Gear Design

Direct Gear Design	Characterstics	Traditional vs Direct Gear
Asymmetric Gears

Do you Know?

Engineers from the ancient era successfully used Direct Gear Designs. They studied and knew the optimum performance parameters like gear ratio, center distance and the type of power source (for example water current, wind) and applied them to define the exact gear parameters such as diameters, number or shape of teeth). After thoroughly studying the above consideration they manufactured gears using the materials, technology, and tools of that era.
Ancient Gear

The design of gears is a long process. The traditional gear design offers benefits like:

Interchangeability of the gears
Low tool inventory
Easy gear design process

Traditional gear design that is based on standard tool parameters gives universality. However, it does not necessarily provide the optimum gear performance for a particular gear application. This is due to the fact that it is limited by predefined tooling parameters. Actually traditional gear design relies heavily on operating conditions and performance parameters. These necessitated the need for an alternative gear design.

Direct Gear Design
Direct Gear Design is more or less a process for gear development that is application driven. Here, the primary emphasis is on two things. They are performance maximization along with cost efficiency. They defer from the traditional approach, by showing no concern for the previously defined tooling parameters. Direct Gear design can be put to use to get the optimum gear performance from all the type of involute gears like helical, spur, bevel, worm, and others.

The following geometric parameters are there in the gear design:

Pitch Diameter
Root Diameter
Outside Diameter
Base Diameter
Addendum
Dedendum
Tooth Thickness
Tooth Thickness Angle
Size of Fillet
Pressure Angle
Diametral Pitch = Pitch Diameter + (2 * Addendum)
Circular Pitch = pi/Pitch Diameter
Number of Teeth = Diametrical Pitch * Pitch Diameter

Characteristics of Direct Gear Design
Following are some of the major characteristics of the direct gear design. They collectively decide the gear performance.

Gear Mesh Synthesis
In case of Direct Gear Design, a gear tooth is not defined by using the typical generating rack parameters such as module, diametral pitch or a pressure angle. Instead the gear tooth is represented by 2 involutes of a base circle(see the image) and the circular distance (also called base tooth thickness) between them. The tooth height is restricted by the outside diameter for avoiding a pointed tooth tip and making available a preferred tip tooth thickness. Fillet, which is the non-involute portion of the tooth profile, interestingly, do not transmit torque. However it is a crucial element of the tooth profile. The fillet's importance can is immense it is the area that has maximum bending stress. This in turn can have a bearing on the strength as well as the durability of the gear.

Sa= tip tooth thickness
Sb= base tooth thickness
da= outside diameter
db = the involutes

Efficiency Maximization
In a Direct Gear Design there is maximization of gear efficiency. This is done by the process of equalizing the gear pair's maximum specific sliding velocities. As compared to traditional gear design, here this is done without in any way compromising the factors like gear strength or stress balance.

Optimization of Fillet Profile
Optimization Fillet Profile

In a Direct Gear Design, For any pair of gears, there is optimization of fillet profile. This is done to minimize the effect of bending stress concentration. The image is a perfect example of the gears that has optimized fillet profile.

Processing and Tooling and for Direct Designed Gears
Direct Gear Design is fully committed to custom gears. This requires custom tooling. This effectively means that for cut metal gears, it must have its own hob or shaper cutter. This invariably leads to higher costs along with increase in gear cutting tool inventory. So it is imperative that the Direct Gear Design approach must be totally justified by a significantly enhanced gear performance. A typical example of direct gear design is the asymmetric gears.

Advantages of a Direct gear design

An increase in the load capacity (15 to 30%)
Weight and size reduction (10 to 20%)
A longer life
Reduction in cost
More reliability
Reduction in noise and vibration (a finer pitch along with more teeth leads to higher contact ratio for a given center distance).
Increase in the gear efficiency (atleast1 to 2% per stage)
Low maintenance Cost

Traditional vs Direct Gear Design
The following table highlights the points of differences between a direct and traditional gear design.

Direct Gear Design	Traditional Gear Design

Basic Principle	Basic Principle
Gear design is governed by application (performance based parameters).	Gear design is governed by manufacturing (based on parameters of cutting tool profile).
Custom Application Gears Metal and plastic moulded, forged, powder metal, die cast gears. High production machined gears. Gears that has special requirements and also gears for extreme applications.	Custom Application Gears Stock gears Gearboxes that has interchangeable gear sets (such as old machine tools) Prototype Mechanical drive Machined gears with low production

Asymmetric Gears
It is a common fact that the two profiles or sides of a gear tooth in almost all the gear drive is functionally different. A significantly higher workload and for a longer period is applied to one profile than the opposite one. This functional difference is well reflected in the design of the tooth shape of an Asymmetric Gear.

Asymmetric Gears Design

Direct Gear Design is perfectly applicable for gears that has asymmetric teeth. This is due to the reason that in case of Asymmetric Gears, no standard is applicable for asymmetric generating racks. The approach of Direct Gear Design for asymmetric gears is identical for symmetric gears. The only difference being asymmetric tooth in Asymmetric Gears, is defined a set of two involutes of two different base circles.

It must be noted the degree or extent of asymmetry and the selection of drive profile for these gears is totally dependant on the application. Direct Gear design of asymmetric profiles has made it possible to effectively manage both tooth stiffness and load sharing. This is done while keeping a suitable pressure angle along with an exact contact ratio on the drive profiles. Unlike traditional gears, Asymmetric gear design is not limited by the effects of standardized tooling or the approach of tool based design.

Benefits of Asymmetric Gears
The basic intent of designing an asymmetric gear teeth is to considerably enhance the performance. Actually the performance improvement is done for the primary contacting profile by way of deliberately degrading the performance of the opposite profile. Generally the opposite profile is unloaded. Sometimes, during a relatively short work period it can also be lightly loaded. Thus this improved performance done on the primary profile offers the following advantages:

Increased load capacity
Reduced size and weight
Reduced noise
Reduced vibration
Reduced contact stress

Application of Asymmetric Gears
Asymmetric Gears are considered for gear systems which requires extreme performance. For example say aerospace applications. Asymmetric Gears gives desired result in mass production transmissions.

However a very encouraging use for Asymmetric Gears is for the molded gears and powder metal gears. As in molded gears there is a need for custom tooling anyway. Using Asymmetric tooth profiles in them do not raise the production or tooling costs.