This article will present an in-depth discussion on plastic gears.
The article will provide more detail on topics such as:
This section will cover an overview of plastic gears, including their production methods and operational mechanics.
A plastic gear is a toothed wheel crafted from specialized engineering plastics, designed to adjust the speed relationship between an engine and its driven components. Common engineering plastics used for these gears include nylon, a type of polyamide resin, and polyacetal.
The production of plastic gears mirrors the process used for metal gears, with the primary distinction being the material involved. Each gear type is crafted to suit specific application requirements, which influences its manufacturing process. Unlike metal, plastic materials lack the same level of strength and are highly sensitive to temperature fluctuations, affecting their strength and stiffness. Plastic gears also exhibit a lower elastic modulus and mesh stiffness, leading to greater deflection under load.
Despite the similarities between metal and plastic gears, the engineering and design of plastic gears involve additional considerations specific to their material properties.
Hobbing uses a hob, a cylindrical tool with teeth on its surface. During the process, a flat plastic disk is positioned next to the rotating hob. As the hob turns, it interacts with the plastic disk, carving gear teeth into its side. By adjusting the contact angle of the hob, various types of gears can be produced.
Gear hobbing is a fast, continuous process that is economical due to the speed at which gears can be cut and processed. With exceptional accuracy, it produces spur gears, helical gears, splines, worm wheels, and sprockets.
Injection molding involves the use of a steel mold with the shape and dimensions of the gear. The key to the process is the accuracy of the mold since it determines the final shape of the gear. Molten plastic is forced into the mold under pressure such that the molten plastic reaches every part of the mold cavity. It is an expensive process designed for high-production runs.
CNC machining is a versatile cutting method capable of creating any type of gear. This process involves removing layers of plastic material from a disc or rod using various tools controlled by G codes that direct the tools' movements. Although CNC machining provides high precision, it is time-consuming and more costly due to its limited gear production capacity per run.
This method is particularly suited for manufacturing gears with precise shapes and tight tolerances. CNC machining also incorporates broaching, where a broaching tool is employed to shape the teeth of the gear. The broaching operation can be programmed into the CNC machine, enhancing the process's efficiency and precision.
For smaller production runs, machining is a faster process. However, when dealing with large volumes of plastic parts over extended periods, injection molding becomes a more effective option. Once the mold is created, injection molding can scale up production rapidly. Despite the initial time and cost required to create the mold, the efficiency and speed of the injection molding process can justify the upfront investment.
Machining tends to be more cost-effective for small production runs, typically involving only a few hundred parts. As production volume rises, machining costs also increase. On the other hand, injection molding offers a lower cost per part for large production volumes. Although the initial expenses for mold creation are higher, these costs are spread out over time as the number of parts produced increases, making injection molding more economical for large-scale manufacturing.
Machining provides a wide range of raw material options, allowing for the creation of parts from high-performance plastics that offer superior structural strength and durability. This makes machining ideal for working with harder materials. In contrast, injection molding is limited to softer plastics such as thermoplastics and thermoset resins, which can be melted and molded without compromising material integrity. Injection molding is best suited for producing parts from flexible and pliable materials.
Machining emphasizes product specifications, ensuring that the final products achieve high precision, accuracy, and tight tolerances. The control exercised by a computer in machining is more straightforward, with fewer variables affecting the outcome. In contrast, injection molding can introduce a wider range of defects, such as flow lines, vacuum voids, warping, and burn marks. This is due to the fact that injection molding focuses on the tolerances of the mold rather than the final part.
Injection molding provides excellent repeatability, which is advantageous for producing multiple parts with the same design. However, if design specifications change, creating a new mold incurs a significant upfront cost, making the process less cost-effective for design alterations. Conversely, machining allows for greater flexibility, as the CAD program can be easily adjusted to accommodate design changes without substantial additional costs.
There is a variety of machines available for plastic gear production, which are crucial in modern industries due to the benefits plastic gears offer, such as cost-efficiency, design adaptability, noise reduction, and resistance to corrosion. These advantages make plastic gears popular across diverse sectors including automotive, milling, and mining. Below is an overview of some prominent manufacturers of machines used for plastic gear manufacturing in the United States and Canada.
