Cold rolling delivers superior gear strength

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Cold rolling delivers superior gear strength and finish
Gears are an important aspect of product design. Often used to modify speed, force and direction, gears can be used in a variety of applications, from mountainous oil rigs to the smallest desk clock. Regardless of shape or size, every gear must have substantial strength, transmit motion uniformly and resist wear over time. The key to a useful gear is in the manufacturing method.

Displacement vs. subtractive processes
The most common method of gear manufacturing is to cut away solid material to create the basic shape. Once the teeth are formed, the gear is heat treated and finished. This process is subtractive in nature, taking material away from the original, raw, stock material.

Comparatively, a displacement process reshapes material instead of removing it. A good example is cold rolling, where rollers forcefully push down into a metal cylinder blank, causing the metal to deform to a mud-like consistency, and fabricating the gear tooth from the force of the downward pressure. The material displaced from area A (Figure 1) forms the tooth, labelled as B.

Displacing metal has several substantive benefits. Most notably, the displacement method rearranges the grain structure of the metal across the entire tooth profile, increasing tensile strength over the gear and its teeth, especially at the root where strength is most critical. The resulting gear is harder and tougher than a subtractively manufactured gear.

This method also produces a high surface finish (Ra), achieving Ra 4 mirror or ASTM No. 8 finish readings. These smooth surfaces support lubrication films, which extend component life when compared with rough surfaces that tend to break up the film, causing premature component wear. The amount of frictional torque exerted on a gear can lead to scuffing damage. This condition decreases significantly when the surface finish improves.

When considering cold rolling, it is important to consider common methods of gear manufacture.

 

Manufacturing methods
The main gear manufacturing methods include hobbing, powdered metal, extrusion, rack rolled and cold rolling. There are two main failure points for gears: tooth breakage, and bending and surface wear. Tooth breakage is usually the result of stress, while bending and surface wear occurs with excessive contact or when a gear’s finish is rough enough to cause tooth failure.

 

Hobbing
Hobbing is the most common process for creating gears. A specialized mill called a hobbing machine is programmed to cut gear teeth into blank stock. The gear teeth are cut into the metal with a continuous rotary motion. The process is accurate, fast and inexpensive. However, hobbing requires an additional number of finishing steps after the gear fabrication is complete.

 

Powdered metal
Powdered metal’s benefits include no material waste, unique shapes, complex designs and low-price tooling. Additionally, it is the least expensive process for moderate-to-high-volumes. However, the strength of the parts are notably weaker and will wear more quickly than other materials. There are also size limitations to maintain gear manufacturability and effectiveness.

 

Extrusion
This process forces material through several dies in an extrusion press, with a final die dictating the gear’s tooth shape. After the extrusion, the bars are transferred to screw machines for finishing.

 

Since the material is displaced by pressure, the outside surface is hardened and smooth. Gears manufactured via extrusion result in reduced production time, no materials waste and improved performance. However, extrusion may require complicated tool designs and increased development time, which may result in non-uniformity in the gear.

 

Rack rolled
With this method, gear racks with straight teeth are cut into one surface of a bar, then rolled into a ring. The distance will change when the stock gear rack is curved. If the design of the gear requires that few teeth engage, this should not be a problem. However, the process of cutting teeth combined with a heat treatment step adds complexity and introduces the chance for inconsistencies.

 

Cold rolling
With a cold rolling process, metal is placed between rollers at room temperature and is compressed and squeezed, causing sufficient strain to yield optimum tooth strength. Benefits include industry estimations of 20% greater strength than other methods, based on improved hardness. This method can be used for complex shapes and tooling, and is relatively inexpensive and flexible. Cold rolling provides for more exact dimensions and substantially better surface qualities. Tolerances range from .002 in on fine gears (20 to 48 pitch) and from .003 to .005 in on coarse gears (6 to 16 pitch), and surface finishes are as good as 4 Ra. While the process is more expensive, eliminating post processing, including honing and grinding tooth flanks, results in a savings in both time and cost.

Grob Inc. and cold rolling
For nearly 70 years, Grob Inc. has provided a revolutionary method of cold forming gear and spline forms. The Grob process has inherent advantages such as strength, durability and an extremely high surface finish.

Application examples include the following.

· Shafting for the air tool industry is typically heat-treated to achieve a sufficient strength. Cold rolling completely eliminates the necessity for heat treating, lowering cost and speeding up the process.

· Several NASCAR teams and a major international truck company use the Grob process for a collapsible steering column, with great durability due to surface finish.

While the Grob process is known for industry-leading strength and surface finish, what often sets Grob apart is the available lengths of stock it offers. Small-diameter material, for example, comes in 5 ft to 6 ft lengths, while large-diameter material comes in 10 ft to 12 ft lengths.

Customization is another major advantage. Custom products represent between 60% and 70% of Grob’s business, and all custom products and processes remain the property of the customer. Given that the actual forming tool itself is quite small, creating unique shapes or adjusting existing designs — such as adjusting for backlash, noise or force per tooth — can be achieved quickly and new tooling can be machined in a matter of hours.

Coupled with tight tolerances, high strength and a smooth surface finish, cold rolled gears continue to fill an important need in many gear and spline applications.

For more information on how cold rolling can improve gear manufacturing and performance, visit Grob Inc.