This article will give an in-depth discussion on plastic gears.
The article will bring more detail on topics such as:
This chapter will discuss what plastic gears are, their manufacturing process, and how they function.
A plastic gear is a toothed wheel made up of engineering plastic materials that work with others to alter the relation between the speed of an engine and the speed of the driven parts. The engineering plastic materials used in manufacturing plastic gears can be nylon, which is essentially polyamide resin, and polyacetal.
Various methods can be used in the manufacturing of plastic gears. The most common methods will be discussed below.
Plastic gears are manufactured by different machining processes, usually milling or hobbing. The machining process is about taking away material from a plastic block that is machined layer by layer in a manner that is precise to fabricate the desired part. These processes mentioned prior are the same machining processes used in manufacturing metal gears.
The manufacturing cost of a plastic gear is reduced by the very low cutting forces that permit high infeed rates. If high infeed rates are used during milling, this may result in a wavy and rough surface. This kind of surface may be seemingly detrimental to long performance. However, during a dry run-in period, the surfaces of the plastic teeth that bear the load are smoothened. Lubricant pockets may even be offered by the waviness, but the waviness does not lower the quality of the gear. Plastic gears have a load bearing surface that is somewhat pliable. This allows wider tolerances due to the elasticity of the plastic. The quality of the plastic gears can be reduced to two AGMA grades compared to steel gears. Compared to gears made of steel material, plastic gears have an increased backlash by one to two quality grades. This helps to allow for the effect of moisture and temperature on the plastic.
This is an alternative way of manufacturing plastic gears. This type of process is about the addition of material. It involves melting stock material such as thermoplastics, and through a high pressure nozzle, the materials are injected into runners. The material is then allowed to cool in the molds and then ejected, and the process starts afresh. The injection molding process produces plastic gears that have good precision. There are several factors to consider in a successful production, such as material used, the type of equipment used, and the process conditions. The amount of post processing is always needed to be reduced. This reduces the time that is spent on each product and this has a direct impact on revenue. These factors also affect the quality of the product.
When contrasting between machined plastic gears and injection molded plastic gears, the following parameters provide the differences or contrasts:
The machining process is faster when there are low volume parts to work with. It can be considered as a solution for smaller goals. However, on the other hand, if there are high volumes of plastic parts to be consistently manufactured for a longer duration, injection molding is a viable solution as the manufacturing process can be scaled once the mold is ready. The injection molding process has faster manufacturing capabilities that offset the delay of creating a mold and the upfront cost.
For the machining process, the cost appears to be cheaper when there are only a few hundred parts to work with. Expenses are only incurred in the setup. However, when the volume increases the cost also increases. In contrast, the price is significantly low for each part with injection molding. Cost is accrued in the initial stages of injection molding. But once large volume production of parts begins, this cost distributes with time.
With the machining process, there are a lot of options for raw materials to choose from, for the creation of parts. The structural strength and durability of the final product is imparted by the core material, therefore machining is the way to go if you are working with harder materials such as high performance plastic material or specific plastics. With injection molding, there is a limit on the type of material selected. Only softer plastics like thermoplastics and thermoset resins can be worked with because they can be melted and molded without compromising material strength. This process is used to fabricate the most flexible and pliable materials.
The machining process delivers products that can meet expectations of precision and accuracy with high tolerances. Only fewer variables are involved in the level of control exercised by a computer. To add on, the primary focus of the manufacturing process is on the product specifications. But with injection molding, a greater scope and room for defects exist. The defects are flow lines, vacuum voids, warping, burn marks, etc. This is so because injection molding does not consider the tolerances of the part, but of the mold.
With injection molding, a high degree of repeatability is offered, but this is an advantage if you are fabricating parts with the same design. If the design specifications are changed, there will be an upfront cost of making the mold. This, in turn, would make the injection molding totally cost-inefficient as you would start from scratch once again. With machining, the CAD program can be tweaked to incorporate the design changes.
This chapter will discuss the design details of plastic gears and the types of materials that can be used in plastic gears.
