This article contains information regarding spur gears, their use, and their benefits.
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A spur gear is a cylindrical toothed gear with teeth that are parallel to the shaft and is used to transfer mechanical motion and control speed, power, and torque between shafts. They are the most popular types of cylindrical gears and have a simple design of straight parallel teeth positioned equally around the circumference of the cylinder barrel.
There are several different designs of spur gears that change according to the shape and thickness of the gear hub, which changes without changing the face of the gear.
The popularity of spur gears is due to their use in mechanical applications where they increase or decrease a device’s speed or multiply torque by transferring motion and power from one shaft to another. Spur gears are normally mated in a series to change motion, speed, and torque quickly and efficiently.
The performance of a spur gear is determined by its design, construction, and materials. An essential part of developing a spur gear is how they are fabricated, which includes the use of high quality materials and exacting and precise dimensional compliance that is necessary to determine a spur gear’s function.
Although there is a wide range of spur gears from very small ones to ones that operate conveying systems and motors, there are factors that are common to all spur gears, which are:
Pitch Circle: The pitch circle is the distance from the face of one tooth to the next tooth. It divides a gear's teeth into the top of a tooth, the addendum, and the bottom of the tooth, the dedendum. If a gear system is designed correctly, pitch circles between gears will be tangent to one another.
Diametral Pitch: The diametral pitch is a function of a gear’s pitch circle and is equal to the number of teeth per inch. It is used to determine what size and type of gear are needed to interlock with another gear.
Pitch Diameter: Pitch diameter is the outer diameter of a gear as if a circle was drawn around the midpoint length of each tooth of a gear. It is used to determine the distance between mating gears, as seen in the diagram below.
Center Distance: The center distance is the distance between the centers of two meshed gears and is the sum of the pitch diameters divided by two.
Spur Gear Addendum: The addendum is the height of the teeth of a gear from the pitch circle to the top of the teeth.
Gear Dedendum: The dedendum is the depth of a tooth below the pitch circle and is greater than the addendum for clearance.
Outside Diameter (OD): The outside diameter is as if a circle were drawn around a spur gear and touched the tops of the teeth. Its diameter is the distance to the outermost points of a spur gear’s teeth across the center of the gear.
Root Diameter: The root diameter is the diameter of a circle that could be drawn around a gear and coincides with the bottoms of all teeth. It is a diameter that could be drawn across that circle.
Pressure Angle: The pressure angle is the angle between the pitch line and the pressure line where the pressure line is tangent to the pitch point and is normal to the tooth surface. A diagram of it can be seen in the image below.
Whole Depth: The whole depth is the sum of the addendum and dedendum.
Module: The module measures the tooth size, which is normally measured in millimeters. When teeth have the same size and module, they can be easily paired or meshed. The sizes of gear teeth are indicated by the symbol “m” such as 1m, 2m, and 4m, where the sizes get larger as the numeric value increases. The module of a gear can be found by dividing the pitch of a gear by pi (π).
In the examples below, the left gear has a standard tooth profile while the gear in the center is a high pressure angle gear. The gear on the right is a high module gear.
The configuration of an external spur gear is typical of all types of gears with the teeth of the gear on the cylinder’s surface. When it meshes with another gear, the gears will rotate in the opposite direction. The drive gear is normally smaller than the driven gear.
The teeth for an internal spur gear are cut on the inner surface of the gear. The external surface is smooth in the shape of a perfect oval or circle. The inner gear teeth mesh with a pinion or smaller gear. The gears rotate in the same direction.
A type A spur gear does not have a hub and is flat with a small hole in the center. The circle of the root diameter is solid with the small hole placed in its center.
A pin hub spur gear has a set screw used to secure a pin on the gear to the shaft. They can be connected to a shaft using a dowel, spring, roll, or taper pins. The hole in the gear is drilled to the exact size of the pin for a tight connection. The set screw used to pin the gear to the shaft is removed to avoid loosening and falling into the mechanism. The difference in the various types of pin hub spur gears is the size of the hole where the gear connects to the shaft.
