How To Design a Gear ?
How To Design a Gear ?

How To Design a Gear ?

September 29, 2020
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Are you facing difficulties in Gear design?  I’m going to help you to design gear for your desired application through this gear design blog series, Here I’ll share all the design aspects of gear design from calculation to CAD and analysis. This is the first blog of the gear design series, and in this particular blog we will discuss Gears, Different types of gears, Gear terminologies & Working of gears, this information will help you to understand the gear design process.

Gears and their mechanical characteristics are used in industry to transmit power and motion in a variety of mechanical devices, for better understanding, Let’s take the example of vehicle transmission.

The basic definition of transmission, it is a power transmitting device used after a flywheel and before an axle and it is used for the reduction of RPM to torque.

[sc_fs_multi_faq headline-0=”h2″ question-0=”Why we need to reduce RPM to torque in cars?” answer-0=”The power we get from the engine is constant, and according to this simple expression POWER = TORQUE*RPM, If we increase torque, rpm will decrease and vice-versa. We need more torque at starting to overcome the static friction and inertia of the vehicle and more rpm at the time when the vehicle is running on the road. That is why we need to reduce rpm to torque, to overcome initial inertia and friction with the help of higher gear ratios.” image-0=”” count=”1″ html=”true” css_class=””] Transmission is a combination of gears arranged in a specific order

What is Gear?

In more technically correct terms, a gear is a toothed wheel that is usually, but not necessarily, round. Gear is an analogy of belt and rope drive. The purpose of gearing is to transmit motion and/or power from one shaft to another. This motion transfer may or may not  be uniform, and may also be accompanied by changes in direction, speed, and shaft torque. They are also known as positive drive due to less slippage. The history of gears is old and the use of gears already appears in ancient Greece in B.C. in the writing of Archimedes.

The gear which provides initial rotational input is called driver and the driven gear rotates by the impact of the driver gear.

Classification of Gears-

There are three categories of gears in accordance with the orientation of axes :

  • Parallel
  • Intersecting
  • Non-parallel, non-intersecting

Parallel Axes configuration:

As the name suggests, Parallel axes configuration involves parallel shafts of gears mating each other within the same plane. The direction of the rotating shaft and driven gear is opposite to that of the driving gear. Spur gear, helical gear, internal gear, and some variants of rack and pinion gears come under parallel axes configuration. Parallel  axis gears transmit power with greater efficiency than any other type or form of gearing.

Spur Gear

Spur gear has straight teeth that are parallel to the shaft axis and it transmits power and motion between rotating parallel shafts. These are not subjected to axial thrust due to tooth load and has high load handling with high-efficiency rates. Spur gears are widely used gears that can achieve high accuracy and easier manufacturability. Some of the disadvantages of spur gears are the amount of stress experienced by the gear teeth and noise produced during high-speed applications due to less contact ratio. The most common  pressure angles used  for spur gears are 14.5, 20, and 25 degree.

In general, the 14.5 degree pressure angle is not used for new designs; however, it is used for special designs and for replacement gears. Lower pressure angles have advantage of smoother and quieter tooth action because of the large profile contact ratio. Lower pressure angle gears also have lower bending strength and surface durability ratings and operate with higher sliding velocity than their higher pressure angle counterparts. High Pressure angles have  the advantage of better load-carrying  capacity, with respect to both strength and durability, and lower sliding velocities. Most common tooth form for spur gears is the involute, other tooth forms are possible as long as they provide conjugate motion.

Helical Gear

Similar to spur gear, helical gear transmits power and motion between parallel shafts but teeth on gear are twisted around the cylindrical gear body at an angle to the gear face. They have better teeth meshing than spur gears and have superior quietness and can transmit higher loads, making them suitable for high-speed applications. Helical gears are produced with right-hand and left-hand angled teeth with each gear pair comprised of a right-hand and left-hand gear of the same helix angle. For meshing two mating gear should have the same helix angle but with opposite directions. The efficiency of helical gear is lesser than spur gear.