The design details of plastic gears include:
The face width of a plastic gear tooth can be as wide as the diameter of the gear. The axial stability of the gear wheels is the one that limits the minimum width. From experience, the face width should be at least six to eight times the gear module. When mating plastic and steel, the face of the plastic gear should be slightly narrower than the steel gear face. The plastic gear then touches the load with its whole width, and no groove can be worn into the face of the plastic.
The choice of module and pressure angle directly affects the load bearing capacity of plastic gears. If the module or pressure angle is increased, the root strength of the teeth also increases but the transmitted force stays the same. With plastic gears, it is impossible to engage several teeth simultaneously due to the decreased contact ratio. This, in turn, makes the increase in root strength smaller with plastic gears. If the contact ratio is high, this can be better for increasing the load bearing capacity rather than increasing the root strength of the individual tooth.
A small module is preferred (several teeth engage simultaneously and contact ratio increased) for tough elastic thermoplastics. A large module is preferred (a higher contact ratio is impossible with the hard material; therefore root strength of the teeth is increased). At 20f, the pressure angle for the involute plastic teeth is defined. Pressure angles less than 20f produce thinner teeth of less load capacity, with steep tooth profiles but with a running, low noise. Pressure angles that are greater than 20f result in more pointed, thicker teeth that have greater root strength. An integer multiple is not allowed as a number for the ratio of the number of teeth in high speed gears. Different teeth must engage in order to reduce accelerated wear.
Plastic materials have high thermal expansion rates, therefore the material-specific designing of backlash and clearance is important. A guarantee of a minimum backslash must be made. A minimum face clearance of 0.04 x module is recommended and an initial clearance of 0.3 x module is recommended.
Temperature plays a key role in establishing the load capacity of a plastic gear. The tooth body temperature sets the allowable loading and deformation of the tooth base. An approximation of the rate of wear is allowed by the tooth face temperature. Determining both temperatures accurately is difficult. Only an approximate estimation of the heat transmission coefficient can be made. Thus, the tooth face temperature value calculated may sometimes be a very high number, higher even than the plastic’s melting temperature.
Intermittent operation of plastic gears increases the load capacity because of the generation of less friction heat. In the following equation, the relative duty cycle ED is considered by a correction factor f. This relative duty is defined as a percentage of the load time t and the overall cycle time T.
Where: t = total operating time under load within cycle time T in min; T = cycle time in min.
The root, flank, and wear strength are parameters of large importance in the estimation of gear life. The number of load cycles determines the specific parameters (root pulsating strength and flank strength) for both metal and plastic gears. For plastic gears, these parameters are strongly dependent on temperature and the type of lubrication (oil, grease, or dry running) used. Whereas for steel, only one value for the tooth strength calculation is sufficient.
Comparing plastic to steel, it is clearly seen that Young's Modulus for plastic is about two magnitudes smaller than for steel e.g. about 2800N/mm2 for POM compared to 206,000 Nmm2 for steel. The permissible bending stress of plastic is also about one magnitude smaller than steel: 25 Nmm2 compared to 250-450 Nmm2 for steel. Since many gears are sized from their strength requirements, the relative deformation of metal gear teeth is lower than that of plastic gears.
However, Plastic gears can absorb shock better than metal gears, and most plastic gears have a mechanical stop design to avoid extra wear and tear on the gear teeth and the motor itself. Plastic gears have a longer wear period and a longer life expectancy than many metal gears. Metal gears grind down faster than plastic gears.
Plastic gears do not require backlash adjustment as is the case with metal gears. Most steel gears require frequent backlash adjustment in order to avoid backlash issues from the shock of operating the gear and motor vibrations. Since plastic gears can absorb these vibrations, the gears can often last for a much longer period.
For gears like PowerCore, the size of the injection-molded gears is usually limited by the size of the press, to about 5” to 6”. In the US, about 80% of the gears, metal or plastic, are selected from a catalog. In contrast, in cooperation with customers’ engineers, the designed gears solve specific problems or address specific design challenges, such as inertia or product contamination with grease. (This is particularly the case with open gearing in e.g. paper, food, or semiconductor processing equipment).
