Introduction
Power Screws are one of the common machine elements used in various applications that require converting the rotating motion (Torque input) into a linear motion (Force output). The best example of a power screw is the Lifting Jack used for changing the car wheel. Power Screws are also called lead screws and are used in machines such as actuators, Production Machines, Automation, lifting mechanisms, and height adjustment systems (like in a Mechanical Press), Laboratory equipment, Medical equipment, etc. Due to their design principle, they are capable of very large mechanical advantage so as to move or lift extremely heavy loads with relatively smaller amounts of torque. Fasteners are not capable of taking such a high load as they are designed for joining two parts in stationary condition so the strength of their Threads are not sufficient.
Type of Thread (of Power Screw)
Power screws are (generally) of three types based on the shape of the thread- Square Thread, Acme Thread, and Buttress Thread
Square Threads have the highest efficiency and performance. Moreover, theoretically, there is no radial component of force between the screw and the nut. But it is not possible to manufacture as they have a perpendicular face (cutting tools can not cut such shapes on a lathe machine). Not only this, if a screw requires heat treatment then it will not be possible in the case of the square thread due to the sharp edge (corner). So Acme and Buttress threads are designed for manufacturing feasibility. Both Acme and Buttress threads have included angles and also allow the use of the Nut in 2 parts (split nut) so that the assembly process becomes easier. In the long term, when the nut gets worn off (due to high friction), it is easy to squeeze the nut radially against the thread. Acme threads can take loads in both directions whereas Buttress threads can take loads in only one direction. However, the strength of the Buttress thread is higher due to the larger area at the root.
Online Calculator for Deisng of Power Screw
Use the Below program to calculate the Torque required to Lift & Lower the screw and the Von-mises stress generated inside the screw. This calculator can be used to calculate the parameters to start the design process. Actual Von-mises should be calculated by Finite Element Method for complex shapes and analyze the stress in small regional areas of components from where failure can start.
Follow the guidelines carefully to use the below program for accurate results.