14 Nov 2001
PEEK™ polymer provides high performance and design freedom for electric parking system
TRW, a global technology, manufacturing and service company, 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 180ºC, 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: -40°C - +120°C
peak temperature: +180°C
- 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 150ºC. 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 250ºC. 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.
Reader enquiries
Helen SwarbrickVictrex plc
Victrex Technology Centre
Hillhouse International
Thornton Cleveleys
Lancashire FY5 4QD
United Kingdom
Tel +44 1253 897700
Fax +44 1253 897701
United Kingdom
Notes for editors
Victrex plc, which has its sales, technical and production headquarters at Thornton Cleveleys in Lancashire, UK, had a turnover of more than £58m in the last financial year. The company exports 98 percent of its VICTREX® PEEK™ polymer output to OEMs in the automotive, industrial, medical, food and electronics market sectors in Europe, North America and Asia Pacific.
Information on products, technical support services and sales and distribution contacts worldwide, can be accessed through the company’s web site: www.victrex.com
The PEEK™ Trademark
For many years Victrex plc has been using the PEEK™ trademark to refer to its brand of Polyaryletherketone. Victrex plc is the sole manufacturer and supplier of the PEEK™ brand polymer world-wide. PEEK™ is a trademark of Victrex plc.
Related images
Photo caption:
TRW chose VICTREX® PEEK™ polymer to provide high performance and design freedom for a new electrically controlled parking brake
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