To engineer a solution for an application, all the operational parameters, including torque, RPM, shock load, backlash requirements, inertia, chemical exposure (semiconductors), operating temperature, etc are considered. Alternative solutions, such as tooth modification or increased gear width can be proposed to make sure the plastic gear will work in a given application.
An integral part of the gear engineering process is the use of our proprietary gear life calculation. The calculation will give the number of hours in which the gear will last for the specified conditions. Conversely, we can design a gear to last a certain number of hours. So, for example, medical equipment manufacturers require 5 years of component life. Over the last 20 plus years, the calculation has proven to be a reliable predictive tool, and it allows us to quickly assess if a plastic gear will work in a given application or not, especially when replacing a metal
The various types of materials used in plastic gears include:
Power-core is a type of material with a unique, tension-free, homogeneously crystalline structure, which is achieved through a unique casting process. Power-Core (PA12GC) shapes are made from laurolactam resin. Power-Core exhibits a unique combination of the physical properties of plastics, metals together with ceramics. This secures Power-Core’s domain among polyamides. Power-Core non-metallic gears allow transmission of the full torque found on the keyway, machined into the metal core. Power-Core outperforms all other polyamide and acetal based plastics (e.g. nylon and delrin).
This type of material exhibits several characteristics such as being wear resistant and absorbing impact even though it’s under rough conditions. However, this material is not much suitable for small precision gears.
This extruded polyamide has better wear resistance than the PA 6, except against mating surfaces that are of high quality. It absorbs less moisture and is dimensionally more stable, but like the PA 6, it is not much suitable for small precision gears.
These polyamides exhibit wear resistance that is similar to PA 6 G, and is engineered toward toughness.
This type of material exhibits very good dry running and wear resistant properties due to the addition of lubricating oil into the PA 6 G.
This acetal is more suitable for precision gears due to its little absorbance of moisture, but it must be continually lubricated under high loads.
PE is dimensionally stable and does not absorb any moisture. It is also resistant to chemicals and dampens vibrations. However, this material is only suitable for low loads. There are speed limitations and load limitations for plastic gears. Metal gears have an operating temperature range in which they operate well, which is the temperature limit of the material. Plastic gears are, however, designed in such a way to take into consideration the increase in temperature, which is a result of friction, pressure, and speed.
There are various types of plastic gears which include:
The injection molded bevel come with characteristics such as:
Injection molded spur gears can be applied in the same industries as other plastic spur gears.
Characteristics of injection molded plastic spur gears include:
Molded flexible racks come with characteristics such as:
This type of gear has a conical shape with straight or spiral cut teeth. Plastic Bevel Gears transfer motion between two axles that are intersecting, changing the rotation axis. These types of gears are mostly utilized in power tools and automotive applications. On all designs, the spiral cut version can be smoother and less noisy than other designs.
Plastic bevel gears come with characteristics such as:
The plastic bevel gears can be applied in industries such as:
The plastic bevel gears can have specific applications. Examples include:
In these types of gears, two helical faces are positioned next to each other having a gap separating them. They are a variation of helical gears. The faces of the gears have identical helix angles, but opposite to each other. Double helical gears eliminate thrust loads and they allow greater tooth overlap and a smoother operation. Double helical gears are also used in enclosed gear drives, just like the helical gear.
Plastic double helical gears are used in the following industries:
The plastic double helical gears can have specific applications. Examples include:
These types of gears have teeth that are oriented at a certain angle to the shaft, which is a different case in spur gears. This increases the number of teeth in contact during operation to more than one. Plastic helical gears can carry more load than plastic spur gears. Plastic helical gears can share load between the teeth. This arrangement in turn allows plastic helical gears to operate smoother and quieter than spur gears. A thrust load which needs to be considered when they are used is produced by plastic helical gears during operation. Plastic helical gears are mostly used in enclosed gear drives.
Plastic helical gears can be used in different industries such as:
The plastic helical gears can have specific applications. Examples include:
These types of gears are similar to double helical gears, but their helical faces do not have a gap separating them. Compared to the double helical gears, herringbone gears are typically smaller. This makes them ideally suited for high shock and vibration applications. These types of gears have manufacturing difficulties and high costs, so they are not often used.
