The present invention is directed to a lift truck-mounted load-handling clamp adapted for handling stacked loads of different sizes simultaneously. More particularly, the invention is directed to a clamp for handling stacked paper rolls of abbreviated length, referred to as split paper rolls.
A relatively common requirement in the paper industry is the handling of split paper rolls which, because of their short length, are normally handled by a lift truck roll clamp in pairs having different diameters, stacked one atop the other. Lift truck paper roll clamps specially adapted for handling such stacked split rolls have been available in the past and consist of a pair of separately-actuated clamp arms on one side of the clamp, in opposed relation to a single, larger clamp arm assembly on the opposite side of the clamp. The separately-actuated arms give the clamp the ability to apply clamping force to two cylindrical objects of different diameters stacked one atop the other. Similar clamping capability can be required with respect to other types of loads, such as stacked bales or cartons of different sizes.
A problem common to such previous clamps is their inability to attain the required clamping force on one of the separately-actuated clamp arms without attaining it also on the other separately-actuated arm. Stated another way, the resistance to clamping force from a load engaged by one of the clamp arms must be matched by a corresponding resistance to clamping force on the other arm before any clamping force can be applied. For example, some of the prior split roll clamp structures merely have each separately-actuated arm powered by a separate hydraulic cylinder, the cylinders being connected in parallel to a source of pressurized fluid such that the pressure build-up in the two cylinders during clamping must be identical. The problem with such a structure is that, if only a single split roll is to be handled, clamping pressure on the roll-engaging arm cannot be attained until the other arm has closed to its maximum extent, which is very time-consuming. Conversely, on opening of the clamp arms to release a load, the release of both clamp arms is not usually simultaneous due to different frictional resistances in the respective arm mechanisms and, depending on the relative friction in each mechanism, may require full opening of one clamp arm before the other releases sufficiently to disengage the load, which is likewise time-consuming.
Certain alternative types of split roll paper clamps have been marketed in an attempt to solve these problems. These alternative designs operate on a common principle, i.e. a mechanical or hydraulic link is provided between the separately-actuated clamp arms which permits only a limit range of movement between the clamp arms, or between the respective load-engagement pads of the clamp arms. Such mechanical links include simple flexible or articulated tether-type links which prevent the unloaded arm from closing beyond a predetermined position relative to the loaded arm, such that the link supplies the resisting force to the unloaded clamp arm which would otherwise be provided by a load. This allows the required clamping pressure to be built up without complete closure of the unloaded arm. Comparable alternative structures employ either a mechanical or hydraulic balance beam principle between a pair of clamp arms or load-engagement pads, so that the extent to which the unloaded arm or pad must close before clamping pressure can be built up on the loaded arm or pad is likewise limited. All of these structures, however, share the common problem that they are capable of handling a pair of split rolls only if the respective diameters of the two rolls are within a predetermined range of each other corresponding to the limited range of movement permitted between the two arms. Moreover, they share the further problem that, when only a single split roll is handled, it is subjected to twice the clamping force that exists when a pair of rolls are handled since all reaction to the clamping force must be absorbed by the single roll. This places undue and possibly damaging stress on the single roll, as well as on the particular clamp arm or pad which is engaging the roll.
In many types of hydraulic mechanisms designed for a variety of purposes, it is common to use flow divider or combiner valves to regulate the flow of fluid to multiple actuators so as to cause them to move simultaneously and in proportion to each other. Such systems, however, are not adaptable to split paper roll clamps because the varying differences encountered between the diameters of pairs of stacked split rolls, as well as the requirement for engaging only a single split roll if necessary, require different proportions of movement between the respective clamp arms with each different load situation encountered. Flow divider and combiner valves are not readily adaptable to changeable proportions of movement.
Alternatively, it would be possible simply to control the actuation of each clamp arm through a separate directional control valve. However, such arrangement would require too many hydraulic lines passing from the lift truck to the vertically-reciprocating load clamp, and would require too many control valves for the lift truck operator to manipulate while simultaneously maneuvering the lift truck.