With a keyway spur gear, a slot is cut in the gear’s bore that fits the slot milled into the shaft. The keyway allows the shaft and gear to fit snugly together with the proper alignment of the slots. The purpose of the design is to eliminate slippage between the shaft and spur gear.
With a spline spur gear, ridges or teeth are cut into the drive shaft that matches grooves cut in the bore of the spur gear. There are different varieties of spline configurations with some straight like a keyway or involute like gear teeth. The spacing between the splines varies according to the design of the gear. Involute spline spur gears can have any number of teeth up to 32.
The split hub has a split along its axial plane, which allows it to be attached to any shaft by having the split hub tightened with a clamp to snuggly and securely attach it to the shaft. It is one of the easiest spur gears to assemble and disassemble. Split hub spur gears can adjust and configure their position on the shaft with little effort. The drawback to split hub spur gears is their size since they tend to take up more space.
Set screw spur gears can have a keyed or round bore with a set screw in their hub that attaches them to the shaft through the hub. They work like pin hub spur gears, except that the set screw is not removed. Set screw spur gears allow for a certain amount of flexibility in placing the gear and provide a secure tight fit.
A rack and pinion spur gear set aims to change rotational motion into linear motion. The pinion of a rack and pinion spur gear set is a bar of metal with gear teeth. The spur gear is the drive gear of the set and is referred to as the rack gear. The benefits of a rack and pinion spur gear set are their simplicity, large load capacity, and no limit to their length.
The positive characteristics of plastic gears are their lightweight, high rust resistance, silent operation, low cost, and the ability to operate without lubrication. They are very durable, strong, and reliable and can easily be interchanged. Plastic spur gears are the most common form of plastic gear. The main use of plastic spur gears is in low demand applications as an alternative to metal gears.
There are several methods used to produce spur gears, which include forging, blank machining, cutting, casting, powder metallurgy, and computer numeric controlled (CNC) manufacturing to name a few. Regardless of the process, dimensional accuracy and adherence to tolerances is critical since the slightest error can prevent the proper meshing between gears. In cases where exceptional performance is required, spur gears are forged, cut, or machined.
Spur gears are one of the oldest forms of equipment and have existed since the time of the Greeks and Romans. The use of more sophisticated designs began in the 17th Century when the first attempts were made to calculate velocity ratios of involute gears. During the first industrial revolution, form and rotating cutters were introduced, which was followed by the invention of the hobbing process.
Regardless of the cutting process, it begins with a gear blank. The quality of a spur gear depends on the type of material from which the gear blank is made. High quality spur gears can only be made from high quality materials, which is a pie shaped billet.
The milling process can produce all types of gears beyond spur gears. It involves using a milling machine that has a form cutter that passes through the gear blank to cut the tooth gap. In form milling, the workpiece is mounted such that the cutter can move perpendicular along the axial length of the tooth at the correct depth. At the end of each cut, the cutter is withdrawn, and the gear blank rotates for the next cut. The cutter has the shape of the space between the teeth.
In the hobbing process, the gear blank and the hob rotate simultaneously in a continuous motion such that they mesh as the teeth are being cut. The tool or hob is fed inward until it reaches the proper depth and has equally spaced cutting edges. Hobbing is a fast and accurate process.
The gear blank rotates on its vertical axis as the hob cuts horizontally. The gear is created by cutting the facets of each tooth such that it is like the gashes of the hob, which produces a far more precise profile and tolerance.
The gear shaping process can be used to create internal spur gears and has a cutting tool that is a gear with cutting edges that moves axially against the inner diameter of the workpiece. The cutting gear rotates at the same velocity ratio as the gear being made. The motion of the cutting tool is repeated several times until the proper depth is reached for the gear teeth. It is a slow process that involves rotating the tool and workpiece as the tool continues its oscillating motion.
