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Bevel Gears

Designing a bevel gear
by

Brad Kingdon

on 14 June 2011

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Transcript of Bevel Gears

Bevel Gears Pitch Diameter
5" Half of Pitch Diameter
2 1/2" Right Angle Right Angle The pitch diameter
is the basis the bevel
gear is built on. It is
measured at the crown. Bevel gears are used to connect intersecting shafts. They go Around corners Gear teeth are Cut on a cone with the top removed
(called a frustrum) Types of bevel gears: straight
zerol
spiral
hypoid Straight Gear
transmits power around corners
can connect any angle, size, or number of teeth
If both gears have same number of teeth and meet at 90 deg. they are called mitre gears
tooth form is involute Because the tooth diminishes as it moves towards the point (apex) the involute cutter is thinner than a spur gear.
The cutter is 70% as thick as a spur gear cutter
The finished tooth is the same thickness as a spur gear (at the large dia.)
The bevel gear tooth will require widening if made on a milling machine
Typically the tooth form is generated Zerol Bevel Gears
Zerol (zero angle)
Curved tooth aligns with centerline (not cut at an angle)
Meshes smoothly
Operates at high speeds
Little axial thrust
Gear tooth must be generated Spiral Bevel Gear
Curved tooth design meshes gradually
Quiet
Smooth
Used for high speeds and loads
Significant thrust
Must be generated Hypoid Gear
This gear does not intersect (exception)
Used in automotive rear ends (differentials)
Must be generated and ground LET'S MAKE A GEAR! Let's build a straight tooth bevel
gear using a milling machine
(4 DP - 20 tooth miter gear)

Find PD
PD = N/DP
PD = 20/4
PD = 5" Because we are building
a mitre gear the centerline
extends 1/2 the pitch dia
to the apex If 1/2 of the pitch diameter
is 2 1/2" and the pitch cone
intersects with the apex at
45 deg we need to find the length of the pitch cone. 45 deg Line extending to apex Pitch Diameter Pitch Cone The length of the Pitch Cone
hypotenuse = 2 1/2" / sin45
= 3.536" 3.536" The addendum is at right angle
to the pitch cone above the large
diameter. Add = 1 / DP
= 1 / 4
= 0.250" addendum Right Angle This becomes the face angle which the
blank is cut to.
45 deg. + (.250/3.536) x tan-1
= 49.043 deg. Face The dedendum is below the pitch cone
at right angle to the pitch cone.
Dedendum = 1.157 / DP
= 1.157 / 4
= 0.2893" Dedendum The Root Angle is
45deg -(.2893 / 3.536) x tan-1
= 40.324 deg If you draw a line straight up off the pitch diameter And use the addendum as the hypotenuse you can calculate how much larger the outside of the gear is Multiply this number by two
and add it to the pitch diameter
and you will know the size you
need for the outside diameter of
the gear. The back of the gear is commonly faced to the bottom of the dedendum Typically 1/3rd of the gear face is used. Since the pitch cone is 3.536" / 3 = 1.178" of length is used. The gear tooth is the same depth as a spur gear.
All teeth are cut to a full depth of 2.157/N.
After all teeth are cut the teeth will require widening (gear cutter is 70%).
The table will need to be moved to widen the teeth.
The dividing head will need to be "rolled" to get the proper involute form. The cutter required is a #4 - 4DP bevel gear cutter.
Cutter thickness is: [(3.14 /DP) / 2] x .7
=[(3.14 / 4) /2] x .7
= 0.275" Table set over
= (cutter thickness / 2) - (factor from machinery handbook table / DP)
= (0.275 /2) - (0.275 /4)
= 0.06875" The table is moved to the right (.068") and the tooth is cut.
The tooth is blued and the dividing head is rolled left
until the tooth is clean at the large end.
All teeth are cut again then the table is moved left (.068")
and rolled right. The End!
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