Rolled high-tech threads in application
Specialist topic: An exciting overview of thread rolling... The basics, processes, tools, and applications of rolled high-tech threads
Power screw vs. fastening screw
Rolled high-tech threads are mainly used as power screws in threaded or ball screw drives. They are rarely used as fastening screws. While the thread slides directly on the thread flanks of the threaded spindle in a drive screw, the balls are low-friction connecting parts between the threads of the threaded spindle and the nut in a ball screw drive. Nowadays, the ball screw drive dominates.
As a whole, the ball screw drive (with its screw spindle and nut) is a drive screw with balls as the rolling elements. It is used to convert a rotary motion into a straight-line (linear) motion or vice versa. Though the basic structure of the ball screw is quite easy to describe, the designs and practical requirements vary quite a lot.
Fig. 1: Ball screw drive as an example of a motion thread
Fig. 2: Expansion screw as an example of a rolled fastening thread
The ball screw offers the designer almost unlimited possibilities for solving transportation and positioning tasks. These possibilities range from ball screw drives with a driven screw or driven nut to those with double nuts and ball screw drives with rolled precision spindles.
Ball screw drive for a cargo elevator
With the cargo elevator shown in Figure 3, loads of up to 100 kg can be moved safely and without fatigue; up to six of the standard crates shown can be transported from the storage rack to the workstation at the touch of a button. The use of rolled ball screws ensures low wear with consistent positioning accuracy and is characterized by great efficiency. The recessed ball returns in the spindle nut offer the user a high degree of reliability. Compared to other motion options, significantly fewer mechanical parts are required, allowing for a compact design of the drive unit.
Fig. 3: A rolled threaded spindle makes for a really compact drive unit of the cargo lift
Ball screw drive for a coffee machine
Figure 4 shows the piston unit of a coffee machine for the catering trade. The stamp as the central element is moved up and down by two laterally positioned screw drives with a rolled thread.
Fig. 4: The threaded spindles “run” in the two guide columns of the spindle-stamp unit
After the grinder has ground the fresh coffee beans to the required fineness, the required amount of grounds is fed through the inlet funnel into the brewing chamber. The stamp is at the top at this point. The threaded spindle drives lower the stamp with the required force and the stamp presses the coffee powder together. This process must be extremely even. If the coffee layer does not have a homogeneous thickness, the hot water cannot flow through it evenly. The aroma would vary from coffee cup to coffee cup, resulting in a loss of quality. After brewing, the filter cake is ejected via a stamping movement.
High-safety thread for the suspension of cable car cabins
Cable car manufacturers differentiate between reversible aerial tramways (large cabins in shuttle service) and circulating cableways (many small cabins are connected to a cable). The cabins of modern cable cars are suspended from bolts with rolled threads – experts refer to these bolts as tie rods. These tie rods are high-safety parts.
In the past, cable car manufacturers connected the cars by welding. Nowadays, these constructions no longer meet the much stricter safety requirements. In addition, welding has become far too costly due to the associated change in material structure and the prescribed tests.
Fig. 5: The large cabin of the gondola lift on the 3,000-meter-high Titlis (Central Swiss Alps) rotates 360° during the ride
The large cabins of the reversible aerial tramway shown in Figure 5 are each suspended from four tie rods. Each of these brackets extends through the cabin into the cabin floor. The cabin floor is attached to these four bolts. The cabin walls and glazing are there to protect passengers from the weather and unintentional “exit” from the cabin.
Fig. 6: Cable car in the Matterhorn region: four bolts made of high-alloy steel with rolled threads support the cabin and ensure safety.
The cabin suspension of the cable car shown in Figure 6 has a different design. The small cabins also have four rolled threaded bolts, but they do not pass through the cabin as a tie rod. Instead, forged parts with a threaded insert in each corner of the cabin roof accommodate one of the four bolts. These straps are connected to the cabin floor, which supports the walls and glazing, via corner braces. The force flows here from the cabin suspension via the threaded bolts into the ceiling brackets. From there, the force is transferred via four corner profiles into the floor lugs – also forged parts – and thus into the floor structure. Because the corner struts are required as a connection between the suspension and the cabin floor, this is referred to as a self-supporting cabin.
Rolled threads are mandatory for the tie rods of large cabins (Fig. 7). The base material – high-alloy steel with manufacturing certificate – and the shape and manufacture of the thread are subject to very strict standards and regulations for the required material tests (X-ray, ultrasound). Approval for installation is only given with the relevant certificates. In this application, the tensile and notched impact strength are important, as well as the proof that the parts can still perform reliably at temperatures of -20 °C. If production with forming is required, it is carried out exclusively by water jet cutting. This is because cold forming on its own produces no structural changes in the material.
