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Introduction
This article takes an in-depth look at Helical Gears. After
reading this article, you will be able to understand more
about Helical Gears, including:
What are Helical Gears?
How Helical Gears Work
Parts of a Helical Gear
Considerations in Helical Gear Selection
How Helical Gears are Manufactured
Types of Helical Gears
Applications of Helical Gears
Advantages and Disadvantages of Helical Gears
And more…
Chapter One: Helical Gears, How They Work, and How They’re
Selected
What are Helical Gears?
A gear is a particular kind of simple machine that controls the
strength or direction of a force. A gear train is made up of
multiple gears that are combined and connected by their teeth.
These gear trains allow energy to move from one component of a
system to another. High-quality helical gears are necessary for
advanced industrial machine gearboxes, which are present in most
mechanical manufacturing, fabricating, and construction
machinery.
The main purpose of helical gears as power transmission devices
is to enhance torque and decrease speed between rotating shafts.
They can be broadly split into two types: the ones transmitting
mechanical energy between parallel parts and cross-axis gears
transferring energy between non-parallel parts. While the
features and advantages are comparable to spur gears, they may
be preferable when higher velocities are required.
Helical gears are cylindrical gears with teeth bent into a helix
shape; these teeth are positioned at an angle to the gear axis
called the helix angle. A helical gear has the same involute
tooth geometry as a spur gear in section view, despite being
cut. With proper design, the larger overall contact ratio in
helical gears can reduce vibration and noise. Helical gears
feature stronger teeth and a higher load-carrying capability
than spur gears. With a high degree of component and
sub-assembly interchangeability, the modular design and
fabrication of helical gears in gearboxes offer several
engineering and performance benefits. This provides for
cost-effective construction while maintaining the highest level
of component integrity.
How Helical Gears Work
The mechanical advantage, also known as the ratio of output
torque to input torque in a system, is the guiding principle
behind helical gears. The gear ratio, or the ratio of the last
gear's speed to the initial gear's speed in a gear train,
determines the mechanical advantage of gears. The law of
conservation of energy plays a key role in this relationship for
gear trains. This concept can be simplified when analyzing gear
trains by examining the system's saved power. In addition, this
analysis relates the angular velocities of the gears to their
torques.
Special teeth in helical gears are positioned at a specific
angle to the shaft and the gear face. When two teeth in a
helical gear system make contact, the initial point of touch is
at one end of the tooth, and as the gears turn, the contact
gradually expands until the two teeth are fully engaged. Since
more than one tooth makes contact during the action, the gear
can withstand a greater load.
Due to the load-sharing between teeth in this design of gradual
engagement, helical gears can operate more quietly and smoothly
than spur gears. Because of this, helical gears are utilized in
practically all automobile transmissions. In addition, helical
gears' bent teeth force them to be staggered, which means they
must be stacked in a zigzag pattern or otherwise unaligned. The
next gear's teeth are oriented differently from the first gear
so they can mesh.
However, the sliding contact between the teeth brought on by the
inclined angle of the teeth also generates axial forces and
heat, reducing efficiency. Helical gears' angled teeth cause a
thrust load to be placed on the gear when it meshes. Helical
gear devices have bearings aiding in rotation that can withstand
this thrust force. Inside the equipment, the bearings support
the revolving shaft. Helical gears require thrust or roller
bearings, often larger and more expensive than the plain
bearings used with spur gears since they must endure both radial
and axial forces. The size of the tangent to the helix angle
determines how the axial forces change. The helix angle is
normally limited to 45 degrees because of the generation of
axial forces, although bigger helix angles offer better speed
and smoother motion.
Selecting a Gear Type
A few crucial measurements should be considered when choosing
the equipment for a project, such as the number of teeth, pitch
diameter, outer diameter, and center distance. In general,
applications requiring high speeds, significant power
transmission, or noise reduction call for using helical gears.
The majority of automobile transmissions use them because of
this.
Chapter Two: How Helical Gears are Manufactured
Basic Parts of Helical Gears
Normal Circular Pitch
The distance between similar profiles of neighboring teeth along
a pitch circle or pitch line is known as the circular pitch (p).
Circular Thickness
Circular thickness (t) refers to the arc length that separates a
gear tooth's two sides on the pitch circle.