When using helical gears, they create thrust force in the axial direction, which necessitates the employment of thrust bearings in any application which uses single helical gears. The design of helical teeth is complex which increases the degree of difficulty in manufacturing and also the cost of manufacturing.

Helix angle from only a few degrees up to about 45 degrees are practical. As the helix angle increases from zero, in general, the noise level is reduced and the load capacity is increased. At angles much above 15 to 20 degrees, however , the tooth bending capacity generally begins to drop off.

Double Helical and Herringbone Gear

Double-helical gear is equivalent to a pair of helical gears attached together, one having a right-hand helix and the other left-hand helix. If the left and right-hand helix are separated by a groove then it is called double helical gear and if the left and right-hand helix meet at a common apex then it is called herringbone gear. Double-helical or herringbone gears are frequently used to obtain the noise benefits of single-helical gears without the disadvantage of thrust loading. While double -helical or herringbone gears do eliminate the net thrust load on the shaft, it is important to note that the two halves of the gear must internally react to the full thrust load.

Internal Gear

Internal gears have teeth cut on the inside of cylinders and are paired with external gears. The teeth may be made either spur or helical. There are limitations in the number of teeth differences between internal and external gears due to involute interference, trochoid interference, and trimming problems. The rotational directions of the internal and external gears in the mesh are the same while they are opposite when two external gears are in mesh. The main use of the internal gear is for planetary gear drives and gear type shaft couplings.

The teeth of an involute form internal gear have a concave shape rather than a convex shape. Because of concave nature of the internal tooth profile, its base is thicker than an equivalent external gear tooth. The tooth strength of an internal gear is greater than that of an equivalent external gear.Internal gears are generally more efficient since the sliding velocity along the profile is lower than for an equivalent external set.

Intersecting Axes configuration:

In intersecting axes configuration, the axis of the gear shaft intersects with each other within the same plane. This configuration includes Bevel gear(Straight, spiral bevel), and miter gears, these gear configuration employs intersection configuration. This configuration is used to change the direction of motion and torque generated by the source. While the involute is the tooth form  of almost universal choice for parallel axis gears, most gears that operate on nonparallel, coplanar axes do not employ involute profiles.

Bevel Gears

Bevel gears have a cone-shaped appearance, these gears are used to transmit torque between two shafts which intersect each other. They are usually mounted on the shafts which are at 90 degrees apart, but configurations with lesser or greater angles are also manageable. Bevel gears are more costly then spur and helical gears and are not able to transmit as much torque, per size, as a parallel shaft arrangement, which means the same size parallel shaft arrangement can transmit more torque comparison to bevel gears.

There are four basic types of bevel gears : Straight, Zerol, Spiral, and Skew tooth. In addition, there are three different manufacturing methods face milling, face gobbing, and tapered hobbing. All bevel gears impose both thrust and radial loads in addition to the transmitted tangential loads on their support bearings.

Straight Bevel Gear

This is the most commonly used bevel gear tooth design due to its simplicity and ease in manufacturing as compared to other bevel gears, Straight bevel gear has the same tooth design as the spur gear, spur gears are constructed with straight teeth cut on a cylinder but straight bevel gears are constructed with straight teeth cut cone-shaped. So these gears have the same problems as straight spur gear like when a tooth engages, it impacts the tooth all it once, which creates noise during high-velocity application. This impact also produces stress on the tooth which decreases the durability and life span of gear.

Zerol Bevel Gear

An improvement over straight bevel gears in terms of contact conditions, noise level, and power capacity is the zerol bevel gears. This gear is similar to a straight bevel except that the teeth are curved along their axis; however, the mean spiral angle is zero, thus the bearing reaction ;loads are the same as for straight bevels. Zerol bevel gears may be compared to double helical or double herringbone parallel axis gear teeth in that they have no more thrust load than their straight counterparts but provide advantages related tot he improved contact ratio.

