Gears design

Plastic gears design for power transmission

Most of gears used worldwide is intended to transmit movements, even very precise movements, although with very small power transmission.
Actually, the mechanical characteristics of some thermoplastic resins allow the manufacture of gears, which can transmit important powers, even at operating temperatures of 120 – 150°C.  Therefore, such gears can be used in very heavy conditions, such as inside internal combustion engines.
Under these conditions of use, the comparison between steel and plastic gears makes the functional benefits of plastic gears even more important, i.e.

• weight reduction
• noise reduction
• reduction of friction and then of the dissipated power

In the most heavy applications, these functional benefits can have equal or greater importance than saving money, which is always the case for medium to low production volumes.
In order to design plastic gears for power transmission, an extremely precise calculation of the stresses on the teeth is required, as well as using all the opportunities offered by involute toothing to find the geometry that ensures the maximum resistance of the teeth.
To do this, the standard toothing sizing needs to be set aside and looking for the optimum configuration of all the geometric parameters (number of teeth, module, pressure angle, addendum, dedendum, profile shifting, tooth thickness, etc.) is required too, to get any gears, which can meet all the functional needs they require.
Unfortunately the standard gear calculation programs underestimate the real plastic toothing resistance, sometimes even over 30%, especially when toothing parameters deviate from standard values and contact ratio is close to/above 2.
The reason, why the standard calculation programs underestimate the real plastic gears resistance is that they have been developed according to mechanical properties of steel, which differ substantially from those of thermoplastic resins.
The elastic modulus plays a decisive role in the toothing, which as for steel is 20 – 25 times higher than for the resins.
A low value of elastic modulus allows the gears having a better force distribution, being generated by the drive torque, on the teeth, not only on the ones already in contact with each other but also on those coming in contact.
In this way the real driving torque, which can be transmitted, is higher than the one we can have, for the same material stress, if the elastic modulus was similar to the steel one.
This effect of redistributing force on multiple pairs of teeth becomes as important as the contact ratio approaches or exceeds the value 2 this involving a larger number of teeth meeting together at the same time.
To overcome the unreliability of traditional programs, which effectively limits or prevents the use of plastic gears for power transmission, Mr. Morini developed a set of programs, which allow the extremely precise calculation of the forces exchanged by each pair of teeth in contact with each other and then the maximum torque the gears can transmit.
These new programs allow him to design optimized gears for each application, by choosing geometric parameters to have  high contact ratio and low wear factor (pV).
By using these programs, Mr. Morini can easily dimension gears with different materials, for example with a resin gear and a steel one.
In these cases, the toothing parameters must be selected according to the mechanical properties of the resin being chosen and the toothing of steel gear must be adapted to the toothing of the plastic gear.
In this way, resin gear applications have been made, which would have had no chance of running according to standard programs.
By using these programs, steel gears have been produced too, both by mechanical machining and by sintering, for applications where noise reduction and weight reduction were required, besides mechanical strength.