April 24, 2001
The years have brought a host of improvements to stamping presses as technology has made presses more efficient, safer, and easier to use.
Model Ts are an example of a once-modern mode of transportation that have become a collector's possession and museum lore. Industrialist Henry Ford gave the world access to mobility and freedom to travel; however, vehicles today have improvements that make the formerly innovative Model T an obsolete and cumbersome passenger car choice.
Improvements in the metal forming industry have been just as amazing over the past 50 years, and stamping press modernization is one of those improvements. Today's maintenance and plant engineers can implement improvements with each press outage or maintenance repair.
This article outlines the top 10 improvements available to modern pressroom managers.
Lubrication is key to the upkeep of any mechanical system and is the main cause of stamping press downtime.
All primary bearing and sliding surfaces must receive lubricant to avoid scoring and overheating. When an upgrade is scheduled or downtime occurs, some maintenance managers consider switching to a modern lubrication system.
Modern lubrication systems overcome the failures of earlier systems in a number of ways, including delivery, accuracy, and monitoring. Earlier systems relied on gravity and hand pumps to deliver the lubricant to the bearing surfaces. Operators, maintenance personnel, and other employees sometimes overlook these systems, or the systems are failure-prone. New systems can deliver pressurized lubricant via pumps powered by air, electricity, or motion from the mechanical components of the press.
Accuracy is another consideration for lubrication systems. Before the modern oil crisis mentality--thinking that wasting oil is not allowed because of cost--some people thought that the more lubricant supplied, the better. This not only was wasteful, it also caused problems with lubricant spillage and contamination of the machine, the flooring around it, and the parts being manufactured. It also was the cause of many workplace accidents.
By design, modern systems deliver the correct type, amount, and pressure of lubrication for the size and type of bearing or slide surface being lubricated. Thorough analysis of each bearing allows for accurate delivery at each point. Environmental cleanliness, shop safety, avoidance of part contamination, and safety factors all play into the justification for using a modern lubrication system.
Job shops first drove stamping press manufacturers to design presses with the ability to change speeds. Differing requirements of parts that ran at different strokes per minute gave rise to demand.
Early press modifications were made with makeshift gearboxes. Later, variable, cone-type speed drives allowed the press to be turned to the desired speed. Eddy current drives allowed the press to speed up and slow down using the magnetic field of the motor.
All these methods were both costly and required much maintenance. Mechanical gearboxes did not allow for infinite variation of speed. Variable-speed cone drives consumed a lot of energy and broke down frequently. Eddy current drives also consumed large amounts of energy, were expensive, and did not address the maintenance headaches of the other drive systems.
Presses entered a new phase of reliability when variable-speed AC drives were introduced and combined with programmable logic control technology.
Today, a press can be converted from another type of drive to a variable-speed drive, which allows for full torque at any speed. Drives now cost less than a motor did a generation ago, and they include programmable controllers. These new drives boast low maintenance costs, high performance, and low energy consumption.
Gone are the days of sweatshops. Gone with them are the antiquated methods of controlling stamping presses. Originally, simple single-relay systems offered little operator safety or options.
Systems developed since 1980 offer simple to complex options in a compact package. Not only do these systems cost less than older systems, but they also can be compact and easy to attach to the press. Options available include self-diagnostics, programmable functions for press attachments, integration of speed control, setup parameters, feeds, lubrication, and pneumatic systems.
Press safety was not a major concern during the early part of the 20th century. New systems, however, are compliant with the Occupational Safety and Health Administration (OSHA) and the American National Standards Institute (ANSI). Today's safety improvements resulted from improved worker conditions and OSHA regulations.
Technology improvements in stamping presses not only have made safety modifications common, but also offer previously unimaginable protection from metal forming hazards. Pressroom safety upgrades include safety light curtains that guard workers' fingers and hands with an infrared barrier that, if broken, immediately sends a stop signal to the press control.
Dual pneumatic valves, which control press engagement, eliminate the possibility of a valve sticking. This causes a press to run repeat strokes accidentally. Palm buttons now can eliminate accidents with anti-tiedown and antirepeat functions.
These safety improvements are so common that metal forming press injuries usually happen because machines have not been upgraded with current safety technology.
