Gears design

Design of Plastic Gear Wheels for Power Transmission

Most plastic gears used worldwide today are intended for motion transmission, often very precise, but with negligible or very modest applied power. In reality, the mechanical properties of some thermoplastic resins allow the creation of gears capable of transmitting significant power, even at operating temperatures of 120 – 150°C, making them suitable for use in harsh environments such as those inside combustion engines.

 Under these operating conditions, the functional advantages offered by plastic gears over steel ones become even more important, namely:

- Weight reduction

- Noise reduction

- Reduction of friction and therefore dissipated power

In the most demanding applications, these functional advantages can be as significant or even greater than the economic advantage that resin gears always ensure when compared to metal gears, even for medium to low production volumes.

To design plastic gears intended for power transmission, it is necessary to perform a highly precise calculation of the stresses to which the teeth are subjected and to use all the possibilities offered by involute gear profiles to identify the geometry that ensures maximum tooth strength. This requires abandoning standard gear sizing and seeking the optimal configuration of all geometric parameters (number of teeth, module, pressure angle, addendum, dedendum, profile shift, tooth width, etc.) to achieve gears capable of meeting all functional requirements.

Unfortunately, standard gear calculation programs underestimate the actual load capacity of resin gear teeth, sometimes by more than 30%, especially when the gear parameters deviate from standard values and the contact ratio is close to or exceeds 2.

The reason standard gear calculation programs underestimate the strength of resin gear teeth is that they were developed based on the mechanical characteristics of steel, which differ substantially from those of thermoplastic resins. In gear teeth, both the elastic modulus, which in steels is 20-25 times higher than in resins, and the ratio between elastic modulus and tensile strength, which in steels is 3-4 times higher than in resins, play a crucial role.

A low value of the elastic modulus allows gear teeth, already in contact or about to come into contact, to better distribute the force generated by the transmitted driving torque between the two gears. In this way, the actual driving torque that can be transmitted is higher than it would be, assuming the same material tensile strength, if the elastic modulus were similar to that of steel.

This effect of force redistribution over multiple pairs of teeth becomes increasingly important as the contact ratio approaches or exceeds the value of 2, i.e., with the increase in the number of teeth that can be simultaneously in contact.

To overcome the unreliability of traditional programs, which effectively limits or prevents the use of resin gears for power transmission, Ing. Morini has developed a series of programs that allow for the extremely precise calculation of the forces exchanged by each pair of teeth in contact and therefore the maximum power that can be transmitted by a pair of gear wheels. These new programs enable the design of optimized gear profiles for each specific application, selecting the geometric parameters to achieve a high contact ratio and a low wear factor (pV).

Using these programs, it is easy to size gears with different materials, for example, when one gear is made of resin and the other is made of brass or steel. In these cases, the choice of gear parameters must be made based on the mechanical characteristics of the chosen resin, and the metal gear profile must adapt to the plastic gear profile.

In this way, resin gear applications have been realized that, according to standard programs, would have had no chance of functioning.

Using these programs, steel gears have also been created, obtained either through mechanical processing or sintering, and intended for applications where, in addition to mechanical strength, noise reduction and weight reduction were required.