Arburg provides a variety of injection molding machines suitable for producing plastic gears, with the Arburg Allrounder series being particularly notable. These machines are renowned for their flexibility, accuracy, and sophisticated control systems. Key features include high repeatability, efficient energy use, and precise temperature regulation, all of which contribute to the production of high-quality plastic gears. The Allrounder series is favored for its capability to accommodate diverse production needs, enhance cycle times, and consistently deliver reliable outcomes in plastic gear manufacturing.
Engel is a well-known manufacturer of injection molding machines ideal for producing plastic gears, with the Engel Victory series standing out in this area. This series is celebrated for its precision, energy efficiency, and advanced capabilities tailored to plastic gear production. The Victory series features servo-electric drives that provide superior control and energy savings, ensuring accurate and repeatable molding processes. Additionally, the incorporation of multi-component technology allows for the creation of gears with intricate designs or multiple materials. These attributes make the Engel Victory series a popular choice for applications in plastic gear manufacturing.
Sumitomo (SHI) Demag offers injection molding machines that are well-suited for plastic gear production, with the Sumitomo (SHI) Demag IntElect Multi series being a key model in this domain. The IntElect Multi series is designed for multi-component and overmolding applications, making it ideal for producing plastic gears. These machines are recognized for their high-speed operation and precise capabilities, which facilitate the creation of detailed and accurate plastic gears. The IntElect Multi series also features advanced automation and provides flexibility in handling various material combinations and mold configurations. This versatility, coupled with high-performance and advanced automation features, enhances the efficiency and reliability of plastic gear production, contributing to the popularity of the Sumitomo (SHI) Demag IntElect Multi series.
Milacron provides a selection of injection molding machines ideal for producing plastic gears, with the Milacron Elektron Multi-Shot series being a notable example. The Elektron Multi-Shot series is tailored for high-performance multi-shot molding, making it particularly effective for plastic gear production. These machines are known for their precise control, rapid cycle times, and consistent repeatability, which enhance the efficiency and cost-effectiveness of gear manufacturing. Advanced features like servo-electric drives and accurate shot control systems enable the creation of complex gear designs with multiple materials or colors. The blend of precision, speed, and versatility of the Milacron Elektron Multi-Shot series makes it a popular choice for plastic gear manufacturing.
Nissei produces injection molding machines suitable for plastic gear production, with the NEX Series being a notable example. The NEX Series is distinguished by its precision, dependability, and advanced technology, making it an excellent choice for producing high-quality plastic gears. These machines leverage Nissei's expertise in injection molding, offering high repeatability, energy efficiency, and ease of use.
These manufacturers are prominent in the industry and are known for providing high-quality machines for plastic gear production. For specific model details, unique features, and the most current information, it is advisable to contact the manufacturers directly or review their product catalogs.
In the US, approximately 80% of gears are chosen from catalogs. While this approach is common, the optimal method is to collaborate with the customer’s engineers to design gears that address specific issues or design challenges, such as inertia or contamination. For instance, open gearing in industries like paper, food, or semiconductor processing can introduce substantial contaminants.
To engineer the ideal solution for an application, it is crucial to consider various operational parameters, including torque, RPMs, shock loads, backlash requirements, inertia, chemical exposure, and operational temperatures. Modifications to gear teeth or increasing gear width may be necessary to ensure that a plastic gear performs effectively in its intended application.
The face width of a gear tooth refers to the width of the tooth's top surface that is parallel to the gear's axis. Increasing the face width enhances the gear's bending and surface strength, contributing to its overall durability.
It is essential for the face width of a gear to be smaller than the space where it engages with another gear, a condition known as effective face width. This should not be confused with the face of a gear tooth, which is the surface above the pitch surface. Remember, the face width is specifically the top portion of the gear tooth that interfaces with the space between the teeth of a mating gear.
The size of a gear is represented by its module, which defines the gear's scale. The module is calculated as the ratio of the gear's diameter to the number of teeth. When selecting a gear, both the module and the number of teeth are crucial factors that determine the appropriate gear for the application.
The pressure angle of a gear is defined by the angle between two tangent lines that intersect at the inner circle of the gear and the top arc of a tooth. Traditionally, a pressure angle of 14.5° was the standard; however, advancements in technology have led to adjustments in this standard. A larger pressure angle results in larger and stronger gear teeth, enhancing the gear's durability and performance.
