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Posted on Aug 5, 2020
A rack and pinion gear system converts rotational into linear motion. A perfect example is the steering system for many cars. The steering wheel rotates the gear meshed with the rack. When the gear rotates, it will slide the rack left and right according to the way the wheel rotates. Some gears also use rack and pinion gears to turn a dial that displays a certain weight. But of course, rack and pinion gears are used in a wide range of machine tools such as for linear guideway systems and others. In this introductory blog entry, we will provide an overview of rack and pinion gears. Let’s go!
Although this is not always the case, most rack and pinion systems are based on standard gear tooth forms identified as 20-degree involute spur gears. For involute gears only, the pressure angle will remain the same during the meshing of each tooth. The genius behind the involute tooth profile is that it can provide a theoretically constant speed ratio even if the center distance setting is not ideal. The need for a constant speed ratio is sometimes referred to as the basic law of gear transmission.
The 20° line on the rack is easy to identify because it is a straight line. A rack is a useful tool for making circular gears because these straight lines are easy to machine. After the rack is hardened and sharpened, it can be used to cut the curved tooth surface in the round blank. This process is called gear generation and is one of several processes for manufacturing gears. This is the second advantage of involute teeth. Spur gear racks and pinions and helical gears are common.
The helical gear is cut into a certain angle with the surface of the blank, thereby producing gradually meshing teeth, so it runs quieter than a spur gear, and has a longer contact length and can withstand higher loads. One disadvantage of helical gears is that gears on parallel shafts have opposite pointers, which creates thrust components on the rack and pinion. Thrust will increase the side load of the mechanical structure.
Spur gear racks, pinions, and helical gears are very common. The helical gear is cut at a certain angle with the surface of the blank, thereby gradually forming meshing teeth, so it runs quieter than the spur gear, has a longer contact length, and can withstand higher loads. One disadvantage of helical gears is that the gears on the parallel shafts have opposite pointers, which generates thrust components on the rack and pinion.
Thrust will increase the side load of the mechanical structure. The spiral rack with the best helix angle is preferred because it has a higher tooth contact ratio, so it can run more quietly at a higher speed and has a higher load carrying capacity. Pinion profile offset or tooth tip modification can prevent undercut; it can also balance bending stress to achieve higher load capacity. Helical gears mesh smoothly and quietly-for example, when machining parts with strict tolerances, this helps to improve surface finish.
Compared with linear motors, rack and pinion systems can provide similar performance, but at a much lower cost. They are smaller, allowing a more compact and simple mechanical design. The lack of magnetic force greatly reduces the need for the support structure to absorb high normal forces, so standard rails can be used.
The overall efficiency of linear motors is as high as 90%, although sometimes the efficiency is much lower. Due to this inherent low efficiency, linear motors usually require water cooling. In contrast, the rack and pinion do not require a cover. The guidance system may be exposed to metal particles, and the safety restrictions are small. Better rack and pinion systems often do not require costly linear scales and external brakes.
Standard engine feedback equipment and brakes are adequate. In certain cases, linear motors need a complete redesign of the unit, partly because the enormous normal force produced by the attractive force between the primary and the secondary can have a profound effect. The out-of-the-box rack and pinion system is an easier option that can facilitate blind assembly, thereby further saving costs and reducing assembly time to about 10 minutes per meter of stroke length.
:: Read More : What is a Linear Motor?
Compared with the traditional bolted design, the rack and pinion transmission system has many advantages. The rack is designed to be completely located in the T-slots of any eight aluminum extrusion production lines so that a compact linear slide can be constructed without the use of a bolted drive mechanism. No part of the frame protrudes above the contoured surface. The groove ensures that the rack is aligned parallel to the guide mechanism, without unnecessary snaking that often occurs with replacement racks. Any number of individual rack segments can be used to construct a rack of variable length.
Compared with the longer length, the use of 80mm parts can obtain higher accuracy. In this case, the pitch error may be large and waste can be reduced because the longer frame must be cut to adapt to the application. Assembly couldn't be easier. The end part is fixed in place with a single positioning pin, and the patented spring clip can hold continuous parts without further processing.
The second end part is fixed with a set screw. The matching 18-tooth pinion is installed in the housing through double ball bearings, and the housing is neatly fixed on the lower side of the movable carriage plate. The coupling module is connected to the pinion gearbox, and the key shaft of the pinion gear meshes with the coupling group contained therein. Before securing the housing firmly in place, the housing has been adjusted to eliminate gaps.
In order to install the drive motor, the housing was processed as needed. Maintenance only requires a few drops of light oil to the pinion gearbox, which contains a pair of felt discs for transferring lubricant to the pinion gear. The drive system is designed for applications with a maximum driving force of 1,000N, a maximum motor torque of 23Nm and speeds up to 3m/s.
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