Tech Briefs Magazine - December 2021 - MD-8

Hypoid Gears -
A Long Way from
Truck Axles to
Motion Control
H
ypoid gears are a unique category
of spiral bevel
gears
that are used to transmit rotational
power between two
shafts at right angles. Invented by Ernest
Wildhaber in the early 1920s, hypoid
gears were used predominantly in the
drive trains of heavy trucks.
At the time, the genius of the invention
was the offset between gear shafts,
which allowed the truck frame to be lowered.
This development had a profound
impact on vehicle designs, leading to
significant improvements in available
space within the vehicle, roll stablility,
vehicle handling, safety, aerodynamics,
and gas mileage. While hypoid gears are
still used in this capacity, they also have
a growing presence in the industrial
power transmission and motion control
industries.
What Are Hypoid Gears?
Hypoid gears can be classified as a
type of bevel gear used to transmit rotational
power, or torque, between two
shafts at right angles. There are two primary
categories of bevel gears: straight
teeth and spiral teeth. Spiral teeth bevel
gears have proven to run smoother and
quieter than straight teeth bevel gears
and it is these spiral bevel gears that
hypoids most closely resemble.
In contrast to standard bevel gears,
for hypoid
gear
shafts
Wormwheel
Wormshaft
Worm
range of gear ratios and the ability to
transmit higher amounts of torque.
Additional
benefits resulting from
the increased spiral angle and contact
area between hypoid gear sets include
reduced wear and tear, lower friction,
lower energy loss, and higher efficiency,
all of which help extend the life of hypoid
gears and gearboxes.
Benefits of Hypoid Gears Over
Standard Bevel Gears
Worm gears can provide single-stage ratios
up to 100:1 but are less efficient than hypoid
gears.
the mating
8
MD Power Transmission 1221_1.indd 8
gear sets do not intersect because the
smaller gear shaft (referred to as the
pinion) is offset from the larger gear
shaft (referred to as the crown). It
is these non-intersecting axes that Mr.
Wildhaber designed with the rear axle
in mind, leading to the vehicle performance
improvements described above.
The axis offset allows the pinion diameter
to be larger and have a greater spiral
angle than would otherwise be possible,
which increases the contact area and
tooth strength. The spiral angle between
the pinion and crown causes the gears to
mesh very smoothly. Increased contact
area and tooth strengh allow for a wider
Due to their increased spiral angle
and tooth strength, hypoid gears can
carry load across several teeth simultaneously.
The average number of hypoid
gear teeth in contact at one time
is 2.2:1 to 2.9:1. Bevel gears, on the
other hand, are loaded one gear tooth
at a time. More tooth-to-tooth contact
allows hypoid gears to transmit
greater torque compared to similarly
sized bevel gears. In addition to transmitting
more torque, hypoid gears and
gearboxes tend to be smaller and more
compact than bevel gearboxes with
equivalent torque ratings, making them
a better choice for applications involving
tight spaces.
Bevel gears and bevel gearboxes are
most common in ratios of 1:1 and 2:1,
with some manufacturers providing ratios
up to 5:1. By contrast, hypoid gears
do not provide solutions for 1:1 and 2:1
applications. However, they efficiently
Motion Design, December 2021
Cov
ToC
11/16/21 5:00 PM
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Tech Briefs Magazine - December 2021

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