Frequently Asked Questions
The advanced whirling method that Bornemann has developed is a machining production process. With this method, a cutting ring equipped with evenly spaced cutting edges turns quickly around the workpiece, which turns slowly in the same direction. The parallel axes of rotation are offset from each other during the process. The offset causes favourable chip formation, which helps to dissipate heat away from the workpiece and enables the production of very precise threaded components as a result.
Tilting the whirling ring allows the thread pitch to be produced. We differentiate between external and internal whirling. In the case of external whirling, the cutting edges on the whirling ring are directed inwards, which means that the outside of the workpiece is machined. Threaded spindles are an example of such a workpiece. In the case of internal whirling, the cutting edges are on the outside of the tool. The tool penetrates into the borehole on the workpiece and machines it from the inside. The principle is the same in this case. An example of an internally whirled part is a nut.
The main advantages of whirled threads are the better lubrication conditions, high precision and comparatively low manufacturing costs for smaller quantities of pieces.
Bornemann’s advanced thread production method can achieve greater dimensional precision in comparison to thread rolling, particularly in terms of pitch accuracy. This is due to its clean and even cut and the fact that the material is not subjected to any stress during the process. The surface quality of Bornemann threads is equivalent to that of ground threads and pitch accuracies of 0.03 mm to 300 mm can be achieved.
In contrast to rolling, Bornemann threaded spindles are polygonal instead of circular. The small elevations on the polygon are similar to a scraped surface and are just a few µm in height. They provide a noticeable improvement in the lubrication conditions on the thread, because the thread flanks on a rolled thread are technically very smooth and the lubricant is pushed or forced out relatively quickly by movement and surface pressure between the friction surfaces.
The slightly polygonal thread flanks on Bornemann threaded spindles are different: the lubricant is deposited in the microscopic recesses and these recesses create pockets of lubrication. As a result, Bornemann threads are generally better lubricated than rolled threads.
The better lubrication reduces the occurrence of stick-slip and the tendency to gall the thread.
Different materials, diameters and pitches
Bornemann’s advanced manufacturing process is very flexible and can be quickly adapted to different thread geometries, sizes and lengths. Any machinable material can be used. Even exotic materials such as Hastelloy, Incoloy, Inconel, Monel, titanium, hardened steels, plastics or anti-magnetic steels can be processed with ease.
Lower risk of micro-cracks
More and more customers in the field of lifting technology need their lifting spindles to be tested for micro-cracks and request the exclusive use of crack-tested primary materials.
Threads that are pressed into shape through work hardening may push down existing micro-cracks in the primary material and conceal them. These cracks are then no longer visible and cannot be detected by the usual test methods.
In the Bornemann manufacturing process, the material fibre is cut and the material is not subjected to any additional stresses. As a result, the material can be checked for cracks at any time later and micro-cracks can be completely excluded.
The surfaces of Bornemann threaded spindles are comparable to ground thread surfaces, and pitch accuracies of less than 0.03 mm to 300 mm can be achieved. Apart from expensive thread grinding, no other competing manufacturing process achieves such precision.
In addition to the pitch accuracy, tolerances of within a percentage are achieved during end machining of the threaded spindles and during nut machining.
Bornemann’s optimised thread whirling method is particularly suitable for precision threads and special threads that are required in small to medium quantities (less than 5,000 pieces). The individual cutting edges for special threads are inexpensive to produce. To do so with thread rolling would involve producing very expensive rolling dies.
In addition, Bornemann threads have “pockets of lubrication”. These pockets are produced by the interrupted cuts made during the manufacturing process. These cuts create a very small polygonal shape on the thread flanks, and deposits of lubricant form in their recesses. As a result, Bornemann threads are especially suitable for applications in heavy-duty lifting technology where the high levels of surface pressure mean that constant lubrication is essential. Furthermore, Bornemann threads are less susceptible to stick-slip thanks to the improved lubrication conditions.
In principle, stick-slip is the jerking motion caused by surfaces that constantly and alternately stick and slip over each other. A system where surfaces adhere to each other and then begin to slide over each other may cause stick-slip. This phenomenon occurs at low sliding speeds in particular, and when the static friction is significantly greater than the dynamic friction. It is similar to an earthquake, where tension is produced between two plates and force is built up to overcome the static friction.
Once this static friction is overcome, much less force is needed for the sliding motion. The excess force is released in the form of a jerking motion, vibrations and the noise they produce. The phenomenon has similar effects in a system with threaded spindles and nuts. During start-up, the system jerks and may cause vibrations, which produces an unpleasant and sometimes screeching sound in a resonating body. Another example of stick-slip is a creaking door. The occurrence of the phenomenon is fundamentally undesirable because it interferes with movement and increases wear. In the worst cases, the phenomenon can cause cold welding throughout the system.
Stick-slip usually occurs where the level of static friction is
significantly greater than that of dynamic friction. To avoid the
phenomenon, the level of static friction must be reduced. Let us
consider the formula for static friction:
FS = µS x FN
FS = static friction force or static friction
µ= coefficient of static friction
FN = normal force
Static friction is the product of normal force and the coefficient of static friction. The normal force is the force applied horizontally to the thread flanks, which results from the weight and the load (the predominant forces in the thread). You do not want to change this force if possible.
The coefficient of static friction indicates how well a surface slides or adheres. Ideally, we want to minimise this coefficient. The easiest way to do so is by lubricating the thread flanks.
The pockets of lubrication on Bornemann threaded spindles do this particularly well, providing constant lubrication.
