the reason why their teeth are cut in a manner,
for making the leading edges remain parallel to the line of axis of
rotation. Not withstanding the basic limitation of center distance,
spur gears are able to provide a constant and positive speed drive. The
speed can be varied by increasing or decreasing the teeth number in the
driving gear. Spur Gears are used typically in applications where noise
control does not matter. But in cases where noise does matter, but
higher speeds are also essential a nylon or non-metallic gear is best.
These can be then be operated easily up to speeds of around 2400 feet
per minute. This way noise and vibration can easily be prevented. The
following image shows the basic geometry of Spur Gears.
Given below in the table are Tooth Proportions of Spur Gears
of various standard diametral pitches.
| Diametral
Pitch |
Circular
pitch(inch) |
Thickness of Tooth on Pitch
Line(inches) |
Depth to be cut in gears
(inch)(Hobbed gears) |
Addendum(inch) |
| 3 |
1.0472 |
0.5236 |
0.7190 |
0.3333 |
| 4 |
0.7854 |
0.3927 |
0.5393 |
0.2500 |
| 5 |
0.6283 |
0.3142 |
0.4314 |
0.2000 |
| 6 |
0.5236 |
0.2618 |
0.3565 |
0.1667 |
| 8 |
0.3927 |
0.1963 |
0.2696 |
0.1250 |
| 10 |
0.3142 |
0.1571 |
0.2157 |
0.1000 |
| 12 |
0.2618 |
0.1309 |
0.1798 |
0.0833 |
| 16 |
0.1963 |
0.0982 |
0.1348 |
0.0625 |
| 20 |
0.1571 |
0.0785 |
0.1120 |
0.500 |
| 24 |
0.1309 |
0.0654 |
0.937 |
0.0417 |
| 32 |
0.0982 |
0.0491 |
0.708 |
0.0312 |
| 48 |
0.0654 |
0.0327 |
0.478 |
0.0208 |
| 64 |
0.0491 |
0.0245 |
0.0364 |
0.0156 |
Materials of
Spur Gears
While coming to manufacturing materials for Spur gears, a wide variety
is available. These includes steel, nylon, aluminum, bronze, phenolic,
cast iron, bakelite and now also available in plastics.
Spur
Gear Design
A single spur gear usually has a ratio range between 1:1 and 1:6. Pitch
line velocity can be up to 25 m/s. The spur gear shows an excellent
operating efficiency that is between 98-99%. The pinion of the spur
gear is always made from a harder material as compared to wheels. An
ideal Spur Gear pair gear pair is the one that has the highest number
of teeth that is totally consistent with factors in strength and wear.
Suitable number of teeth on Spur gear is a follows:
| 12 |
13 |
14 |
15 |
16 |
18 |
20 |
22 |
24 |
25 |
28 |
30 |
32 |
34 |
38 |
40 |
45 |
50 |
| 54 |
60 |
64 |
70 |
72 |
75 |
80 |
84 |
90 |
96 |
100 |
120 |
140 |
150 |
180 |
200 |
220 |
250 |
In the case of spur gears, design for Gears are made by taking into
account the specific velocity ratio and the distance between central
shafts. Which is represented by the following equations, the distance
calculated between the center of two shafts is given by:
x = (d1 + d2)/2
The speed or velocity ratio is calculated by:
N1/N2 = d2/d1 = T2/T1
Where,
X = the distance between the center of the two
shafts
N1 = The speed of the driver
T1 = Number of teeth found on the driver
d1 = Driver's Pitch circle diameter
N2 = Speed of the follower
T2 = Number of teeth of the follower
d2 = Pitch circle diameter of the follower
Pc = Circular pitch
From the above equations, calculations of d1 and d2 (or T1 and T2) and
circular pitch (Pc) are made. It is to be noted here that, the values
obtained of T1 and T2, can be a whole number or sometimes may not be
whole number. But in gear design, as the number of teeth is invariably
a whole number, there is a need to for a slight change in the values of
x, d1, and d2. This is to ensure that the number of teeth in the gear
design always comes as a complete number.
Spur
Gear Cutting 
Today lots of options are available for manufacturing of Spur Gears.
Methods include Machine cutting(that includes operations of Hobbing,
Milling, Shaping), Blanking, Grinding, Molding, Forming,Casting,
Stamping and the relatively new method of Wire EDM. It is essential to
pick a suitable method depending upon the factors of quality,
production quantity, cost, Gear material and last but not the least
application methods.
Types
of Spur Gears
External
Spur Gears
External Spur Gears are the most popular and common type of spur gear.
They has their teeth cut on the outside surface of mating cylindrical
wheels. While the larger wheel is referred to as the gear and the
smaller wheel is known as the pinion. Single reduction stage is the
most basic type of arrangement of single pair of spur gears. Here the
output rotation is in opposite direction to that of the input. In other
arrangements of multiple stages higher net reduction can be achieved
where the driven gear is connected rigidly to a third gear. This third
gear in turn drives a mating fourth gear. This serves as the ideal
output for the second stage. In this way, many output speeds on
different shafts are produced starting from a just single input
rotation. The image given below shows the inside of an External Spur
Gear.
