| 摘要 |
A core includes coaxial inner and outer tubes having a plurality of ribs extending therebetween defining a plurality of empty pockets. The inner tube has a thickness which is at least as large as the thickness of the outer tube for increasing crushing resistance of the core. In a preferred embodiment, each of the pockets includes four corners having a radius greater than the thicknesses of the inner or outer tubes or both.
|
