Ball screw drives
Ball screw drives (KGT) consist of a spindle, a nut and a large number of balls. Here, the balls serve as a connecting element between the spindle and the nut and are used as force- and motion-transmitting rolling elements. The rolling balls roll very precisely in the revolutions of the threads on the spindle and in the nut with the highest running quality. KGT are primarily intended for the safe transmission of axial forces. The force is transmitted via the effective direction of the pressure lines through the balls. In the case of an O arrangement of the balls, they run with the tip pointing outwards and in the case of an X arrangement, they run inwards. The respective direction of action of the pressure lines is important both for the force flow and for the generation of the preload.
KGTs, together with their bearings and drive elements, are highly loadable, very dynamic and extremely precise drive units for generating the feed movements of machine tools, for example. Due to their high load-bearing capacity and enormous precision, they have proven themselves optimally over the course of time and are still a very innovative machine element with a future.
Ball screws and there precision
In general, KGT are differentiated into two basic constructive principles:
- Ball screw with driven spindle and stationary nut
- Ball screw with driven nut and stationary spindle
The majority of KGTs are designed with a driven spindle. Here, the rotation of the spindle is transformed into a linear displacement movement of the nut relative to the spindle. This design is called the normal configuration.
However, under certain circumstances, the driven nut version can also be beneficial. In this case, the rotary drive movement of the nut is converted into a translatory movement of the spindle relative to the nut. Compared to driven spindles, longer travel distances, higher rotational speeds and thus higher travel speeds can be achieved with driven nuts. Furthermore, it is possible to place several nuts on one spindle and to move them separately with separate drives.
Two basic principles of ball screw drives
Applications of the ball screws
The design and manufacturing process of the KGT are ultimately determined by its use. Ball screws can be used not only in the usual areas of application but also in the high-speed range, in the heavy-load range, in the short-stroke and oscillation range and in the smooth-running range. In addition, KGT can also be used under special environmental conditions such as under water or at high temperatures. The technological requirements and the expectations in the respective areas are strongly dependent on the application.
KGT are very specific and sometimes even contradictory. For example, if a ball screw can bear a high load, it will never have to realise very high travel speeds. In contrast, a highly dynamic KGT is loaded by relatively low forces. If a large mass is to be moved in small steps, high dynamics are usually not required, but rather a low breakaway torque and very low friction are expected. Therefore, special constructive designs are necessary that correspond as exactly as possible to the respective requirement profile.
Precision ball screw drives
Precision ball screw drives require perfect interaction between nuts, balls and spindles. Here, the individual parts are ground to very tight tolerances and are brought to the highest precision through manual assembly by highly qualified specialists. Only the highest quality materials are used in the manufacture of the components. The high load-bearing capacity and precision are guaranteed by the lock-resistant, long-term nitrided surfaces of the entire spindle as well as the through-hardened nuts and balls, optimum feedback systems and effective wipers. Furthermore, the installation and load direction of precision ball screws is basically arbitrary and has no influence on their functionality. The respective designs and the connecting dimensions of the precision ball screws comply with DIN ISO 3408 or DIN 69051 or, as special designs, with individual customer requirements.
Ball screw nuts
The ball screw nut is the most extensive component of the KGT. The smooth functioning of the KGT depends primarily on the nut, as it contains the ball tracks, the ball returns and the wipers. The preload is generated via the nut, which on the one hand serves to avoid the reverse span and on the other hand significantly increases the stiffness.
The double nut is the primary nut design in machine tool construction, as almost only preloaded systems with 2-point contact are used here. The two nut body parts are mounted so that the balls are clamped between the grooves of the spindle and the nut. Axial displacement can be achieved either by ground-in spacers or by twisting the nut parts relative to each other. In both cases, the distance between the thread revolutions of the two nut parts is increased or decreased.
The double nut usually consists of two parts, where one part takes the external load and the other nut part creates the preload. The load nut is always the nut through which the axial force is transmitted in direct force flow. Furthermore, it depends on the force flow whether O or X stresses are generated on a double nut. For manufacturing reasons, the load-bearing nut parts of double nuts can also be made in two parts, so that a triple or quadruple nut is also possible.
