Lead screw is a power transmission linkage used in modern machines. It can generate very high forces with a small moment, thus providing a large mechanical advantage. It can be thought of as a wedge wrapped around a cylindrical rod.In applications such as screw jacks and mechanical presses, it is used to create large forces. They also find use in applications needing extremely precise motion transfer and control, such as linear actuators and linear stages.As the sliding contact area between the screw and the nut is high, a lead screw has more friction losses compared to other alternatives such as gear trains and chain drives. This characteristic generally limits a lead screw’s use to light- and medium-duty applications.How Does a Lead Screw Work?A lead screw works in one of two ways:
The shaft is stationary and the power is supplied to the nut
The shaft rotates and transfers power to the nut
In the first case, manually applied force or a motor rotates the nut. This pushes the nut along the shaft’s axis. Ultimately, the torque applied to the nut is transformed into linear motion.In the second case, the nut’s rotational motion is restricted and the screw shaft rotates. The nut moves along the screw axis in the process. Thus, the torque on the screw shaft converts into linear motion of the nut.Lead Screw ComponentsThe lead screw itself is a small component in many complex assemblies. But even a basic lead screw can be broken down into three main components/features. These are:
The screw shaft is a cylindrical rod with threads or grooves along its length. At times, it may be referred to as an ACME rod, buttress rod or square-threaded rod as per the thread geometry. The most common materials used for lead screw shafts are carbon steel, stainless steel and aluminum. PTFE-based coatings are often used in harsh environments for durability and to remove the need for oil and grease.The threads are present on the screw shaft and the nut. Although it is not a separate component in itself, the thread’s structure is responsible for converting rotational motion into linear motion. The external threads are in direct contact and mesh with the nut’s internal threads.The nut is a fairly simple component but does not offer much use by itself. Typically, it will have some means, such as tapped or through holes, to connect it to supporting components like a clamp. The clamp connects the nut to a guide rail, restricting the nut’s rotation and only allowing linear motion. Nuts can be made from plastic or bronze to add self-lubricating properties.Apart from these components, lead screws may require additional parts depending on the function. For instance, to control backlash, precision lead screws are fitted with loaded springs that create an axial load to prevent unintended axial movements.