Rotary-cam type reclining devices are suitable for use in automobile reclining seats.  The device is lockable by way of a rotary motion of the rotary cam mechanically linked to a reclining lever for forward and backward angular adjustment of a seat back relative to a seat cushion. 
A typical rotary-cam type reclining device includes a base fixed to a seat cushion and having a recessed portion formed with two opposing parallel sidewalls, an arm fixed to a seat back and rotatably supported by the base via a pivot shaft and two toothed blocks slidably guided in the two opposing parallel sidewalls in the base.  A reclining lever is integrally connected to the pivot shaft and a rotary-cam is fixed to the pivot shaft and is operative to mesh or unmesh the radially sliding toothed locks with or from the inner toothed portion of the arm by way of rotation of the rotary cam. 
All known prior art rotary-cam type reclining devices have radially slidable locking members.  For example, U.S. Pat.  No.  5,873,630 includes a base fixed to a seat cushion, a toothed arm fixed to a seat back and rotatably supported on the base, and a radially slidable toothed block slidably disposed between the base and the arm and having an outer toothed portion and a cam contour surface slidably guided by two parallel sidewalls of a recessed toothed block guide.  A rotary cam produces outward and inward sliding motions of the toothed block to mesh and unmesh the outer toothed portion of the toothed block. 
The '630 patent recognizes the need for improved mechanical strength in such typical rotary-cam type reclining devices, and also recognizes the undesirability of simply increasing the thickness of various components to increase the mechanical strength of the reclining device, as well as the undesirability of using an expensive high strength steel.  In order to improve mechanical strength, the '630 patent provides an auxiliary interlocking member which is responsive to deformation of the toothed block resulting from impact applied to the arm.  The interlocking member locks the rotary-cam type reclining device by meshing engagement with the inner toothed portion of the arm.  However, this design merely adds additional parts to the assembly, thereby resulting in increased manufacturing and assembly costs. 
It is therefore desirable to provide an improved rotary-cam type reclining device wherein mechanical strength is improved without increasing manufacturing costs or weight.