Gear shaping can produce all of the different varieties of spur gears with high surface finishes internally and externally. The shaping tool is similar to a cylindrical gear or rack type gear.
In the stamping process, a sheet of metal is placed between the top and bottom dies where the upper die is pressed down on the lower die cutting out the gear from the sheet. It is a low cost efficient method for producing lightweight spur gears for medium duty with no loads. Stamping is restricted by the thickness of the metal, which ranges in thickness from 0.25 mm or 0.010 inches to 3 mm or 0.125 inches.
Cold drawing is capable of forming different types of tooth configurations. In the cold drawing process, a bar of metal is pulled, drawn, pushed, or extruded through a series of dies with the final die having the required shape of the teeth of the gear. As the rod passes through the dies, it is squeezed to the shape of each die. The pressure of the process places greater stress on the surface of the bar.
Once the bars have passed through the dies, they are called pinion rods, which are put into screw machines to finish the individual gears. The cold working process increases the strength of the gear and reduces its ductility. The process may require several dies aligned in a series to achieve the desired shape.
The roll forming of spur gears requires several rotations of the forming rolls where the rolls are fed slowly during several revolutions. Top lands of roll formed teeth may not be smooth or perfect in shape. Since the top land does not affect the action of the teeth, this deformity does not cause any difficulty.
Spur gears that are roll formed must have at least 18 teeth or more since spur gears with fewer teeth perform badly.
The main use of forging in the manufacture of spur gears is the production of gear blanks for the cutting process. The process includes an open die, closed die, and hot upset forgings. Aside from the production of blanks, forging is also used to produce precision forged spur gears that require little to no finishing.
Precision forging of gears uses closed die hot and cold forging. In the majority of cases, spur gears are shaped with one blow with the velocity of the ram supplying the major forging force. Pancaking is used to produce spur gears, which causes lateral flow in the die. Of the various types of gears, spur gears are the easiest to forge. Low alloy steel, brass, aluminum alloys, stainless steel, and titanium are the metals used in the forging process.
The goal of precision forging is to produce gears at near net shape, which means that the gears will not need finishing after being forged. With hot forging, there is very little waste and involves casting or injection molding. Compared to cut gears, forged spur gears have greater load carrying capacity than cut gears. The grain flow in forged spur gears follows the contour of the teeth.
The processes described above are a few of the methods that are used to manufacture spur gears. The choice of how a gear is made is dependent on several factors including the type of material, time involved, use of the spur gear, and the manufacturer. What has not been included is the production of plastic gears, which can be molded, cast, or extruded.
The strength, endurance, and performance of spur gears are dependent on their construction. As with all products, design is important but relies on the quality of the materials to produce a product. Nearly every form of material used in the manufacture of other products can be used to make spur gears, including steel, brass, plastics, aluminum alloys, grades of stainless steel, and titanium. Of the wide array of choices, hardened steel is the most commonly used since it can be honed to prevent premature wear.
Each type of metal used to make spur gears has a unique and distinct purpose. Plastic spur gears are noiseless while steel and stainless steel spur gears are more durable and long lasting. The choice of materials is based on the torque the gear will be sending. In those instances, plastic gears are not a choice since they are not desirable for high torque applications.
There are several types of plastics used to produce plastic spur gears, including polyacetal (POM), nylon, polyethylene (UPE), and polyetheretherketone (PEEK), a ketone polymer. Plastic spur gears are lightweight, rust resistant, inexpensive to produce, and can operate without producing friction. They are widely used in food production, electronics, toys, and medical instruments.
Polyacetal (POM) – POM is a very strong plastic commonly used to produce spur gears. It can be easily molded, shaped, and formed. Once it hardens, POM becomes extremely stiff, strong, and abrasion resistant. The malleability and resilience of POM make it an ideal material for spur gear manufacturing.