Large, separate clutch and brake units were the basis for machine transmission during the early- to mid-1900s. As a result, repair costs consumed a large portion of maintenance budgets, and the units were slow-acting and did not have inherent safety features in their design.Today's modern, interconnected clutch brakes can be an alternative to original equipment manufacturer (OEM) units, whose replacement parts often can be expensive and hard to find. As an alternative, a new, fast-acting clutch brake can be retrofitted to help lower maintenance costs and extend press life.
Though a lack of lubricants is the primary cause of press downtime, heat also is a major cause of machine failure. Component seizure because of overheating of the main bearings can be a critical failure. Touch and smell were the only heat-monitoring options for earlier pressroom personnel.
The introduction of heat-sensing probes, sensors, and detection devices has allowed for simple machine monitoring. Avoiding catastrophic machine failure can be accomplished with a simple, low-cost upgrade that pressroom employees can perform. Simple monitor sensors can be attached using a drilled and tapped hole located at critical bearing points. Tied into press controls, these sensors can send a stop signal immediately when operating temperature elevates to dangerous levels.
Other temperature monitors include heat guns, which are portable, hand-held devices. When pointed at suspected areas of heat, heat guns can obtain accurate temperatures from a safe distance.
A counterbalance ensures that all components keep positive contact as the press proceeds throughout the stroke. This device eliminates excessive snap-through, which damages crankshafts, gearing, bearings, and other press components. Originally designed using springs or counterweights, pneumatic counterbalances were developed with variable die weight adjustments and the necessary flexibility for variable-speed machines (see Figure 1).
Counterbalance systems now are available to adjust pneumatic pressure automatically, and they can be added to existing press counterbalances.
Die cushions have been added to the press bed, where springs and rubber devices originally were located. Die cushions now are available with fully automated pressure and stroke lengths. Die cushions use air pressure to accomplish work-holding in the metal stamping press. Because die cushions are designed with a certain amount of stroke, the current technology available makes it possible to automate the pressure and stroke features of the cushions.
The most dramatic improvement now available is the low-cost, energy-conserving air-bag die cushion. This unit uses a pneumatic air-bag-design die cushion that is maintenance-free and conserves energy.
Attaching components such as gears, couplings, hubs, and other devices to shafting historically has been the domain of mechanical keys, rectangular pieces of steel machined in such a way as to act as a positive driving and/or holding device when assembled to a keyway in two mating components. Whether tapered, straight, feathered, or Woodruff, keys maintained their status as drive and fastening mainstays throughout the industry. Failure of shafting and other components caused by loose keys, rocked-out key fits, and stress risers developing into cracks at the machined keyway became commonplace maintenance headaches.
Keyless locking devices now eliminate the need for keys (see Figure 2). These devices can provide high torque values, ease of installation, and can be cost-competitive with key-type drives. Using a double-taper lock concept, the locking devices develop an interference fit by tightening an annularly located series of bolts to a predetermined torque rating.
Maintenance engineers now can eliminate keys with a simple part modification. Adding a keyless device for locking a shaft and component together can reduce assembly and disassembly time, eliminate the need to fit keys, avoid the stepped key requirements of poorly machined parts, and can allow simple timing on multiple-gear presses.
Today's quick die change systems can improve production (see Figure 3). Simple systems include:
More complex systems vary from rolling bolster conversions to full die carts that can be programmed to make die changes automatically.
Previously, the signs of an overstressed press were broken components or poor-quality parts. When a press was overloaded, the elasticity point of the metal composition was surpassed, so the part was damaged. The designed tonnage for an overloaded press was surpassed, but damage did not necessarily occur. The inability to monitor press loading was a major cause of press shutdown and emergency maintenance.
With the advent of load monitoring, low-cost sensors can be attached to press frames and components, which determine the load that the machine is under during the metal forming operation. These devices can read the specific amount of tonnage at each monitored point of the press and allow the press operator to interpret the readings. Die wear, bearing clearances, ram parallelism, and press failure all can be predicted from the readings of modern load-monitoring devices.
It is critical to understand the difference between overload monitors and overload protection devices. A monitor can read an overload condition only after it occurs. The damage caused by overload already has occurred once the overload reading appears.
An overload protection device, in contrast, is designed to prevent overload damage. By applying current valve technology, overload devices can be retrofitted to existing presses to prevent component failure. The valving is used in conjunction with hydraulic cylinders located at each connection. When an overload condition occurs, the valve reaches its preset pressure relief point, allowing the cylinder to dump its fluid and avoid damage to machine parts.
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