Constructive design of plastic gears in high-load applications

EPS column. Source: IMS Gear

If you replace a metal gear with a plastic gear without adapting the design accordingly, you will quickly turn to steel again. Why? Because success does not only depend on the right material, but is rather an interaction of material choice, wheel body design and tooth design. Without knowing the relationship between these factors and their interaction, it is almost impossible to use plastic gears successfully in high-load applications.

But how should a highly loaded plastic gear be designed? In the following four different concepts are considered and evaluated.

“Who knows plastics, takes steel”, is a saying in mechanical engineering. However, the correct sentence should read: “Anyone who knows plastic knows where, how and when it is to be used.”

If you replace a metal gear with a plastic gear without adapting the design accordingly, you will quickly turn to steel again. Why? Because success does not only depend on the right material, but is rather an interaction of material choice, wheel body design and tooth design. Without knowing the relationship between these factors and their interaction, it is almost impossible to use plastic gears successfully in high-load applications.

But how should a highly loaded plastic gear be designed? In the following four different concepts are considered and evaluated.

Non-reinforced plastic materials with a small hub

First of all, a large, non-reinforced plastic gear is to be investigated. If you evaluate the material alone, the advantages of the non-reinforced plastic are clear: Due to the homogeneous structure of the material, good manufacturing tolerances can be achieved, the material has low friction and good damping properties (NVH).

However, if the gear body is also included in the consideration, the result looks quite different.

The larger the diameter of the plastic gear, the greater the dimensional change due to temperature and humidity. Depending on the size and material, the dimensional changes can exceed the manufacturing tolerances by more than a factor of two. The resulting gear mesh that occurs during the application no longer corresponds to the design and can lead to interference, acoustic abnormalities and premature failure.

Non-reinforced plastic gear. Source: IMS Gear

In order to give the unreinforced gear the necessary stability, high wall thicknesses and complex rib structures are required. The large wall thicknesses not only lead to longer injection moulding processes, but also increase the risk of the formation of leakage, which has a negative impact on quality. Despite the rib structure, it is not possible to prevent axial deflection of the gear, especially in the case of helical gears. This leads to premature tooth interventions and non-complete flank contact. This results in increased tooth root stresses and contact pressure, which can ultimately lead to premature failure of the gear.

Thus, it is clear that the advantages mentioned at the beginning are offset by dimensional changes, voids and axial deformation. All these factors lead to significant changes in the gear mesh situation in the application. Therefore, large gears made of non-reinforced plastics are not suitable for high-load applications.

Non-reinforced plastic materials with a large hub

In order to counteract the change in dimensions of an unreinforced gear and to minimise axial deflection under load, the gear can be designed with a large metal hub.

Non-reinforced plastic gear with large hub. Source: IMS Gear

This increases weight and costs, but also increases the stability of the gear. Good gearing qualities can still be achieved with non-reinforced plastics, which are not strongly influenced by dimensional changes of the plastic due to the larger hub. Due to the higher stability of the wheel body, the gear mesh can take place on the entire surface. This optimal distribution of force over the entire tooth flank thus reduces the tension. However, the increased stiffness of the gear impairs the damping properties of the plastic. Nevertheless, this solution seems to represent a good compromise for the application.

If the wheel body design is considered in connection with the manufacturing process, another difficulty arises, because it becomes clear that due to the large hub, the plastic is very much hintered in shrinkage behaviour. Thus, residual stresses are introduced into the gear during the injection molding process. These overlap with the stresses of the operating load and can ultimately lead to cracking and thus to premature failure of the gear. Due to the risk of cracking, this solution is also not suitable for highly loaded gears.

Reinforced plastics with optimized rib structure

As non-reinforced plastics do not have the necessary stability and plastic gears with large metal hubs tend to shrinkage cracks, reinforced plastics could be used. Glass-fibre-reinforced plastics are usually used. Due to the higher strength of the material, the gear has a higher stiffness of the wheel body and deforms less under load. While this is a significant advantage for axial deformation, this point is rather disadvantageous for gearing. Due to the stiffer teeth, the increase in contact ratio under load is less than with non-reinforced plastic gears. This effect is exacerbated by the poorer gearing qualities. Due to the filler, the plastic exhibits an anisotropic (directional) shrinkage behaviour, which reduces the gear quality by at least one quality level compared to non-reinforced gears. This means that more toothing clearance must be taken into account in the design in order to be able to compensate for the tolerances.

Reinforced plastic gear. Source: IMS Gear

However, the advantage of reinforced plastics is that the dimensional change can be reduced due to temperature and humidity. Nevertheless, the increased manufacturing tolerances in combination with the lower damping result in less favourable noise behaviour.

However, the decisive point for reinforced plastics is the wear behaviour. After a run-in phase, the glass fibers reach the surface of the gear and begin to abrasively abrasive off the matrix material. This abrasion reduces the tooth thickness and thus leads to increased tooth root stresses, which ultimately lead to premature failure of the gear.

Fibre-reinforced plastics in high-load applications would therefore be quite conceivable, but are not recommended due to their poor wear behaviour.

Two-component gear wheels

It is clear from the observations made so far that a combination of these approaches is the optimal solution. A soft, deforming, wear-resistant, damping and unfilled plastic is an advantage for tooth design. This allows the load to be better distributed among several teeth and reduces the noise and wear behavior. As far as the wheel body is concerned, however, a material that is as rigid as possible with an optimised rib structure is necessary.

In order to combine both advantages, IMS Gear uses so-called two-component gears (2K wheels). These 2K wheels are made of two different plastics, which are used application-optimized.

Two-component gear wheel. Source: IMS Gear

The 2K wheel consists of a metal hub, a support (web) and a toothed ring. The web is made of a highly filled and rigid plastic and receives an optimized ribbing to give the gear the necessary stability and ensure optimal filling. The tooth ring consists of an unfilled plastic, which is selected to optimize the application.

The advantages of such a design are obvious: The unreinforced toothed rim leads to good manufacturing tolerances and low wear and NVH behaviour. The change in dimensions can be reduced to a reasonable level by the design. The reinforced web results in high wheel body stiffness, reduced axial deformation and minimised dimensional change due to temperature and humidity.

But what about the hindered shrinkage behavior? After all, this solution also involves overmoulding a large hub, doesn’t it? Although it is a large plastic hub, does it not hinder the shrinkage behaviour of the tooth ring?

Since the 2K wheel consists of two plastic components, the order of manufacturing can be arranged as desired. For this reason, the tooth ring is manufactured in the first step. It can shrink freely and no residual stresses enter the component. In the second step, the tooth ring and metal hub are inserted into a second injection mold and overmolded with the web. This design is therefore the optimal solution for high load applications.

This principle of wheel body design has been used at IMS Gear for years, mainly in the area of steering assistance. A transfer of this technology to other areas of application is not only possible, but also useful. Especially in areas where high dynamic loads affect the gears, the damping properties of the plastic can not only improve the NVH behavior, but also absorb the peak loads and thus lead to the overall damping of the system.

Authors

Author: Veronica Labriola / B.Eng / IMS Gear SE & Co. KGaA / R&D / Project Engineer

Co-Author: Stephan Oberle / Dipl. Ing. / IMS Gear SE & Co. KGaA / R&D / Director

Web: www.imsgear.com