GEARS OR GEAR DRIVE
Gears are used to transmit motion from one shaft to another shaft or between a shaft and slide when distance between them is small. in this blog we study about types of gears.
here velocity of point p = w1R1=w2R2 and (w1/w2= R2/R1)= constant, when there is a slip w1/w2 not equal to constant and the drive is known as negative drive.
in gear slip is impossible, due to interlocking of teeth, therefore gear drive is a positive drive.
CLASSIFICATION OF GEARS
According to the axis of their shaft
when the teeth are straight and parallel to the axis of shaft then the gear is known as spur gear.
- there is no axial thrust.
- but huge impact stress and noise are generated due ti sudden engagement and disengagement.
- not used at high speed. and velocity ratio is restricted to 6.
- low speed gear.
- used in toys, watches etc.
When the teeth are straight but inclined to the axis of shaft then the gear is known as helical gear.
- Axial thrust is present.
- impact stress and noise is absent due to gradual engagement and disengagement.
- due to the generation of axial thrust it also not used at very high speed.
- velocity ratio is limited to 10.
Double helical gear
Actually double helical gear is used to minimize the axial thrust. A double helical gear is equivalent to a pair of helical gear separated together one having right hand helix and other having left hand helix.
Special case:- if the left hand and right hand inclination of a double helical gear meet at common apex and there is no groove the gear is known as herringbone gear. which is also a types of gears.
For all the above type of gear generating surface is cylinder.
Types of gears when the axis of shaft is intersecting
The motion between two intersecting shaft is equivalent to rolling of two cones without slipping, the gear in genral is known as bevel gear. therefore for bevel gear the generating surface is cone. When the teeth are straight then the gear is known as bevel gear. and when the teeth are inclined the gear is known as helical/spiral bevel gear. the advantage of is used to gradual load and it have low impact stress.
SPECIAL CASE:-in bevel gear if generating surface is same and axis of their shaft are right angle to each other the gear is known as mitre gear.
Types of gears When axis are neither parallel nor intersecting
The which are neither parallel nor intersecting are called skew shaft. here pure rolling motion is impossible therefore, (rolling possible= rolling + partial sliding). ex- skew bevel gear, hypoid gear, worm and worm wheel.
LAW OF GEARING
According to the law of gearing the ratio of angular speed must remains constant for power transmission between the gear and pinion. when two mating surfaces satisfy this law of gearing then it is known as gear. any tooth profile which satisfy the law of gearing is known as conjugate tooth profile. to satisfy the law of gearing the common normal to the both tooth profile must pass through the pitch point. that is w1/w2= constant. the common normal must intersect to the centre line at a fixed point and that point is known as pitch point.
Important points about helical gears Helix angle– the angle made by inclination of teeth and the axis of gear is known as helix angle.in types of gear it is the important point of helical gear. Normal pitch- the shortest distance between two adjacent teeth is known as normal pitch. the noraml pitch of two mating gear must be same. Circular pitch-the perpendicular distance measured between the two adjacent face is known as circular pitch.
Terms used in types of gears
pitch circle diameter-the diameter of circle produced by the pure rolling action of gear is known as pitch circle diameter. Pitch point- in two mating gears the point of contact of two pitch circle is known as pitch point. Circular pitch- circular pitch is the distance measured along the circumference of pitch circle from one point of teeth to another adjacent teeth. it is also defined as it is the ratio of pitch circle circumference ti the number of teeth on gear. Diametral pitch– it is defined as the ratio of number of teeth to the per unit pitch circle diameter. Module- it is the reciprocal of diametral pitch. the ratio of pitch circle diameter to the number of teeth is known as module. it is generally expressed in millimeter. Addendum- the vertical distance between the top of tooth to the pitch circle is known as addendum. generally addendum equal to one module. Dedendum- it is the radial distance between the pitch circle and base of the tooth. this radial distance is known as dedendum. dedendum equal to generally 1.157 module.