“Galling” or “adhesive wear” occurs on threads when the nut and spindle repeatedly adhere to and break free from each other due to a lack of lubrication. This effect often occurs in threaded components that are subject to a high level of tribological stress and breaks occur in their lubricating film.
It can lead to cold welding or cold pressure welding. It is extremely difficult and sometimes even impossible to release such galled or cold-welded threads. Even if a galled thread can be released again, the support flanks are usually so damaged that they are unusable and must be completely replaced.
Thread flanks that are too rough are susceptible to corrosion and produce a high level of adhesive force, which promotes galling. A thread path that is not properly deburred or a non-constant pitch may cause the thread to become wedged and therefore to seize or cold-weld.
Similarly, however, a surface that is too smooth may cause the thread flanks to “adhere” to each other like two glass sheets that are pressed together. On threads with very smooth contact surfaces, the lubricant is squeezed out and there is no longer a lubricating film between the sliding elements. As a result, at an atomic level, a very large number of metal atoms make contact with each other at the interfaces. Particularly when these interfaces come under pressure, this leads to the formation of stable atomic lattices that can no longer be separated without destroying them. This phenomenon is also referred to as cold welding.
Threads that are produced by rolling processes and that therefore have very smooth surfaces tend to “gall” more frequently than the Bornemann precision thread.
Another cause is an insufficient difference in hardness between the spindle and nut.
In addition, excessive stress can cause galling or even cold welding.
At high temperatures, the lack of lubrication and high forces may also cause the parts to become welded.
A summary of the causes of thread galling:
- a) Inadequate lubrication/breaks in the lubricating film
- b) Unfavourable material combinations
- c) High load (high surface pressure in the thread)
- d) High temperatures
Contamination of threaded spindles is often the cause of serious failures. Tiny particles act like sand paper and cause high wear or blockages of entire spindle/nut systems, especially if bellows are not used to protect the threaded spindles.
To identify the cause as quickly as possible in the event of damage, we always recommend analysing the lubricating grease so that contamination can be ruled out as the cause of the damage from the outset.
We can send our customers an analysis set for taking swabs for this purpose. We analyse all the iron particles, water content, additives and any impurities from the sample taken and send our customers a detailed report with recommendations for action within a few days.
In one piece, 6,000 and 7,500 mm in the standard range. Special dimensions of up to 12,000 mm are available in some cases. Otherwise, we can produce spindles with multiple parts. These parts are pinned, screwed and bonded. The longest spindle that we have produced with this method is over 80,000 mm long.
We can produce threads with a maximum diameter of roughly Ø450 mm. The maximum diameters also depend on the availability of the material and the outer contour of the threaded spindles.
Our custom-built thread whirling unit gives us the capabilities to produce extremely long threaded nuts and extremely steep internal threads. It allows us to produce internal threads with a length of up to 2,500 mm.
The pitch accuracy of a threaded spindle determines the purposes for which it can be used. The greater the pitch accuracy, the more precise the application can be.
Our threaded spindles offer a high level of pitch accuracy from 0.03 mm to 300 mm. Our precision threads are therefore particularly suitable for applications where a high level of positioning precision and a long service life are required.
The pockets of lubrication that Bornemann intentionally introduces during the manufacturing process can store and distribute lubricant particularly effectively. Good lubrication is especially important in heavy-duty lifting applications because the spindle may weld to the nut when subjected to the enormous surface pressure. Thanks to the pockets of lubrication, the threaded nut does not completely displace the lubricant. In addition, the risk of stick-slip, which has major effects with such loads, is drastically reduced.
Effective lubrication often allows smaller drives to be used for the threaded spindle, because the level of adhesive force to be overcome is significantly lower thanks to the lubrication.
Many of our machines and tools have been specially designed and produced by us in house for the production of special thread profiles. Thanks to our uniquely flexible fleet of machines, we can produce practically any kind of thread profile. That includes, for instance, trapezoidal threads, buttress threads, ACME threads, fine-pitch threads, metric threads, high-helix threads, round threads, sharp threads, self-reversing screws, module threads and, of course, their corresponding threaded nuts. We can also produce customized flank angles that are tailored to the requirements of your individual application.
ACME threads have a flank angle of 29° and are mainly used in the United States. Trapezoidal threads in accordance with DIN 103 and with a flank angle of 30° are predominant in Europe. Both threads are self-locking.
The service life of a threaded spindle depends on the application, the use, the material combination, the lubrication and other external factors.
For most components in mechanical engineering, fatigue damage (breakage) is used to calculate the service life. However, wear is the decisive factor in determining the service life of sliding threads. The tribology/friction is in this case significantly influenced by lubrication and temperature (= external factors) and, surprisingly, there is still no method to accurately calculate or predict friction forces at present.
The level of wear therefore ultimately has to be reproduced through physical tests. For this purpose, we have developed a method for reproducing the wear that occurs over many years in just a few weeks. This method allows us to compare different types of threads and lubricants with each other for the individual customer in order to identify the optimal solution. You can find more details here
Self-reversing screws or level winder screws with one direction of rotation are used to wind ropes in crane construction, cables in offshore winches or yarns in textile production, for example. Whether in XXL or small-format applications, we can produce self-reversing or level winder screws based on your individual drawing, from any machinable material and outside the usual standard dimensions.
Multi-start threads consist of several offset threads that run in parallel to each other. The number of threads corresponds to the number of starting points. The higher number of engaging threads allows force to be distributed more effectively.
It also allows for high axial strokes. Multi-start threads are often referred to as high-helix threads. High-helix leadscrews translate a small radial movement into the largest possible axial movement.