Working of External Spur Gears
Actually the working of External Spur Gear is best explained with the
help of Gear meshes. In the external mesh, the gears are made to rotate
in directions that are opposite. The Figure below shows a simple spur
gear mesh where the gears are meshing externally.
Internal
Spur Gear
This is actually a type of Spur Gear. Internal Spur Gear is not much
different from a regular spur gear. These gear by appearance shows
pitch surface that is cylindrical. Here the tooth is parallel to the
axis. In case of Internal Spur Gear, the gears are positioned to make
internal contact. It is also referred to popularly as Ring gears. The
output rotation produced by the Ring gears is direction wise same as
that of input rotation.
As is clear from the figure the gear tooth are cut from inside. A
typical Internal Spur Gear or Ring Gear consists of typically three or
four larger spur gears referred to as planets. That surrounds a smaller
central pinion referred to as sun. Normally, the ring gear remains
stationary. This is quite like our own Planetary system, where the
planets orbit round the sun in the same rotational direction. It is
quite obvious that this class of gear is known as a planetary system.
It is through a planet carrier that transmits the orbiting motion of
the planets to the output shaft.
In a different planetary arrangement, the ring may be left to move
freely. This is done by restricting the planets from orbiting round the
sun. This action results in the ring gear rotating in an opposite
direction to that of the sun. Thus a differential gear drive is
effected as a result of rotation of both the ring gear and the planet
carrier. The output speed of the shafts are interdependent.
Advantages of Internal Spur Gears
As compared to External Spur Gears, the Ring Gears or internal Spur
gears offer two distinct advantages:
- Center distance is much shorter in an internal spur gear.
- Greater tooth strength.
- Shows the remarkable ability for achieving a higher
contact and drive ratios in shorter center distances as compared to
standard spur gear.
- It is also possible to enable a velocity change without
undergoing a directional change.
- Shows good surface endurance because of the convex profile
surface that works against a concave surface.
Limitations of Internal Spur Gears
There are also certain limitations to the way in which a Spur Gear
works.
- Complicated housing and bearing supports, because the
external gear is nestled within an internal gear.
- Low ratios are sometimes not suitable and in many cases
just impossible due to interferences.
- Fabrication is somehow limited to the process of shape
generation. Most of the times special tooling is required.
- Center distance is much shorter in an internal spur gear.
Rack
and Pinion Gears
Rack and pinion Gear are another variety of Spur gears. Actually a Rack
is defined as a straight bar that has teeth which are cut straight and
across. Basically, The Rack is considered as a spur gear that is
unrolled and laid flat. Pinion is the normal gear. A rack and pinion is
really a very special example of spur gearing. The rack-and-pinion has
been found to be specially useful in conversion of rotary motion into
linear and vice versa. Rotating a pinion produces a linear motion of
the rack. Alternately moving a rack causes rotation in the pinion.
Working of a Rack and Pinion Gear
To describe the working of a rack and pinion gears system is not very
easy. In fact the rack and pinion gear system looks a bit unusual.
However you will be amazed to know that it is composed of two gears.
The ‘pinion’ is what you call a normal round gear while rack's shape is
straight or flat. It is the teeth of the ‘rack’ which meshes with
pinion gear's teeth.
Actually it is the pinion that rotates and is able to move the rack in
a straight line. In other words, ‘rotary motion’ is converted into to
‘linear motion'. The perfect example to describe a Rack and pinion gear
system is the trains moving on the tracks, on steep inclines. The
wheels of a train are made up of steel and there is no way to grip the
steel tracks. What happens is that the heavy weight of the train is
sufficient for allowing the train to travel on the tracks. But consider
a situation where a train has to travel a steep incline or hill. It is
quite likely that would slip backwards.
Here comes the magnificent technology of ‘rack and pinion’ system that
exactly provides solution to the trains to overcome this problem. As
the image shows a big gear wheel is added at the center of the train.
While in the track a “Rack” an extra track with teeth, is added. As the
train moves over an inclined hill or slope, the gear is lowered
instantly to the track where it meshes perfectly with the ‘Rack’. As a
result there is no chance of the Train slipping backwards and is
carried along smoothly over the steep slope.
Types of Rack and Pinion Gears
Basically there are three variations of Rack and Pinion Gears. They are
helical teeth modular pitch, straight teeth metric pitch and straight
teeth modular pitch. Rack and pinion gear variations are also made
available in various qualities. For example 9/10 milled teeth or
hardened and milled quality. 7/8 precision cut or precision cut and
hardened quality and lastly the quality of 5/6 teeth hardened and
ground.
Application of Rack and Pinion Gears 
- Machine tools
- Heavy Machinery
- Scales for displaying weight
- The rack-and-pinion is used extensively. The following
photograph illustrates the rack and pinion gears used widely in a
household scale.
Selecting the Right Rack and Pinion Gear
System
For optimal efficiency it is very important to select the Rack and
pinion Gear System that comes with optimal features. For this keeping
in mind the following features would be really useful:
- Tooth Profile: If it is offers full
depth
- Pressure Angle
- Material Used
- Heat treatment: This includes
tempering, stress relief annealing, induction hardening of tooth flanks
- Hardness: Hardness at the center,
teeth
- Surface treatment
- Gear and Rack Precision