Plastic herringbone gears are used in the following industries which are:
The plastic herringbone gears can have specific applications. Examples include:
Plastic hypoid gears are similar to spiral bevel gears, but their operating shafts do not intersect, as is the case in spiral bevel gears. In plastic hypoid gears, the bearings support the shafts on either end of the shaft because the arrangement has the pinion set on a different plane than the gear.
Plastic hypoid gears are used in the following industries:
The plastic hypoid gears have applications such as:
Planetary gears have teeth that are cut around the internal diameter of one gear and a spur gear (or several) meshes with these center teeth. This, in turn, allows it to run around internal diameter. Teeth can also be designed in such a way that they are on the outer of the larger gear, so additional gears can run around its outer diameter. These types of gears are commonly used in automotive gearboxes.
Plastic miter gears come with characteristics such as:
The rotational motion is converted into linear motion by these types of gears. The arrangement of the teeth is along a straight bar, which has a cylindrical mating gear. One gear axis is fixed. These gears offer short oscillating strokes and are used in steering systems, conveyors, and machinery used for lifting.
Plastic racks come with characteristics such as:
Plastic screw gears come with characteristics such as:
These types of gears are the most used and they are easy to recognize because of their teeth that protrude out from the perimeter. Their teeth and the shaft axis are parallel to each other.
Plastic spur gears do not generate thrust force in the axial direction. They are mounted on axes that are parallel to transfer movement between two shafts that are also parallel.
Characteristics of spur gears include:
The plastic spur gears can be applied in industries such as:
The plastic spur gears can have specific applications. Examples include:
Their applications are generally the same as the plastic spur gears without a steel core.
However, characteristics of spur gears with steel core include:
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. Worm gears are often used in lower horsepower applications due to their low efficiency.
Plastic worm wheels come with characteristics such as:
Plastic worm gears are used in the following industries:
The plastic worm gears can have specific applications. Examples include:
This chapter will discuss the advantages and disadvantages of plastic gears.
One major advantage of plastic gears is their ability to handle more load due to their tooth bending and load sharing properties. Bending stress can try to bend the teeth of the gear and shear the teeth from the bulk material of the gear. These forces cause fatigue and static loading, leading to failures by tooth breakage. Contact stress can cause failure in the surface of the gear teeth and wear. In other words, plastic gear teeth spread the load over more teeth by deflecting more under load. The load sharing capacity increases the load bearing capacity of the plastic gears.
Generally, the production of plastic gears is less expensive than that of metal gears. Plastic gears do not require secondary finishing; they typically represent a 50% to 90% saving relative to metal gears.
Molding plastic offers a greater diversity of efficient gear geometries than metal. Molding is an ideal process for producing shapes such as internal gears, worm gears and cluster gears, where cost of production in metal can be prohibitive.
Plastic gears can achieve high levels of precision. This can be done using consistent material quality and accurate molding process control.
Plastic gears are immune to corrosion, unlike metal gears. They can be used in water meters, chemical plant control because of their relative inertness. They can also be used in any other applications in which metal gears can corrode or degrade.
Plastic gears are lighter in weight than gears of the same size but made out of metal. The specific gravities of nylon and acetal are closer to 1.4, while the specific gravity of steel is 7.85.
Plastic gears can deflect to absorb impact loads than metal gears. Plastic gears also distribute localized loads that result in misalignment and tooth errors.
Plastic gears reduce noise levels because of the plastics’ noise-dampening properties, resulting in a quiet running gear. This is the reason why plastics are essential for tooth shapes with high precision and flexible materials required for quieter drives
Plastic gears have inherent lubricity, making them ideal for printers, toys, and other applications that require low load and dry gears.
However, more specifically, plastic gears offered by companies like PowerCore have advantages such as:
However, 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 process for manufacturing depends on the volume of the parts needed to be produced and other factors like the required strength of the parts, etc. 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 a plastic gear for a particular application, for successful and efficient performance.
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