Heavy loads under control – Heavy-duty trapezoidal lead screws for lifting systems
The design of heavy-duty trapezoidal screw drives is the basis for the safe and smooth operation of lifting systems—and places high demands on designers.
Bornemann Gewindetechnik, a company well known among experts, has been manufacturing threaded spindles and nuts for many branches of industry for more than 25 years. One focus of production is the manufacture of ready-to-install trapezoidal screw drives, consisting of spindles and nut sets, for heavy-duty lifting systems. To name a few examples: lifting systems for rail vehicles, systems in theaters, stage construction, and special machine construction. The design and construction of these drives are prerequisites for the safe and smooth operation of the systems and place high demands on the designers.
But what should be considered when using threaded spindles for heavy loads? For screw jacks that reach a surface pressure of > 20 N/mm2 under load and also operate at very low peripheral speeds, constant lubrication of the bearing surfaces of the thread profiles is absolutely essential. This is because inadequate lubrication can lead to increased wear, excessive heat generation, and also to the notorious stick-slip effect.
It all depends on the lubricant
The stick-slip effect, also known as the adhesive sliding effect, describes the jerky sliding of solids moving against each other. The effect can occur whenever static friction is greater than sliding friction. Depending on the tribological system, this leads to the excitation of vibrations, which are emitted as noise by a resonant surface (screeching and squeaking spindles).
The effect usually disappears as soon as the friction partners are separated by an intermediate medium or lubricant. The stick-slip effect is often undesirable in technical applications. Negative influences caused by the stick-slip effect can be observed in bearings, guides in linear technology, and threaded spindles in lifting technology, among other things. Noises such as the creaking of a door, the squeaking of trains when cornering, and rattling windshield wipers on car windows are the generally known consequences of this effect.
If this effect occurs, it may be due to the choice of lubricant (grease). Ralph Wuertele, Manager Application Engineering at Klüber Lubrication, explains: “In heavy-duty lifting equipment in particular, unsuitable lubricants can easily lead to insufficient lubrication, which causes increased wear and thus premature failure of the equipment.”
This phenomenon is often caused by the design of the spindles. The lack of lubrication pockets on the bearing flank of the thread profile prevents the applied lubricant from remaining in place when the nut profiles slide over the spindle with high surface load. Either the lubricant is pressed off the flank or it is pushed in front of the nut. In either case, the lubricating film breaks down, initially causing a rise in temperature and stick-slip with sometimes deafening noises, and later leading to the destruction of the threaded drive.
At circumferential speeds of a few cm/s, the presence of lubricant or lubrication pockets on the threaded spindles is absolutely necessary to prevent this stick-slip effect. The lubrication pockets prevent the lubricant from simply being pushed away by excessive edge pressure and generally high specific surface pressure. The lubricant should ideally be a grease with a consistency of 0 or 00 and must be drawn into the lubrication gap.
How can this problem be addressed? The foundation for preventing lubrication problems and the resulting issues is laid early on in the designers’ requirements for thread production. Threaded drives can be manufactured using various production methods: Thread turning and thread milling are methods that are rarely used for the aforementioned threaded drives for economic reasons. In thread rolling (or rolling), threaded spindles are produced by cold forming without cutting. This manufacturing process is quite economical, but due to the very high tool and setup costs, it is usually only applicable in large series production.
Soft cut with smooth surface
Furthermore, rolled threaded spindles promote lubricant film breakup due to the absolutely bare thread flank surface. The rolled trapezoidal thread has an excessively high flank surface quality due to the deformation process in stamp polishing/press polishing, in which the lubrication pockets are missing. This can lead to the dreaded stick-slip or lubricating film breakup between the spindle and nut. The lubricant is squeezed out of the thread grooves by excessive edge pressure and generally high specific surface pressure.
This squeezing out is possible if the materials of the plain bearing pairing do not have lubrication pockets in which the lubricant can be stored. The result is serious consequential damage to the affected systems in the form of screeching spindles and seizing nuts, and even complete thermal welding of entire lifting systems.
The frequently cited argument that rolled threaded spindles have a longer service life cannot be confirmed without exception. The company Wirths-Werres was able to prove this in a continuous stress test. “Even after many hours of operation, the whirled threaded spindles showed only minimal signs of wear,” emphasizes design engineer Christian Zahn. According to Zahn, the stresses in the material caused by the rolling process often lead to costly rework. In a direct comparison, Wirths-Werres opted for high-quality spindles in a whirled design, as the price difference between rolled and whirled spindles is now negligible for many diameters.
Bornemann Gewindetechnik manufactures threaded spindles using the whirling process. The whirling tool for the production of spindle threads consists of a driven whirling ring with a set of profile tools and works in what is known as envelope cutting, as the workpiece rotates within this tool ring during machining. Since 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, a very smooth cut with a very smooth surface is produced.
Since a tool set consists of several individual cutting edges that work in succession in interrupted cuts, microscopic interruptions occur between the individual cuts in the workpiece surface, creating polygonal, concave facets in the μ range (lubrication pockets). These lubrication pockets are the secret behind the excellent lubrication film properties of whirled threaded spindles.
Comparisons of the various thread manufacturing processes reveal the following: Rolling and grinding thread profiles are modern manufacturing processes that are recommended for many applications. However, for the use of trapezoidal thread drives for lifting spindle systems, the whirling process practiced at Bornemann Gewindetechnik should be preferred.