Backlash is the space between the meshing teeth of gears. It occurs when there is a slight gap, allowing movement or play. During gear design, it is possible to minimize backlash to ensure a tighter mesh between the teeth, which improves precision and efficiency in gear operation.
Temperature is a critical factor affecting the performance of plastic gears. Selecting appropriate materials during the design phase helps address temperature concerns. However, accurately predicting the working temperature of a gear can be challenging. It's beneficial to monitor gear operation to ensure that the temperature remains within safe limits. Many thermoplastics can function at temperatures up to 500 °F (260 °C).
The root, flank, and wear strength are crucial parameters for estimating gear life. These parameters, including root pulsating strength and flank strength, are influenced by the number of load cycles and vary depending on temperature and lubrication type (oil, grease, or dry running). For plastic gears, these factors significantly impact performance, whereas a single value is typically sufficient for steel gears.
Comparatively, Young's Modulus for plastic is approximately two magnitudes lower than for steel, with POM around 2800 N/mm² versus 206,000 N/mm² for steel. Similarly, the permissible bending stress for plastic is about one magnitude lower: 25 N/mm² compared to 250-450 N/mm² for steel. As a result, metal gears experience less relative deformation compared to plastic gears.
Despite this, plastic gears excel in shock absorption compared to metal gears. They often feature a mechanical stop design to minimize wear and tear on both the gear teeth and the motor, leading to longer wear periods and extended life expectancy. Metal gears tend to wear down more quickly than plastic ones.
Plastic gears typically do not require backlash adjustments like metal gears do. Most steel gears need regular backlash adjustments to counteract issues caused by gear and motor vibrations. In contrast, plastic gears can absorb these vibrations, which often extends their operational lifespan.
For gears like PowerCore, the size of injection-molded gears is constrained by the press size, typically up to about 5” to 6”. Unlike catalog gears, custom-designed gears developed in collaboration with engineers address specific challenges such as inertia or grease contamination, particularly in applications like paper, food, or semiconductor processing equipment.
Engineering a solution involves evaluating all operational parameters, including torque, RPM, shock load, backlash requirements, inertia, chemical exposure (especially in semiconductor applications), and operating temperature. Based on these factors, alternative solutions such as tooth modifications or increased gear width can be proposed to ensure the plastic gear performs effectively in the application.
Calculating gear life is a critical component of the engineering process. This calculation predicts the number of hours a gear will operate under specified conditions. Conversely, gears can be designed to meet specific longevity requirements, such as the five-year lifespan required by medical equipment manufacturers. Over the past two decades, this calculation has proven to be a reliable tool for assessing whether a plastic gear will be suitable for a given application, particularly when replacing metal gears.
| Input Data | ||
|---|---|---|
| z1 | 21 | Number Teeth Pinion |
| z2 | 210 | NUmber Teeth Gear |
| n1 | 1000 | RPM Pinion |
| n2 | 100 | RPM Gear |
| DP | 10 | Diametral Pitch |
| b | 1" | Face Width |
| g | 20° degrees | Pressure Angle |
| t | 65°C | Operating Temperature |
| P | 0.75 hp | Transmitted Power |
| Cs | 1.10 | Shock Load Factor |
| Geometry Data Output | ||
|---|---|---|
| dw1 | 2.10° | Pitch Pinion Diamter |
| dw2 | 21.00° | Pitch Diamter Gear |
| dk1 | 2.30° | Outside Diameter Pinion |
| dk2 | 21.20° | Outside Diameter Gear |
| Hk1 | 0.10° | Addendum Pinion |
| Hk2 | 0.10° | Addendum Gear |
| ea | 1.74 | Contact Ratio |
| i | 10.00 | Transmission Ratio |
| a | 11.55 | Center Distance |
| Operational Data Output | ||
|---|---|---|
| V | 2.79 m/sec | Pitch Line Velocity |
| T1 | 3.9 ft lbs | Torque at Pinion Shaft |
| T2 | 39.4 ft lbs | Torque at Gear Shaft |
| Load and Safety Data Output | ||
|---|---|---|
| Sb2 | 4.15 | Gear Tooth Root Stress Safety |
| Sg2 | 2.34 | Gear Tooth Flank Pressure Safety |
| SigmaW2 | 6.0 N/mm2 | Gear Tooth Root Stress |
| Kw2 | 0.6 N/mm2 | Gear Tooth Flank Pressure |
One of the advantages of using plastic for gear manufacturing is the broad range of polymers available. This includes various specialized polymer blends designed to enhance the strength and durability of plastic gears. Typically, crystalline plastics are favored due to their superior strength, durability, and resilience.
