Addressing potential hazards before they occur
By David L. Fischer, Engineering Manager
for the HILMA Division of Carr Lane Roemheld Mfg. Co.
Article reprinted with permission of Stamping Journal.
Many manufacturing concepts have changed in the last 10 years or so. One way many metal stamping companies are attempting to remain competitive is by setting goals for reduced inventories. To help accomplish these aims, many are turning to shorter production runs, which often require more frequent die changes.
The traditional method of using strap clamps to accommodate variously sized dies will not meet today’s requirements for Just-In-Time manufacturing. Stampers are demanding faster and safer ways to move and clamp dies. A Quick Die Change system can help achieve these goals.
Many factors must be considered to ensure that a new die change system will meet a company’s goals for faster part-to-part die changeover times while providing a safer work environment.
Die Movement
One facet of providing a safe die change environment involves moving and locating dies under controlled conditions with minimal effort. However, moving dies at a press traditionally has meant prying, pushing and pulling dies into and out of the press using fork lifts, chains, prybars and sledgehammers.
Moving a 1,000-pound die in the traditional manner requires about 700 pounds of force, or 70 percent of the die weight, because changing dies involves skidding metal against metal. Moving a die this way often requires more force than is otherwise necessary. If only a fraction of this effort were required to move a die, it seems reasonable that the alternate method might also be safer.
Ball-type lifters can accommodate movement in any direction while reducing the amount of force required to move the same 1,000-pound die to 20 to 40 pounds, or about 2 to 4 percent of the die weight (see Figure 1). A set of roller-type lifters can allow movement in one or two directions and can reduce the effort required to move the same die to 10 to 30 pounds of force, or about 1 to 3 percent of the die weight.
Some variations in the amount of force required or rolling resistance encountered can be attributed to the condition of a die surface that comes into contact with the balls or rollers. A lesser amount of force is required to move a die that has a smooth, hard surface and is free of holes or pockets. For roller-type lifters, die movement should be parallel with that of the roller direction.
Roller- or ball-type die lifters that are operated hydraulically must include a circuit relief valve to protect personnel and equipment from a potential pressure increase if the die lifters are overloaded. If die lifters are loaded beyond their rated capacity, the pistons that raise the rollers can act as pumps and generate higher pressure in the lift circuit. Without a safety valve to relieve the pressure, seals, tubing or fittings can burst, and the die lifter can be damaged.
Clamping Force
The clamping force on a die must be sufficient to overcome the die weight and the acceleration, stripping and ejector forces to which the die may be subjected. If forces working against the clamps are greater than those provided by the devices, clamp pistons can move, causing a pressure increase and die movement. A rule of thumb is that the total clamping force should be about 20 percent of a press’s capacity. For a 200-ton press, for example, 40 tons of clamping force should be used.
Clamping forces must be applied consistently to a die — in the same locations and in the same manner every time a die is changed. A properly designed hydraulic system can provide consistently safe and measurable clamping forces. The force is created by the pressure applied to each clamp, and pressure can be monitored with a gauge and a pressure switch that is tied into a press’s emergency stop circuit. If a 20-percent drop in clamping pressure occurs, the press shuts down.
A review of a stamper’s current clamping methods and the locations of the devices on a press can provide guidelines for safely holding a die in place and eliminating possible die deflection when a new clamping system is being considered. The number, sizes and locations of the clamps should be evaluated so that clamping forces can be placed as close as possible to those that they must overcome.
In many applications, clamps can be applied externally along the edge of a die. Others may require the installation of internal clamps if the loads are greatest internally.
Safety Circuits
Anyone who works on, in or near stamping presses knows that unexpected events can occur. Those workers may ask questions such as, “What if I lose power? What if I break a hydraulic line to a clamp? What if I break a hydraulic line from the press column to the slide that supplies pressure to all of the slide clamps? What if we lose the input power to the air- or electric-powered hydraulic pump?” If a clamping system is designed with appropriate safety circuits, a die will not move if any of those circumstances arise.
Clamping pressure can be maintained with zero-leakage directional control valves and pilot-operated check valves. Pilot-operated check valves allow flow-through, but they lock pressure and fluid downstream until pressure is applied to a separate line to open the valve and release the pressure at the clamp (see Figure 2). This in turn releases the clamp so the die can be removed.
