Ball screw drives - accuracy by preloading (12 4960)

Ball screw drives convert rotary motion into precise linear motion or vice-versa. They are often used as a drive element. Positioning accuracy is critical when using ball screw drives. One way to increase the positioning accuracy of a ball screw drive is to reduce the play between the spindle and nut with a preload. However, since this preload also changes the ease of movement of the system, there are ball screw drives with adjustable preload. The system can be preloaded to such an extent that the permissible axial play between the ball nut and the threaded spindle is achieved without overly limiting the free movement. Preloading can achieve positional accuracy within a few micrometers. In this article, you will learn how this works and what accuracy classes there are.

What is a ball screw drive?

A ball screw drive, also called a recirculating ball lead screw, converts rotary motion into linear motion. In its simplest form, a ball screw drive consists of a lead screw and a nut. Balls roll between them in a recirculating track to reduce friction. More complex systems for example consist of a double nut system or have protective devices. Ball screw drives are highly precise and capable of transferring high forces with low friction losses, making them ideally suited for motion control in industrial applications and machines such as CNC machines or robots. With their help, for example workpiece fixtures can be positioned very accurately. The correct lubrication is essential for operating ball screw drives (more information in lubricating ball screw drives).

For more detailed information on the function, design and precision classes of ball screw drives, see our article on Ball screw drives: function, structure, types, precision classes.

What is preload?

Preloading is understood as the purposeful introduction of a force into a system or component before it is exposed to an external load. It guarantees stability and functional reliability. A preload can be applied e.g. on threaded joints, springs, lead screw bearings, but also on ball screw drives.

Ball screws tend to be resilient and have axial play in the drive train. Axial play is the unused movement range or backlash of a lead screw, which is created by the gap between the balls of the bearing and the thread of the lead screw and the tracks in the threaded nut.

Due to the axial play, the ball screw drive shifts between the positions (+) and (-). For preloads in one direction, this does not yet lead to positioning inaccuracy because the balls are pressed against one side of the thread flank. However, as soon as there is a change in direction or a reversal of the axial load, the absolute positioning accuracy is no longer given.

• F_r = Radial force effect
• M = bending moment, e.g. due to misaligned installation

A preload is introduced to minimize this undesirable effect. Consideration of dimensional tolerances is essential when adjusting the preload. Bearing or thread tolerances are adjusted such that there is no more play, which optimizes the positioning accuracy and stiffness of the system. It is usually introduced between the lead screw and the double nut, e.g. by using two nuts that are clamped against one another. The spring tension between the nuts can usually be adjusted using a threaded ring.

The preload term is also defined in standards. JIS 1192 for example defines the preload as the force internally exerted on ball screw drives by using a group of axially offset steel balls or a nut pair to reduce the play and increase the stiffness of the ball screw drive. However, excessive preloading can for example also affect the service life of the ball screw drive. The amount of preloading must therefore be considered thoroughly.

Types of preload

In general, there are two types of preload: radial and axial. Radial preloading is a force applied perpendicular to the axis of rotation, e.g. on bearing rings in a rolling bearing. It ensures uniform contact between rolling bearing and race. The axial preload is applied along the bearing or spindle axis and prevents play in the longitudinal direction. Axial preloading is used for ball screw drives.

Importance of preload force

Using a preload has the following advantages:

• The stiffness improves, which in turn optimizes the load capacity and repeatability.
• The service life increases. However, care should be taken to ensure that the preload is not too high, as it can otherwise have a negative effect.
• Vibrations and noises are minimized, which is for example relevant for applications requiring quiet motion.
• The play is minimized, resulting in more precise motion control and positioning. This is important, among other things, for CNC machines or in robotics.

When calculating the necessary preload force, the required tolerance classes of the components to be manufactured must also be taken into account (see tolerance classes iaw. ISO 22081 and DIN ISO 2768).

Calculating the preload force

The preload force of the nut always also represents a constant load on the ball screw drive. It is therefore important to find a preload force that at the same time does not exert excessive stress on the ball screw drive while still fulfilling its purpose. For this purpose, the expected operating load in particular can be considered. The preload force can for example be calculated from the dynamic load rating. This indicates what loads the ball screw drive can withstand in order to achieve a service life of 1 million revolutions.   It is specified by the manufacturer. Load ratings are for example standardized in ISO 281. For ball screw nuts with 4-point contact, 5-8% of the dynamic load rating makes sense for preloading, and 8-10% for 2-point contact. Other values apply for determinations iaw. ANSI standard.

Various preloading methods

There are various methods for adjusting the preload, such as ball screw nuts with adjustable preload. In this variant, a spring is mounted between two nuts, which presses the nuts against the left and right side of the spindle race. The preload can be further adjusted with a threaded ring. Preloading can also be achieved automatically by using hydraulic systems. Various methods are presented in detail below:

Spring preloaded double nut

Selecting the correct spring tension is crucial for optimizing the preload (see selecting tension springs and compression springs). The spring-loaded double nut is a good option for small ball screw drives. The two nuts are located in a housing in a rotationally fixed manner, where they are pushed apart by a spring clamped between them. The spring ensures that the preload always remains the same, even in the event of wear or due to manufacturing tolerances. The preload itself is also comparatively small. Disadvantages of the spring-loaded double nut are that it is generally more complex and larger and the alignment is more difficult. The forces can also no longer be transferred in all directions: the spring may yield if forces are above the preload force.

Installing balls with a defined oversize

Balls can be mounted with a defined oversize if individual standard nuts are used in the ball screw drive.   This results in a so-called four-point contact between four points of the ball and the nut. This leads to a significant increase in friction.

• (1) Lead screw (cross-section)
• (2) Nut (cross-section)

Due to the high friction, this method should be used with a small preload or play. The efficiency deteriorates somewhat and the response to production tolerances is also higher with this type of preloading. For particularly long lead screws, it therefore makes no sense to install preloaded single nuts. The advantages are: efficiency and compact design.

Installing a single nut with pitch offset

A pitch offset can still be selected for individual nuts. This involves installing a nut with a thread pitch that deviates slightly from the spindle. This defined offset causes the position of the rolling elements to shift in the spindle thread, resulting in a two-point contact. This method is suited for medium preloading.

• (1) Lead screw (cross-section)
• (2) Nut (cross-section)

Installing a spacer element for double nuts

A more complex preloading variant is to install a double nut system with a spacer element.

• (1) Lead screw (cross-section)
• (2) Nut (cross-section)
• (3) Spacer element

Two threaded nuts are clamped against one another by means of a spacer element, which results in a two-point contact. Friction increases very little as a result. The axial play is almost completely eliminated, which contributes to the stiffness and precision of the ball screw drive. This method is viable for medium and high preloading. The spacer elements allow simple adjustments of the preload, which is why they are suited for variable loads.