About Gear, Treatment & Material
About Gear
A gear is a mechanism used to transmit force from one part of a machine to another. Gears are present in virtually all machines that spinning parts, like car engines and watches. The origin of gears predates Roman Civillization. Presently, gear enjoy widespread use in industries from aerospce to toy manufacturing. These versatile and powerful components come in a veriety of forms to suit every mechanical need.
Gear Basic
Gears are used to reverse rotational reduction direction, increase or decrease speed of rotation, transfer rotation to a different axis, or synchronize rotation across two or more axis in a machine or engine. A key benefit of gears is their ability to utilize the mechanical principles of to turn speed into power, otherwise referred to reduction. Gears are used in what is called a group of two or more gears that work together produce torque. The basic gear train is comprisedfof gears, one large and one small. They rotate in directions from one another, with their teeth and driving the rotation. A gear in a train is a driver (the gear that effects the rotation), (the gear last in the train), or an adler (any gear between the first and the last gear). The power output or produced by a gear train is determined by the gear ratios and the by output direction (which gear drives which).
Gear Design
In order to maintain precise gear ratios and output speeds, gears have specially design cogs. The most popular design used to achieve this is to involute profile. It uses the mathematical concept of the involute curve to space gear cogs in a manner that efficiently keeps gear rotation perpendicular to the flat plane.
Gear Types
Here is a brief overview of the most popular gear types :
> Spur Gear : characterized by their straight cogs, these gears are mounted on parallel shaft. You’ll find examples of spur gear trains in wathes and clocks.
> Bevel Gear : Characterizied by cogs cut in a cone shape. The gear shafts are generally mounted at 90 deg angles to each other.
> Worm Gear & Wheel : a gear comprised of a whell gear with inclined cogs that is rotated by a screw thread (a single-cogged gear called a worm).
> Helical gear : Characterized by cogs that are cut at an angle to the face of the gear. Can be mounted perpendicularly or parallel. Automobile transmissions make use of the gears.
> Internal Gear : an inside out gear. A small gear is set within a larger gear that has its cogs on the this setup protects the cogs of the gears from contaminants.
> Rack & Pinion gear : are essentially a linear shaped variation of spur gears The spur rack is a portion of a spur gear with an infinite radius.
Heat Treating
Fundamentally, heat treating is a process which changes the microstructure of steel in order to achieve an even distribution of carbide crystals throughout the base structure. Carbide starts out in “large globs” which are diffused into smaller crystal groupings when the part is heated to a high temperature. Quenching influid immediately after heating couses a very rapid reduction of temperature which “freezes” the carbide crystalsin a well distributed condition. After quenching, heat treated parts should be temepered (reheated to a lower temperature and slow cooled) to increase ductility (reduce brittleness) by the lowering the post-quench hradness a few points. The resulting structure of small, evenly distributed grains gives heat treated stell is hardness. There are three processes which are used to do the majority of sprocket and gear hardening – flame, induction & carburizing.
> Flame Hardening, apart is simply with any gas torch antil is red hot, then it is quenched, usually in water. This process is dificult to do well and is probably the easiest to do poorly.
> Induction Hardening is a good general propose heat treating method which requires the use of medium carbon steel (.030 – .050 C) Parts are heated by placing the target hardening area (usually the teeth in this context) in close proximity to a cooper coil which surrounds the entire part, or a pointed tip the shape of one tooth space in the case of the larger parts. An electrical current is run through the coil or tip and the magnetic resistance between the work piece and the coil or tip causes the work to heat up at a very rapid pace. When the work reaches its hardening temperature it is then quenched. Part distorsion are similar to or slightly better than that of flame hardening.
> The Carburizing method of heat traeting results in the highest hardenesess of the three methods mentioned here, typically R/C 57 ~ 63, but the depth to which parts can be hardened is less than with either flame or induction hardening (.030 ~ .060 inches as opposed to .100 ~ .200). This method utilized low carbon steel (0.10 ~ .030 C). The part is heated in a carbon rich athmosphere, usually a furnace filled with a gas high in carbon content. As the part heats up it absorb carbon into the exposed surface areas After enaough time has passed for the part to have absorbed an appropriate amount of carbon, and to have reached its hardening temperature, it is quenched. Unlike flame or industion hardening, parts are hardened all over rather than strictly on certain target areas such as the teeth of a sprocket or gear . Because the entire part is heated to a high temperature the distortion rates seen with carburizing are higher than with other method. This can cause problems for gearing which requires highly accurate tooth forms, thus resulting in the need for post-heat treat finishing such as grinding of teeth or other critical surfaces such as bores or bearing journals.
Gear Materials :
In order for gears to achieve their intended performance, life and reability, the selection of suitable gear material is very important.
1. Steel Gears, are divided into carbon steel & alloy steel. Carbon steel offers low cost and can be hardened. A major disadvantage is the lack of resistance to corrosion.
Alloy steel is elements like stainless steel, and element other than carbon. They offer high strength and a wide range of heat treatment properties. they provide highest strength & durabillity. Stainless steel that is non-magnetic and has good corrosion resistance; that can be easily hardenend by heat, is magnetic and have reasonable corrosion resistance. Alloy steel are used in industrial field.
While making gears many times few minerals are added to steel. These minerals are Nickel ( increases hardness & strength), Chromium ( increases hardness & strength but the loss of dictility is greater.It refines the grain and imparts a greater depth of hardness. It has high degree of wear resistance), Manganese (it gives greater strength and a high degree of toughness than chromium), Vanadium (the hardness penetration is greatest. The loss ducility is also more than any other alloys), Molybdenum (increases strength without affecting the ducility), Chrome-Nickel Steels (the combination of the two alloying element adds the beneficial qualities of both).
2. Cast Iron Gears, the gears made of cast iron have low cost of manufacture, are easy to machine with high dumping. Cast iron has good machining characteristics like dry cutting, better dimensional stability, longer cutter life, and superb surface finished. Cast iron gears are made by heating a blanket of cast iron and having a tooth forming part of such a temperature that compatible to the blanket. Cast Iron has a good strength and has ability to obtain hardness. Gears made of cast iron have some weaknesses also. The casting is subjected to internal weakness frominclusions or blowholes that cannot be detected except by ultra sonic inspection. Cast Iron Gears are widely used in grinding mills, sugarcane crushers, refining machine, filter press, power mills, and other.
3. Plastic Gears, are made of inexpensive commodity plastic are use in low power drives in products. Plastic Gears have greater consistency and are easily molded to various shapes. They are less expensive and are lighter in weight. Plastic gear is chemical and corrosion resistant and have quieter and smoother operation. They deflect to compensate for ineccuracies by absorbing tiny impacts of small tooth errors and gear misalignment. They more efficient drive geometry. Plastic gear retios are feasible to allow for wider gears that transfer more power in single stage, there by increasing load capacity. The most common Plastic materials used for making gears are acetal, polybutylene terepthalate (PBT), nylon, polyphenylene sulfide (PPS), and lyquid crystal polymer (LCP).
Technical Plastic/Polymer data
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