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 of rod and operates with a pinion, which is a small cylindrical gear meshing with the gear rack. Generally, gear rack and pinion are collectively called “rack and pinion”. 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.
To provide many variations of rack and pinion, 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 tooth cutting process and the heat treatment process 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 for using racks in place of ball screws. The advantages of a gear rack are its mechanical simplicity, large load carrying capacity, and no limit to the length, etc. One disadvantage though is the backlash. The advantages of a ball screw are the high precision and lower backlash while its shortcomings include the limit in length due to deflection.
Rack and pinions are used for lifting mechanisms (vertical movement), horizontal movement, positioning mechanisms, stoppers and to permit the synchronous rotation of several shafts in general industrial machinery. On the other hand, they are also used in steering systems to change the direction of cars. The characteristics of rack and pinion systems in steering are as follows: simple structure, high rigidity, small and lightweight, and excellent responsiveness. With this mechanism, the pinion, mounted to the steering shaft, is meshed with a steering rack to transmit rotary motion laterlly (converting it to linear motion) so that you can control the wheel. In addition, rack and pinions are used for various other purposes, such as toys and lateral slide gates.
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NOTICE: Use of CAD Drawings
The tooth profile detailed in the CAD drawing differs from that of the actual gear.
Also, please note that the details of any chamfer, fillet, or slotted groove on the CAD drawing may differ from the true values or shape on the actual product.
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
Gears transmit power by rotating one gear to move the gear that is meshed with it. On the other hand, in rack and pinion, the combination of rack gear in the form of a gear stretched in a rod form and a small diameter gear (pinion gear) converts rotational motion into a linear motion to transmit power. For example, in a case where the pinion gear is stationary and the rack moves, the pinion is often connected to the output shaft of motors. The driven side of the rack is supported by a separate structure of machine elements. The pinion gear’s repetitive rotational motion produces a repeated forward-backward motion of the rack.
For the power transmission mechanism, the gear is attached to a shaft by means such as with a key, and its shaft is supported with ball or sleeve bearings. In case of a rack and pinion when the driven member is the rack, more creativity in design is needed since the rack is in the form of a rod (circular or rectangular).
When the rack is circular, sleeve bearings on the market can be used and the bearing support structure is relatively simple. On the other hand, to insure the constant meshing of the pinion and the rack, it is necessary to provide for means to stop the rack from rotating. The round racks have the gear teeth cut on the rod so that the cross section is different from the normal gears. They have the shape of a crescent moon with its shoulders shaved off. As a result, their strength is less than that of rectangular racks.
When the rack is rectangular, it becomes necessary to make suitable bearings. In this case, they can also act as the means to stop rack rotation, Also, the cross sections of rectangular racks are, unlike that of round racks, the same as those of gears with the same strength as the gears of the same specifications.
Rack and pinion has the characteristics of its function being drastically altered depending on whether the rack is stationary or movable.
When the rack is movable, its motion is in a straight line, and its use is mainly to take advantage of this behavior. For example, it is used as a jack or clamping system or, by modifying the tip of a rack, utilized as the pusher of a work piece.
When the rack is stationary, the pinion gear rolls on the rack and its application method varies widely. Positioning of machines, hand press, horizontal transport mechanism and elevating mechanism, etc. can be used as examples.
Also, if two racks are laid facing each other and a pinion is placed between them, the repeated forward-backward motion of the pinion will produce an alternating advance and retreat motion of the racks. For applications of this mechanism, work escapement mechanisms and air driven rotary actuators can be listed.
Racks can be placed midstream in conveyor transport mechanisms. By incorporating freely rotating pinions on transport pallets which engage the racks, the items on the pallet can be flipped or rotated. This is one special application example.
As shown, pinion and rack possesses a high degree of freedom in its applications depending only on the users’ ideas.
The steering mechanism is used to change the direction of automobiles and are mainly classified into rack-and-pinion and ball nut types.
Of these two, the rack and pinion type steering mechanism has become the mainstream used in many small cars. Its construction is simple with other characteristics such as lightweight, high strength, low friction, superior responsiveness, etc.
The rack and pinion type steering mechanism consists of a pinion attached to the tip of the steering shaft on which the steering wheel is mounted. The pinion is meshed with a rack so that the movement of the handle rotates the pinion which in turn moves the rack sideways. The wheel is moved left and right through the system of tie rods connected to the ends of the rack.
(Caution : Currently, KHK does not supply rack and pinion for automotive steering mechanisms.)
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
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
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 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
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
Related links :
齿条 - 中文版
CP Racks and Pinions - A detailed description of CP Racks and Pinions
“Raw Material” and “Gear Precision Grade” Equivalent Tables
Gear Nomenclature
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