Helical Angle
The helical angle is formed by the involute tooth shape and the
transverse plane (plane of rotation) at the pitch radius.
Pitch Diameter
The pitch diameter is the pitch normal to the tooth or at a
right angle to it.
Helix's Direction
Also known as Lead, this term refers to the axial advance of the
tooth throughout one rotation (as in thread pitch).
Pitch Circle
The pitch circle is the circumference used to represent the gear
teeth size. Its distance is equal to the number of teeth times
the circular pitch. Contrary to the tip and root circles, the
pitch circle is an imaginary circle that cannot be seen.
Circle Pitch Size
This is the diameter of the pitch circle (also known as the
pitch circle diameter). A gear is a friction wheel with teeth,
and the pitch circle, which is the reference circle for figuring
out the pitch of the gear teeth, corresponds to the friction
wheel's outside circumference.
Transverse Pressure Angle
The projection of the load onto the plane concerning the shaft
axis forms the angle known as the transverse pressure angle.
Centre Distance
This is the extended or contracted standard center distance, the
desired operating center distance.
Addendum (A)
The gear's addendum (A) is the measurement between the pitch
circle and its tooth tip circle. The tooth height (h) is the
measurement between the gear's root circle and tip, and the
gear's module (m) determines the overall height of the gear.
Outside Diameter
The circle's circumference formed by joining the tooth tips is
known as the outside diameter (also called the tip diameter).
Dedendum
The dedendum of the gear is defined as the length from the pitch
radius to the root radius at the midpoint of one gear tooth.
Whole Tooth Depth
The addendum and dedendum are added together to form the entire
depth, which is the height of the tooth measured from the root
circle to the tip circle.
Root Diameter
The diameter of a circle surrounding the bottom (root) of the
gear tooth gaps is known as the root diameter (R.D.).
Contact Ratio
This figure is greater than possible with straight spur gears
since it represents the total of the involute tooth overlap and
the helical overlap.
Manufacturing Process of Helical Gears
The precision required in gear production makes the
manufacturing process rather difficult. Gear manufacturing is a
separate business today that depends on several historical and
contemporary procedures to maintain the ideal balance between
cost, quality, and operations. There are different ways in which
gears can be manufactured. These are highlighted in this
section.
Helical Gear Casting
While gear teeth are manufactured by machining, blanks or
cylinders for gears are often prepared through a simpler method
called casting. During a typical casting process, liquid
material is poured into a hollow mold in the desired shape, then
allowed to harden. A casting, which is the term for the
solidified component, is ejected out of the mold to complete the
procedure. Due to its potential for mass production and relative
simplicity, it is a suitable method for producing gears for
numerous purposes. Very large helical gears are typically
produced through casting. Large dimensions make machining
techniques and other gear-forming techniques less practical.
Helical Gear Forging
During the forging process, metal is hammered, pressed, or
rolled with a press, die, or hammer. Essentially, forging is the
process of heating and shaping hot metal into a design or shape
appropriate for a particular use. Depending on one's needs, this
shaping technique can provide both blanks and ready-to-use
gears. With simple gears, forging is a very viable option.
Theoretically, forging is a great method for producing helical
gears for heavy-duty applications. However, the gears’ size and
thinness are constrained by the enormous force needed for the
forging process. Heat treatment is also necessary during forging
so that the finished gear has improved fatigue characteristics.
Helical Gear Extrusion
Extrusion is a process where a material experiences plastic
deformation through the application of a force that causes the
material to flow through an aperture or die. Extrusion is
different than the cold drawing method, in which tubes or wire
are passed through progressively smaller dies without first
heating the material to reduce the cross-sectional diameter
while increasing the product's tensile strength. Extrusion
requires fewer tools but is not necessarily the most
cost-effective method.
Powder metallurgy
When compacted metal powders are heated to just below their
melting temperatures, this process, called powder metallurgy, is
used to create metal. The field of powder metallurgy has
advanced significantly in recent years. These days, it is
employed in various manufacturing procedures, including gear
creation.
The process begins with metal powder. The initial stage shapes
all of the powder into the desired form. Afterward, the next
stage compacts the setup to ensure better mechanical qualities.