Check valves can be provided for each clamp (see Figure 3), or they can be located in the bed and slide circuits near the clamps. Several methods are used to apply these safety devices in a clamp circuit, and each offers a different level of safety and a different cost factor.
If a press has eight clamps equipped with integrated check valves on the slide and a pressure line breaks at one clamp, the press shuts down because the break is detected by a pressure switch at the pump. All eight clamps remain locked in place to hold the die.
Another option also involves providing separate check valves for each clamp. Rather than integrating valves into individual clamps, however, they are located in separate blocks attached to the slide or bed (see Figure 4). With this type of circuit, a press will shut down, and the die will remain locked in place with all but one clamp if the line to that clamp breaks. One advantage to this system is that one hydraulic line rather than two runs to each clamp. This may be an important consideration if a clamp is to be manually positioned.
If a check valve is not provided for each clamp, a diagonal clamping circuit with dual check valves can be used (see Figure 5). In this circuit, one pressure line runs into a connection block mounted on the slide or the bed, where it splits past two pilot-operated check valves to create two diagonal clamp circuits. Every other clamp is on either circuit A or B.
If a hose to a clamp on circuit A fails, the pump pressure switch senses the break and shuts down the press. The die remains locked in position, but it is held by only half of the clamps — those in circuit B. If a power or pump failure occurs, or if the pressure line from the press column to the slide fails, the die is held with all of the clamps locked in place.
A similar diagonal clamping circuit can be provided using two pressure lines to the slide, two to the bed, and doubling the number of clamping valves at the pump. If a failure occurs in one circuit, the other circuit remains locked up with the pressure held at the pump control valves rather than at the check valves on the slide.
If a clamping system is designed without safety checks, clamping pressure must be maintained with the integrity of the hydraulic system, relying on zero-leakage clamp seals and control valves.
This type of clamping system is suitable if used with hydraulically actuated and mechanically locking clamps. Various types of mechanically locking clamps are available that use the wedge lock principle, which offers a high level of safety. A hydraulic piston drives a tapered wedge inside such a clamp, creating the clamping stroke and mechanically locking the clamp.
A system equipped with manual control valves must include pressure switches in each clamp circuit to ensure that the valves have been shifted and each clamp circuit is pressurized before the press is enabled. Before the clamps are actuated, the die lifters must be down, the bolster slide must be at bottom dead center and the slide must be down on the tool.
Automating a die clamp system can help to provide additional levels of safety. Electrical controls, interlocks and sensors can help ensure that the various moving parts of the die change system are at the right place at the right time.
Sensors can indicate, for example, whether a traveling clamp is at the die or at its home position, or whether a clamping piston is in the proper clamp or unclamp position. In a press, sensors can indicate that a press slide is down on the die, that a die is located horizontally and resting on the bed, or that the slide is at bottom dead center.
Pressure switches can signal the pump that the clamping circuits are at proper pressure. Hydraulic reservoir sensors can monitor oil and temperature levels. Key-operated clamp/unclamp switches are available, as are key-locked pressure switches and pressure relief valves to ensure the pump pressure settings will not be reset without approval. Solenoid valves should be normally open so they do not shift if a power failure occurs.
Maintaining Safety
If problems develop with a sensor, repairs should be made immediately. Although emergency jumpers are sometimes placed across proximity switches to keep a press running, this temporary “fix” can cause other problems in the clamping system. When sensors are bypassed, press controls do not “know” what is happening inside a clamp.
If temporary jumpers must be used, operators should be warned to make certain that the function that had been monitored by the bypassed proximity switch, such as clamp location, operates properly.
A Quick Die Change system must also be maintained properly to ensure that the level of safety meets expectations. Some companies have spent large sums of money to retrofit an existing press or buy a new press with a Quick Die Change package, only to have the equipment fail to perform properly or deteriorate because of a lack of personnel training and maintenance.
Conclusion
As plants and presses are updated, automated Quick Die Change systems can be incorporated to help achieve production goals and maintain competitiveness. In the process of reducing part-to-part die changeover times, safety levels in the plant can also be increased with minimal additional cost.
If the Quick Die Change system on a press develops a problem, the press should be equipped so that the worst thing that can happen is that the press shuts down and production is delayed. When that is the case, the goal of incorporating a safe Quick Die Change system has been achieved.