Image Credit: Atlanta Drive Systems
What are Rack and Pinion Drives?
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.
Rack and Pinion Drive. Image Credit: Wikipedia
The GlobalSpec SpecSearch database allows industrial buyers to select rack and pinion drives by type, configuration, performance specifications and features.
There are two gearing configurations for rack and pinion drives,
- Straight tooth gears contain teeth that run parallel to the axis of the gear. Load movement or transfer is manual or walk-behind.
Straight Tooth. Image Credit: mechanicalmania.blogspot.com
- Helical tooth gears provide continuous engagement along the tooth length and are often quieter and more efficient than straight tooth gears. Helical tooth gears resemble spur gears in the plane of rotation, but include teeth that are twisted along a helical path in the axial direction.
Helical Tooth. Image Credit: i-automation.com
- Roller Pinion drives use bearing supported rollers that mesh with the teeth of that rack in order to provide minimal backlash.
Roller Pinion. Image Credit: engineeringenuity.blogspot.com
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.
Selecting rack and pinion drives requires an analysis of performance specifications,
- Axis drive force is the maximum drive force for which devicesare 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.
Design Tip: In general, the wider the pressure angle, the smoother the pinion will roll.
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.
Design Tip: Pinions should be made with as large a number of teeth as is possible, and practical. The larger the number of teeth, the larger the radius of the pitch line, and the more teeth are engaged with the rack, either fully or partially. This results in a smoother engagement and performance
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 vehicales.
Standards and Regulatory Bodies
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 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.