One can now carefully heat the entire arrangement. Powder
metallurgy is very effective, straightforward, and practical for
huge numbers. There is no need for post-processing, and the
finished product will be usable immediately. However, there are
size restrictions and weight constraints.
Gear Machining
Conventional machining was relatively prevalent for cutting and
manufacturing gears, but CNC machining has increased its
usability.
Here are the most common helical gear-cutting methods:
Hobbing
Hobbing uses a conical cutting tool called a hob. The hob
revolves around the gear blank while the workpiece turns.
Hobbing has only been used to make external spur and helical
gears.
This method is quick and flexible. Processing several stacks at
once will also boost production rates. However, the process does
call for more precision and ability.
Shaping
With the cutting-edge manufacturing technique known as shaping,
gears can be created that cannot be made with the hobbing
process. Any shape, such as a pinion, rack shape, or
single-point shape, is acceptable for the cutter. The tool cuts
through the blank to form a shape resembling the desired gears.
With the shaping procedure, the machine can produce internal or
cluster gears.
Broaching
The easiest way to cut helical gear forms is by broaching. The
process uses a tool with several teeth and embedded cutters that
dig deeper than tools used in shaping. This leads to
easier-to-make, smaller-incremental cuts that quickly shape the
product into the desired form without sacrificing precision.
Milling
This is a straightforward technique for cutting helical gears
that can progressively create each gear tooth. Milling is a
highly adaptable process, particularly when using a CNC milling
machine. Designers can use a milling machine to create any gear,
but the level of precision sometimes suffers. Due to this,
milling is less popular than it once was.
Post-Manufacturing Processes
After manufacturing, the designers can apply the following
surface finishing methods.
Grinding - Grinding is a typical surface finishing
technique that produces a surface with a smooth finish. It
doesn’t matter if grinding is done continually or
sporadically; the outcome remains the same.
Lapping - This procedure is used for delicate gears
requiring high precision. Lapping is a low- to medium-speed
process that uses tiny abrasive particles to smooth a surface.
Honing - This is another typical technique that
polishes and smooths the surface. Additionally, tiny
corrections can be made to the shape of the teeth.
Shaving - This technique involves removing incredibly
thin layers from the surface to create a smooth profile. Since
shaving is typically expensive, it is rarely used to
manufacture gears.
Burnishing - In its simplest form, burnishing uses
compression to smear a surface smoothly.
Leading Manufacturers and Suppliers
Chapter Three: Types of Helical Gears
Double Helical Gear
The forces needed to overcome axial thrust can be neutralized or
counteracted by double helical gears. The entire face is divided
into two equal parts with opposite hands and the same helix
angle. The forces are contained in the gear and are not
transferred to the bearing because the axial thrusts oppose one
another. Therefore, these gears have the advantages of high
loading capacity and reliable transmission. Double helical gears
are used frequently for power transmission in gas turbines,
generators, prime movers, pumps, fans, and compressors in
maritime ships and construction machinery due to their benefits.
Typically, a special generator is used to produce the huge
double helical gears. However, the tooth arrangement constrains
the machining of the gears, and the engineers must manage the
phase difference of approaching gears with great accuracy. The
invention of a machine tool with many axes of control and
several functions has made the complex shape possible for the
machine. This development process has been suggested as the
bevel gear manufacturing process.
Helix angle adjustments are made to many single and double
helical gears with wide face widths to compensate for the
teeth's bending and twisting under operating loads. For the
helix angles on two mating gears to be the same when the design
load is applied during these adjustments, they are purposefully
cut differently.
Herringbone Gear
A herringbone gear is a particular kind of double helical gear.
The herringbone gear has two sets of gear teeth—one set on the
right hand and one on the left hand—on a single gear. When there
are two sets of gear teeth, one set's thrust cancels the other.
When visible of the top, each of this gear's spiral grooves
resemble the letter V and form a herringbone pattern. As a
result of this pattern, herringbone gears do not produce a
further axial load.
Since there will always be more than two teeth entangled at any
given time, these gears have the advantage of transferring power
quietly, smoothly, and at faster speeds. In addition, since the
side thrust of each half is balanced by the other, they have an
advantage over helical gears. Torque gearboxes can use
herringbone gears without a significant thrust bearing. As a
result, double-helical planetary gear sets in heavy-duty,
high-speed mechanical transmission, particularly in ship
turbines and internal combustion engines, are common.
