What is a Lead screw?- Definition, Parts, And Working

A lead screw is a type of mechanical power transmission used in machines for high-precision actuation. It converts rotational motion into linear motion. Other names for a lead screw (or leadscrew) are power screw and translation screw.

Lead screws are sometimes confused with threaded rods. Although they might look identical to an untrained eye, the threads on a threaded rod are a lot finer. Thus, increasing the risk of thread deformation under load.

In this article, we’ll take a closer look at different lead screw types, their design, use-cases, advantages and disadvantages. So let’s get to it.

What is a Lead screw?

A leadscrew (or lead screw), also known as a power screw or translation screw, is a screw used as a linkage in a machine, to translate turning motion into linear motion. Because of the large area of sliding contact between their male and female members, screw threads have larger frictional energy losses compared to other linkages.

They are not typically used to carry high power, but more for intermittent use in low power actuator and positioner mechanisms. Leadscrews are commonly used in linear actuators, machine slides (such as in machine tools), vises, presses, and jacks. Leadscrews are a common component in electric linear actuators.

Leadscrews are manufactured in the same way as other thread forms (they may be rolled, cut, or ground).

A lead screw is sometimes used with a split nut (also called half nut) which allows the nut to be disengaged from the threads and moved axially, independently of the screw’s rotation, when needed (such as in single-point threading on a manual lathe).

A split nut can also be used to compensate for wear by compressing the parts of the nut.

A hydrostatic leadscrew overcomes many of the disadvantages of a normal leadscrew, having high positional accuracy, very low friction, and very low wear, but requires a continuous supply of high-pressure fluid and high precision manufacture, leading to a significantly greater cost than most other linear motion linkages.

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 Parts

Every screw requires a nut, and lead screws are no exception. The lead screw nut is a part with an internal thread that mates with the external thread of the lead screw and provides a means of attaching the nut to the machine and transmitting the force generated by the rotation of the lead screw.

The components of a lead screw are the following:

• Screw Shaft: The screw shaft is a cylindrical rod that has a single or series of grooves running helically around its length; this is referred to as the external thread.
• Thread: The thread is the structure responsible for converting rotational motion into linear motion as the screw shaft and the nut slide with each other.
• Nut: The lead screw nut is a cylindrical section that has an internal thread that matches the external thread of the screw shaft.
• Major Diameter: The major diameter is the largest diameter of the thread. The major diameter of the screw shaft is the distance between two opposite crests, while the major diameter of the nut is the distance between two opposite roots.
• Minor Diameter: The minor diameter is the smallest diameter of the thread. The minor diameter of the screw shaft is the distance between two opposite roots, while the minor diameter of the nut is the distance between two opposite crests.
• Crest: A crest is the raised helical structure in an external thread (screw shaft) and the recessed helical structure in an internal thread (nut).
• Root: A root is the recessed helical structure in an external thread (screw shaft) and the raised helical structure in an internal thread.
• Tread Depth: The thread depth is the distance from the root to the crest, measured radially.
• Flank: The flank is the surface that connects the root to the crest.
• Pitch Diameter: The pitch diameter, or the effective diameter, lies concentrically and approximately halfway between the major and minor diameters. It is the diameter of the imaginary cylinder whose circumference intersects half of the thread pitch.
• Pitch: The pitch is the axial distance between two adjacent threads measured parallel to the axis. It is equivalent to 1/number of threads per inch.
• Lead: The lead is the linear distance traveled by the screw shaft or nut along its axis in one complete revolution (3600 rotation). As the lead increases, the linear speed also increases, but the load capacity of the lead screw decreases.
• Thread Starts: The number of starts refers to the number of independent threads running around the length of the thread. The lead of a screw is determined by multiplying the number of independent threads by the pitch.
• Helix Angle: The helix angle is the angle formed between the helix of the thread and the line perpendicular to the axis of rotation. Generally, a lead screw with a higher helix angle has lesser frictional losses and therefore has higher efficiency. This is because the number of revolutions to rotate such a screw is lower than a screw with a lower helix angle for the same linear distance covered. However, it requires more torque to rotate the screw.
• Lead Angle: The lead angle is the complementary angle of the helix angle. It is the angle formed between the helix of the thread and the line parallel to the axis of rotation.
• Thread Angle: The thread angle is the angle formed between two adjacent threads.
• Screw Handedness: Screw handedness refers to the direction in which the thread runs along the length of the screw. A lead screw may be right-handed or left-handed. In right-hand and left-hand screws, the thread runs around the screw length in a clockwise direction and counterclockwise direction respectively.

Pitch vs Lead

Pitch is the distance between screw grooves and is commonly used with inch-sized products and specified as threads per inch. Lead is the linear travel the nut makes per one screw revolution and is how ball screws are typically specified. The pitch and lead are equal with single-start screws.

For multiple start screws, the lead is the pitch multiplied by the number of starts.

lead screw examples

Here is a list of real-world examples of a lead screw:

• Lathes
• Vices
• Jacks
• CNC machines
• Engraving machines
• Fluid handling machines
• Linear actuators
• Linear stages
• Screw jacks
• Mechanical presses
• Appliances such as printers, disc drives, lifting equipment, and robots.
• Scanners
• Recorders
• Wire bonders
• Machine slides (such as in machine tools)
• Visas
• Heavy lifting applications when combined with a metallic (typically bronze) nut.
• Laboratory and life sciences equipment such as DNA sampling, scanning, and fluid handling devices
• Rapid prototyping

What is a lead screw used for?

Lead screws are commonly used as a component within linear motion control systems. They are well suited to instrument grade applications such as;

• Laboratory and life science equipment including DNA sampling
• Fluid handling devices
• Engraving
• Rapid prototyping
• Data storage
• Inspection
• Heavy lifting applications – a metallic nut must be used for this type of application

When selecting a lead screw, it is important to take into consideration the speed, thrust, accuracy, and repeatability of the application.

Material Selection for Lead Screw

The screw is subjected to torque, axial compressive load, and bending moment also, sometimes. Screws are generally made of alloy steel.

As the failure of lead screws may lead to serious accidents, a higher factor of safety of 3 to 5 is taken. Threads may fail due to shear, which can be avoided by using a nut of sufficient height. Wear is Another possible mode of thread failure is as nuts and bolts rub against each other.

Nuts are made of a softer material than screws so that if at all failure takes place, the nut fails and not the screw, which is the costlier member and is also difficult to replace.

Plastic, bronze, or copper alloys are used to manufacture nuts. Plastic is used for low-load applications and has good abrasion and wear properties. Bronze and copper alloys are generally used for high-load-carrying applications.

Advantages of a lead screw

• Cost Less
• Self-Locking, Need No Braking System
• Better For Vertical Applications
• High load-carrying capacity
• Compact
• Easy to design into a system
• Minimal number of parts
• Smooth operation
• Quiet
• Require little maintenance

Disadvantages of Lead Screw

• Require More Torque
• Require A Larger Motor And Drive
• Generate More Friction
• Generate More Heat
• Slower
• Less Efficient
• Shorter Lifespan

FAQs.