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Case-Hardened Precision Gear Racks

Case-Hardened Precision Gear Racks

An innovative carburized case-hardening process that increases the strength of precision gear racks, including the tooth flanks and root, maximizes power density, stiffness and performance. The Strong Line class of gear racks from Atlanta Drive Systems Inc. enables designers to use smaller rack sizes, compared to traditional gear racks, and can increase the performance of existing systems.

"What is revolutionary is that this is the first rack on the market that is completely case-hardened with root-hardened teeth. It achieves the maximum power density, since there isn't anything more you can do to the rack to increase the power it can transmit," says John Entwistle, vice president of engineering with Atlanta Drive Systems. "For design engineers, it decreases the size of the rack required in applications. The more power-dense rack makes it possible to have smaller designs, and the rack required for an application typically drops down a size. In existing applications, the new rack increases the capabilities and performance of the design."

"By completely case-hardening the rack, you harden the root as well as the sides and back of the rack and the mounting holes. This allows the back surface of the racks to be used with cam follower roller bearings for guiding the axis," Entwistle continues. The racks are precision ground on the teeth and all sides to achieve extremely accurate pitch deviation and parallelism. Rack mounting holes are optimized for maximum holding force to insure the rack remains stationary under high forces.

Racks are typically sized based on the acceleration and torque needed for a machine to ramp up-to-speed. The new racks allow the design to push harder during acceleration and carry more mass. With root hardening, the rack is also stiffer and there is less deflection under load.

Case-Hardened Precision Gear Racks

Entwistle says there is more demand for hardened racks now, especially with machine tool and aerospace customers who need the stiffness and power density that the rack provides. With this new product, we are showing a way to increase the forces that a rack can transmit and increase the performance of a machine.

With a simple, soft milled gear rack, the limits to the amount of force that can be transmitted are based on the bending and pitting strength of the teeth. Because the teeth are soft, the limit is the pitting strength or contact pressure between teeth. Hardening the teeth increases the allowable contact pressure and changes the limit from pitting strength to bending strength.

There are racks available on the market where the flanks are hardened to increase the allowable contact pressure. So the only way to further increase the capacity of the rack is to root-harden the base of the teeth. But it's very difficult to reach between the teeth with an induction hardening process, and is expensive and difficult to control.

With rack-and-pinion applications, Entwistle says there are always several solutions for a machine, at several different price points. The hardened racks provide a high-end solution for engineers designing high-performance machines. The Strong Line gear racks are available in straight and helical versions, and can be combined with precision pinions and servo reducers to create an ultra-high-precision rack-and-pinion drive system.
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