Helical Rack and Pinion
A particular kind of linear actuator known as a helical rack and
pinion transforms the circular pinion's rotating motion into
linear motion at the rack. A rack is just a straight bar with
gear teeth, yet it may also be conceptualized as a part of a
gear with an infinite radius. Helical racks and pinions are
affordable for linear motion with movement lengths greater than
2 meters. They transform rotational motion into linear motion
when combined. The rack is driven in a line when the pinion is
rotated. On the other hand, if the rack is moved linearly, the
pinion will turn.
Helical gears are quieter and more effective than gears with
straight teeth. This is due to the more progressive way their
teeth mesh with the rack. Helical gears can also support larger
loads because of the longer contact length. In addition, the
rack gear and the pinion gear have a thrust component due to the
opposing hands of helical gears on parallel shafts. Rack and
pinion gears are most frequently used in automobiles' steering.
In a car, the steering wheel's rotational input is translated
into a linear motion that pivots the wheels.
Screw Gear
When engaged, the screw gears exhibit a screw action, or a
permanent sliding of the flank, rather than a simple rolling
movement. As a result, no points on the reference bodies of
crossed helical gears may be attributed to a pure rolling
process, and the circumferential speeds of the gears are not
identical at any point. Screw gear reference bodies are
rotational hyperboloids. A skew straight line is rotated around
a rotational axis to produce a hyperboloid. Screw gears are made
for moderate speeds and torques, such as those used in machine
tool drives.
Screw gears in the medium load and speed range emit low amounts
of noise. To prevent additional wear from the frequent sliding
of the flanks, hypoid gear oil is typically used as a specific
lubricant for the screw gears. However, strong lateral forces
are also generated by the screw tooth path, which needs to be
constructively absorbed by the right bearing.
In addition to the oblique orientation of the gear axes and low
noise operation, screw gears can also be moved axially within
rather broad limits without significantly degrading power
transfer. However, using screw gears harms transmission
efficiency due to the flank sliding motions. Worm gears are an
uncommon type of screw gear. Worm gears give a line-shaped
contact of the flanks as opposed to the standard case of a screw
gear, enabling the transmission of greater torques.
Helical Worm Gears
Helical worm gears are cylindrical objects with an external
spiral thread that meshes with another gear to turn it. A worm
or a screw collides with a gear in this particular gear system.
Various industries use worm gears to increase torque and when
significant gear reductions are required. Worm gears frequently
have reductions of 20:1 and sometimes even 300:1 or more.
Helical worm gears frequently have high gear reduction, which
indicates self-locking; the worm can turn the gear, but the gear
cannot turn the worm. The worm's shallow angle prevents it from
spinning due to friction when the gear tries to turn it. Helical
worm gears are frequently used in high-speed reduction gearing.
Conveyor systems are one example of an implementation where the
locking mechanism also serves as a brake. The Torsen
differential, which boosts torque for some high-performance
automobiles and trucks, also uses worm gears. Torsen®
differentials are torque-biasing, which means that they work
without requiring a loss of traction by distributing torque
across the tires and biasing more torque in the direction where
it is most useful. They function by controlling the friction
that results from applying torque to the helical gearing.
The worm wheel in this gearbox has a large diameter and is
connected to the worm shaft's outer teeth. The worm wheel's
non-intersecting and perpendicular axis is how the engine
produces rotational energy. The meshing gears may cause a large
reduction in speed since they pass through one another, which is
advantageous for a wide range of applications. They are also
widely used to calibrate tools, elevators, and gates. Helical
worm gearboxes are ideal for situations involving shock loading
as well. Heavy-duty devices, including conveyor belts, packing
machinery, and crushing equipment, are included in this
category. Worm gearboxes can also be employed in instances where
noise is a problem. Worm gears’ low-power, low-speed
applications are well known, but they can only transmit a small
amount of power.
Bevel Helical Gears
Although they can be made to operate at other angles as well,
helical bevel gearboxes are angular gearboxes in which the
output shaft of the gear unit rotates 90 degrees concerning the
motor's rotor shaft. Shafts can be solid or hollow. When a shaft
needs to rotate in a different direction, bevel gears come in
handy. Applications involving angular geared motors that require
high power density and output torque should use gearboxes with
helical bevel gears. Bevel helical gearboxes are characterized
by curved teeth enclosed within a cone-shaped base at the
device's edge. By creating rotating motion between non-parallel
shafts, this design achieves a stable and silent operation. As
is customary, the spiral teeth mesh with other helical gears.
