PP, PE, PA, POM, PC, PEEK, PPS ...... Gears of different materials and their properties

PP , PE , PA, POM , PC , PEEK , PPS ...... Gears of different materials and their properties

The main function of plastic gears is to transmit motion and power,Compared with metal gears, plastic gears have many advantages such as light quality, low running noise, good wear resistance, good self-lubricating properties, corrosion resistance, etc., and have the characteristics of easy molding, low manufacturing cost and flexible design.

Under different working conditions, the materials used to make plastic gears need to be considered comprehensively, such as gear performance, process performance and economy. Plastic gear molding materials can be divided into general-purpose plastics, engineering plastics and special engineering plastics and fiber-reinforced plastic composites with their matrix, etc.

General purpose plastic gears

Common general-purpose plastics used for gears are Polyethylene, Polypropylene, PVC , Polystyrene, etc., as shown in the figure below

They have high output, low price and wide use, but at the same time have many disadvantages, such as low surface hardness, easy to be scratched; cracking under the action of environmental stress, leading to failure; in the process of gear meshing, due to the large coefficient of thermal expansion, a large amount of heat will be generated, more likely to creep and permanent deformation. Therefore, general-purpose plastic gears are not suitable for applications with high load-bearing capacity, but are suitable for manufacturing gears for toys and medical devices.

Engineering Plastic Gears

Engineering polymer is a special, high-performance synthetic plastic with excellent overall performance, high rigidity, low creep, high mechanical strength, good heat resistance, good electrical insulation, and can be used for a long time in harsh environmental conditions. Properly designed, they can be molded into semi-precision parts or structural parts with strong mechanical functions, and offer the possibility of replacing metal gears with "plastic instead of steel".

Engineering plastics can be subdivided into general engineering plastics and special engineering plastics 2 categories, which can be molded gear engineering plastics are polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), acrylonitrile / butadiene / styrene copolymer (ABS) and thermoplastic elastomers, Polyimide ( PI ), Polyphenylene Sulfide (PPS), polyester (PBT), Polyetheretherketone (PEEK) and liquid crystal polymers (LCP) and so on.

PA Gear

Polyamide (PA) materials have the hardness of thermoplastic materials with good impact resistance and impact toughness. It has better properties than metals in some cases and has been widely used in the production of gears, cams and bearings. However, its heat deflection temperature is relatively low; the product has a large water absorption after placing, and the residual stress generated will cause surface defects on the product, change the thickness of the surface, increase the yield strength and cause a reduction in toughness; creep will also occur during use

POM Gears

With the increasing use of plastic gears, the application requirements have shifted from gear accuracy to load-bearing performance. The good mechanical properties of polyoxymethylene (POM) material can be applied to the injection molding of gears by combining precision and load-bearing. As a highly crystalline linear polymer, it is an excellent example of a heat resistant plastic. By injection molding, POM material can be made into hard and dense fatigue and wear resistant gears with good impact, low coefficient of friction and good self-lubricating properties, therefore, POM is the engineering material of choice for polymer gears. The disadvantage is the tendency to shrink when molding products and the tendency to produce defects.

PC Gear

Polycarbonate (PC) material is a colorless glassy amorphous polymer with high impact strength, good dimensional stability, low creep and good processing and molding properties. However, PC is susceptible to aging under the influence of higher temperatures, so the use of such materials at higher temperatures requires protection of the product surface, thereby extending the service life of PC.

Due to the colorless and transparent material, gears made of PC can be very beautiful in appearance and can be installed in precision instruments for gear transmission or as craft decorations, but the gears made of them will have radial tooth deformation, for which accurate gear simulation is required to obtain such defects.

PEEK Gears

As polymer gears become more demanding, there is a need to use engineering plastics to meet gear service requirements. polyether ether ketone (PEEK) has excellent mechanical, chemical and thermal properties, which give it an advantage over other specialty engineering plastics. PEEK materials remain high in tensile strength and flexural modulus at high temperatures and exhibit reliable creep and fatigue resistance for long-term use at temperatures up to 200°C. PEEK gears can also achieve power transmission under higher load conditions, making them the top engineering material in today's molded gears.

PEEK gears are excellent in performance and have many potential uses in dentistry. PEEK has a low Young's (elastic) modulus and excellent tensile properties all close to human bone, making it suitable as a replacement for teeth.

PPS gears

Polyphenylene sulfide (PPS) has the advantages of high hardness, good dimensional stability, fatigue resistance and chemical resistance, is the preferred material for gears in high temperature, corrosive environments, and now PPS gears have been applied to harsh operating conditions in automobiles, special fluid pumps, etc. PPS can be combined with other polymers with each other to achieve good mechanical and frictional properties, gear wear and transmission performance can also be improved.

Fiber-reinforced plastic gears

Plastic gears have the advantages of light weight, easy molding, and low noise, but it also has the disadvantages of low strength and poor wear resistance, so the manufacture of plastic gears requires higher performance materials, which can be improved by adding different additives to the material. Plastic gear additives are generally divided into two categories, one is to reduce friction, and the other is to improve heat resistance and mechanical properties.

