What Is A Lead Screw?

A 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.

This is another simple machine that can generate very large forces. The screw can be thought of as a wedge or ramp that has been wrapped around a shaft. Holding a nut and rotating the shaft will allow the nut to slide either up or down the shaft via the key.

In this way, a relatively small moment on the shaft can exert very large forces on the nut.

They are usually not used for high power transportation, but rather for intermittent use in low power actuator and positioning mechanisms.

Lead screws are commonly used in linear actuators, machine slides such as machine tools, vices, presses, and jacks. Lead screws are a common component in electric linear drives. Leadscrews are made in the same way as other thread forms. They can be rolled, cut or sanded.

Sometimes a lead screw is used with a split nut, also known as a half nut. This allows the nut to be loosened from the threads and, if necessary, moved axially independently of the rotation of the screw (e.g., single-point threading on a manual lathe. A split nut can also be used to reduce wear by compressing the parts of the nut balance.

A hydrostatic lead screw overcomes many of the disadvantages of a normal lead screw with high positional accuracy, very low friction, and very little wear, but requires a continuous supply of high-pressure fluid and high precision manufacture, which results in significantly higher costs than most other linear motion linkages.

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 of transmitting the force generated by the rotation of the lead screw.

• Screw,
• Nut
• Part to hold either the screw or the nut in its place.

Application Of Lead Screw

Common applications of power screws are: Screw jacks: to lift weight. Lead screw of lathe: for axial movement of tool and its precise positioning. Tensile testing machine: to exert large force.

Also power screws operate in two different ways, depending on the mechanism of holding.

• The power screw typically uses square threads, but ACME or buttress threads may also be used. Power screws should be designed for the smooth and noiseless transmission of power with the ability to carry heavy loads with high efficiency.
• Either the screw or nut is placed comfortably and the other member rotates as it moves axially. A typical example of this is a screw clamp.
• Either screw or nut rotates but does not move axially. A common example of this is the press.
• Other applications of lead screws are, vices, screws for presses, jack screws, lathe lead screws, etc.

Lead Screw Mechanism

A lead screw uses the helix angle of the thread to convert rotary motion into linear motion, combining a screw and nut with the screw thread in direct contact with the nut thread. In the case of roller screws, the rollers are in direct contact with the screw thread instead of the nut, which offers greater efficiency.

The performance of a lead screw depends heavily on the friction coefficient between nut and screw, which in turn depends on the material used for nut and screw.

The main terminology of the lead screw can be defined as follows

• Pitch(p): It is the distance measured parallel to the axis of the screw from a point on the thread to the corresponding point on the adjacent thread.
• Lead(l): It is the distance measured parallel to the axis of the screw It is the distance measured parallel to the axis of the screw which the nut will advance in one revolution of the screw. For the single-threaded screw, the lead is the same as the pitch while for the double-threaded screw, the lead is twice the pitch, and so on.
• Nominal diameter (d): It is the largest diameter of the screw. It is also called the major diameter.
• Core diameter(dr): It is the smallest diameter of the screw thread. It is called the minor diameter.
• Helix Angle (λ): This is the angle formed by the helix of the thread with the vertical plane on the axis of the screw. The helix angle is related to the lead and means the diameter of the screw.

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 the 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.

Types Of Screw Thread

External threads are on bolts or screws. Internal threads are on nuts. There are many forms of threads but two types are in common use on fasteners. Machine Screw Threads used on bolts, setscrews, machine screws and designed to mate with preformed threads in nuts or tapped holes.

There are three standard thread series in the Unified screw thread system that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).

On the other hand, the purpose of power transmission threads is to reduce friction between the screw and mil. Therefore, V threads are not suitable for lead screws. So, the following types of threads used in the design.

1. Square Threads

Square threads can be used to transmit power in any direction. It have maximum efficiency and there is no radial or bursting pressure on the nut, which increases the life of the nut and makes it uniform. But it is difficult to construct square threads.

These are difficult to cut with taps and dies and are usually cut on a lathe with a single-point cutting tool, making it expensive. Also, it is not possible to compensate for wear in square threads as the split nut cannot be used with it.

Therefore, a nut or screw has to be replaced, when worn out. Square threads are used in screw jacks, presses, and clamping devices.

2. Acme Or Trapezoidal Threads

Wear can be compensated in this case by using a split nut, which is a nut cut into two halves along its diameter. These two halves are tightened together after certain intervals to take care of the wear taken place.

3. Buttress Threads

Buttress thread is designed to take large loads in one direction. It is the strongest thread form due to the greater thickness of the root. Its efficiency is comparable with the square threads, which is easier to cut and is compatible with the split nut also. The buttress thread finds its application in lightweight jack screws and vices.

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 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 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.

FAQs.