Spiral Bevel Gear

To solve the noise and stress problem of straight bevel gear, spiral bevel gear has curved and angled teeth design to increase the contact ratio, efficiency, and strength. On the other hand, they are difficult to manufacture and are very costly. These spiral gear teeth engage just like helical gears: the contact starts at one end of the gear and progressively spreads across the whole tooth which decreases the stress on the tooth as compared to a straight bevel gear. As the gear has curved teeth, it creates the thrust force in the axial direction like helical gears. Like helical gears, spiral bevel gears are available with right-hand or left-hand angled teeth. The most common spiral  angle in use is 35 degree.

Skew Bevel Gear

The final type of bevel gear is the skew tooth. This is similar to a spiral bevel gear. Actually, the skew tooth has no length curvature; rather the teeth are simply cut straight but at an angle to the shaft center line. This provides an improvement in load capacity when compared with a straight bevel gear. Skew tooth gears are used primarily in large sizes only. They are produced on planing generator machines.

Face Gear (On-Center)

A face gear set is actually composed of a spur or helical pinion that is mesh with a “face” gear. Face gear have teeth cut into the blank such that the axis of the teeth lie in a plane that is perpendicular to the shaft axis. The mating pinion is either a spur or a helical gear.The pinion and face gear axes most often form a 90 degree shaft angle. The load capacity of face gears, compared with that of bevel gears, is rather small; thus they are used mostly for motion transmission rather than as power gears. Face gears are easy to make and somewhat less expensive as well.

Non-parallel & Non-intersecting Configuration:

In this type of configuration, the Axes of the gears shaft do not intersect and are nonparallel to each other. These gears are complex, both in terms of geometry and manufacturing. This configuration includes Crossed Axis Helicals, Hypoid Gears, Spiroid and Helicon Gears, Face Gear (Off-Center), Crown gear, Screw gear, Worm gear, and Rack and Pinion Gear.

Hypoid Bevel Gear

Hypoid gears are a type of spiral bevel gear but these gears are used for non-parallel, non-intersecting configurations. The hypoid gear can engage with the axes in different planes. This allows the input pinion to be mounted lower than the axis of the ring gear which allows more space in the sections above. These gears are used in car differential to lower down that driveshaft to make more space in the rear passenger cabin of the car.

Curved and angled teeth similar to those used in spiral bevel gears make hypoid gears even more complex and, consequently, more difficult (and costly) to manufacture. As hypoid gears use angle and cured tooth design, which lower the noise during operation, and that is why hypoid gears are suitable for the drive train.

Pressure angle on each side of hypoid gears are different. The pitch surfaces of hypoid gears are hyperboloids of revolution. The teeth in mesh have line contact; however, under load, these lines spread to become elliptical regions of contact inclined across the face width of the teeth. One condition that must exist if a hypoid gear set is to have conjugate action is that the normal pitch of both members must be the same.

The number of teeth in a gear and pinion are not, however , directly proportional to the ratio of their pitch diameters. This makes it possible to make large pinions while minimizing the size of driven gear. In operation, hypid gears are usually smoother and quieter than spiral bevel gears due to their inherent higher total contact ratio. The efficiency of hypoid gears is thus much less than that of similar set of spiral bevel gears. Hypoids generally have greater tolerance to shock loading and can frequently be used at much higher single stage ratios than spiral bevel gears.

Crossed Axis Helical

Crossed helical gears are good for the normal range of ratios used for single reduction helical gears. These gears provide both speed reduction and extreme versatility of shaft positioning at a relatively low initial cost. Crossed axis helical gear are cut in the same manner as conventional helical gears using same tooling.

It is interesting to note that, while mating parallel axis external helical gears must have opposite hands of helix, crossed axis helical can have either the same or opposite hands of helix, depending on shaft angle and the relative directions of rotation of the driver and driven gear. The reduction ratio between pinion and the gear is a function of their relative numbers of teeth but not directly of their  pitch diameters. This provides a great deal of flexibility in choosing the ratio, center distance, and diameters of the gears.

Worm Gear

Worm gears are used when large gear reductions are needed, these gears comprise a worm (a screw-type gear) and a worm wheel (a cylindrical gear like a spur and helical). These transmit motion and torque between two non-parallel and non-intersecting shafts. The efficiency of worm gears is less as compared to other gears but many worm gears have self-locking characteristics (the worm can easily turn the gear, but the gear cannot turn the worm). This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

These gears require continuous lubrication due to higher friction between the worm and the worm wheel. Due to low efficiency, they are often used in lower horsepower applications, and the manufacturing and design of worm and worm wheel is complex as well as the cost of gear.

