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Posted on Oct 7, 2020
Spur rack and pinions are used to convert rotary motion into linear motion. The straight rack has straight teeth that are cut into one side of a square or triangular cross-section of the rod and driven by a pinion gear, which is a small cylindrical gear meshed with the straight rack. Generally, spur gears and pinion gears are collectively referred to as "rack and pinion".
When the spur gear rack moves, the pinion gear will cross, otherwise, the pinion gear will rotate along the fixed axis. The former is widely used in conveying systems, while the latter can be used in extrusion systems and lifting applications. As a mechanical element that converts rotational motion into linear motion, Spur rack and pinions are usually compared with ball screws. There are advantages and disadvantages to using a frame instead of a ball screw. Spur rack and pinions have the benefits of mechanical flexibility, wide carrying capacity, and infinite range. Nevertheless, one of the disadvantages is backlash. In this article, we will answer the most frequently asked questions about spur rack and pinion technology.
Spur rack and pinions are used for lifting mechanism, horizontal movement, positioning mechanism, brake, and allow synchronous rotation of multiple shafts in general industrial machinery. On the other hand, they are often used to adjust the course of cars in steering systems. The features of the steering rack and pinion system are as follows: basic construction, high rigidity, compact scale, lightweight, and strong reactivity. Through this mechanism, the pinion gear mounted on the steering shaft meshes with the steering rack to transmit rotational motion laterally, thereby controlling the wheels. Besides, racks and pinions are also used for various other purposes, such as toys and side sliding doors.
:: Read more : Everything you need to know about Rack and Pinion Gears
Spur rack and pinion is, in fact, an old invention, but that isn't a downside. First, high precision isn't needed for all applications. For instance, applications of material handling can have very low demands on precision. Second, developments in gear processing technologies have significantly improved the performance and load-carrying capability of rack and pinion transmissions, making them attractive components for any linear axis drive use. Also, low feedback and pitch errors on the rack and pinion drives are likely.
The reality is the spur rack and pinions have a theoretical center distance where optimal meshing takes place. The resistance can be minimized by moving the pinion onto the rack-but this can also make the tooth mesh weaker, and will result in unnecessary wear and rubbing. The easiest way to reduce the rack and pinion transmission backlash is by enhancing the efficiency of the teeth and their performance in deployment. To mcompletely eliminate the backlash of the rack and pinion transmission, you can use a split pinion or double pinion transmission, one of which is driven by a pinion, and the other eliminates the backlash.
Spur rack and pinion gears are considered open gears because they are not enclosed in the housing, so some external matter may enter the gear holes and cause damage. However, it can be minimized by turning the rack teeth sideways or facing down. Linear bellows or covers can also be used to protect the drive. Any debris on the tooth flanks can be cleaned off by using an automatic lubrication device with a felt gear applicator, thus avoiding injury. Similarly, the use of stainless steel racks and pinions for flushing or damp conditions can reduce corrosion.
In fact, in order to obtain the desired stroke length, multiple racks can be installed end-to-end. During the assembly process, errors can be caused in the drives that are partially docked together. However, the use of a "mate" rack that engages between the two rack parts can minimize this error. Otherwise, a rack assembly kit can be used to achieve a perfect rack connection, which can accurately measure the connection and eliminate any errors.
For long-axis drives, multiple racks can be installed end-to-end to achieve the required stroke length. Although each rack has a total pitch error associated with it, the error can be controlled or even eliminated, so that no cumulative error occurs over the entire stroke length. The pitch error per meter in length may be between 0.012 -0.2 mm, depending on the quality of the rack teeth. The racks can also be mounted in a predefined order such that the cumulative error in pitch is still kept close to zero.
Ball screws usually have low backlash and pitch errors, thereby providing sufficient performance for shaft drives that require precise positioning and repeatability. If necessary, zero backlash can be achieved by using pre-tightened nuts. Spur rack and pinions are also capable of accurate positioning and repeatability.
Lower backlash and pitch errors can be achieved by using the hardened and ground tooth surfaces. The use of single pinion or double pinion drives will produce zero backlash. Racks and pinions will potentially outperform ball screws for long-axis and heavy-axis drives, as the latter's stroke length can be influenced by its buckling power and whipping force cap.
Rack and pinion drive systems can accommodate higher linearity than ball screws for high-speed applications too. Spur gear rack and pinion transmission can achieve zero backlash by using split pinion or double pinion to preload the shaft. The pre-tightening force can be set mechanically or electrically, and always exists, regardless of the installation accuracy of the rack.
Every system that wants to turn rotary motion into linear motion will use spur rack and pinion. From basic, low-precision assignments to the most demanding high-precision activities, these can be chosen and implemented without compromise to achieve the necessary shaft efficiency.
Typical pinion and pinion applications include but are not limited to CNC milling cutters, pick-and-place robotics, moving portholes, material handling, automation, machine tools, construction, woodworking, friction stir welding, carbon fiber positioning, and seventh-axis robotic slide sheets.
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