The use of Polytetrafluoroethylene, graphene and siloxane can reduce friction during gear meshing; the addition of carbon and glass fibers can improve the strength and thermal properties of gears; to improve wear resistance and reduce the friction coefficient of gears, aromatic polyamide fibers can be used. Fibers have the advantages of high tensile strength and good absorption of impact energy, which is a simple and effective way to improve the strength of gears and provide a solid foundation for polymer gears to replace metal gears.

Carbon fiber reinforced plastic gears

Carbon fiber (CF) has high tensile strength and modulus, excellent tensile properties, low density, high thermal stability, and good thermal and electrical conductivity, and high freedom of design to meet requirements. Carbon fiber is widely used in various fields. By adding carbon fiber's to plastic gears, the frictional properties of the gears can be improved due to carbon fiber's own rigidity, and the gear stability and wear resistance of the gears can be improved on the basis of achieving gear lightness.


Depending on the carbon fiber content and type, the enhancement effect on gears and wear loss performance vary. kurokawa et al. studied and evaluated different CF-reinforced PEEK gears, CF-reinforced gears can cause different wear on tooth surfaces depending on the type of paired gears and the presence or absence of lubricant. The wear rate of the gears in the dry steady state is small; with the application of lubricant, there is also different wear depending on the mating gear. The researchers speculate that it is the affinity between PEEK and CF, the difference in CF abrasive chip intervention in the meshing zone, and the characteristics of CF that affect the gears, but further verification analysis is needed.

In terms of filling materials in gears, different fibers and substances added to polymer gears can have different effects. chroeder et al. in tribological tests of PEEK, CF-reinforced PEEK and filled graphite, PTFE and CF-reinforced PEEK, found that unfilled PEEK exhibited greater wear in the experiments; CF-reinforced PEEK also exhibited very low wear resistance, with higher sliding and micro-abrasive wear resistance; adding PTFE and graphite back into the CF-reinforced PEEK gears resulted in a dramatic decrease in the friction coefficient of the gears, which exhibited high scuffing and almost no wear resistance, mainly due to the transfer of the protective friction layer of the carbon fiber PEEK gears filled with graphite and PTFE, from the composite to the other gear.

Glass fiber reinforced plastic gears

Due to their small diameter, glass fibers (GF) exhibit high strength, excellent performance and freedom of product design when combined with the base material. Under specific stress levels and speed conditions, GF-filled reinforced gears exhibit excellent strength, modulus and thermal conductivity and extended fatigue life compared to unfilled reinforced gears. The uniform distribution of fibers within the injection molded products, the improved tensile properties and flexural strength are widely used in various mechanical components in the automotive, aerospace and mining fields.

GF filling content has different effects on gear performance enhancement. mao et al. found that 28% GF reinforced POM gears have significantly enhanced performance compared to POM gears without GF filling. in terms of load capacity, the load bearing of gears is increased by 50%; in terms of surface crystallinity, the crystallinity of POM gears decreases by 20%, and the crystallinity of GF reinforced POM gears obtained from SEM images has no change. After the experimental tests, the fiber length inside the POM gears will decrease, which is caused by fiber breakage under high load, resulting in a significant decrease in local bending resistance and rapid thermal failure of the gear teeth.

The different GF orientations affect the meshing wear of the gears. In order to obtain the best performance of fiber-filled gears, Kunishima et al. conducted tribological studies on GF-reinforced Pa66 . Under high contact pressure, sliding and lubrication conditions, greater erosion was observed when GF was oriented along the vertical orientation, causing fiber flaking and tooth surface scoring on the corresponding steel meshing material, which is mainly due to increased friction and creep; wear when fibers were aligned parallel to the sliding direction increases; in the presence of lubricating oil, the wear due to debris from abrasion is significantly less than that due to increased contact temperature.

Aramid fiber reinforced plastic gears

Aramid fiber is a synthetic fiber with the advantages of ultra-high strength, high modulus and light weight, which can not decompose and melt at high temperature and is a good insulating and anti-aging material. Aramid fiber filled gears can reduce the mass of gears, making gears toward miniaturization and high performance.

In terms of the effect of aramid fiber, which can be obtained in comparison with GF and CF, in Kukureka et al.'s double-disk machine test of aramid fiber, GF and CF reinforced PA66, GF and CF reinforced materials can make a significant decrease in friction coefficient, while aramid fiber does not change the frictional properties of PA66 material. The wear of aramid fibers is linear with time, which leads to the increasing wear rate of the products made of reinforced PA materials with time.

On the contact surface of the two discs, the aramid fibers are rapidly removed from the contact surface, leaving only the matrix material, an effect similar to that of increasing the friction coefficient of the material, resulting in a significantly higher wear rate than the unfilled reinforced material. It is possible that the aramid fibers only inhibit crack expansion in certain areas, resulting in wear pieces that are large, thin flakes.

The wear characteristics of gears determine the application conditions of gears. Gordon et al. performed sliding-rolling contact tests on PA46 and PA46/aramid fiber composites, and at 2% slip, PA46 + 15% aramid fiber had the lowest coefficient of friction, but a stable wear rate at higher loads and speeds. It was found that the PA46+aramid fiber composite showed pitting and large cracks and eventually fractured and failed under any load and rate conditions. Therefore, PA46+aramid fiber gears are suitable for use under low load and low speed conditions.