Starting at one end of the gear, the contact gradually increases
throughout the length of each tooth.
Applications needing a high torque output and strong efficiency
ratio are ideally suited for these gears. Bevel helical gears
are also programmable. This industrial gearbox is used
extensively in the concrete, steel, plastic, automobile, and
mineral industries because of its strength and heavy-duty
applications. Bevel helical gearboxes are often employed in
industrial mixers, rope lifters, and baggage conveyors. When the
teeth are engaged, stable power and energy transmission are
possible. There are numerous applications made possible by the
Bevel Helical Gearbox. Compared to worm gearboxes, they can
transfer power more effectively. Bevel helical gears also offer
a high-efficiency ratio.
Chapter Four: Applications, Advantages, and Disadvantages of
Helical Gears
Helical Gear Applications
Fast-paced Industries
Since helical gears experience less wear and friction than other
gears while still having a substantial force-transfer capacity,
they are perfect for high-speed applications.
Pumps with Helical Gears
The overlapping of subsequent discharges from intervals between
the teeth is increased by the helical gear design over the
herringbone arrangement. The discharge flow is thus smoother. As
a result, gears with an increased capacity can be made with
fewer huge teeth without sacrificing smooth flow.
Industrial Chemistry
Centrifugal compressors and turbines are slowed down using
helical gears to match the nominal speeds of motors and
generators. These gears must be properly cooled and lubricated
to function properly.
Automotive Industry
Automotive helical-type gears are more durable than spur gears
because they have more teeth that can mesh together, creating a
larger surface area that can support the weight. Due to this,
helical-type gears are an excellent choice for heavy-duty
automobile applications like transmission operations.
Production Industry
Helical gears' teeth enable axial forces to withstand twisting
or spinning motions. Therefore, these gears are advised for use
with machinery that needs quicker rotational speed, carries
heavy loads of items, or runs continuously.
Advantages of Helical Gears
One of their most appealing features is that helical gears are
quieter than other gears. They are highly sought after for
large production operations. It results in a smoother, more
regulated machine transition that effectively mutes vibration
and shock.
One should also consider whether their project calls for power
transfer between shafts that are not parallel. Helical gears
enable this, although sometimes at the expense of efficiency.
Helical gears' teeth enable axial forces to withstand twisting
or spinning motions. Therefore, these gears are advised for
use in machinery that needs to rotate at a quicker rate,
transport big amounts of goods, or run continuously.
Construction projects and facilities with heavy machinery
frequently use this kind of equipment. This is because helical
gears can handle a greater torque shift than other gear types.
They can do this because of their balanced, well-designed gear
teeth, which are excellent for those demanding jobs.
The strength output is the last benefit helical gears have
over spur gears. Since the spiral gear tooth is diagonally
positioned and effectively larger, helical gears can take more
load than spur gears. Helical gears will offer greater
strength for the same tooth size and corresponding width.
Helical gear design offers flexibility while still being
durable. Depending on the purpose of the machinery, these
gears' shaft connections might be either parallel or
perpendicular. They make it feasible to adapt machinery to
specific requirements, maximizing productivity.
Helical gears and gearboxes are typically strong and ideal for
high-load applications.
Automobile industries can use these gears to transmit force
and motion between shafts with a right or parallel angle.
Disadvantages of Helical Gears
Due to the helix angle of the gear teeth, when a pair of
helical gears mesh, an axial thrust load is created on the
gear, so the gearbox designer must use bearings that can
sustain and absorb this load.
The mating gear teeth produce sliding action when using a
helical gear, and more heat is generated than with a spur
gear. As a result, helical gears need high-quality lubricant.
The efficiency of a mating pair of helical gears will be lower
than that of a matching pair of spur gears of equivalent size.
Helical gear train operation will result in a greater power
loss than spur gear train operation.
Helical gear manufacturing and design costs will be higher
than spur gear manufacturing and design costs.
Leading Manufacturers and Suppliers
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