Ready to install and highly resilient
Bornemann-Gewindetechnik has been producing threaded spindles and nuts for almost all branches of industry for more than 25 years. The main focus of production is the manufacture of ready-to-install trapezoidal screw drives, consisting of spindles and nut sets for heavy-duty lifting systems, such as those used in lifting systems for rail vehicles, in theater and stage construction and in special machine construction.
For screw jacks that achieve a surface pressure of more than 20 newtons per square millimeter under high load and operate at very low circumferential speeds, constant lubrication of the load-bearing flanks of the thread profiles is absolutely essential. This is because inadequate lubrication leads to increased wear, excessive heat generation and also to the undesirable stick-slip effect.
Stick-slip effect
The stick-slip effect, which is also known as the stick-slip effect, describes the jerky sliding of solid bodies moving against each other. The effect always occurs when the static friction is greater than the sliding friction. Depending on the tribological system, this leads to the excitation of vibrations that are emitted from a resonant surface as noise. The effect usually disappears as soon as the friction partners are separated from each other by an intermediate or lubricant. The stick-slip effect is generally undesirable in technical applications. Negative influences caused by the effect can be observed in bearings, guides in linear technology or threaded spindles in lifting technology. Noises such as the creaking of a door, the squeaking of trains when cornering and rattling windshield wipers on car windows are well-known consequences of this effect.
Ralph Wuertele, Manager Application Engineering at Klüber Lubrication: “Especially in heavy-duty lifting systems, unsuitable lubricants can easily lead to lubrication deficiencies that cause increased wear and thus lead to premature failure of the system.” The reason for failures is often due to the design of the spindles themselves. Missing lubrication pockets on the bearing flank of the thread profile prevent the applied lubricant from remaining when the nut profiles slide over the spindle with a high surface load. Either the lubricant is pressed off the flank or it is pushed in front of the nut. In both cases, the lubricant film breaks off, which initially causes an increase in temperature and a stick-slip effect with sometimes deafening noises, and can later lead to the destruction of the screw drive.
Turning, rolling or whirling?
Lead screws can be manufactured using different production processes. Thread turning and thread milling are processes that are rarely used for these screw drives for economic reasons, while thread rolling or thread rolling is used to produce threaded spindles by means of chipless cold forming. This manufacturing process is very economical, but due to the very high tool and set-up costs, it can usually only be used in large series. In addition, rolled threaded spindles promote lubrication film breakage due to the absolutely bright thread flank surface. The rolled trapezoidal thread has too high a flank surface quality due to the deformation process in stamp polishing or press polishing, which also lacks lubrication pockets. This can lead to the above-mentioned stick-slip effect or lubricant film breakage between the spindle and nut.
The frequently cited argument of the longer service life of rolled threaded spindles cannot be confirmed either. Wirths-Werres provided this evidence in an endurance test. “Even after many hours of operation, the whirled threaded spindles showed only minimal signs of wear,” explained Christian Zahn, design engineer at Wirths-Werres. According to Christian Zahn, the stresses in the material caused by the rolling process also often lead to cost-intensive reworking. In the whirling process, the whirling tool for the production of spindle threads consists of a driven whirling ring with a set of profile tools. It works in the so-called envelope cut, as the workpiece rotates within this tool ring during machining.
The envelope cut starts at the outer diameter with a chip thickness and chip depth of 0 and continuously increases to the calculated chip thickness and chip depth or thread depth, before exiting the workpiece again at 0. This produces a very soft cut with a very smooth surface. As a tool set consists of several individual cutting edges that work one after the other in an interrupted cut, microscopically small interruptions occur in the workpiece surface between the cuts, which create polygon-shaped, concave facets in the E area, the lubrication pockets.
These lubrication pockets ensure the good lubricating film properties of whirled lead screws, as the lubricant is not touched or displaced by the high surface loads of the bearing flanks of the thread profiles in the valleys of the polygons. As a result of the comparison of the manufacturing processes, it can be stated that although the rolling and grinding of thread profiles are modern manufacturing processes that are recommended for many applications, the whirling process used by Bornemann Gewindetechnik should be preferred for the use of trapezoidal screws for screw jack systems.