How to avoid the stick-slip effect in spindle lifting jacks?
The avoidance of the dreaded stick-slip effect is key when heavy loads are lifted. In mechanical lifting systems where trapezoidal spindles are used it is important to consider certain factors already in the design phase in order to avoid serious problems in the aftermath.
For the reliable operation of threaded spindles in lifting jacks, several factors need to be considered right from the start. Hans Gereke-Bornemann (Bornemann-Gewindetechnik, Managing Director): "If we are told of problems with lifting jacks, one can generally assume that one of the following factors is part of the problem: surface quality of the spindles, contamination inside the system, constructional faults or the selection of the lubricant. In many cases, the problem is a complex mixture of several of these factors. It is therefore important to consider all these factors already in the design phase in order to avoid the dreaded stick-slip effect.”
The stick-slip is caused by two surfaces alternating between sticking to each other and sliding over each other, with a corresponding change in the force of friction. Typically, the static friction coefficient between two surfaces is larger than the kinetic friction coefficient. If a force applied is large enough to overcome the static friction of the material, then the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement. This leads, depending on the tribological system, to vibrations that are emitted as noise ("squeaky" spindles). The effect usually disappears when the friction partners are separated by an intermediate or lubricant. The stick-slip effect is often undesirable in technological applications as it can also lead to cold welding of materials.
Ralph Würtele (Kluber Lubrication, Manager Application Engineering): "Especially in applications for heavy-lifting equipment unsuitable lubricants can easily lead to insufficient lubrication conditions that cause increased wear and thus lead to premature failure of the system." As a directly observable side effect of this lack of lubrication the lifting systems will emit screaming and screeching noises. In the worst case, a lack of lubrication can even lead to the destruction of complete lifting systems.”
If the quality of the spindles is not good the lubrication will suffer. Either the lubricant is squeezed from the flank, or it is pushed in front of the nut. In any case, the lubrication film will be destroyed which will lead to an increase in temperature of the complete system at first, then to "squeaky" spindles and finally to the destruction of the complete screw drive.
At low speeds and high pressures on the flanks the presence of a lubricant on the spindles is absolutely necessary in order to prevent the stick-slip effect from occurring. Bornemann spindles feature lubricating pockets on the flanks which help to keep the lubrication on the spindles. These lubricating pockets are added through the “whirling” production process which features several advantages compared to spindles manufactured by thread rolling machines. Here the thread is not forced into the material by pressure but the thread is directly cut into the material which moreover leads to the creation of an uneven surface of the flanks which in turn helps to keep the lubrication in place.
Bornemann-Gewindetechnik provides threaded spindles for various applications in all kinds of industries for more than 25 years. One focus of the production lies in the production of trapezoidal screw drives, consisting of spindles and nut sets for heavy-lifting equipment, such as those used in the field of lifting equipment for rail vehicles, in the theater or stage construction and special machinery. The standard sizes of screws are in the range of 10 to 180 millimeters in diameter and up to a length of 6 meters. Spindles are delivered within 2-3 weeks after the receipt of the order.
Released by Bornemann Gewindetechnik (09/2010)