Cast iron, like POM, can be molded into any shape and is resistant to rust. The composition of cast iron involves the use of different ingredients each of which gives cast iron a different degree of strength and durability. Cast iron is commonly used to produce machine parts because of its low cost, rust resistance, and ability to be easily molded and shaped. It can be unbelievably strong or very weak, depending on the types of added mixtures.
Stainless steel comes in several levels, grades, and characteristics, with all types having at least 11% chromium. Other alloys include nickel, manganese, silicon, phosphorus, sulfur, and nitrogen. Ferritic stainless steels are magnetic, while austenitic stainless steels are nonmagnetic, martensitic, and precipitation hardened. Austenitic stainless steels are in the 300 series of stainless steels, while ferritic stainless steels are in the 400 series.
The stainless steel that is used the most is 304, which contains 18% chromium and 8% nickel. The most common stainless steel used for manufacturing spur gears is grade 303, which has a chromium content of 17% and a 1% sulfur content. The slight addition of sulfur makes 303 machinable.
When an application needs corrosion resistance, grade 316 is normally the first choice. It has 16% chromium, 10% nickel, and 2% molybdenum. Grades 303 and 316 are the most common stainless steels used to manufacture spur gears.
Steel is an alloy of iron, carbon, and various other elements. There are four major steel types: carbon, alloy, stainless, and tool steel. Of the various types, carbon steel is used the most to manufacture spur gears because it is easy to machine, is wear resistant, able to be hardened, available, and inexpensive.
Carbon steels are categorized as mild, medium carbon, and high carbon, with mild carbon steel having less than 0.3% carbon content. Each of these types of steel are good for the manufacture of spur gears.
Induction hardening or laser hardening are used to harden carbon steel to a Rockwell Hardness scale of 55. The addition of various alloys to carbon steel makes it stronger, easier to machine and creates corrosion resistance. All of the types of alloyed carbon steel are used to manufacture spur gears.
Aluminum alloys are used to make spur gears for applications that need a high strength to weight ratio since aluminum is one third the weight of steel and has a surface that is resistant to corrosion. It is more expensive than steel but less expensive than stainless steel. Aluminum alloys are easy to machine, which offsets its higher cost.
Typical aluminum alloys used to produce spur gears are 2024, 6061, and 7075. Aluminum alloy 2024 is a close relative to bronze since it is made up of aluminum and copper. The copper in 2024 gives it strength but lowers its corrosion resistance. Alloy 7075 has zinc and magnesium alloyed with aluminum, which gives the aluminum increased strength and resistance to load stress.
Aluminum alloys 2024, 6061, and 7075 can be heated to improve their hardness. They are commonly used in the manufacture of spur gears.
Spur gears are the most common form of gear and are found in a wide range of applications. They perform several important functions, the most critical of which is providing gear reduction to mechanical motorized devices. Once a spur gear is mounted on a parallel shaft, it meshes perfectly with other gears.
Since the teeth of a spur gear are parallel to the rotational axis, they do not produce axial thrust, which makes it easy to mount them with ball bearings. They are cost effective, and precision engineered products that are easy to use and install.
Spur gears are used to increase or decrease the torque or power on a device and are found in washing machines, blenders, clothes dryers, construction equipment, pumps, and conveyors. In power stations, groups of connected spur gears, or trains, are used to convert energy, such as wind or hydroelectric power, into electrical power. Spur gears in a train have the same sized teeth with adjacent gears rotating in opposite directions.
In applications that require the increase or decrease of speed, spur gears are an ideal solution. They transfer motion and power from one shaft to another, which alters the operating speed of machinery, multiplies torque, and allows for fine tuned control of positioning systems. It is for this reason that they are found in clocks to adjust the speed of second, minute, and hour hands.
Since spur gears control the speed of applications, they are used in washing machines to control the rotating motion of a washing machine. Depending on what cycle the machine is performing, spur gears assist in increasing or decreasing torque.
In a road roller, a set of spur gears changes the fast rotational speed of the engine into a slow rotational speed for the wheels. This change makes it possible for a road roller to be able to move its heavy roller.