Fig. 7: Bolt made of high-alloy steel: only a rolled thread meets today's safety standards for arial railways
Nowadays, mainly only bolts with rolled threads are approved for the suspension of small cabins. Since the longitudinal fibers of the material are not cut but only deflected during thread rolling, unlike during milling or turning, they meet the requirements for the prescribed impact strength. Milled or turned threads do not meet the safety standards.
Silent threaded spindle drive for a swing/sliding door
Some sliding doors on public transport make up to 200,000 movement cycles per year and have been in use for over 20 years. This requires high-quality components. In the double-leaf swing/sliding door shown in Figure 8, a screw drive with a rolled thread ensures that the door leaves swing open smoothly and, above all, safely.
Fig. 8: Swing/sliding door for buses, rail and commercial vehicles when closed
The leaves are opened and closed by a longitudinal shaft onto which both a left-hand thread (for the left-hand door leaf) and a right-hand thread (for the right-hand door leaf) have been rolled. The threaded spindle is actuated by a DC motor via a toothed belt and a torque limiting clutch.
The two nuts in the carriages (Fig. 9) – one with a left-hand thread, one with a right-hand thread – move on the threaded spindle and thus cause the door leaves to move sideways. Curved guide rails ensure that the door swings outwards or inwards. The door turns using levers. A rotary axis is built into the carriage for this purpose.
Fig. 9: Carriage with built-in nut: when the threaded spindle rotates, the doors move sideways. The carriage also forms the pivot point for swinging the doors.
The emergency opening function is especially important. Passive safety standards stipulate that the doors must be unlocked both in an emergency and in the event of a system failure and must be easy for passengers to open, even in a panic situation. After pressing the red emergency button, the electric motor is disconnected from the threaded spindle (coupling). The air reservoir – which is permanently pressurized during normal operation – suddenly loses pressure. Once the excess pressure has been released, two strong springs open the door leaves sufficiently to allow them to be pushed fully open by hand.
Fig. 10: Door opening system: the carriage with the pivot axis, the threaded spindle, and the spring for emergency opening (at the top) can be seen
The pitch of the rolled thread is crucial to ensure that the door can be opened manually in an emergency. If the pitch is too small, it would self-lock. With the selected pitch of 100 mm, the two nuts can be easily moved on the threaded spindle (Fig. 10).
Counter-rotating threaded spindle for a cable assembly device
Before the ends of an electrical connection cable can be connected with plugs or terminals, it must be cut to length and stripped. Nowadays, cables are almost exclusively assembled fully automatically.
The cable assembly device shown in Figure 11 accommodates cables of different diameters. The insulated cable is fed from a feed roller from the right via a roller system into a guide tube (the swivel guide). The machine stops after a preset distance. Two blades (one from below and one from above) approach the cable and cut through the insulation all the way around, leaving one end blank. The cable is then pushed further until the required length is reached.
Fig. 11: Fully automatic machine for cutting and stripping electrical cables
The blades then cut the copper cable all the way through. The insulation is slit open, stripped, and falls down. The cut and stripped cable is then placed in a storage container for further processing.
To carry out the vertical cutting movements, the lower blade moves upwards while the upper blade moves downwards. The two blades are moved using a rolled threaded spindle.
Fig. 12: The two counter-rotating parts of the rolled threaded spindle can be clearly seen on the traversing unit of the assembly device. They generate the opposite vertical movement of the blades (top) via the two nuts
More precisely, the threaded spindle consists of two screwed half-spindles, one with a left-hand thread, and the other with a right-hand thread. When the threaded spindle is turned, the two nuts on which the blades sit move in opposite directions (Fig. 12). The screw connection in the center of the spindle does not loosen because the fastening thread is tightened during the cutting process; this is where the greatest forces occur. The return movement does not exert any force on the screw connection.
Ball screw drive for a telescope
On the Canary Island of La Palma, 2225 meters above sea level, is a telescope that was developed under the leadership of the Max Planck Institute for Physics (Fig. 13). It can detect gamma rays from galaxies up to eight billion light years away with extremely high sensitivity. With a diameter of 17 m, this telescope is the largest of its kind in the world. The pivoting frame is made of carbon fiber tubes.
Fig. 13: “Magic” telescope on La Palma
Almost 1000 aluminum mirror pieces, mounted on 247 rigid support plates, result in a mirror surface of 240 m2. The researchers can align the telescope to any point in the sky within 20 seconds. Each of the carrier plates is set to a pre-adjusted laser point by two ball screws (thread with a nominal diameter of 10 mm and a pitch of 2 mm) (Fig. 14). Hybrid stepper motors drive the almost 500 ball screw drives.
Fig. 14: The spindle drives are mounted in the corners of the individual mirror elements
Ball screw drives without backlash for an embroidery machine
The high-tech embroidery machine shown in Figure 15 is capable of embroidering precise patterns on huge fabric webs using thousands of needles. The stretched fabric web is moved left and right and up and down with high speed and precision using just one movement and control unit.
