Tonnage Monitoring for Hydraulic Forging Presses
There are many advantages to a hydraulic forging press that are unavailable to those who operate mechanical presses. One of those is the ability to monitor the process and respond in “real time.” With the proper high-speed electronics monitoring the process, an intervention can prevent a press disaster.
Once the footswitch is pressed on a mechanical press, the crankshaft is either going to come around 360 degrees or will stop dead at 180 degrees with the dies stuck together. There is no way to respond in real time to a mechanical forging-press cycle.
In contrast, when a hydraulic press is instrumented properly, the distribution of forces can be monitored during the forging process. This is done by measuring the strain levels of the load-bearing machine components of the press in real time. The load-bearing members can be tie rods or cast machine housings. The strain levels are measured using linear variable differential transformers (LVDTs) and related hardware. LVDTs are used because of their ability to measure displacement and strain without the need to be recalibrated. Figure 1 shows the physical configuration for measuring strain for the surface-mount hardware. Figure 2 shows the same for tie-rod hardware. It is important to note that the LVDTs measure the strain over a distance of 50 inches or more.
When the analog voltage signals from the LVDTs are piped into a high-speed data-acquisition board, they can be compared and analyzed in a combination of ways to determine if a destructive distribution of forces is developing. The signals are compared at a respectable 50 Hz, and alarm outputs can be triggered at this same frequency. This system does an amazing amount of math very fast. It can signal your press to stop within 1/50th of a second. The system constantly runs in the background until there is a problem. When a problem occurs, the system can trigger an output bit to stop the process. At this point, it is also possible to create a data file to investigate the problem.
Comparison of Strain SignalsThe strain signals in the press are compared three different ways: front-to-back distribution of forces; left-to-right distribution of forces; and a check to see if the force in any one column exceeds the average of the other three combined. For each of the three ways the forces are monitored, there are six different alarm conditions that correspond to them. Each alarm condition has two different thresholds designated high and low.
In a left-to-right alarm, for example, the computer takes the average of the left two columns and the average of the right two columns and subtracts the two values. The magnitude of the difference is compared with the alarm thresholds. If the difference exceeds the setpoint of the low alarm, a digital output bit will be activated. If the difference should exceed the high-alarm setpoint, a different digital output bit will be activated. These digital output bits are wired to Opto22 modules to communicate with the external control system. The front-to-back and column alarm conditions are computed in the same way as the left-to-right alarms.
One type of control scheme is to wire the low-alarm output to a buzzer or light and to wire the high-alarm output to a process-intervention routine. Figure 3 depicts an example of a left-to-right distribution of forces high-alarm condition. Notice that the tonnage values are displayed on the computer monitor in the form of bar graphs as the process is happening. A numeric value is also displayed under the bar graph that will read in any unit specified – metric tons, tons or kiloNewtons, for example.
Figure 4 shows the calibration-settings window where the production supervisor can use a password to set critical alarm thresholds and other press-specific parameters. The system has the capability to create files during the forging stroke. The production supervisor can select to create files for each forging stroke or select to create files only for the forging strokes that trigger one or more of the six alarm conditions.
Closed-Die HydraulicsIn closed-die applications, bulk tonnage can convince you that you have made a forging that meets the specifications for that particular component. You may be able to produce 50,000 concentric forgings in a closed-die hydraulic press, and then a screw falls out of the bolster. If the screw falls into the die area, it will cause a huge distribution-of-forces problem. You don’t want the press to finish the forging stroke, and you don’t want to ship that forging.
When forging non-concentric parts in a closed-die application, although bulk tonnage is not exceeded by the main cylinder of the press, an off-center load can develop. Off-center loading is common when forging parts of complex shapes. Also, it becomes more difficult to calculate the force vector when making more than one forging per press cycle. When a press operator places three sets of dies in the machine, the distribution of forces can surprise you. When a distribution-of-forces system is installed on the press, there is no more guesswork when setting up jobs on the press.
Open-Die HydraulicsOpen-die forgers can also benefit from distribution-of-forces monitoring technology. If an operator adjusts a workpiece between the dies and executes a ram down with the joystick, there is the potential for the workpiece to be slightly off-center and cause a destructive distribution of forces. The fix for this is simple. Adjust the workpiece with the manipulator and try the forge stroke again. The monitoring system constantly runs in the background until there is a problem. Upon encountering a problem, the system will trigger an output bit to stop the process.
Open-die forgers can also use the system for deflection compensation. Knowing the distance between dies under load is critical to knowing the size of the workpiece being forged. The LVDT system measures over 50 inches of press deflection/strain. This translates into superior deflection measuring accuracy. When this is coupled with a linear scale that will determine ram position, an accurate distance between dies can be determined under load.
Open- and Closed-Die HydraulicsThere is an unexpected benefit of the tie-rod hardware shown in Figure 2 when installed on a hydraulic press. The tie-rod hardware can be used to ensure that there is ample preload in the tie rods that comprise the press frame. Proper tie-rod preload is critical to ensuring the press will last its expected lifetime. Improper preload will cause the tie rods to stretch beyond their intended strain rate every time the press is cycled, a condition that may eventually cause the tie rod to fatigue and inevitably fail.
The LVDT tie-rod hardware can measure the strain rate of all the tie rods in the press. The strain rates can be checked very easily once a week by applying the full force of the hydraulic system and recording the raw strain values for each tie rod. The strain rates will increase if preload has decreased to less than 100% of column capacity. The strain rates will also increase for tie rods that start to fatigue. The strain rates can be plotted over time. Checking the strain rate allows you to eliminate one more variable on the list of all the things that can go wrong with a hydraulic forging press.
In addition to strain rate change over time, the strain-per-force-applied data will be collected to determine a calibration constant for each tie rod. If the correlation between tie-rod strain and force applied is non-linear, this is an indication of preload that is not at least 100% of column capacity of the press. When initially installed, the system can detect “loose” tie rods before the system monitors the forging process for the first time.
ConclusionThere are ways to improve hydraulic-press life and increase uptime by using available distribution-of-forces technology. There is more information necessary than just knowing the pressure behind the cylinder of a hydraulic forging press. Monitoring the distribution of forces within the die bed can solve a multitude of problems that can occur during a forging cycle. You wouldn’t drive a sports car without a tachometer. You wouldn’t fly an airplane without a gyroscope. Similarly, you shouldn’t operate a hydraulic forging press without a tonnage distribution monitoring system.
As we like to say, an ounce of prevention is worth a ton of cure!
Author Steven F. Rasmussen is President and Engineer at Angstrom Corporation, Twinsburg, Ohio. He may be reached at 330-405-0524, 1-888-73-GAUGE, or at email@example.com