A gear (or cogwheel) it is the main element in a transmission system, in which teeth are cut around cylindrical or cone shaped surfaces with equal spacing; or in the case of a cogwheel, inserted teeth (called cogs). By meshing a pair of these elements, they are used to transmit rotations and forces from the driving shaft to the driven shaft.
For efficient transfer of speed and power, gears should conform perfectly to the designed profile and dimensions. Misalignments and gear runout will result in vibrations, chatter, noise, and loss of power.
Types of gear
Gears can be classified by shape as involute, cycloidal and trochoidal gears. Also, they can be classified by shaft positions as parallel shaft gears, intersecting shaft gears, and non-parallel and non-intersecting shaft gears.
- Spur gear
- Helical gear
- Gear rack
- Herringbone gear
- Worm gear
- Bevel gear
- Hypoid gear
Spur gears, or straight-cut gears, are the simplest and the most widely used gears that can achieve high accuracy with relatively easy production processes. The gear teeth are cut on the periphery and are parallel to the axis of the gear. Though the teeth are not straight-sided (but usually of special form to achieve a constant drive ratio, mainly involute but less commonly cycloidal), the edge of each tooth is straight and aligned parallel to the axis of rotation.
Spur gears are used to transmit power and motion in the same plane or when the driving and driven shafts are parallel to each other. They have the characteristic of having no load in the axial direction (thrust load). The larger of the meshing pair is called the gear and smaller is called the pinion.
Helical or “dry fixed” gear, is a type of gear that offers a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Each tooth has a helical or spiral form. Since the gear is curved, this angling makes the tooth shape a segment of a helix.
Helical gears can be meshed in parallel or crossed orientations. The former refers to when the shafts are parallel to each other; this is the most common orientation. In the latter, the shafts are non-parallel, and in this configuration the gears are sometimes known as “skew gears”.
These gears can deliver higher torque since there is more number of teeth in a mesh at any given point of time. They can transmit motion between parallel or non-parallel shafts.
The gear rack have the same sized and shaped teeth cut at equal distances along a flat surface or a straight rod. A gear rack is a cylindrical gear with the radius of the pitch cylinder being infinite. By meshing with a cylindrical gear pinion, it converts rotational motion into linear motion. Gear racks can be broadly divided into straight tooth racks and helical tooth racks, but both have straight tooth lines. By machining the ends of gear racks, it is possible to connect gear racks end to end.
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.
Design and application
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 ensure 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 have 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 workpiece.
When the rack is stationary, the pinion gear rolls on the rack, and its application method varies widely. The 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 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. Pinion and rack possess a high degree of freedom in its applications depending only on the users’ ideas.
Herringbone gears have two sets of helical teeth, one right-hand and the other left-hand, machined side by side.
A worm gear is similar to a screw having single or multiple start threads, which form the teeth of the worm. The worm drives the worm gear or worm wheel to enable transmission of motion. The axes of worm and worm gear are at right angles to each other.
Worms and worm wheels are not limited to cylindrical shapes. There is the hour-glass type which can increase the contact ratio, but production becomes more difficult. Due to the sliding contact of the gear surfaces, it is necessary to reduce friction.
For this reason, generally, hard material is used for the worm, and soft material is used for the worm wheel. Even though the efficiency is low due to the sliding contact, the rotation is smooth and quiet. When the lead angle of the worm is small, it creates a self-locking feature.
Bevel gears are used to connect shafts at any desired angle to each other. The shafts may lie in the same plane or in different planes.
Hypoid gears are similar to bevel gears, but the axes of the two connecting shafts do not intersect. They carry curved teeth, are stronger than the common types of bevel gears, and are quiet-running.
These gears are mainly used in automobile rear axle drives.
From a metrological point of view, the major types of errors are as follows:
- Gear blank runout errors
- Gear tooth profile errors
- Gear tooth errors
- Pitch errors
- Runout errors
- Lead errors
- Assembly errors
Gear blank runout errors
Gear machining is done on the gear blank, which may be a cast or a forged part. The blank would have undergone preliminary machining on its outside diameter (OD) and the two faces. The blank may have radial runout on its OD surface due to errors in the preliminary machining.
In addition, it may have excessive face runout. Unless these two runouts are within prescribed limits, it is not possible to meet the tolerance requirements at later stages of gear manufacture.
Gear tooth profile errors
These errors are caused by the deviation of the actual tooth profile from the ideal tooth profile. Excessive profile error will result in either friction between the mating teeth or backlash, depending on whether it is on the positive or negative side.
Gear tooth errors
This type of error can take the form of either tooth thickness error or tooth alignment error. The tooth thickness measured along the pitch circle may have a large amount of error. On the other hand, the locus of a point on the machined gear teeth may not follow an ideal trace or path. This results in a loss in alignment of the gear.
Errors in pitch cannot be tolerated, especially when the gear transmission system is expected to provide a high degree of positional accuracy for a machine slide or axis. Pitch error can be either single pitch error or accumulated pitch error. Single pitch error is the error in actual measured pitch value between adjacent teeth. Accumulated pitch error is the difference between theoretical summation over any number of teeth intervals and summation of actual pitch measurement over the same interval.
Pitch and index errors are both caused by three things: problem with machine 1001s, cuttillg tool problems and gear blank and mounting errors.
This type of error refers to the runout of the pitch circle. Runout is a characteristic of gear quality that results in an effective center distance variation. As long as the runout doesn’t cause loss of backlash, it won’t hurt the function of the gear, which is to transmit smooth motion under load from one shaft to another. However, runout does result in accumulated pitch variation, and this causes non-uniform motion, which does affect the function of the gears. Runout is a radial phenomenon, while accumulated pitch variation is a tangential characteristic that causes transmission error, vibrations, noise, and reduces the life of the gears and bearings. This error creeps in due to inaccuracies in the cutting arbour and tooling system. It is also possible to have a gear with accumulated pitch variation, but little or no runout.
In fact, runout affects, every other characteristic of gear quality, such as involute or tooth form, index or pitch variation, lead or tooth alignment variation, and noise and vibration. It is quite common for one to have problems trying to meet specifications for index or pitch variation when the cause is actually runout. The various measures of gear quality are not independent parameters. They are influenced by runout.
This type of error is caused by the deviation of the actual advance of the gear tooth profile from the ideal value or position. This error results in poor contact between the mating teeth, resulting in loss of power.
Errors in assembly may be due to either the centre distance error or the axes alignment error. An error in centre distance between the two engaging gears results in either backlash error or jamming of gears if the distance is too little. In addition, the axes of the two gears must be parallel to each other, failing which misalignment will be a major problem.