Examining the choices in roll and gripper feeds
January 10, 2002
The two basic press feed types commonly in use today are roll feeds and gripper feeds. Roll feeds, as the name implies, use rollers to move the material into the tool, while gripper feeds employ gripping clamps and a linear motion to move the strip.
The two basic press feed types commonly in use today are roll feeds and gripper feeds. Roll feeds, as the name implies, use rollers to move the material into the tool, while gripper feeds employ gripping clamps and a linear motion to move the strip. Both types of press feeds can be powered by the press, by a self-contained drive system (such as air- or hydraulic-powered), or by a servo-drive system.
Press-driven. Feeds that are driven by the press, such as rack-and-pinion or cam feeds, always are synchronized with the press rotation. They begin their motion at some predetermined point in the press cycle and finish 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 to the press.
This synchronization feature makes press-driven feeds suitable for high-speed indexing, feeding in-die transfers, or for use with unloaders and other applications that require feed motion to be tied to press rotation to avoid a collision. When the press stops, the feed motion stops as well.
Conversely, because most press-driven feeds lack timing adjustment, the feed motion for all dies cannot begin until after a point in the stroke at which the deepest draw die disengages.
Inherent in the design of press-driven feeds is smooth motion, called an S-curve move profile, in which the rate of acceleration varies throughout the index. These feeds make gradual transitions in velocity, with high acceleration and deceleration in the interim. Eliminating sharp velocity transitions, called jerk points, helps these feeds make high-speed indexes with good accuracy.
Most press-driven feeds are limited in feed length and feed length adjustment. They cannot perform inching and use no controls interface. Most require that gear sets, rollers, or mechanical linkages be changed or adjusted to modify feed length.
Also, since these feeds are coupled directly to the press rotation, they lack the ability to jog the strip for threading. Without electrical controls, mechanical feeds cannot accept setup information from or provide feedback to press control or automation systems.
Self-powered. Self-powered units begin their motion in response to a signal from the press but have a finite, minimum amount of time in which they are capable of indexing based on the amount of power available and the load that is encountered.
Thus, the point at which the feed finishes varies with the press speed. The faster the press runs, the later in the stroke a self-powered unit will finish. These feeds always finish the feed motion once it has been started, regardless of what the press does.
Self-powered feeds operate independently of the press and can be adjusted for each application to begin feeding as soon as the die opens. Most go from a stationary condition directly into a fixed rate of acceleration, resulting in jerk points at the beginning, middle, and end of each move.
Servo-driven. Servo-driven roll feeds use a closed-loop positioning drive, usually a servo but sometimes a stepper, to control the index position of the feed rolls.
These feeds operate at high speeds and take up minimal space. They have unlimited feed length capability and use a microprocessor-based control that provides programmable move patterns, self-diagnostics, autocorrection, and the ability to communicate with automation.
Servo-driven roll feeds are available in a variety of configurations, including conventional two- and four-roll units, feeder-straighteners, unwinder-feeder-straighteners, and zig-zag units. Feed control packages range from single setup controls with thumbwheels or keypads to systems that allow programming of multiaxis move patterns. Some can control auxiliary functions and devices and offer varying levels of memory and communications capability.
Most servo feeds use a trapezoidal move profile, which has four distinct jerk points. Some also are available with controls that can execute S-curve move profiles. Some systems are electronically synchronized with press rotation. These units require a controls package and a feedback device, either a resolver or encoder, that is attached to the press crank to track press rotation. Their top speeds are limited by the available drive power.
Servo-drive technology has matured over the past few decades to the point that these drives are more reliable and less expensive than they were in the past. However, a considerable amount of technical expertise may be required to troubleshoot them.
As mentioned previously, gripper feeds employ a linear motion to move the strip. They are available in a variety of sizes, from compact press-mounted units to large cabinet-mounted models, which include pull-through straighteners.
Gripper feeds are limited to a specific maximum feed length based on the model selected, so the longest feed length requirement must be anticipated at the time of purchase. After that, each additional increment of length costs more, and the longer feed length capability means the machine itself must become longer, thus requiring more floor space.
The tendency is to buy the shortest machine that will fill the need. If the machine must run a feed length that is longer than it was designed for, it must perform multiple cycles on each press stroke, commonly referred to as multistroking. This capability requires an optional controls package, and the press typically is operated in the single-cycle mode because of the time required for the return stroke.Gripper feeds use a pair of clamps: the retainer, which remains stationary, and the gripper, which moves during the feed and return strokes. During the feed stroke, the retainer releases the strip as the gripper holds 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. Since the return and feed strokes take about the same amount of time, gripper feeds are limited to a 180-degree feed window at the maximum operating speed.
The gripper's pulling force can be provided by air- or hydraulic-powered cylinders, a hydraulic motor, a servomotor, or the press. The gripper usually is supported by guide bars or rails and is driven by cylinder rods, chains and sprockets or ball screws, or a mechanical linkage to the press.
Self-powered. With self-powered (air- or hydraulic-powered) units, feed length is adjustable. The gripper moves between an adjustable stop and a stationary stop, and a cushion softens the blow at the end of each stroke. Feed length adjustment may require the use of tools and can involve some trial and error.
Air-powered gripper feeds generally are low in cost and commonly are used in conjunction with pull-through straighteners for applications requiring low to moderate speeds and limited feed lengths.
The cost of the compressed air used by these feeds and the loss of air through leaks, pressure drops, and contamination can offset the savings realized from the feed's lower initial cost. Air-powered gripper feeds have many moving parts and wear components, which can add to maintenance costs.
Servo-driven. Servo-powered gripper feeds use a closed-loop servo drive coupled to a ball screw and nut to position the gripper, and they don't use stops or cushions. Feed setup information is programmed into the control unit via an operator interface, or it can be serially downloaded from another device. The control unit then commands the servo drive to position the gripper accordingly. These feeds also can interface with press automation systems.
A variety of methods are used to unwind and feed coiled stock; only the most common have been covered here. Careful thought must be given to the equipment that is selected to avoid limiting the productivity of the system as a whole.
Jim Ward is general manager of Coe Press Equipment Corporation, 40549 Brentwood Drive, Sterling Heights, MI 48310, phone 810-979-4400, fax 810-979-2970, Web site www.cpec.com. Coe Press Equipment Corporation designs and manufactures coil handling and servo roll feed equipment that includes servo roll feeds, air feeds, power straighteners, coil reels, coil cradles, and accessory equipment.