Stick-Slip Effect
For more than 25 years, Bornemann Gewindetechnik has been manufacturing threaded spindles and nuts for virtually every application across all industrial sectors. A key focus of production is the manufacture of ready-to-install trapezoidal thread 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-purpose machinery manufacturing.
For the reliable operation of lifting equipment, the surfaces of the threaded spindles and the selection of the appropriate lubricant are of great importance. It is essential to avoid the notorious stick-slip effect. Hans Gereke-Bornemann (Bornemann-Gewindetechnik, Managing Director): “When we receive reports of problems with screw jack systems, it can generally be assumed that one of the following factors is part of the cause: surface finish of the screws, contamination, installation position of the system, or, to a particularly significant extent, the selection of the lubricant. In many cases, however, the problem is a complex combination of several of these factors. It is therefore essential to give each of these factors sufficient consideration during the design phase.”
The Stick-Slip Effect
The stick-slip effect (from the English words “stick” and “slip”), also known as the adhesion-slip effect, describes the jerky sliding motion of solids moving against one another. This effect occurs whenever static friction is greater than kinetic friction.
Depending on the tribological system, this leads to the excitation of vibrations that are radiated as noise by a resonant surface (“screeching” and “squeaking” spindles). The effect usually disappears as soon as the friction partners are separated by an intervening substance or lubricant. The stick-slip effect is often undesirable in technical applications. Negative effects caused by the stick-slip effect can be observed, for example, in bearings, linear guides, or threaded spindles in lifting technology.
Ralph Wuertele (Klüber Lubrication, Manager of Application Engineering): “Especially in heavy-duty lifting systems, unsuitable lubricants can easily lead to conditions of insufficient lubrication, which cause increased wear and thus result in premature system failure.” The aforementioned screeching and squeaking noises then typically occur as a directly perceptible side effect of this insufficient lubrication.
In the worst-case scenario, such conditions of insufficient lubrication can even lead to micro-welding or cold welding of entire lifting systems—the so-called “seizing” of the nuts—resulting in serious consequential damage and even failure of the affected equipment.
Here, the material pairing of the lead screw-nut system plays a decisive role. In material pairings of gray cast iron and steel, the iron content in the materials can promote cold welding. With a combination of bronze and steel, cold welding can generally be ruled out. So what should be considered to avoid these dreaded conditions of insufficient lubrication or the so-called stick-slip effect?
Material Compatibility
The lubricant must be demonstrably compatible with the bellows and sealing materials. If this were not the case, the lubricant could adversely affect these materials, leading, for example, to porosity, cracks, or bulging. The result would be leaks with lubricant loss or the ingress of contaminants. In both cases, the bellows or sealing system can no longer perform its function as intended. Compatibility tests between the lubricant and the bellows and sealing materials should be conducted at the maximum permissible operating temperature for at least 168 hours to ensure reliability.
According to the bellows manufacturer MFB-Technik, the correct selection of bellows material and consideration of external environmental influences are important criteria that affect the service life of bellows but are often neglected by customers.
2. Surface of the threaded spindles
Several factors ensure that the lubricant remains where it is needed. These include the surface topography of the threaded spindles and nuts, which Bornemann-Gewindetechnik manufactures using a swirling process. This creates microscopically small lubrication pockets on the threaded surface. These lubrication pockets ensure that the lubricant is not pushed away under high surface loads but remains in the “valleys” of the surface, as it is not touched by the load-bearing flanks of the thread profiles.
In the case of threaded spindles manufactured using the rolling process, the absence of lubrication pockets on the load-bearing flank of the thread profile often prevents the applied lubricant from remaining in place when the nut profiles slide over the spindle under high surface loads. Either the lubricant is squeezed off the flank or it is pushed ahead 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. Moritz von Soden (Sales Manager, Bornemann-Gewindetechnik): “Many of our customers are in the heavy-duty lifting technology sector. Here, swirled-finish threaded spindles are consistently used to avoid the well-known problems associated with insufficient lubrication.”
Furthermore, the adhesion of the lubricant itself plays an important role. This is determined by the lubricant’s formulation, specifically the type and amount of thickener. In addition, cleaning the surface before applying the lubricant is crucial. Residues from metalworking fluids, for example, are not conducive to adhesion.
3. Creep Behavior
The appropriate thread geometry of the nut ensures that it does not push the lubricant forward but rather draws it into the lubrication gap. Nevertheless, the lubricant can be quickly carried away or flung from the friction point—but it should flow back as quickly as possible. For this reason, Klüber Lubrication recommends using grease with an NLGI consistency class of 0, or better yet 00. The spindle systems must, of course, be properly sealed. The amount of grease should fill at least 30% of the volume between the spindle and the bellows.
4. Oil Release
Due to the relatively low sliding speeds and high contact pressures in the spindle-nut tribological system of screw jacks, a hydrodynamic lubricating film cannot form—which could lead to insufficient lubrication. In this operating situation, the additives play a decisive role in determining the lubricant’s performance. Since the additives are dissolved in the oil component of the grease, the grease must release sufficient oil under load. Due to its good fluidity, the oil—including the additives—easily reaches the actual friction point, thereby ensuring reliable lubrication.
5. Corrosion Protection
Another requirement for the lubricant stems from the system’s sealing properties: Since the bellows is generally not airtight, the formation of condensation inside cannot be completely ruled out. For this reason, the lubricant must have high water resistance and provide excellent corrosion protection.
Marc Schillig (Managing Director of MFB-Technik Schillig GmbH & Co. KG): “We, too, are regularly confronted with the issue of ‘condensate.’ In principle, ‘airtight does not work.’ Air exchange must be ensured during movement, as the bellows will otherwise certainly be damaged. High-quality screens or valve flaps are used to address this problem. Condensation is transported to the outside via a separator by the overpressure generated when the bellows closes. We have already implemented various solutions with our customers.”
For 25 years, Bornemann-Gewindetechnik GmbH, located in Delligsen near Hildesheim, has been manufacturing threads in all special shapes as well as turned and milled parts with a staff of 40 employees working in a 3,800-square-meter facility. The company uses all machinable materials as raw materials. In addition, the company offers repair services for defective spindles. The standard sizes for threaded spindles range in diameter from 10 to 180 millimeters and extend up to a length of 6 meters. In modern machining centers, dimensional tolerances of h6 can be maintained. For a spindle diameter of 100 millimeters, h6 corresponds to a tolerance of 99.978 to 100 millimeters, ensuring that ball bearings can be properly mounted on it. Using the whirling process, thread surfaces achieve a grinding finish. Delivery time is 2 to 3 weeks after receipt of the order.