Gear drives

6
Helical Gears

o A helical gear has teeth in form of helix around the gear and may be
used to connect two parallel shafts in place of spur gears (Khurmi;
Gupta, 2005).
o Elements of helical teeth are cylindrical; one end of the tooth is
advanced circumferentially over the other end (Faires, 1965).
o Helical gears are designed to transmit motion and power between
either parallel or right-angle shafts which rotate in opposite directions1.

The teeth of helical gears with parallel axis have line contact, as in spur gearing. This
provides gradual engagement and continuous contact of the engaging teeth. Hence helical gears
give smooth drive with a high efficiency of transmission.

2
Figure 6-1. Helical gears

1
http://www.wmberg.com/products/Helical-Gears.aspx. Retrieved August 12, 2012
2
http://s.hswstatic.com/gif/gear-helical1.jpg. Retrieved August 12, 2012

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GEAR TERMINOLOGIES

Helix angle a constant angle made by the helices with the axis of rotation.
Axial pitch is the distance parallel to the axis between similar faces of adjacent
teeth.
It may also be defined as the circular pitch in the plane of rotation
or the diametral plane.
Normal pitch is the distance between similar faces of adjacent teeth along a helix
on the pitch cylinders normal to the teeth.
It may also be defined as the circular pitch in the normal plane
which is a plane perpendicular to the teeth.

The relation between the normal pressure angle (φN) in the normal plane and the pressure
angle (φ) in the diametral plane (or plane of rotation).

PROPORTIONS FOR HELICAL GEARS

STRENGTH OF HELICAL GEARS

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Velocity factor, Cv can be taken as follows,

Dynamic tooth load

where: b and C are constants

Limiting wear on helical tooth load

where: Dp, b, Q, and K are constant

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PROBLEM
Find the tooth thickness on the tooth circle of a helical gear having 20 degrees full-depth involute
tooth, diametral pitch of 3, circular pitch of 1.0472 and whole depth of tooth of 0.60.
Ans. 13. 29 mm

SOLUTION

PROBLEM
Helical gears having 14.5 degrees normal pressure angle and transverse diametral pitch of 2.36 /
cm. The helix angle is 45 degrees and has 8 teeth. What is the transverse pressure angle?
Ans. 20.10

SOLUTION

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Design Exercises

PROBLEM
Compute the circular pitch of a pair of gears having a ratio of 4 and center distance of 10.23
inches. Each has 72 teeth and pinion has 18 teeth.
Ans. Pc = 0.785 in

SOLUTION

PROBLEM
Design a pair of helical gear to transmit 40 Kw from a shaft running at 1500RPM .to another
shaft with a speed reduction of 6. Select the suitable materials for pinion and gear. Assume
medium shock condition.

SOLUTION

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PROBLEM
For a helical gear drive an expression for the virtual no of teeth Nf in terms of helix angle φ and
actual no. of teeth N.

SOLUTION

PROBLEM
A pair of helical gears is used to transmit 15Kw. The teeth are 20o stub in diametral plane and
have a helix angle of 45 degree. The pinion has an 80mm pitch diameter and operates at
10000RPM. The gear has a 320mm pitch diameter. If the gears are made of cast steel
(s0=100MN/ m2), determine a suitable module and face width. The pinion is heat treated to BHN
of 3000 and the gear has a BHN of 200.

SOLUTION

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PROBLEM
A 10 Kw, 750RPM induction motor is to drive a pump at 150 RPM through a pair of helical
gear. The starting torque of the motor is taken as 2.5times rated torque. Design the gear pair
completely and check for dynamic and wear loads.

SOLUTION

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PROBLEM
A compressor running at 360RPM is driven by a 150 Kw, 1440-RPM motor through a pair of 20o
full depth helical gears having helix angle of 25 degree. The center distance is approximately
400mm. the motor pinion is to be forged steel and the driven gear is to be cast steel. Assume
medium shock conditions. Design the gear pair.

SOLUTION

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Assignment # __
Find machinery or device/machines that have helical gear application. Have a picture of it and
identify the details of the gear (i.e. tooth thickness, face width, pitch diameter, etc.) and compute
the power needed to operate such machine. Submit this requirement(s) on ___________.
Notes

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DESIGN PROBLEM # 6

Name: Rating:
Course/Yr: Date:

HELICAL GEARS DESIGN

PROBLEM
Two parallel shafts are connected by a pair of steel helical gear. The pinion transmits 10Kw at
400RPM of the pinion. Both gear made of same material, hardened steel with allowable stress
(s0=100MN/ m2). If the velocity ratio is 4.5:1, determine the smallest diameter gears that may be
used having sufficient strength. No less than 30 teeth are to be used for either gear, the teeth are
of 20 degree stub in diametral plane and the helix angle is 45o.

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DESIGN PROBLEM # 6
5 helical gear continuation

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