Abstract: Based on the operation of the main gear reducer used for the Φ90 mm pipe rolling mill located at Steel Rolling Plant No.4 of Anyang I & S Co., Ltd., the assembling characteristics of the herringbone gear reducer is analyzed. To eliminate machining and installation errors and take the advantage of the fact that the axial forces resulting from the right and left pitch angles of the herringbone gears counteract to each other in case of normal engagement transmission, relevant modification is carried out to get the two herringbone gears self-aligned during transmission to reduce the axial force towards zero.