Spur Gears

Types	Materials	Spur Gear Design
Spur Gear Cutting	Rack and Pinion Gears

Spur Gears: Product Specifications: Materials Used: Alloy Steel, Manganese Steel, Carbon Steel Standards: ASTM, BS, IS, AISI, SAE Customer's requirements are also taken into consideration. » Send Enquiry

Spur gears are the simplest form of Gears available. They are also the most commonly used gears in the market. Spur Gears are found generally in the form of a cylinder or disk. These gears are applied for varying the force and speed of a rotating axle. These gears 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 spur 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 gears. 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 gears are 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 these gears.

Given below in the table are Tooth Proportions of Spur Gears of various standard diametral pitches.

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
Single spur gears 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 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:


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

• Internal Spur Gear
• External Spur Gear

Internal Spur Gears
This is actually a type of Spur Gear. Internal Spur Gears are 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 Gears, 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.
  

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 External Spur Gears.

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.


Rack and Pinion Gears
Rack and pinion gears 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