Plastic Gears for Power Transmissions
Plastic gears have been used for decades in various applications such as consumer products or electromechanical actuators in cars. Plastic specific material properties such as low density and high damping characteristics as well as the possibility of mass production through injection molding are advantageous and contribute to the increasing application of plastic gears. In most cases the gears are running dry or under starved lubrication. In the context of these operational conditions the transmission of motion is often of principal importance as the potential to transmit power is limited due to the high level of frictional heat in combination with limited capability for heat removal.
Recent development trends, however, include considerations of an increased use of plastic gears for oil-lubricated transmission systems. This enables transmitting significantly higher amounts of power.
In recent research activities at the Gear Research Centre (FZG) of the Technical University of Munich (TUM), experimental gear tests using state of the art injection molded test gears are conducted. Beside POM, several adequate thermoplastic materials such as PA46, PEEK and carbon fiber reinforced PA66 are investigated in back-to-back testing. Material specific damage mechanisms are identified and load carrying capacity is evaluated. The results indicate that the load carrying capacity of cylindrical steel-plastic pairings can be significantly higher than expected according to the state of knowledge (VDI 2736). Extensive investigations using POM gears revealed that, depending on the gear geometry, the transmittable torque is considerably higher than the prediction according to the bending strength calculation. The reason for this is that the tooth deflections under load have noteworthy influence on the tooth root stresses. These load-induced deflections are, however, not yet considered in state of the art calculation methods. Therefore, a modified calculation method is derived to consider elastokinematic effects and so improve the correlation with both numeric calculations and experimental results.
Since the strength of plastic materials is limited, potential for higher power densities of plastic gear stages arises from higher revolution speeds. Presuming oil as a lubricant as well as a coolant, considerably higher amounts of continuous power, compared to dry running, can be transmitted without the occurrence of thermal damages.
In this context, the influence of dynamic effects on the load carrying capacity of plastic gears gains importance. For steel gears, the operation in or near the main resonance speed should be avoided respectively dynamic tooth forces must be considered regarding the gear design. Experimental investigations show that dynamic effects can also have a noteworthy influence on the load carrying capacity of plastic gears. The effect of dynamic tooth forces is, however, not yet considered in current calculation methods.
In a nutshell, recent studies have revealed significant amounts of research potential for oil-lubricated plastic gears as the bearable mechanical loadings shift to higher levels due to the improved tribological and thermal conditions. Within a lubricated transmission system, the damages that are relevant for the endurance of the plastic gear change. Damages like those known from steel gears, such as tooth root breakage and pitting-like damages, gain importance.
Compared to dry running conditions, the reliability of oil-lubricated plastic gears is significantly increased as an equalized heat balance allows continuous operation. Therefore, plastic gears may be considered for future high-order applications like auxiliary drives of vehicle drivetrains, industrial gear units as well as even the main power transmission of light vehicles.
The results of the investigations in the bending strength and the dynamic tooth forces of plastic gears will be presented and discussed in a contribution to the VDI International Conference on High Performance Plastic Gears, which takes place simultaneously with the International Conference on Gears 2017 in Garching.
Authors of this article
Christian Hasl; Christopher Illenberger; Dr.-Ing. Thomas Tobie; Prof. Dr.-Ing. Karsten Stahl;
Technische Universität München - Forschungsstelle für Zahnräder und Getriebebau (FZG)