The invention relates to a hoist clamp, constructed for clampingly gripping plate-shaped pieces to be hoisted.
Hoist clamps for plates are known, which comprise a smooth eccentric round clamping segment constituting part of a spring operated toggle lever system, on which at the other end the hoist eye operates, and which co-operates with a smooth flat counter clamping segment which is adjustable in the direction of the thickness of the plate material to be gripped. The counter clamping segment constitutes part of a movable body realized as a wedge, which co-operates--by means of toothed faces--with a stationary block and which urges the counter clamping segment under spring action in the direction of the smallest thickness of the plate after a locking mechanism acting upon the toggle lever system has been released. A double locking mechanism is provided, to be operated by means of a handle, and which in the locking position acts upon the toggle lever system to lock it in its most-folded condition in which the eccentric clamping segment is inoperative, and which is further capable of blocking the wedge in the non-operative position. The construction is such that the blocking of the wedge is first released, without change of position of the toggle lever, before the unlocking of this toggle lever can be brought about.
Such a hoist clamp is disclosed in U.S. Pat. No. 3,843,186 of the same inventor.
As is disclosed in the specification of the prior patent that invention aimed at providing a hoist clamp which is adapted for gripping plate material of which the surface of which should not be damaged. Therefore the eccentric clamping segment may not, as is known with other hoist clamp types, be toothed, but it should have a smooth surface. In order to nevertheless obtain a sufficient clamping force it is then necessary to change the dimensioning in this way that the ratio between the path of displacement of the hoist eye and the change of radius of the eccentric clamping element is selected at a greater value. This implies, however, that the maximum extent of the change of radius of the eccentric clamping element is reduced to a fraction, so that it is not possible anymore to process any plate thickness value.
This purpose is served by the special wedge shaped structure of the counter clamping segment with toothed faces, a control lever and a locking mechanism, by which the counter clamping segment always finds the correct position for each and any value of plate thickness to be gripped before, in hoisting, the toggle lever system is able to develop the definitive clamping force.
Further there was introduced a double locking. With this it is possible to release in a first stage during the unlocking only the wedge, so that this wedge is able to confine the thickness of plate present at that moment, and to subsequently have the toggle lever stretch itself forcefully and suddenly, whereby clamping force is being built up and the wedge will, with complete certainty, engage the teeth which have come to lie opposite it in the first stage. The strong spring acting upon the toggle lever is not involved in the play of forces in the first stage of unlocking. Only in the second stage the unlocking of the toggle lever system will take place, whilst at that same time the compressed spring will exert a force upon the toggle lever in order to finally bring the teeth of the wedge into engagement.
The locking mechanism in the prior structure was comprised by two locking cams, operated by a same operating lever at the outside of the frame of the clamp, and one of which is acting upon the toggle lever system and the other upon a separate lever for the wedge. The operation of this system is excellent but it is sensitive, amongst other things to fouling and to certain deformations which, irrespective of how robust such a hoist clamp is usually realized, do occur sometimes in practice, for example when it is overloaded. Also, with the prior embodiment, it may occur that, if the clamp is being put onto the plate to be hoisted in a slanting position, the counter clamping segment will not confine the plate well and flatly, which means a loss of part of the effectivity of the eccentric clamping segment.