What is a Lead screw?
A lead screw (or lead screw), also known as a power screw or translation screw, is a screw that is used as a linkage in a machine to translate rotary motion into linear motion. Because of the large sliding contact area between their male and female elements, screw threads have greater frictional energy losses compared to other linkages.
They are typically not used to transmit high power, but rather for intermittent use in low-power drive and positioner mechanisms. Lead screws are widely used in linear actuators, machine slides, vices, presses, and jacks. Threaded spindles are a common component in electrical linear actuators. Screw spindles are manufactured like other thread forms.
A lead screw is sometimes used with a split nut, also called a half nut, which allows the nut to be loosened from the threads and moved axially, if necessary, regardless of the rotation of the screw. 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 with high positional accuracy, very low friction, and very little wear, but requires a continuous supply of high-pressure fluid and high precision manufacturing, which results in a significantly higher cost than most other linear motion linkages.
How Does A Lead Screw Work?
Lead screws are threaded bars of metal and a threaded nut which is in direct contact with the screw; this generates sliding friction as opposed to rolling friction from other alternative devices (such as a ball screw). A rotational motion will turn the screw, causing the nut to move along in a linear motion. This, therefore, converts the motion from rotary to linear.
A lead screw works in one of two ways: the shaft is stationary, and the power is supplied to the nut, or the shaft rotates and transfers power to the nut.
The 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: screw shaft, threads, and nut.
The screw shaft is a cylindrical rod that is wrapped with a wedge. The threads are the grooves that wrap around the screw shaft, and the nut is a component that contains the threads that match the screw shaft.
When the screw shaft rotates, the nut moves linearly along the screw shaft. The sliding contact area between the screw and the nut is high, so 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.
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:
- 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.
- Wire bonders
- Machine slides (such as in machine tools)
- 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
Lead Screw Used in Real Life
Lead screws are widely used in instrumentation applications where smooth, precise, clean, and maintenance-free operation is required. The nut material is typically made from an internally lubricated polymer.
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.
Some examples would be laboratory and life science equipment such as DNA sampling, scanning, and liquid handling equipment. Other applications are engraving, rapid prototyping, inspection, and data storage. Lead screws are also used in heavy-lifting applications when combined with a metallic (typically bronze) nut.
Lead Screw Mechanism
A lead screw converts rotary motion into linear motion that combines a screw and nut, with the screw thread in direct contact with the nut thread. In the case of roller screws, the rollers and not the nut is in direct contact with the thread, which offers greater efficiency.
In the case of roller screws, the rollers and not the nut is in direct contact with the thread, which offers greater efficiency. There are three types of screw manufacturing processes, either rolled, ground, or whirling. Power Jacks uses all methods in the manufacture of our lead screws.
Lead screws are used in a very wide range of applications, sold as a single product or built into screw jacks and electromechanical actuators. As with screw jacks, industrial automation, medical, defense, and transportation applications are particularly popular.
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 Power screw
- A power screw has a large load-carrying capacity.
- The overall dimensions of the power screw are small, resulting in a compact construction.
- A power screw is simple to design.
- The manufacturing of a power screw is easy without requiring specialized machinery. Square threads are turned on the lathe. Trapezoidal threads are manufactured on a thread milling machine.
- A power screw provides a large mechanical advantage. A load of 15 kN can be lifted by applying an effort as small as 400 Therefore, most power screws are used in various applications such as screw-jacks. The clamps, valves, and vice are manually operated.
- A power screw provides precisely controlled and highly accurate linear motion required in machine tool applications.
- A power screw provides smooth and noiseless service without any maintenance.
- There are a few parts in a power screw. This reduces cost and increases reliability.
- A power screw can be designed with a self-locking property. In screw-jack application. self-locking characteristic is required to prevent the load from descending on its own.
Disadvantages of Power Screw
- It has poor efficiency; Therefore, it is not used in continuous power transmission in machine tools, with the exception of the lead screw. the lead screw is mainly used for intermittent motion that is occasionally required for lifting the load or actuating the mechanism
- High friction in threads causes rapid wear of the screw or the nut. In the case of square threads, the nut is usually made of soft material and replaced when worn out. In trapezoidal threads, a split type of nut is used to compensate for the wear. Therefore, wear is a serious problem in lead screws.