The invention relates to a line element composed of an inner and outer element. Such line elements are preferably installed in exhaust pipes in motor vehicles and flexibly connect individual, rigid line units with each other.
In the exhaust system of motor vehicles, vibrations are always produced, which are mostly caused by imbalances of rotating elements in the engine, turbo or ancillary units. Furthermore, vibrations are excited by the pulse-shaped pressure curves of the internal combustion engine. In addition to these higher-frequency vibrations, frequencies in the single-digit and lower two-digit Hertz range can be excited by the driving movements in conjunction with road bumps and their feedback into the chassis.
The line elements described above have the task of decoupling such vibrations and movements in the exhaust system of motor vehicles. In addition, they compensate for possible installation tolerances.
In the case of flexible line elements which are insufficiently damped, the vibrations induced in the exhaust system during operation cause excitation of natural frequencies, which when severe have a service-life-reducing effect on the flexible line element. Their vibration behavior is characterized by their masses and spring stiffness. In the case of metal bellows with corrugations formed symmetrically with respect to the rotation axis, the natural modes associated with the small natural frequencies are frequently standing longitudinal waves in the bellows structure, with the number of oscillation nodes increasing with the order of the natural frequency. The amplitude of these natural frequencies decreases with decreasing rigidity. Therefore, the natural frequencies of such components generally decrease with increasing diameter and length. Especially in the commercial vehicle sector, where the diameters and lengths of the flexible line elements are larger due to the system design, the use of metal bellows without a damping system adapted to the specific configuration is not promising.
Flexible line elements therefore have damping units which when, subjected to vibrational loads, preferably operate on the basis of frictional contact. In this case, metal mesh or metal braids are primarily used.
The patent DE 196 41 963 C1 discloses a damped line element, which includes in the exterior region as well as in the interior region a metal braid that is in contact with the turns of the line element over the entire component length. In this case, the contact area between the line element and metal mesh extends in the radial direction in a completely enclosing manner.
Line elements attenuated by an outer metal mesh are known from the published patent applications DE 199 24 476 A1, DE 40 42 291 A1 and DE 10 2005 052 204 A1, wherein a portion of the turns is in frictional engagement with the mesh for producing the damping, while other turns are positively or non-positively connected specifically with the mesh. The outer contact region extending over the entire component length is completely encircling in the radial direction. The line elements with damping known from the prior art have the disadvantage that, to provide the damping function, additional components must always be installed in the line element, which represents an increase in weight and material.
Furthermore, DE 199 13 562 A1 discloses a flexible hose composed of an outer bellows and a metal hose arranged therein, which are in contact with one another for the purpose of damping vibrations.