Ball screw drives - function / structure / types / precision classes

Ball screw drives are manufactured according to the selection criteria and specifications of DIN ISO 3408 or JIS B 1192, etc.

Ball screw drives are high-precision components used in various applications to convert rotary motion into linear motion. The areas of application for ball screw drives are wide-ranging and include areas such as CNC machines, 3D printers, robotics, machine tools and many other industrial applications.

Styles of lead screws

There are different versions of lead screws. They differ, for example, as follows:

• By design principle (rotating, stationary lead screw), linear guide of the nut
• By ball return (inside or outside)
• Left or right-hand start
• Single or multiple
• Precision: Transport or positioning screw drive

Single ball screw drives

Single ball screw drivers consist of a single lead screw with one ball screw nut. They are widely used in industry and are used primarily in applications with low loads and high speeds.

Multiple ball screw drives

Unlike single ball screw drives, multiple ball screw drives consist of a ball rotation nut with several ball rows. The additional rows of balls lead to a higher load capacity and stiffness and are often used in heavy-duty applications.

With linear guide

A ball screw drive with a linear guide has one or more guide rails on which the ball nut (which carries the carriage or the load) slides. The function of the guide rail is to guide and support linear motion and to achieve precision and stability. The advantages are high precision, repeatability and stiffness. Ball screw drives with linear guides are particularly suited for applications that require precise positioning and stability, such as CNC machines and machine tools.

• 1 Lead screw
• 2 Ball screw nut with flange
• 3 Bearing
• 4 Carriage
• 5 Motor with gear box

Without linear guide

For ball screws without linear guides, the ball screw nut moves directly on the lead screw without a separate linear guide rail. The nut slides along the lead screw. The advantage is that screws without separate linear guides are used in applications that have space limitations or require simple designs. They may offer a less expensive solution in less demanding applications.

Precision classes

The precision classes of ball screw drives are specific classifications that describe the accuracy and tolerances. They are specified in relevant standards such as the Japanese JIS B 1192 (ISO 3408). ISO 3408 specifies, for example, the difference between the target path and the nominal path over the useful path. The following tolerance classes result:

• C0 - C5: Defined by the mean path deviation and the tolerance of the path fluctuation
• C7 - C10: Defined by the mean distance deviation over a length of 300 mm.

The ISO 286 standard provides an alternative classification for tolerance levels used in some standards and provides similar degrees of accuracy as the C classes: IT 1 et seqq.

With regard to axial play, ball screw drives can be either preloaded (no play) or with play. Ball screw drives with a high precision class are usually preloaded. A ball screw drive with pre-load is recommended for precision positioning, for position measurement applications, for scientific device construction and generally for applications that require the highest possible rigidity of the assembly. Ball screw drives with a low precision class are usually subject to play. Ball screw drives subject to play are recommended if torque resilience (smooth travel) is a key requirement, if any tolerance deviations are compensated by a measuring system or if position errors caused by the axial play is irrelevant.

Advantages of ball screw drives

Ball screw drives offer high precision and rigidity, as friction is significantly reduced. This feature provides increased accuracy and efficient transfer of rotary motion to linear motion. The uniform load distribution by the balls over the raceway ensures stability and an effective distribution of the load. Ball screw drives are available as ready-to-install assemblies and achieve high efficiency levels of up to 95%.

Advantages over trapezoidal screws

A ball screw drive offers several advantages over trapezoidal screws:

• Higher precision
• Lower friction: Ball screws have less friction due to the use of balls or ball bearings as roller contacts.
• Higher speeds due to reduced friction and efficient rolling contact
• Lower self-blocking
• Reduced wear
• Reduced drive force due to reduced friction and more efficient energy transfer
• More compact design: Ball screws often allow for a more compact design because they allow the use of smaller motors or drive units due to their higher efficiency and precision.

Specifying matching ball screw drives

If a new ball screw drive is required, it is important to know certain parameters and properties. Ball screw drives are available with metric thread and inch thread. The commercially available designation provides information about which thread is required. Ball screw drives with inch dimensions are specified as type 1004 (corresponds to a lead screw diameter of one inch and a 1/4 inch pitch) and ball screw drives with metric dimensions, e.g. as type 12x3 (corresponds to a 12 mm lead screw diameter and a 3 mm pitch).

Ball screw drives are labeled with their ball center circle diameter and their pitch. The outer diameter of the lead screw can be measured to determine the diameter of the ball center circle. To determine the pitch, it is firstly necessary to determine whether a single or multi-start ball screw drive is required. On single start ball screw drives, the groove segments are located directly next to each other; and there are corresponding gaps between the starts on multi-start ball screw drives. One can determine the type most easily by winding up a cord:

the pitch results from the distance between a cord segment and the next cord segment, e.g. 3 mm.

The stick-slip effect and how to minimize it

The stick-slip effect (or self-exciting vibration system) occurs when two surfaces alternately move and bond instead of gliding smoothly. The gripping force of an object counteracts the weight force with which the object is to be moved. Until the force required to overcome the gripping force is exerted, the object remains stuck and is then released at an increased speed and begins to glide. Once the state of the sliding friction has been reached and the object continues to be moved with constant pressure, it no longer grips.

Nevertheless, a stick-slip effect that occurs leads to oscillations and vibrations in the system, which are generally undesirable. In contrast to trapezoidal threads or screw threads, which exhibit static friction, the stick-slip effect hardly plays a role in ball threads since these substantially exhibit rolling friction. However, if there is a stick-slip effect on ball screw drives, various measures can be taken, including:

• Use of lubricants
• Precise production and material selection
• Avoidance of excessive stress
• Use of assemblies without axial play or with adjustable axial play
• Tightening the assembly to increase stiffness

Calculating the service life of ball threads

The service life of ball screw drives can be calculated using the dynamic load rating. It is defined by the total number of revolutions, time or distance after which the ball bearing surfaces or balls begin to delaminate. The following formula applies for operating hours L_h:

• L_h = service life in hours (h.)
• C = Dynamic load rating (N): An axial load that acts on a group of equal balls and at which 90% of the test specimens are able to achieve 1 million rotations without the rolling surfaces delaminating
• P_m = Average axial load (N)
• N_m = Average speed (min^-1)
• f_w = Load factor, examples: Operation without shock load f_w = 1.0 to 1.2 | Normal operation f_w = 1.2 to 1.5 | Operation with shock load f_w = 1.5 to 2.0

The following information is customary for the useful life: 10,000 hours for industrial machinery, 20,000 hours for machine tools, 15,000 hours for automation equipment and measuring instruments.

Selecting ball screw drives

The selection of the suitable ball screw drive depends on several factors, including load requirements, speeds, accuracy requirements and the environment in which it is used. On CNC machines, ball screw drives ensure precise engraving and milling, while 3D-printers achieve accurate layer-by-layer print quality. Robotics benefits from precise motion control of joints, machine tools from highly precise machining processes.

The operating conditions of the ball screw drive, such as positioning accuracy, stroke, travel speeds, service life, etc., should be specified first. In the next step, a preselection of the ball screw drive is made based on the guide accuracy, the axial play and the expected loads. Lastly, the allowable axial load, the allowable speed of rotation and the expected service life must be checked. Use our selection process to size your MISUMI ball screw drive. MISUMI supplies suitable ball screws drives, compensating couplings and ball bearings for many applications.