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 rod section and operates with a pinion, 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, equipment is used to rotate a parallel shaft with the gear rack.
HZPT has many types of gear racks in stock to provide many racks and pinion variations. If the application requires a long length requiring multiple gear racks in series, we have shelves 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 methods.
There are applications where the gear rack is stationary while the pinion traverses, and others rotate 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 to using racks in place of ball screws. The advantages of a gear rack are its mechanical simplicity, sizeable load-carrying capacity, 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 direction, 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. With this mechanism, the pinion, mounted to the steering shaft, is meshed with a steering rack to transmit rotary motion laterally (converting it to linear motion) so that you can control the wheel. In addition, racks and pinions are used for various other purposes, such as toys and lateral slide gates. The characteristics of rack and pinion systems in steering are as follows: simple structure, high rigidity, small and lightweight, and excellent responsiveness.
Gear Rack and Pinion – the creation of linear motion
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 by combining as many racks with machining operations on end faces when necessary. A helical rack is a bar-shaped gear with a diagonal linear tooth trace used when silence and high-speed rotation are required, and it can mesh with a helical gear.
Design and Application Examples of Rack and Pinion Mechanisms
Gears transmit power by rotating one gear to move the meshed equipment 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 signal 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 the motors. A separate structure of machine elements supports the driven side of the rack. The pinion gear’s repetitive rotational motion produces a repeated forward-backward movement of the frame.
For the power transmission mechanism, the gear is attached to a shaft by a key, and its shaft is supported with the ball or sleeve bearings. In the case of a rack and pinion, when the driven member is the rack, more creativity in design is needed since the stand 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 ensure the constant meshing of the pinion and the rack, it is necessary to provide 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 have the characteristics of their 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 workpiece.
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, 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 that engage the racks, the items on the pallet can be flipped or rotated. This is one particular application example.
Pinion and rack possess a high degree of freedom in their applications, depending only on the users’ ideas.
Rack and Pinion Used in Automotive Steering Mechanisms
The steering mechanism is used to change automobiles’ direction and is mainly classified into rack-and-pinion and ball nut types.
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 wheel is moved left and right through the system of tie rods connected to the ends of the rack. The pinion has meshed with a rack so that the movement of the handle rotates the pinion, which in turn moves the rack sideways.
(Caution: Currently, KHK does not supply rack and pinion for automotive steering mechanisms.)
Materials and heat-treating racks and pinions
As for the materials of 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, equal 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, equal 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 (equal to Duracon and POM), polyamide, are used.
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 materials and purposes.
Overall length and cross-sectional shape of a rack
The overall size of standard frames on the market is generally not more than 2000mm and is offered in 500mm units such as 500mm, 1000mm, and 1500mm.
Furthermore, the cross-sectional shape is often a square or rectangle, and some are round, called round rack type.
Lubricating Racks and Pinions
As for the lubrication of a rack and pinion, several methods are 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 line.
The rack and pinion in this system are lubricated as the lubricant is dispersed 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. In the case of helical rack and pinions, the rack and pinion must be of the same helix angle and opposite in direction.