What is a Ball Screw?

Ball screws are mechanical linear actuators that consist of a screw shaft and a nut that contain a ball that rolls between their matching helical grooves. The primary function of ball screws is to convert rotational motion to linear motion. Ball nuts are used in transmitting forces to a stationary or dynamic load with high accuracy, precision, and repeatability.

The unique element of ball screws is the rolling balls in the helical groove which reduces the mechanical contact inside the screw assembly and replaces sliding friction with rolling friction. This mechanism significantly lessens the friction generated, which results in highly efficient power conversion. The efficiency of screws is measured by their capability to transform power utilized in exerting rotational force to the linear distance covered.

Ball screws have more complex structures and components and appear to be bulkier than other types of screws. They are more expensive than other screw types, but their benefits and capability outweigh their cost.

Components of the Ball Screw:

The main components of ball screws are the screw shaft, the nut, and the ball bearings (or balls). We will discuss their important specifications and their impact on the operation.

Screw Shaft

The screw shaft is the component of the ball screw that receives rotational force for it to rotate about its axis; this is translated to linear motion. The power to rotate the shaft is supplied by a motor, which is situated on its end.

The screw shaft is a long cylindrical shaft that has a continuous groove, called the ball groove, that runs helically around the length of its shaft, which is referred to as the thread of the screw.

The ball groove serves as the pathway for the rolling ball bearings. The ball groove profile of a ball screw may either be a semi-circular arc or a gothic arc. The semi-circular arc profile is formed from a single arc, while the gothic arc profile has an ogival shape formed from two arcs. The two groove profiles differ on the contact points they touch on the ball.

The ball screw is identified as a right-hand ball screw if the groove is traced in a clockwise direction around the screw shaft and slants to the right. It is a left-hand ball screw if the groove runs in a counterclockwise direction and it slants to the left.

Specifications of the Screw Shaft:

Nominal Diameter

The nominal diameter is the maximum diameter of the screw shaft excluding the ball bearings.

Ball Circle Diameter

Ball circle diameter (or pitch circle diameter) is the center-to-center distance of two opposite balls measured when the ball bearings are in contact with the grooves.

Root Diameter

Root diameter is the distance between the bottommost section of the groove to the bottommost section of the opposite groove. It is the minimum diameter of the screw shaft.

The nominal diameter, ball circle diameter, and root diameter are used in calculating the application characteristics and in sizing the screw.

Pitch

Pitch is the axial distance between two adjacent threads.

Lead

Lead is the linear distance along the axis of the screw that is covered by one complete rotation (3600) of the screw. It is an important specification of a ball screw which determines linear travel and speed and load capacity.

As the lead of ball screws increases, the linear distance covered and the speed increase. However, the number of balls accommodated around the screw shaft decreases, resulting also in a decrease in load capacity.

Start

The start is the number of independent helices that run around the screw shaft. Screws typically have one, two, or four starts. The lead of a screw is equivalent to the number of starts multiplied by the pitch. Single start screws are the most common. In this type of screw, the lead is equivalent to the pitch.

Multiple start screws are used when a rapid linear movement is for a lesser number of rotations and high load capacity screws. This solves the consequences of using higher leads. For instance, in double-start screws, the lead is equivalent twice its pitch; this means that the axial distance covered is two-pitch units for one full rotation completed by the screw. The higher number of starts has the higher linear distance covered in one revolution.

Nut

The nut of a ball screw is a cylinder that houses the ball bearing and its recirculation system. Ball grooves are also present in the internals of the nut that match the grooves of the screw shaft.

Specifications of the Ball Nut:

Circuit

Circuit refers to the closed path in the recirculating system of the ball nut. Multiple circuit ball nuts have two or more independent closed paths. They are capable of carrying heavier loads than single circuit ball nuts.

Turn

The turn of the circuit refers to the number of trips the ball travels before being recirculated in the circuit. The relationship between the turn and the circuit depends on the recirculation method of a ball screw.

The ball recirculation system in the nut allows the balls to be "recycled" in the operation by returning them to their starting point in the circuit. Deflectors, return tubes, and end caps are devices used to feed the ball back to its original position.

Ball Bearings

The ball bearings, or the balls, are the most prominent component of the ball screws that moves between the clearance of the nut and the shaft. They make up the component dedicated to reducing the friction generated by the nut and the moving screw; this friction would be too great if the balls were not present in the screw assembly. The balls are usually made from steel.

The ball is in contact with the screw shaft and the nut. The contact points of the ball between the screw shaft and the nut are distinguished by the groove profile of the two components where the ball touches. The common groove profiles are the gothic arc and the circular arc:

Gothic Arc Profile.

This is the most common groove profile. It is made by two intersecting arcs. With this profile, the ball has two contact points on the screw shaft and two contact points on the nut.

Semi-Circular Arc Profile.

This groove profile has the shape of a semicircle. With this profile, the ball has one contact point on the screw shaft and one contact point on the nut.

The arc in both profiles has slightly larger radii than the ball used. With this design, clearance between the ball and the ball or nut is inevitable. This clearance is unwanted because it causes backlash. It can be prevented by preloading the ball screw.

The seal is a minor component of a ball screw that protects the entire ball screw assembly. It preserves the efficiency of the ball screw by keeping contaminants and foreign materials from entering the clearance between the ball and the nut; it also retains the lubrication of the ball screw assembly.