Gear Rack – creation of linear motion
Gear racks are utilized to convert rotating movement into linear motion. A gear rack has straight teeth cut into one surface of a square or round section rod with a pinion, which is a small cylindrical gear meshing with the gear rack. There are many ways to use gears. For example, as shown in the picture, a gear is used with the gear rack to rotate a parallel shaft.
KHK has many types of gear racks in stock. If the application requires a long length requiring multiple gear racks in series, we have racks with the tooth forms correctly configured at the ends. These are described as “gear racks with machined ends”. When a gear rack is produced, the teeth cutting process and heat treatment can cause it to try & go out of true. We can control this with special presses & remedial processes.
There are applications where the gear rack is stationary, while the pinion traverses and others where the pinion rotates on a fixed axis while the gear rack moves. The former is used widely in conveying systems while the latter can be used in extrusion systems and lifting/lowering applications.
As a mechanical element to transfer rotary into linear motion, gear racks are often compared to ball screws. There are pros and cons. The advantages of a gear rack is its mechanical simplicity, large load carrying capacity, no limit to the length, etc. One disadvantage though is the backlash. The advantage of a ball screw is the high precision and lower backlash while the shortcomings include the limit in length due to deflection.
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What is a rack and pinion ?
A rack and pinion are used when converting rotational movement to linear motion (or vice versa). A bar shaped gear with an infinite (flat surface) radius of a cylindrical gear is called a rack, and a meshed spur gear is called a pinion. A rack can be used by extending it combining as many racks with machining operation on the end faces when necessary. A helical rack is a bar shaped gear with slanted linear tooth trace that is used when silence and high speed rotation are required, and it can be meshed with a helical gear.
Image : Plastic / Steel racks
Materials and heat treatment
As for the materials of a racks and pinions, strength, abrasion resistance, and absorbency are considered.
With steel, S45C (1045 in AISI/SAE, C45 in ISO, equivalent to C 45K in DIN), SCM440 (4140 in AISI/SAE, equivalent to 42CrMo4V in ISO and DIN), 16MnCr5 (name in ISO, equivalent to 17Cr3 in DIN), with stainless steel, SUS303 (303/S30300 in AISI/SAE, 13 in ISO, equivalent to X10CrNiS18-9 in DIN), SUS304 (304/S30400 in AISI/SAE, 6 in ISO, equivalent to X5CrNi18-10 in DIN), with plastic material, reinforced nylon called engineering plastic, polyacetal (equivalent to Duracon and POM), polyamide, are used.
Regarding the heat treatment of a rack and pinion, thermal refining, carburizing and quenching, tooth face high frequency hardening, and tooth face laser hardening are used depending on the kinds of materials and purposes.
Image : Material of racks
Overall length and cross-sectional shape of a rack
The overall length of standard racks on the market is generally not more than 2000mm, and offered in 500mm units such as 500mm, 1000mm, 1500mm.
Furthermore, the cross-sectional shape is often a square or rectangle, and some are round called round rack type.
Image : Cross sectional shape of racks
Production of racks and pinions
Although the difficulty in production varies depending on the accuracy classes and specifications, the basic production of the rack and pinion is possible with a rack cutting machine and gear hobbing machine. They are produced by many gear manufacturers and rack specialized manufacturers in the world. They are especially produced on a large scale by famous rack manufacturers in Germany and Taiwan.
The rack and pinion are used mainly for carrier devices, steering gear mechanisms for vehicles, machine tools, lifting apparatus, and printing machines.
When attaching a rack to a machine, mounting holes drilled though the bottom or through the side are frequently used, additional methods include counterbored bolt holes and tapped holes.
Image : Rack and pinion used for manual lifting device
The helix angle of helical rack
The helix angle of a helical rack is set at 19 degrees 31’42” by many rack manufacturers. When the helix angle is set like so, the movement when a pinion rotates (for instance one rotation) becomes an exact distance in mm, thus making it user-friendly.
Image : Helical rack
As for the lubrication of a rack and pinion, there are several methods used.
One such lubrication system consists of a lubricating device (main body), a tube to deliver the lubricant, a check valve and a special urethane pinion at the end of the tube.
The rack and pinion in this system are lubricated as the lubricant is dispered by the pinion.
Since the pinion injects the lubricant into the mesh as it rotates across the gear rack, the pitch of the rack must be the same as the module of the pinion, and in the case of helical rack and pinions, the rack and pinion must be of the same helix angle and opposite in direction.
Image : Pinion made of urethane
Comparison with a ball screw
As a mechanical element that produces linear motion, a rack and pinion system is often compared with to ball screw.
When compared with a rack and pinion, generally a ball screw has superiorities of accurate positioning, smooth motion with a little friction, and no backlash, on the other hand, ball screws have the disadvantages of higher cost, the difficulty of producing ball screws with long lengths due to deflexion, and unsuitability for heavy loads.
Image : Ball screw in machine tool