Interference, undercutting and backlash in gear Design

 What is the Interference ?

• Gear tooth has involute profile only outside the base circle.  
• The involute profile begins at the base circle. 
• In some case the dedendum is so large that it extends below the base circle. 
• In such situation ,the portion of the tooth below the base circle is not involute. 
• The tip of the tooth on the matting gear,  which is involute, interferes with this non involute  portion of dedendum. 
• This phenomenon  of tooth overlapping and cutting into each other is called ‘Interference.’  

Methods of Elimination of Gear tooth Interference 

• Removing the cross hatched tooth tips i.e., using stub teeth. 
• Increasing the number of teeth on the mating pinion. 
• Increasing the pressure angle 
• Tooth profile modification or profile shifting 
• Increasing the centre distance as illustrated in Figure.

        

             Figure :  Method of elimination of interference in spur gears 

What is the Undercutting  ?

                            

• When the gears are generated by involute rack cutters, this interference is automatically eliminated because the cutting tool removes the interfering portion of the flank. This is called ‘Undercutting’.
• Undercutting solves the problem of the interference.
•  But, it weakens the tooth and remove the small involute portion adjacent to the base circle.  

What is the Backlash ?

                   

• It is the difference between the tooth space and the tooth thickness, as measured along the pitch circle of the gear.
• In general, backlash is the play between mating teeth and it occurs only when teeth are in mesh.

The objectives for providing backlash are as follows

• Backlash prevents the mating teeth from jamming together. The mating teeth do not make contact on both sides simultaneously.  This makes the teeth roll together freely and smoothly.
• Backlash compensates for machining error.
• Backlash compensates for thermal expansion of teeth.

Methods to provides Backlash 

• The teeth of the gear are cut slightly thinner. This is obtained by setting the cutting  tool deeper into the blank resulting in thinner tooth and wider space.
• The center distance between mating gears is slightly increased.