In a sports car, a smaller rotational force is needed for the wheels because a sports car is very light. Spur gears used to move a sports car also change the engine's speed into a slower rotational speed for the wheels. The wheels are able to turn faster with a smaller turning force.
Conveyor systems have to move at a controlled speed, producing high torque. Spur gears are used as a reliable means for altering the torque of the system. In some types, a self-locking worm gear is used to power and move the spur gear, where the worm gear is the drive gear.
In tuning a radio, precision is necessary to hit the correct position on the dial. The accuracy and precision of spur gears allow them to perfectly tune a radio to the chosen channel.
The calculation of the torque and speed of a system is essential when selecting the proper spur gear. This refers to the input speed and input torque and the output speed and output torque.
Pitch is an identifier for spur gears and includes the diametrical pitch, circular pitch, and module. In the United States, the designation of pitch is the diametrical pitch (DP), which is infinite. To make choices easier, a specific set of DPs has been developed.
Gear teeth geometry is determined by the pitch, depth of the teeth, and pressure angle. A complex set of formulas are required to make the teeth geometry calculation.
Idler gears are used to change the directional rotation of the output gear. When designing spur gears, it is necessary to know whether an idler gear is required.
Gear trains are used to pass power between shafts and are a series of spur gears with similar gear shapes and teeth.
Several forms of stress calculations must be made regarding a spur gear system. The purpose of the calculations is to ensure the safety and strength of the system.
The type of material determines the longevity of a spur gear and its strength, reliability, and endurance. This aspect of the selection process is normally the first consideration and is carefully planned by designers and engineers.
Spur gears are one of the most popular types of gears and are used in an endless number of applications, from simple toys to complex industrial machinery. They have a simple straight tool design that can be configured and shaped to fit the needs of any application. The parallel teeth are placed an equal distance apart around the circumference of a cylinder body with a core that fits over a shaft.
The simple compact design of spur gears makes them easy to create, design, and configure. They can fit easily into restricted and tight spaces with few limitations.
This aspect of spur gears is one of the major reasons for their wide usage. Spur gears increase and decrease speed with exceptional precision and accuracy at a constant velocity.
It is very unusual and unlikely for a spur gear to fail during use. Their durability and strength make it next to impossible for them to slip, break, or malfunction.
This is another factor that has made spur gears so indispensable. Their simplicity makes it easier to manufacture them, which greatly decreases their production cost. Large volumes of spur gears can rapidly be produced with little waste.
Efficiency works in conjunction with reliability. During the useful life of a spur gear system, the gears are able to transfer large amounts of power across several gear trains without any or minimal loss of power.
The straight teeth of spur gears eliminates the likelihood of axial thrust since power is transmitted in a straight line at the pitch angle of a spur gear’s teeth.
A spur gear is a cylindrical toothed gear with teeth parallel to the shaft and is used to transfer mechanical motion as well as control speed, power, and torque between shafts.
The performance of a spur gear is determined by its design, construction, and materials. An essential part of the development of a spur gear is how they are fabricated, which includes the use of high quality materials and exacting and precise dimensional compliance that is used to determine a spur gear’s function.
There are several methods used to produce spur gears, which include forging, blank machining, cutting, casting, powder metallurgy, and computer numeric controlled manufacturing to name a few. Regardless of the process, dimensional accuracy and adherence to tolerances is critical since the slightest error can prevent the proper meshing between gears. In cases where exceptional performance is required, spur gears are forged, cut, or machined.
Nearly every form of material used in the manufacture of other products can be used to make spur gears, including steel, brass, plastics, aluminum alloys, grades of stainless steel, and titanium. Of the wide array of choices, hardened steel is the most commonly used since it is honed to prevent premature wear.
Spur gears are one of the most popular types of gears and are used in an endless number of applications, from simple toys to complex industrial machinery. They have a simple straight tool design that can be configured and shaped to fit the needs of any application.
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