拧紧螺母或螺钉时,螺杆将受轴向预紧力T(图1),这时加在扳手上的力矩M为:M= 式中:P——作用在扳手上的力(通常人作用在扳手上的臂力约15～20公斤); L——扳手力臂长度,一般L≈14d:毫米(d,为螺纹中径)。该力矩M不仅需要克服螺纹工作面上的摩擦阻力矩M_p,而且还要克服螺母与被连接件支承面之间或螺母与垫圈之间的摩擦阻力矩M_o。这两个力矩的大小由机械原理可知。对螺纹工作面上所产生的摩擦阻力矩为: Tighten the nut or screw, the screw will be subject to axial preload T (Figure 1), then added to the torque on the wrench M is: M = where: P - the role of the force on the wrench Wrench on the arm of about 15 to 20 kg); L - wrench arm length, the general L ≈ 14d: mm (d, for the diameter of the thread). The moment M not only needs to overcome the frictional resistance moment M_p on the threaded working face, but also overcomes the frictional resistance moment M_o between the nut and the connected bearing face or between the nut and the washer. The size of these two moments is known by mechanical principle. The friction resistance torque on the screw face is: