Polymer moulded over metal insert in gearbox

Edited by the Engineeringtalk editorial team Dec 14, 2001

The use of Victrex Peek polymer has allowed TRW to economically develop a high-efficiency gearing concept with a transmission ratio of 150:1 within a limited space

TRW has selected Peek polymer as a cost-effective, innovative design solution for an electrically controlled parking brake.

An actuator unit, which consists of an electric motor, and a gearing unit activate the brake.

The use of Victrex Peek polymer allowed TRW to economically develop a high-efficiency gearing concept with a transmission ratio of 150:1 within a limited space.

Furthermore, the polymer provided lighter, quieter running parts than could be achieved with metal.

The gearing unit had to meet highly demanding performance requirements since high levels of torque were to be transmitted in a chemically aggressive environment through components within a limited space.

The subassembly sits directly on the calliper in the rim, as a result of which the construction space available for the gearing was only 52 mm in diameter and 30 mm in height.

In addition to being cost-effective, these performance criteria had to be met at high temperatures of up to 180C, since the actuator is connected directly to the cast housing of the brake.

The key criteria can be summarised as follows: mechanical load: peak torque: 25 Nm; nominal torque: 15 Nm; load cycles: 100,000; temperature load: use temperature: -40C to +120C; peak temperature: +180C; chemical resistance: long-term resistance to fuels, oils, brake fluid, etc., over the given temperature range.

The gearing assembly was developed initially by Oechsler AG.

This established supplier of precision injection moulded components is specialised in the development and production of complete subassemblies, especially gears, including conventional spur and planetary gears, and gears with special kinematics.

Several alternative construction concepts were considered.

The chosen specification was for wobble gearing, an unusual gearing concept that has critical demands on design and material choice.

The overall ratio of 150:1 was distributed over a toothed belt stage and a wobble gearing with a ratio of 50:1.

Particular attention was paid to the choice of materials for the gear wheels, the drive and the wobble wheel.

The levels of torque to be converted and transmitted demanded a high strength material.

The wobble plate also required two pegs for support.

These pegs run into corresponding guides in the metal housing and are subjected to very high mechanical stresses.

High running speeds, together with a pointed contact surface, demanded that the material have excellent tribological properties.

Abrasion resistance had to be guaranteed over 100,000 cycles, while a low coefficient of friction between the wobble wheel and metal housing was needed to aid the efficiency of the gearing.

Several materials and material combinations were evaluated in an extensive series of tests by Oechsler.

Victrex Peek polymer 450FC30 grade was chosen for its unique balance of properties, notably a flexural E- modulus that can exceed that of aluminium, and is maintained at temperatures of up to 150C.

Even beyond this temperature Peek polymer retains its long-term resistance to the relevant automotive chemicals, as well as outstanding abrasion resistance.

By selecting the lubricated Victrex Peek 450FC30 grade, it was possible to increase efficiency to 75%, a significant achievement for gearing with such a high transmission ratio in one stage.

This grade provides a coefficient of friction that is two thirds less than that of the base material and remains virtually constant at temperatures up to 250C.

Peek to Peek pairing also allowed an abrasion resistance several times that of other combinations of thermoplastics tested and therefore guarantees a considerably longer life.

The design of the part, in which Peek polymer is injection moulded over a metal insert, provided significant advantages over alternatives using only thermoplastics.

In addition to further enhancing the strength of the part, it allowed proven mechanisms for fixing the wheels to the axle to be used.

The injection moulding process provides economic high volume production of narrow-tolerance, high strength toothing.

Another advantage of this design is the considerably lower and more easily controllable shrinkage of the thermoplastic component, which once again greatly improves the tolerances that can be achieved.

The mould design and process technology further influence the tolerances that can be achieved and ultimately ensure that the strength and wear requirements can be met.