Rack and pinion drives use a rotational motor to affect linear motion via a rack and pinion combination. They are used frequently in long-travel applications, such as machine tool table positioning, that require high stiffness and accuracy.

They are composed of a two components. A rack, which is basically a gear that has been cut on a horizontal plane is used to facilitate horizontal motion. The second component is a matching pinion, which is a source of power and drives the rack for locomotion. Motion occurs from either the pinion moving over a fixed rack or vice versa.

Types of Rack and Pinion Drives

There are three common gearing configurations for rack and pinion drives:

Configurations

There are three preload configurations for rack and pinion drives:

Split pinions contain a torsion spring or other anti-backlash mechanism that enables the pinion to load both sides of the rack simultaneously.

Mechanical preloading consists of a drive pinion and a second preload pinion and gearbox.To maintain preloading and eliminate backlash, the preload assemblyapplies an opposing force to the drive assembly.

Electrical preloading consists of adrive pinion and motor,as well asa second pinion/motor assembly that applies an opposing or braking force. The opposing motor can take over during deceleration,smoothing carriage stops and reversals.

Specifications

Selecting rack and pinion drives requires an analysis of performance specifications:

• Axis drive force is the maximum drive force for which devices are rated. The rating represents the torque of the motor transmitted through the pinion to the rack.
• Axis weight rating is the rated weight for the driven axis.
• Pressure angle describes the shape of the teeth and the angle they make with the spaces in the rack.

Teeth Features

The teeth of the pinion fit into the grooves of the rack,

• Integral rack and rail include teeth that are machined or ground directly onto the moving element to increase power transmission efficiency.
• Ground teeth have a gearing feature with hardened teeth that provides a precise fit and smoother operation.

Applications

Rack and Pinion drives are used in many applications. They are used in industrial shops in drill press spindles and lathe beds as well as in construction applications to move a temporary elevator.

They are also commonly found in the steering mechanism of wheeled, steered vehicles.

Standards

The American Gear Manufacturers Association (AGMA) publishes quality-level standards for gears based on pitch variation, profile error, and tooth alignment accuracy. Quality levels are defined in the AGMA 2000-A88 Gear Classification and Inspection Hand Book. In general, higher quality levels indicate higher quality gears. For example, AGMA 8 and AGMA 9 describe lapped gear teeth. AGMA 10 describes automotive-quality gears with high degrees of strength and dimensional refinement. AGMA 11 describes gears with hardened and ground teeth that are particularly well-suited for rack and pinion drives.

Resources

The Straight Track on Gear Racks (PDF)

Image Credits:

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