Chapter10 Helicalgears

Helical Gears
• For parallel shafts opposite hand gears are needed
Left-hand
gear
Right-hand
gear

• For perpendicular shafts, same hand gears are needed

Right-hand gears Advantages of Helical Gears
Over Spur Gears

Geometry

• Smoother engagement of the gear teeth
• More teeth carry load at a given time so th are more efficient they ffi i t – carry more load l d for f a given size

Transverse Plane

Tangent Plane
Normal Plane ψ = Helix angle – 15°- 45° φt = Transverse Pressure angle φn = Normal Pressure a angle tan φn = tan φt * cos ψ

Pitches
• Circular Pitch – distance from a point on a tooth to corresponding point on next tooth p = πD/N
• Normal Circular Pitch – distance between corresponding di points i on adjacent dj teethh measured on pitch surface in normal direction pn = p cos ψ
• Diametral Pitch – ratio of number of teeth in the gear to the pitch diameter.
Pd = N/D

Forces on Helical Gear Tooth

More Pitches
• Normal Diametral Pitch – diametral pitch in normal direction
Pnd = Pd / cos ψ
• Axial Pitch – distance between corresponding points on adjacent teeth measured in the axial direction. Need Px ≥ 2 for full helical action.
Px = p / tan ψ = π / Pd tan ψ

Wr
Wn

Wx ψ ψ

Wt
Tooth line at pitch diameter

Wt =

33000(P) vt Wt = Tangential g Load
P = Power transmitted (hp) vt = Pitch line speed (ft/min)

Wx = Wt tan ψ
Wr = Wt tan φt

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Forces on Helical Gear Tooth

Forces on Helical Gear Tooth

Driving Gear
Wt from the pinion on the gear pushes the gear in the driven di direction. i nP

LH
RH

Forces on Helical Gear Tooth

Driving Gear
Wn
Wx
Wt

Wr

Wx

Wr

nP

Wt

Wt

Wt

Wr

Wt nG Driven Gear

Wt from the gear on to the pinion is a reaction force

nG nG Wn is ALWAYS perpendicular to the tooth

Driven Gear

Forces on Helical Gear Tooth
Wt

nP
Wx
Wr

Wr

Biggest Disadvantage of Using
Helical Gears
• Axial Force, Wx – called axial thrust load

Driven Gear

Helical Gear Design
• Everything for helical gear design is essentially the same as it was for