求翻译：PDJ型皮带输送机是将一条有端（或无端）的橡胶输送带，装在输送机两端的主动滚筒和从动滚筒上。在两个滚筒中间安装有若干个平行的托辊和托板，以支撑物料和输送带的重量。主动滚筒、从动滚筒和托辊均装在机架上。主动滚筒由驱动装置带动旋转。依靠旋转的滚筒与输送带之间的摩擦力使输送带运动，从而达到运送物料的目的。为了使输送带有合适的张紧度，还设有张紧装置。是什么意思？

PDJ-type belt conveyor is a port (or unwarranted) rubber conveyor belt, mounted on each end of the conveyor drive roll and driven roller. In the middle of two rollers installed have several parallel roll and pallet, with supporting material and weight of the conveyor belt. Active roller, a driven roller and roller is mounted in the rack. Active roller driven by driver rotation. Relying on rotating friction between rollers and conveyor belt conveyor belt movement, so as to achieve the purpose of delivering materials. In order to make transportation with an appropriate degree of tension, also with tensioning device.