Polymer offers design freedom in novel gear

Edited by the Engineeringtalk editorial team Mar 18, 2002

TRW of Koblenz, Germany chose Peek polymer for the actuator unit of an electrically controlled parking brake.

TRW of Koblenz, Germany chose Peek polymer for the actuator unit of an electrically controlled parking brake.

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

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

"The gearing unit had to meet highly demanding performance requirements", explains Kevin Jennings, General Manager for Victrex North America.

"For example, 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 52mm in diameter and 30mm in height".

In addition, the performance criteria had to be met at high temperatures of up to 180C, as the actuator is connected directly to the cast housing of the brake.

A number of gearing concepts were considered with the chosen specification being wobble gearing - an unusual concept that places critical demands on design and material choice.

With this design, 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 wheel and the wobble wheel", says Jennings.

"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, required the material to 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", explains Jennings.

Several materials and material combinations were evaluated before selecting Peek polymer 450FC30 grade.

"This lubricated grade was chosen for its unique balance of properties, notably a flexural E- modulus that can exceed that of aluminum, and is maintained at temperatures of up to 150C", says Jennings.

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

By selecting Peek polymer it was possible to increase efficiency to 75%, a significant achievement for gearing with such a high transmission ratio in one stage.

"The polymer provides a coefficient of friction that is two thirds less than that of the base materials and remains virtually constant at temperatures up to 250C, says Jennings.

" Peek to Peek pairing also allows 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, provides significant advantages over alternatives using only thermoplastics.

"In addition to further enhancing the strength of the part", says Jennings, "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 the design is the considerably lower and more easily controllable shrinkage of the thermoplastic component, which 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", adds Jennings.