Polyoxymethylene (POM), also referred to as acetal, polyacetal, or polyformaldehyde, is a semicrystalline thermoplastic known for its suitability in creating precision components that demand stiffness, low friction, and dimensional stability. Produced through the polymerization of formaldehyde, POM is available in two main forms: homopolymer and copolymer, each offering distinct properties. Additionally, there are six different grades of polyacetal, each tailored with specific characteristics, though all share fundamental attributes of strength and dimensional stability.
PPS, or polyphenylene sulfide, is highly regarded as a high-performance thermoplastic due to its excellent temperature resistance and dimensional stability. Like polyacetal, PPS is a rigid, opaque semicrystalline thermoplastic with a melting point of 536 °F (280 °C). It is synthesized through a reaction between sodium sulfide and dichlorobenzene in a solvent. The primary reasons for choosing PPS in gear production are its exceptional strength, rigidity, and minimal degradation even at elevated temperatures.
Nylon is a polymer known for its toughness and wear resistance. Gears made from nylon are used in applications with increased torque and power, such as conveyors and automated equipment. A major factor in determining the use of nylon gears is the limited amount of noise and vibrations they produce.
Similar to nylon, polyamide is selected for gear production due to its capacity to handle high torque at low speeds. Polyamide gears can tolerate temperatures up to 248°F (120°C) and are well-suited for environments involving acids, gases, and saltwater. In specific applications, polyamide gears are preferred over metal gears because of their lighter weight and cost-effectiveness.
Polycarbonate is a clear thermoplastic renowned for its strength and resistance to impact and breakage. This eco-friendly polymer is processed into gears through pressure molding. Polycarbonate boasts impressive strength with a melting point of 311°F (155°C), though its cost is relatively high compared to other gear polymers. Common manufacturing methods for polycarbonate gears include extrusion and injection molding.
Polyurethane is an excellent material for gears due to its desirable properties. It operates with minimal noise and is highly resistant to chemicals and corrosion, providing zero backlash. Polyurethane is versatile, suitable for a range of gears including spur, helical, worm, and pinion types. Its durability and low noise levels make it a preferred choice over other plastics and metals for extended service life.
The six polymers mentioned are just a few examples of the many plastics used in gear production. The selection of a specific plastic depends on the application requirements, including necessary strength and expected service life. The adoption of plastic gears is increasing as advancements in plastic technology continue to enhance their performance and suitability as alternatives to metal gears.
There are various types of plastic gears, which include:
These types of gears are the most commonly used and are easily identifiable due to the teeth that extend from their perimeter. Their teeth and the shaft axis are aligned parallel to each other.
Plastic spur gears do not produce axial thrust force. They are designed to operate on parallel axes, transferring motion between two shafts that are aligned parallel to each other.
These applications are typically similar to those of plastic spur gears but without a steel core.
Injection molded spur gears are suitable for use in the same sectors as other plastic spur gears.
This type of gear has a conical shape with straight or spiral-cut teeth. Plastic bevel gears transfer motion between two intersecting axles, changing the rotation axis. These types of gears are mostly utilized in power tools and automotive applications. The spiral-cut version can be smoother and less noisy than other designs.
Plastic racks are designed to convert rotational motion into linear motion. The teeth are aligned along a straight bar, working with a cylindrical mating gear. With one gear axis fixed, plastic racks provide short oscillating strokes and are commonly used in steering systems, conveyors, and machinery for lifting applications.
Characteristics of Injected Molded Bevel Gears
These types of gears transmit power through right angles on shafts that are non-intersecting. Worm gears produce thrust load and are mostly suitable for high shock load applications. However, worm gears offer very low efficiency compared to other gears. Therefore, they are often used in lower horsepower applications.