This nut design is the most compact and cost-effective variant. For this reason, it is very widespread and consists of only one nut body part with a continuous thread. In the simplest version, single nuts have a slight axial play. Due to the variety in the thread turns and/or a ball sorting as well as the constructive design, backlash-free, preloaded single nuts are also possible. Due to the often suboptimal running behaviour of these designs, the preload is usually associated with increased friction, increased operating temperature and increased wear.
Different nut designs
Another distinguishing criteria is the outer shape or the type of fastening for the nut. In addition to the cylindrical design, the flange design is also relevant. In the case of the pretensioned design via a double nut, the two nut body parts must always be mounted in a housing. In this case, the preload is applied from the outside to the inside via pressure flanges. The respective limitation of the preload takes place via a spacer washer between the nut parts. This always results in an X-preload.
In the flange designs (with side or centre flange), on the other hand, the mounting flange is always a fixed part of the nut body. In this case, the pre-tension is created via the locknut part by twisting the two nut parts towards each other or via an adjustable intermediate washer from the inside to the outside. The respective locknut is only held in the grooves by the balls and the acting preload force. The adjustment of the variable pretensioning force is an individual assembly process that can be very well adapted to the application and creates perfect conditions for high load-bearing capacity and long durability.
The ball screw is the component of a ball screw drive that carries the nut. It is called a cylindrical shaft and basically consists of at least two parts. One part carries the helical ball track, via which the movement kinematics of the KGT is implemented in its complete length. The spindle thread is only ever loaded sequentially in the current ball contact area. The spindle bearing and, if necessary, the drive pin are located on the other part. This is where the forces and torques are introduced into the spindle or dissipated into the machine environment.
Different spindle designs
The constructive design and geometric implementation result in some basic spindle variants that differ greatly from each other not only technically but also economically. In part, the individual distinguishing features are very striking.
Depending on whether the ball screw is driven or non-rotating, certain functional or manufacturing reasons may require special designs that do not correspond to a “normal” solid screw. These screw designs include:
- Hollow spindles
- Collar spindels
- Coupled spindles
- Corrosion resistant spindles
The majority of ball screw spindles are designed as so-called full spindles. Here, the spindle cross-section corresponds to that of a solid cylinder (without bore). The force transmission takes place over the entire cross-sectional area with corresponding tensile, compressive and torsional stresses. With the full spindle, the required machining effort is the lowest, which is why the full spindle is the simplest and also the most cost-effective variant in terms of production technology.
If the centre of the spindle is to be provided with a central bore, e.g. for cooling or lubrication purposes, the ball screw can also be designed as a hollow spindle. In this case, the spindle cross-section corresponds to that of a tube with a corresponding wall. In this case, however, the tube wall must be dimensioned in such a way that the axial load and the torque are safely exceeded without deformation, taking into account the thread groove depth.
Usually, the outer diameter of the ball screw is the largest diameter from which all other screw contours are machined. However, there are circumstances when larger diameters than the outer diameter of the ball screw have to be used in individual spindle areas. This can be the case if an enlarged contact shoulder for a spindle bearing is to be generated or more rigidity and stability is to be achieved in the spindle shaft. These spindles are called collar spindles.
Coupled spindles for excess lengths up to 25 m
In order to be able to produce longer spindles if necessary, it is also possible to firmly couple two spindles. In this case, two finish-ground spindles are aligned so that the threads merge exactly. In this case, the threads are not used as a screw connection, but as a claw-type counterhold for the potting material. This claw effect is further enhanced by the opposite pitch direction. In theory, spindles of any length can be produced by using several coupling points.
PACH Systems offers you a wide product range of different ball screw drives, ball screw nuts or ball screws. For this purpose, we work together with our long-standing partners. Please let us know your requirements via our contact form. We will be happy to provide you with a non-binding offer or contact you to discuss the details.