Bevel gears have teeth shaped generally like ordinary spur gears, except that the tooth surfaces are made up of conical elements. The teeth may be straight or spiral Spiral teeth engage gradually (starting at one side, as with helical gears), a feature enabling them to operate more smoothly and quietly. Except for hypoid gears bevel gears are mounted on shafts having intersecting axes. The shaft axes are usually, but not necessarily, perpendicular.
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Straight bevel gears: Teeth elements are straight lines which converge into a common apex
point
Spiral bevel gears: Teeth elements are spiral curves which also converge into a common apex
pointSkew bevel gears: Shaft on which straight bevel gears are mounted are non parallel and non-
intersecting. Apex point of the pinion is offset with respect to that of gear
Hypoid gears: Similar to spiral bevel gears that are mounted on shafts which are non parallel and
non intersecting. Used in automobile differential which allow the drive shaft to place well belowthe centerline of the rear axle thereby lower the center of gravity of vehicle
Face gears: Consists of spur or helical pinion mating with a conjugate gear of disk form
The tooth dimensions are usually specified for thelarge end of the tooth. However, in calculatingbearing loads, central-section dimensions and forcesare used
The simplest type of bevel gear is the straight bevel gear. These gears are commonly used for transmitting power between intersecting shafts. Usually the shaft
angle is 90°, but it may be almost any angle. The gearsimpose both radial and thrust load on their bearing.
In contrast to the cylindrical pitch surfaces of spur and helical gears, the pitch surfaces of
meshing bevel gears are conical. When teeth are cut on the conical pitch surfaces they must be
tapered both in tooth thickness and height from a large tooth profile at one end to a smaller tooth
profile at the other end
Bevel gear teeth have profiles that closelyresemble an involute curve. The shape of astraight bevel gear tooth (in a section normal tothe tooth) closely approximates that of aninvolute spur gear
D p and D g are the pitch circle diameters of pinion andgear respectively. is the pitch angle of the pinion,while is the pitch angle of the gear. Line AB is
perpendicular to line OB. Consider triangle OAB
tan =AB/OB; (D p/2)/ (D g/2)= D p/Dg= mZ p/mZ g
tan = Z p/Zg tan = Z g/Z p + =90
The cone distance A O is given byOA=(AB 2+OB 2)1/2 = (D p
Figure shows the resolution of resultant tooth force F into its tangential(torqueproducing), radial (separating), and axial (thrust) components, designated Ft , Fr
, and Fa, respectively. Note that an auxiliary view is needed to show the true length ofthe vector representing resultant force F (which is normal to the tooth profile).Resultant force F is shown applied to the tooth at the pitch cone surface and midwayalong tooth width b. This conforms to the usual assumption that the load is uniformlydistributed along the tooth width, despite the fact that the tooth is largest
In force analysis, it is assumed that theresultant force between two meshingteeth of a pair of bevel gear isconcentrated at the midpoint along theface width of the tooth
Consider a plane ABCD shaded by dots
and fig b. From triangle BCDtan =BC/CD=P s/P tPs=Separating component
=Pressure anglePs=p t tan
Resultant force P shown by dotted line actsat the midpoint D of the face width of the
pinion. The resultant force has followingthree components
Pt= tangential componentPr =radial componentPa = axial or thrust component
In two dimensional representation of forces, very often (.) indicates a force that is perpendicular to the plane of paper and which is towards the observer. (x) indicates aforce that is perpendicular to the plane of paper and which is away from the observer
Tangential component(P t)
The direction of tangentialcomponent for driving gear isopposite to the direction of rotationThe direction of tangentialcomponent for driven gear is same as
the direction of rotationRadial component(P r )
The radial component on the pinionacts towards the centre of pinionThe radial component on gear acts
towards the centre of gear
Thrust components(P a):The thrust component on pinion is equal and opposite of the radial component onthe gearThe thrust component on gear is equal and opposite of the radial component on the