Toothed Gearing & Its Terminologies

Toothed Gearing 

                            As we know that whenever we use belt and ropes for power transmission between two shafts, the effect of slipping occurs. Due to the sleeping the velocity ratio of the system reduces which indirectly reduces the efficiency. To reduce this drawback we use Toothed Gearing. In addition the gear drive is also provided whenever the distance between the driver and the follower is very small.

In order to avoid the sleeping number of teeth are provided on the wheel, when  such two wheels are mesh with each other , its forms a system known as teethed gearing.

 Advantages of Gear Drive -

• It has high reliability.
• It transmit exact velocity ratio.
• It has high efficiency.
• It has reliable service.

Classification of Toothed Wheel -

1.According to position of the axes of the shaft -

        

• Parallel
• Intersecting
• Non-parallel non intersecting 

• Spur Gear - 

When the two parallel and co-planar shafts are connected by gears.This type of    gear is termed as spur gear.

When the teeth are inclined to the axes such type of spur gear is known as Helical Gearing.

• Bevel Gear -

When two non-parallel or intersecting but coplanar shafts are connected by gears . These gears are known as Bevel Gears.

In bevel gears the teeth are inclined to the axes such gears are termed as Helical Bevel Gears .

2. According to peripheral velocity of the gear -

• Low Velocity 
• Medium Velocity
• High Velocity     

The gears having velocity less than 3 m/s are termed as low velocity gears.

If the gears having velocity  in between 3 m/s and 15 m/s, are medium velocity gears.and the gears velocity more than 15 m/s are high velocity gears

3. According to the type of gearing -

• External Gearing
• Internal Gearing
• Rack and Pinion   

When the gears of the shafts are mesh externally then it is called as External Gearing. Larger of these mating wheel is called Spur wheel  & smaller those two wheel is Pinion wheel.

In external gearing the motion of the two wheels is always unlike .

When the gears of the shafts mesh internally, such type of gearing is termed as internal gearing.Larger of these two wheels is known as annular wheel, smaller of these two wheels is called pinion.

In internal gearing the motion of two wheels is always like.

4.According to teeth on the gear surface -

• Straight 
• Inclined
• Curved     

Terms used in Gears -

• Pitch circle - It is an imaginary circle which by pure rolling action, would give the same motion as the actual gear.

• Pitch circle diameter - It is the diameter of pitch circle. The size of the gear is usually specified by the pitch circle diameter.

• Pitch point - It is a common point of contact between two pitch circles.

• Pitch surface - It is the surface of the rolling discs which the meshing gears have replaced at the pitch circle.

• Pressure angle or angle of obliquity - It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at pitch point.

• Addendum - It is the radial distance of tooth from pitch circle to top of the tooth. 

• Dedendum - It is the radial distance of tooth from pitch circle to bottom of the tooth.

• Addendum circle - It is the circle drawn through the top of the teeth and its concentric with the pitch circle.

• Dedendum circle - It is the circle drawn through the bottom of the teeth.

• Circular Pitch - It is the distance measured on the circumference of the pitch circle from the point of one tooth to the corresponding point on next tooth. 

               Mathematically -

                   

               Circular pitch = 3.142 x (D/T)

                                       

                                    Where D = Diameter of pitch circle.

                                                T = No of teeth on wheel.

• Diametral pitch - It is the ratio of number of teeth to the pitch circle diameter.

                 Mathematically - 

                 Diametral pitch = T/D

• Module - It is the ratio of pitch circle diameter to the number of teeth.

                  m = D/T

• Clearance - It is the radial distance from the top of the tooth to the bottom of the tooth,in a meshing gear.

• Total depth - It is the radial distance between addendum and dedendum circle of the gear.

• Working depth - It is the radial distance between the addendum circle and the clearance circle.

• Tooth thickness - It is the width of tooth measured along the pitch circle.

• Tooth space - It is the width of space between two adjacent teeth measured along pitch circle.

• Face of the tooth - It is the surface of the gear tooth above pitch circle.

• Flank of the tooth - It is the surface of the gear tooth below pitch circle.

• Top land - It is the surface of the top of the tooth.

• Face width - It is the width of the gear tooth measured parallel to its axis.

• Profile - The curve formed by face and flank of the tooth.

• Backlash - It is the difference between the tooth space and the tooth thickness,  as measured along the pitch circle. 

• Length of path of contact - It is the length of common normal cut-off by the addendum circles of the wheel and pinion.

• Arc of contact - It is a path traced by a point on a pitch circle from beginning to the end of the engagement of a given pair of teeth.

• Arc of approach - It is the portion of the path of contact from the beginning of the engagement to the pitch point.

• Arc of recess - It is the portion of the path of contact from pitch point to the end of the engagement.