The Thread Types Available in the Thread Device Are Rectangular, Trapezoidal and Serrated, While the Jack is Commonly Used As Trapezoidal Thread, and Some People Say It is Rectangular Thread. in Fact, This Statement is a Previous Statement, Because the Serrated Thread is Used for Single-sided Stress, and the Rectangular Thread is Rarely Used Due to Poor Workmanship and Low Strength, and then It is Gradually Replaced by Trapezoidal Thread.

Trapezoidal Thread Profile is Isosceles Trapezoid with Tooth Angle α= 30. the Internal and External Threads of Trapezoidal Thread Are Tightly Attached with Conical Surface and Are Not Easy to Loosen. the Root Strength of Rectangular Thread is Low, the Tooth Form of Serrated Thread is Unequal Waist Trapezoid, and the Processing Cost is High. in Consideration of Practicability, Trapezoidal Thread is Selected.

The Ordinary Screw Jack Supports the Heavy Object by the Thread Self-locking Function, So That the Heavy Object is in a Standing State, While the Thread of the Self-lowering Jack Has No Self-locking Function, and the Lifting of the Heavy Object Needs to Be Controlled by the Internal Brake.