Service life of self-locking and self-retarding spindle drives for vehicle lifts

Status:

completed 02/2002

Aims:

Spindle drives without supplementary braking facilities are often used in vehicle servicing lifts under the raised loads of which work is carried out during the repair of vehicles. In accordance with the provisions of the German UVV accident prevention regulations governing hoists (VBG 14) and EN 1493,"Vehicle Lifts", the design of these drives must prevent unintended movements. This means that the spindle drives must possess an adequate self-locking and self-retarding action over their service life. The purpose of the study was to determine the extent to which wear-promoting impurities in the lubricant may reduce the self-locking and self-retarding action of spindle drives. The behaviour of load-bearing nuts manufactured from polyamide in comparison to equivalent bronze nuts was of particular interest.

Activities/Methods:

The self-locking and self-retarding factors, representing the dimensions for the self-locking and self-retarding safety in accordance with VDI Guideline 2185, which governs self-locking and self-retarding drives, were determined on spindle drives with load-bearing nuts manufactured from bronze or polyamide. The stopping distance was also determined as a function of the number of load cycles performed. A grease/oil mixture to which defined quantities of wear-promoting substances had been added was employed for lubrication. Further studies were performed in order to determine the influence of mechanical damage to the screw upon the operational safety.

Results:

Even small quantities of solid impurities in lubricants can reduce the service life of spindle drives by over 90%. Physical damage to the spindle surface has an even more severe effect upon the service life than impurities. The tests performed also show that neither impurities in the lubricant, nor physical damage to spindles results in impairment of their self-locking and self-retarding action. This holds true both for the combination steel spindle/bronze supporting nut and for the combination steel spindle/polyamide supporting nut. In order to ensure long service life, spindle drives must be protected against soiling and damage, for example by suitable shrouds.

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