Gripping safety of eccentric clamps

Status:

completed 06/1995

Aims:

Eccentric clamps must make proof of a sufficient gripping safety. The gripping force depends to a large extent on the clamping force and on the friction between the clamping devices. The project, initiated by the expert committee "Materials handling and load-lifting equipment", was aimed to determine friction coefficients between sheet steel and profiled clamping devices on account of various parameters; the friction coefficients should enable a reliable assessment of the gripping safety of eccentric clamps. In addition, a simple method was looked for to decide on the replacement state of wear of eccentric clamps used in industry.

Activities/Methods:

A test facility was constructed which allows for a separate determination of the friction coefficients of plates and eccenters. The friction coefficients were derived from the normal force and the tangential force acting on the clamping device. The following parameters were changed: design and construction of the clamping devices, clamping force, state of wear of the clamping devices, hardness and surface characteristics of the load.

Results:

According to prEN 13 155 "Cranes - Safety - Non-fixed load lifting attachments" eccentric clamps must make proof of a double gripping safety. The latter depends primarily on the gripping force - a product of load weight and design - and on the friction coefficients between the clamping devices and the load. It could be shown that as a result of micro fitting friction values determined for the eccenter are generally higher than those occurring on the plate. Consequently, the eccenters have a much larger effect on the gripping safety than have the plates. The friction coefficents increased together with the gripping force and thus with the load weight. On the other hand, worn out clamping devices lead to lower friction coefficients and thus to a loss of gripping safety. Gripping safety, however, is also determined by the surface structure of the load. The most important parameters in this context are the hardness, the roughness as well as rust and scale layers. The variety of parameters account of which must be taken in the assessment of gripping safety, makes it difficult to define global criteria for the replacement state of wear of clamps. Free-to-float tests represent a possible solution to the problem; these tests are easy to conduct on the spot using test sheet metals. To make sure that a decline in gripping safety is recognised early enough, rather critical load conditions should be chosen for the test. This means that the weight of the sheet metal should be in the lower range of the clamps' loading capacity and the sheet thickness should be close to the lower limit of the indicated gripping range. As to the hardness of the test sheet, model tests revealed that it should be in the range of the admissible maximum values. The sheet surface should be plane, unscaled, free of rust and as smooth as possible. If slipping of the sheets is observed in the test, it is recommended to replace the clamping devices for the sake of accident prevention, as the gripping safety can be supposed to be considerably reduced.

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