Freudenberg Sealing Tech Develops New Plastic Piston

When automotive manufacturers use smaller hydraulic systems to actuate transmission gears, it helps to optimize energy efficiency. When a steel piston is used in the accumulator, it needs an elaborate sealing package and limits the compactness of hydraulic systems. Freudenberg Sealing Technologies has now come up with a new plastic piston that uses single-part sealing technology thus making installation of smaller hydraulic systems feasible in order to save energy. Instead of a three part sealing package, the new piston sealing package has only one part and is hence lighter and easier to mount and lighter in weight.

Dual clutch transmissions make driving more fun whether they are used in internal combustion engines or as a part of hybrid powertrains. There is no change in the tractive force with gear change and for many years hydraulic accumulators have been used to carry out the hydraulic activation of the gear actuator and to cushion the impact of peak hydraulic demand as efficiently as possible. They comprise a gas side and a liquid side, separated by a piston.

When the vehicle is turned on and the hydraulic pump starts up, the lower pressure point is reached, and the hydraulic accumulator takes in the hydraulic fluid that becomes available. The gas, which is nitrogen in this case is compressed, storing energy. When the pressure goes down, the gas spreads out and this forces the hydraulic fluid into either the transmission circuit or to the actuators, depending on the driving scenario. The hydraulic pump and the electric motor can thus be made smaller. A system having a hydraulic accumulator needs only about one-sixth the energy of a conventional, constantly operating pump.

Thus, Freudenberg Sealing Technologies’ development of the new plastic piston will be a game-changer as it replaces the steel piston with a plastic piston. Steel pistons are produced in a forming process and come with an elaborate sealing package that comprises two guide rings for the necessary slide properties.
The actual piston ring needs to be secured with an additional backup ring which can block the seal’s gap extrusion between the piston and the housing. The new plastic piston, on the other hand is produced in an injection moulding process, and needs just one seal ring to be mounted, thus eliminating the need for the two guide rings that promote a sliding effect.

For making the plastic piston, the material specialists at Freudenberg Sealing Technologies have chosen a material that demonstrates the right slide characteristics between the piston and the housing. They also tweaked the thermal properties of the material making it possible to allow a smaller gap between the piston and the housing and thus eliminating the need for a backup ring in order to prevent the seal extrusion.

As an outcome of this change in materials, the weight of the piston can be reduced by half. With fewer components being used in the installation of a hydraulic accumulator, the procedure becomes a lot simpler and this increases the reliability of the production process.

Another major challenge for the engineering team at Freudenberg Sealing Technologies was to find a plastic for the piston that was almost as gas-tight as the steel that is conventionally used. They came up with a thermosetting composite material that is impermeable to gas. The material also had to be resistant to the entry of transmission oil and hydraulic fluid, exhibit an appropriate expansion coefficient at high temperatures, and offer the slide characteristics that the sealing system needed.

The material also had to be highly durable. The piston seal in a hydraulic accumulator should be capable of tolerating several million operating cycles. With a special seal geometry, the engineers guaranteed long operating lives even under severe loads and temperature cycles.
Working with a German car manufacturer, Freudenberg Sealing Technologies is further refining its hydraulic accumulators with plastic pistons, gearing them for use in a series-production hybrid vehicle. In extensive test stand trials and test drives, the technology has already shown the maturity it will need on the road.