Screw drives are simple driving elements which are often used for vehicle lifts. To avoid occupational accidents, EN 1493 "Vehicle lifts" specifies that the drives must be so designed that the load-carrying device is held automatically in the inoperative position by self-locking. Switching off the drive should lead to a quick standstill of the load-carrying device. It is possible to meet these safety requirements without having recourse to additional gripping devices or breaks, but simply by using self-locking and self-retarding drives. EN 1493, however, does not contain any quantitative information on the extent of self-locking and self-retarding properties drives must possess to ensure satisfactory protection. Consequently, the project which was initiated by the expert committee for materials handling and load-lifting equipment was aimed to quantify these parameters for different spindle/nut-combinations and lubrication situations.
The method described in VDI guideline 2158 "Self-locking and self-retarding drives" was applied to quantify the self-locking and self-retardation safety of screw drives with a trapezoid thread. Determination of the self-locking properties of the static drive is made by a static analysis of equilibrium, while the self-retardation properties of the running drive are determined by an energetic analysis of equilibrium. The characteristics are the locking factor r and the retardation factor b. The numerical values of r and b give information about the self-locking and self-retardation qualities of a drive. The more these values exceed 1, the more safety in terms of self-locking and self-retardation is offered by a drive. Values below 1 indicate that there is no self-locking or self-retardation at all. The investigations were conducted on screw drives with load bearing nuts made from bronze and polyamide. Varying thread pitch angles and lubrication situations were examined.
The method described in VDI guideline 2158 allows a quantitative assessment of the self-locking and self-retardation safety of screw drives. Reliable results, however, can only be achieved by long-term tests. It is also possible to have the retardation safety checked regularly by an expert determining the slowing-down path of the drive. To concretise the requirement from EN 1493 calling for self-retarding drives, constructive suggestions in terms of lubrication and material choice are made, which have an immediate effect on the self-retardation behaviour. Generally spoken, a thread pitch angle of about 2.4° must not be exceeded to ensure sufficient self-locking and self-retardation properties. Polyamide nuts turned out to better resist wear than bronze nuts, especially in the case of insufficient lubrication. The unloaded safety nut did not show any significant wear.
craftsType of hazard:
Transport und Verkehr, Maschinensicherheit, Mechanische GefährdungDescription, key words:
vehicle lift, screw drive, self-locking, self-retardation, wear, load-bearing nut, unloaded safety nut