1. Field of the Invention
The invention relates to a gripper mechanism, i.e. a device used to pull single product such as signatures and newspapers from a pile and transport them to another device for further processing, such as for binding, stitching, collating or insulating.
2. Description of the Related Art
Gripper mechanisms work in conjunction with other mechanisms mounted on a common frame. Such an assembly is referred to as a hopper. The invention of the instant application pertains to the type of hoppers as they are commercially available, from Muller Martini, McCain, AM Graphics and Ferag, to name just a few.
Most of these commercially available hoppers are similar in operation. Paper is placed on a paper tray and aligned at a registration point above the gripper drum. This may be done either manually or automatically. A sucker mechanism, e.g. a vacuum-operated suction cup array, pulls a single paper product downwardly. As a gripper seat passes under the leading edge of the paper product, a cam actuated gripper clamps the product and pulls the same from the pile.
As the gripper drum rotates with a constant angular velocity, the period of angular alignment between the gripper seat and the leading edge of the product is very short. A relative slip occurs between the just-gripped and the completely-gripped condition. That timing, however, is very crucial and, in the prior art devices, leads to an increased number of hopper faults. For instance, the gripper may miss the product altogether, it may pull it out of the pile only partially, it may grab more than one product, or it may rip the product due to the extremely quick transfer of momentum. At higher speeds, these problems are compounded and often result in a significant production loss.
Various attempts at solving these shortcomings have been made in the art. One such solution has been to employ two half-speed hoppers for one location. Other solutions include increasing the gripper force and/or providing different gripper seats and gripper materials.
Gripper apparatus of this general kind are exemplified in U.S. Pat. Nos. 2,425,936 and 2,903,260, for instance. The patents disclose signature feeders which have signature feed mechanisms which move relative to a rotating drum and in a direction opposite to drum rotation. Accordingly, the mechanism is slowed down relative to the rotation of the drum at the time of engagement with a signature.
U.S. Pat. No. 4,901,996 to James Schlough pertains to a gripper drum for removing paper signatures from a stack and for feeding them to a collator conveyor. The signature is drawn into the rotating drum at a speed approximately identical to the outer surface velocity of the drum. Once the signature is fully inserted in the drum, it is accelerated in the direction of drum rotation and, after a given arcuate distance has been traversed, the signature is pushed from the storage rack within the drum and fed to the collator feeder.
Furthermore, prior art gripper mechanisms are provided with a spring system for closing the gripper jaw with a compression or tension spring. The spring, which is pre-biased in the open-jaw position, is unloaded when the gripper closes. This causes a reduction in gripper force at the time when it is most needed, i.e. at the time the paper must be pulled from the stack. Increasing the spring size, however, places increased loads on the drum as the spring is compressed. In one possible solution to the problem, a prior art gripper (AM Graphics hopper) is provided with a separate cam system to move the ground point of the spring. In order to attain sufficient gripper force, however, the cam follower is required to leave the cam.