Two basic feed types are roll feeds (press-driven and servo-driven) and gripper feeds. Compact coil lines that combine decoiling, straightening, and feeding into one machine(3 in 1) can reduce feed line length by as much as half. A roll feed can be either powered by the press (typically viewed as older technology or for high-speed, dedicated press applications), or it can be powered by its own self-contained drive system (the majority of today’s applications).
Press-driven Roll Feed
Roll feeds that are press-driven, such as rack-and-pinion or cam, are always synchronized with the rotation of the press. The feed’s motion begins at a predetermined point in the press cycle and finishes at another predetermined point regardless of press speed or die engagement. Although the index speed must increase or decrease to keep pace with the press, the feed can draw as much power as it needs from the press to accomplish this within the limitations of the mechanical coupling of the feed to the press.
This synchronization feature makes press-driven feeds suitable for high-speed indexing, in-die transfers, and for use with unloaders and other applications that require feed motion to be tied to press rotation to prevent a collision.
Drawbacks to most press-driven feeds are their difficulty in adjusting the feed length, lack of inching capability, and absence of controls interface. Most require that gear sets, rollers, or mechanical linkages be changed to adjust feed length. Because press-driven feeds are coupled directly to the press rotation, they lack the ability to jog the strip for threading. Additionally, because of the lack of electrical controls, mechanical feeds cannot accept setup information from or provide feedback to press controls or automation systems.
Servo-driven Roll Feed
The concept of servo-driven roll feeds involves the use of a closed-loop positioning drive, usually a servomotor, to control the index position of the feed rolls. A servo-powered unit begins its motion in response to a signal from the press. It is capable only of a finite, minimum index time, which is based on the amount of power it can deliver and the load that it sees. As a result, a servo-powered unit’s finishing point can vary with the press speed. The faster the press runs, the later in the stroke a servo-powered unit finishes. Their top speeds still are limited by the available drive power, as opposed to press-driven feeds that can run as fast as the press and tooling.
This combination transfer/progressive line’s primary feeder feeds oscillating shear die and feeding blanks to a transfer operation. A second servo roll feed rolls into place when progressive-die operations are used.
Servo drive technology has been used in press feeding applications for more than a decade, and it has now matured to the point that these drives are more reliable and less expensive than they were in the past. Servo systems’ modularity and self-diagnostic features have improved their functionality.
Servo-driven roll feeds have many of the same advantages press-driven feeds have, such as minimal space requirements, low maintenance, and high speeds.
However, servo feeds also provide benefits that press-driven units cannot because a microprocessor-based control gives them capabilities such as programmable move patterns, self-diagnostics, auto correction, and the ability to communicate with automation. Servo-driven roll feeds are available in several configurations, including:
- Conventional two-roll units
- Four-roll units (sometimes specified when processing thick materials)
- Feeder/straighteners (for space-saving requirements)
- Push-pull units (typically for light-gauge materials)
- Zigzag units (for better material utilization when stamping nested round blanks)
Servo feed control packages vary in sophistication, from simple-to-use, single-setup controls with thumbwheels or keypads to complex systems that allow programming of elaborate multi axis move patterns, control of auxiliary functions and devices, and varying levels of job recipe storage memory and communications capability. Most servo feeds offer some degree of integration with the press control to provide constant contact with the press operations:
- “Hand-shaking” on job setup sand recipe storage
- Simplified operator interface that allows single point of data entry/verification and a touch-screen display that delivers operational prompts and diagnostic information and help screens
- Faster maintenance reminders or fault information via a single control station
- More control of peripheral equipment such as in-press transfer units, safety equipment, and die monitoring systems
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 multi stroking. 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 multi stroking.
- 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.