You may have heard of ball screws before. These are certainly the best known of the screw drives, but they are not the only ones, as trapezoidal screw drives are a second type. This article will focus on the former, but you will still find everything you need to know about both types here.
Design, mode of operation and areas of application of ball screws
But first of all, what are ball screws? Ball screws consist of a spindle with a thread profile, a nut, which also has a thread profile inside and moves on the threaded spindle, and balls as rolling elements between the spindle and the nut. The use of seals is possible as an option. Ball screws are available in rolled, whirled and ground versions (more information on the three manufacturing processes can be found here). Thread rolling has process-related limits in terms of achievable precision. The highest accuracy and precision requirements can be achieved by grounding, but grounding is the most expensive and takes the longest. In practice, thread whirling of ball screws is an effective and cost-effective alternative to ground variants, which can also achieve very high accuracy classes.
Ball screw drives are generally used to convert a rotary movement into a longitudinal movement. However, conversion in the other direction is just as possible, so that the nut is caused to rotate when an axial force is applied to the spindle. Ball screws can also be designed without axial clearance or preloaded. Particularly important: Ball screw drives are only capable of absorbing axial loads and must never be subjected to radial loads or moments. Radial loads or moments will cause the ball screw to fail within a very short time! Vibrations, shocks and short-stroke applications should be avoided or at least minimized when using ball screws. In these cases, dimensioning with sufficiently high static safety is necessary. In addition, the correct preload class and an adequate lubricant should be selected – the manufacturer’s recommendations in this regard should be followed at all times. Preload can be used to increase rigidity or achieve precision; it also serves to keep the balls in rolling contact in the case of vibrations, for example.
The areas of application for ball screws are comparable to those of linear guides and linear axis and are therefore similarly diverse, including the machine tool sector, medical technology, aviation, the food and packaging industry, semiconductors and photovoltaics. Ball screw drives are also used in glass molding machines and measuring machines.
Trapezoidal screw drives as the forerunner of ball screws
Trapezoidal screws have been on the market for much longer than ball screws and are used far less frequently than the latter. Their design differs from that of ball screws in that they have flat running surfaces and no rolling elements. Instead, they realize a sliding movement between two surfaces with a trapezoidal profile. This sliding motion causes friction, which leads to a comparatively shorter service life. Trapezoidal screw drives are also characterized by the fact that they must always have clearance, because if a trapezoidal screw drive is designed without clearance, it will jam. Due to the self-locking properties of trapezoidal screws, they are suitable for vertical applications with drives without brakes. They are also very suitable for applications that are mainly statically loaded or do not require much movement.
Ball screws and trapezoidal screws in comparison
As already mentioned in the previous section, ball screw drives are more important in everyday practice than trapezoidal screw drives. This is not only due to their generally more modern design and – because of the balls – low-friction, almost wear-free running. Due to this low-friction running, the decisive advantage is that ball screw drives are considerably more efficient.
If, for example, a BSH02510 ball screw is compared with a TR02410 trapezoidal screw (both with a pitch angle of 7.1°), it is noticeable that the coefficient of friction for the trapezoidal screw is 0.1 µ. The coefficient of friction of the ball screw is generally only 0.003 µ. Taking the lead angle of 7.1° as an example, this means that the efficiency of the trapezoidal screw is around 55 %, whereas it is 98 % for the ball screw. Due to the relatively high friction, just under half of the energy applied is transformed into heat in the former, whereas almost all of the energy can be converted into active power in the ball screw.
In addition to efficiency, there are other advantages that ball and trapezoidal screw drives have in direct comparison with the other type of screw drive. All information on this is given in the table.
| Advantages | |
| Ball screws | • Higher efficiency • Longer service life due to virtually wear-free operation • Lower drive power • No stick-slip effect • More precise positioning • Higher travelling speed • Less heat |
| Trapezoidal screw drives | • Self-locking (relevant for vertical applications with drive without brake) • more cost-effective solution |
In most applications, ball screws are the better choice than trapezoidal screws.
You can also find more information on the calculation principles and the installation of screw drives on this website.
Configurator for ball screws
NTN provides a new online tool that generates ball screw type codes to efficiently select the right product. Following a step-by-step selection of all possible options, the tool configures the correct type code. In addition, products from other manufacturers can be interchanged by entering the brand, nut type, nominal diameter or pitch. Alternatively, this is also possible simply by entering the type code of the competitor’s product. A login is not required, the configurator can be used directly.
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