This invention is revises the cap nut which the custom knows to suppose it to install trough's outline, will cause to install the dadoing to lead (lead angle to be small) lowly or on two above trundle ditch cap nut will not have the broken tooth's question, when will process should install the trough, only need use the face cutter then to process the forming, will reduce the production cost; And return circuit of this invention all establishes in the backflow part, therefore may promote and the cap nut supposes the backflow hole and the load way grade of fit, causes to roll the moving parts movement to be smoother, and reduces the noise production.