Planetary gears feature teeth cut around the internal diameter of a central gear, with one or more spur gears meshing with these central teeth. This design allows the planetary gear to rotate around its internal diameter. Additionally, teeth can be arranged on the outside of the larger gear to accommodate additional gears running around its outer diameter. Planetary gears are frequently utilized in automotive gearboxes due to their compact design and efficient power transmission capabilities.
Helical gears differ from spur gears in that their teeth are angled relative to the shaft. This design results in multiple teeth engaging simultaneously during operation, which allows plastic helical gears to handle greater loads compared to plastic spur gears. The load is distributed across the teeth, enabling helical gears to operate more smoothly and quietly. However, plastic helical gears do produce a thrust load during operation, which must be taken into account. They are primarily used in enclosed gear drives due to their efficiency and quieter performance.
Plastic Helical Gears Applications
In these types of gears, two helical faces are positioned next to each other with a gap separating them. They are a variation of helical gears. The faces of the gears have identical helix angles opposite to each other. Double helical gears eliminate thrust loads and allow greater tooth overlap and a smoother operation. Double helical gears are also used in enclosed gear drives, just like helical gears.
Plastic Double Helical Gear Applications
Herringbone gears are similar to double helical gears but feature continuous helical faces without a gap between them. Their compact design makes them well-suited for high shock and vibration applications. However, due to the complexity of manufacturing and the associated high costs, herringbone gears are not as commonly used as other types of gears.
Plastic hypoid gears are akin to spiral bevel gears but differ in that their shafts do not intersect. In hypoid gears, the shafts are supported by bearings at each end, with the pinion positioned on a different plane than the gear. This arrangement allows for smooth operation despite the non-intersecting shafts.
Bull plastic gears are used in conjunction with pinion gears to transmit power. A bull gear can drive several pinion gears, allowing for speed adjustments through gear changes. Typically, one gear in the set is larger than the other, with the larger gear known as the bull gear and the smaller gears referred to as pinion gears.
This chapter will explore the benefits and drawbacks of using plastic gears.
One significant advantage of plastic gears is their capacity to handle increased loads due to their tooth bending and load-sharing characteristics. Bending stress can attempt to deform the gear teeth and shear them from the main body of the gear, which can lead to fatigue and breakage. Contact stress can also cause surface wear and eventual failure. In contrast, plastic gear teeth distribute the load across more teeth and deflect more under pressure. This load-sharing ability enhances the load-bearing capacity of plastic gears.
Typically, plastic gears are less costly to produce compared to metal gears. Moreover, plastic gears generally do not require additional finishing processes, which can result in cost savings of approximately 50% to 90% compared to their metal counterparts.
Plastic molding provides the flexibility to create a wider range of efficient gear geometries compared to metal. It is particularly suitable for producing complex shapes like internal gears, worm gears, and cluster gears, which can be prohibitively expensive to manufacture using metal.
Plastic gears can attain high precision levels through consistent material quality and meticulous control over the molding process.
Unlike metal gears, plastic gears are resistant to corrosion, making them suitable for use in environments such as water meters and chemical plant controls. Their inert nature also allows them to be employed in applications where metal gears might corrode or degrade.
Plastic gears are lighter than their metal counterparts of the same size. For instance, nylon and acetal have specific gravities around 1.4, whereas steel has a specific gravity of 7.85.
Plastic gears are capable of deflecting to absorb impact loads more effectively than metal gears. They also distribute localized loads caused by misalignment and tooth errors, reducing the impact on gear performance.
Plastic gears help reduce noise levels due to the noise-dampening properties of plastics, leading to quieter operation. This makes plastics ideal for applications where high precision and flexible materials are needed to achieve quieter drives.
Plastic gears possess inherent lubricity, making them suitable for applications such as printers, toys, and other uses where low-load and dry gears are essential.
More specifically, plastic gears offered by companies like PowerCore offer several advantages, including:
Despite these benefits, the disadvantages of plastic gears are far outweighed by their advantages.
Plastic gears are manufactured from two main methods: the injection molding process and the machining process. The type of manufacturing process depends on the volume of the parts needed to be produced and other factors like the required strength of the parts. As already mentioned, plastic gears offer many advantages over metal gears. Each type of plastic gear has a unique characteristic that makes it suitable for a particular application. Therefore, caution must be taken when selecting plastic gear for a particular application for successful and efficient performance.
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