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Helical Gears, Bevel Gears and Worm Gears
MAE 342 – Dynamics of Machines 2
Helical GearDifferences from Spur Gears
• Tooth profile is still involute when looking from side of gear, but profile twists around according to helix angle.
• Line of contact is at angle.• Initial contact is at point instead of line (gears are quieter).
• Contact ratio is higher (quieter).• Causes sideways (axial) thrust loads.
• Note that meshing gear must have opposite helix direction.
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MAE 342 – Dynamics of Machines 3
Helix Angle and Pitch
MAE 342 – Dynamics of Machines 4
Contact Ratio of Helical Gears
• Contact ratio now also depends on:� helix angle
� gear width
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MAE 342 – Dynamics of Machines 5
Herringbone Gears
• Avoids axial thrust loads• More expensive to manufacture.
• Need to allow movement of one of the gears to allow alignment.
• Helix angles can be large, since you don’t need to account for large thrust forces.
MAE 342 – Dynamics of Machines 6
Crossed-Axis Helical Gears• Normal helical gears can actually be used with non-parallel axes!
• But has point contact (not line contact).
• Must have same pressure angle & normal pitch.
• Helix angles can be different.
• Can be same or opposite hand.
Image from RoyMech.co.uk
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MAE 342 – Dynamics of Machines 7
Bevel GearsDifferences from Spur Gears
• Non-parallel axes
• Pitch cone instead of pitch cylinder (and Spherical tooth ends instead of planar)
• Narrowing teeth
MAE 342 – Dynamics of Machines 8
Bevel Gear Relationships
3
2
3
2
2
3
N
N
R
R==
ω
ω
( ) Σ+
Σ=
cos
sintan
2
32
N
Nγ
( ) Σ+
Σ=
cos
sintan
3
23
N
Nγ
Σ=+
Σ
32
32 anglespitch are ,
angleshaft is
γγ
γγ
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MAE 342 – Dynamics of Machines 9
Some Standards for Bevel Gears
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MAE 342 – Dynamics of Machines 10
Some Standards for Bevel Gears
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MAE 342 – Dynamics of Machines 11
Crown and Face Gears
“Crown Gear” “Face Gear”
(Image from NASA)
MAE 342 – Dynamics of Machines 12
Spiral Bevel GearsSpiral bevel gears are to bevel gears as helical gears are to spur gears.
In “hypoid gears,” the axes do not intersect!
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MAE 342 – Dynamics of Machines 13
Worms & Worm Gears Nomenclature
MAE 342 – Dynamics of Machines 14
Nomenclature
