The main meaning is that the design to make some specific requirements to achieve consistency.
Specification is: can not affect the performance of the product being used, and fully guarantee the effect of fastening.
Easy to install, disassemble; to ensure long-term solid does not fail (life); do not exceed the standard selection, cost savings;
Referred to the selection must be selected in the national standard parts, otherwise the changeability does not exist. Speaking of norms nothing less than the following: function according to national standards (strength, rust, shape ....);
