Gripper Feed. 

A lower-cost option, a gripper feed uses a linear motion, rather than rotary action, to move the strip. Gripper feeds utilize a pair of clamps. One is a stationary clamp called the retainer, and the other, called the gripper, moves infeed and return strokes. During the feed stroke, the retainer releases the strip as the gripper clamps and moves it forward through the top half of the press cycle while the tool is open. On the return stroke, the gripper releases the strip, and the retainer holds it while the gripper retracts from the press through the bottom half of the press cycle while the tool is closed. Because it usually takes about as much time for the return stroke as it does for the feed stroke, gripper feeds are limited to a 180-degree feed window at maximum operating speed.

The gripper and retainer clamps can be air- or hydraulic-powered cylinders, or they can be one-way roller mechanisms that hold the strip in one direction but allow it to roll freely in the opposite direction. With cylinder-powered clamps, the timing of the clamp and release is critical to accurate feeding and can be a limiting factor in terms of speed. If the timing is not correct, the strip can be free at times to fall back, resulting in short feeds. Clamping is actuated by solenoid valves or air logic valves. Timing can be controlled either electrically or through valve porting.

The pulling force for the gripper can be provided by an air or hydraulic cylinder, hydraulic motor, or by a servomotor. The gripper usually is supported by guide bars or rails and is driven by cylinder rods, chain and sprockets, or ball screws. With air- or hydraulic-powered units, the feed length is set by adjusting a positive stop. The gripper moves between the adjustable stop and a stationary stop and employs a cushion of some sort to soften the blow at the end of each stroke. Feed length adjustment may require the use of tools and often involves some trial and error, which usually results in longer setup times.

For applications requiring low to moderate speeds and limited feed lengths, relatively inexpensive air-powered grip feeds generally are used with pull-through straighteners to provide a cost-effective alternative to roll feeds with powered straighteners.

Some gripper feed limitations are:

• The longest feed length requirement must be anticipated at the time of purchase. The disadvantage is that each additional increment of length costs more money and requires more valuable floor space. If there is ever a need to run a feed length that is longer than the machine was designed for, it must perform multiple cycles on each press stroke, commonly referred to as multistroking. This capability requires an optional and more expensive controls package, and because of the time required for the return stroke, the press usually must be operated in the single-cycle mode when multistroking.
   
• Its low initial capital cost may be offset by longer setup time and higher maintenance and energy costs. Compressed air often is an expensive energy source because of losses due to leaks, pressure drops, and contamination. Because of the many moving parts and wear components, maintenance costs can be quite high. These machines require timely maintenance to sustain good accuracy and performance.

Gripper feed sizes can run the gamut from compact, press-mounted models to large, cabinet-mounted models that include pull-through straighteners.
There are many ways to feed a press, but many important considerations regarding the purchase, setup, and operation of this equipment will help determine how productive it will be. If the system is to work at maximum efficiency, then each component of the system must complement the others. An in-depth discussion of each potential application with production schedulers, managers, engineers, and equipment suppliers will net the most advantages.