Fig. 15: Mechanical high-tech embroidery machine with 2 m long threaded spindles
There are several factors in favor of drives with ball screws: accuracy, simplicity, cost, and robustness; this applies in particular to vertical travel. The ball screw drives with a nominal diameter of the screw spindle of 25 mm and a thread pitch of 25 mm have four ball return systems. The “gear unit” is integrated in the nut body (Fig. 16). The conversion of the rotary movement into a linear movement is precise and backlash-free.
Fig. 16: The nut of the ball screw in the carriage can be seen
It would not be possible to achieve zero backlash with a standard ball screw over 2 m long, as every nut has a certain amount of backlash. In most applications, the backlash is irrelevant. In the case of the embroidery machine, this would have a particularly devastating effect when reversing the direction of movement. When the nut moves vertically, the accelerations are greater than the acceleration due to gravity. However, as soon as gravity is removed, the nut would flutter on the spindle. But that should not be the case.
To eliminate the backlash, a single nut is used instead of the usual system with two nuts braced against each other, combining a very short design with high rigidity. The preload is generated via a special thread based on the profile of the Gothic ball screw on the spindle.
Depending on the length of the embroidery machine, between three and eleven vertical ball screw drives can be controlled synchronously. The ball screw drives consist of commercially available individual parts that are inexpensive to manufacture. The drive is easy to assemble, operate and maintain. Only standard elements are used. Even the linear guides for the ball screw drives are standardized purchased parts.
Eichenberger – your service provider right from the start
With Eichenberger, you can implement your individual requirements for a screw thread right from the start of development. Don't hesitate to contact us before you even know what you need. We will be happy to work with you to develop a customized solution for your thread system. Contact one of our experts directly, we are there for you right from the start.
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With state-of-the-art production methods, many years of expertise and a tool inventory of over 1,000 rolling tools, we produce rolled screw drives that meet even the most unusual requirements:
- Gradients up to 6x in diameter
- Slope accuracy class G5
- Spindle lengths up to 6 meters
- Spindle diameter from 2 to 160 millimeters
- All standard profiles (M, Tr, UNC, UNF, UNEF, Whitworth)
- Multi-start threads, also as right/left-hand threads
- Steep thread profiles
- Ball screw profiles
- Special profiles
- Worm thread profiles (special quality and price advantages!)
- Serrations and knurling
- Conical thread
- Threads on prefabricated and/or bulky parts, e.g., also on forged parts
- Freely designed thread geometry
- Responding to customer requirements, such as tailored nut geometry
Eichenberger leaves nothing to chance and places the highest value on quality. This is what has been impressing our customers since 1953. See for yourself!
> 100% Swiss Quality
> Thread specialist since 1953
Literature and sources
Apel, H. (1952). Gewindewalzen: Kaltverformen von Präzisionsgewinden und Spindeln. Hanser.
DeWiki (2022, 3.August). Lexikon Gewinde. https://dewiki.de/Lexikon/Gewinde
Kübler, K. & Mages W.J. (1986). Handbuch der hochfesten Schrauben, (1. Aufl.). Girardet.
Peters, H. (2003). Mathematisch-Technisch-Algorithmisch-Linguistisches Sammelsurium. http://www.hp-gramatke.de
Trösch, B. & Husistein, K. (2007). Bibliothek der Technik -, Band 286, Gewinderollen. Moderne Industrie.
Verein Deutscher Eisenhüttenleute (1984) (Hrsg.). Werkstoffkunde Stahl, Bd. 1. Springer,
Wikipedia (2022, 3. August). Metrisches ISO-Gewinde. https://de.wikipedia.org/wiki/Metrisches_ISO-Gewinde
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Abbildungen: Nr. 1, 23-25 RWT Rollwalztechnik GmbH, Engen; Nr. 2 Foto Deutsches Museum, München; Nr. 3 Musée du tour automatique et d'histoire de Moutier, Moutier (Schweiz); Nr. 16 Fette GmbH, Schwarzenbek; Nr. 18 Meinrad Plaz, Staufen (Schweiz); Nr. 26 Habegger SA, Court (Schweiz); Nr. 34-36 FBT Fahrzeug- und Maschinenbau AG, Thörigen (Schweiz); Nr. 37, 38 Schleuniger AG, Thun (Schweiz); Nr. 39, 40 Max-Planck-Institut für Physik (Heisenberg-Institut), München; Nr. 41 Saurer AG, Arbon (Schweiz); Nr. 42 Line Tech AG, Glattbrugg (Schweiz); alle übrigen Eichenberger Gewinde AG, Burg (Schweiz). Satz: abavo GmbH, D-86807 Buchloe. Druck und Bindung: Sellier Druck GmbH, D-85354 Freising. Printed in Germany 889030.