Single Enveloping Worm Gear

We can achieve better load capacity if the simple helical gear is modified such that it is throated to allow the worm to fit down farther into the gear to achieve greater tooth contact area and thus smoother operation and improved load capacity. This types of gears are called single enveloping worm gear as the gear envelops the worm but the worm remain straight. The contact point of these are theoretically a line varying in length up to full face width of the gear with different tooth design. Under loads, this line becomes a thin elliptical band of contact.

The continuous duty rating is lower, this is due to high heat generation that can raise the lubricant temperature to unacceptable level when the arrangement operate continuously. Worm gear efficiency is quite dependent on operating speed. The same set may show an efficiency of, say, 75 percent at a low speed and 85 percent at a higher speed. Ratio, material. accuracy, and geometric design all affect worm gear efficiency. Typical efficiency run from 35 to 90 percent.

If you need irreversibility then use worm set, since, if the lead angle is less than friction angle, the wheel cannot drive the worm. usually, worms with lead angles less than 5 degree are self-locking. Care should be exercised when designing self-locking worms, since this feature is a static one.

Double Enveloping Worm Gear

We have discussed the capacity of a single enveloping worm gear set so for further improvement we can allow the worm to envelop the wheel as well, such drives are known as double-enveloping. By getting more teeth into contact, tend to provide higher load capacity than do cylindrical or single-enveloping worm sets. This is accomplished by changing the shape of the worm from a cylinder to an hourglass. Because of the shape of worm, this type of worm gearing is more expensive to produce; but where weight or size are considered; the cost differential is relatively small.

Rack and Pinion Gears

As the name suggests, Rack and pinion gears have a pair of two gears one is gear rack and another is pinion. The rack is a bar that contains teeth on one face to mesh with pinion (externally toothed gear). This mechanism converts rotational motion to linear motion. Rack and pinion gears tend to have a greater amount of backlash (i.e., additional space between mated gear teeth). Rack with machined ends can be joined together to make any desired length.

Gear Terminologies-

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The Law of Gearing-

The fundamental law of gearing states that the angular velocity ratio of all gears must remain constant throughout the gear mesh. This condition is satisfied when the common normal at the point of contact between the teeth pass through a fixed point on the line of centers, known as the pitch point.

Pressure Angle                      

The standard pressure angles are 14½°, 20°, and 25°. The increase of the pressure angle from 14½º to 25º results in a stronger tooth, because the tooth acting as a beam is wider at the base. The preferred angle in use today is 20º; a good compromise for power and smoothness.

It is important to note that the gears must have the same pressure angles to mesh. 14½º PA tooth forms will not mesh with 20º pressure angles gears and vice versa.     

Gear Profiles-   

Gear profiles should satisfy the law of gearing. The profiles best suited for this law are:

  1.  Involute
  2.  Cycloidal     

Most modern gears use a special tooth profile called an involute. This profile has a very important property of maintaining a constant speed ratio between the two gears.

Involute Tooth Profile

The involute profile is the path traced by a point on a line as the line rolls without slipping on the circumference of a circle. It may also be defined as a path traced by the end of a string which is originally wrapped on a circle when the string is unwrapped from the circle. The circle from which the involute is derived is called the base circle.

Cycloidal Tooth Profile

Cycloidal gears have a tooth shape based on the epicycloid and hypocycloid curves, which are the curves generated by a circle rolling around the outside and inside of another circle, respectively. They are not straight and their shape depends on the radius of the generating circle. The input and output remain in constant mesh. Cycloidal tooth forms are used primarily in clocks for a number of reasons:
  • Less sliding friction.
  • Less wear.
  • Easier to achieve higher gear ratios without tooth interference.        

In the next article, we will discuss different types of gear train and their design process. If you liked the article then share amongst your friends and on social media and give your feedback in the comment section below.     

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