Page 49 - MetalForming March 2011

Is Your Ram on the Level—Part 2?
Last month we looked at ram parallelism and some basic checks that a metalformer can conduct to ensure its presses meet the specifications from the press manu- facturer. Now let’s take a look at how stampers can use sensors to actually track ram travel over a specific distance. With this information, the stamper can check if the ram is moving in a predictable linear path, or if it is wobbling during travel.
To track ram travel, we can set up a fixture containing four analog LVDT sensors, with each sensor monitoring one cor- ner of the ram’s travel. The ram could be unloaded or it could have an upper die section attached.
LVDT sensors come in many sizes, and resemble an air cylinder in appearance—a cylindrical body and a rod that travels in and out of the body. The LVDT reports this rod trav- el as a variable voltage that can be monitored by a personal computer (PC) with the appropriate analog input card. To monitor ram travel, the stamper must specify the LVDTs to cover the ram travel distance of interest. For example, consider monitoring ram travel during its last 1⁄2 in. of motion down and up. The LVDTs would be internally spring loaded and would have the ability to monitor at least a 1⁄2 in. of travel.
The fixture housing the sensors must be carefully mounted using parallels or some other supporting mechanism to ensure that the sensors monitor the ram where required. To prevent the LVDTs from being destroyed by an inadvertent over-travel by the ram, due to poor setup, we embed each LVDT within a spring-loaded block that travels into the fixture should ram trav- el exceed the LVDTs’ range, protecting the sensors. LVDTs are not very forgiving of lateral forces applied to their moving rods, so stampers must take care to avoid any sideways pressure.
Back to our example—As the ram travels, distance meas- urements are reported to the PC by the four LVDTs, with a res- olution well below 0.0001 in.
As an alternative to LVDTs, stampers can install spring- loaded rectangular probes, each with one of their sides machined to a taper. As the probes move down and up with the ram, the four tapered sides are monitored by four analog inductive proximity sensors whose output would be sent to a PC. The stamper must be sure to select and test the proximity sensors
George Keremedjiev has been writing this column for more than 20 years. He regularly consults with metal- forming companies worldwide and provides metal- formers with training on the application and imple- mentation of sensors for die protection. For more information on his seminars and consultancies, con- tact:
Tecknow Education Services, Inc.
P.O. Box 6448
Bozeman, MT 59771
tel: 406/587-4751 | fax: 406/587-9620 www.mfgadvice.com
gk@mfgadvice.com
to ensure they have sufficient linearity to accurately report the travel of the probes over the full range of interest.
Yet one more method used to monitor full ram travel, via its four corners, is by using magnetostrictive sensors. These have a cylindrical body from which a hollow sealed tube extends, in a variety of lengths spanning a few inches to sev- eral feet. A round doughnut-shaped magnet travels (non-con- tact) along the length of the tube, its position reported by the sensor to the PC. Magnetostrictive sensors are absolute—they report magnet position as a true position, not a relative value. This allows extremely accurate monitoring of ram travel during its full down and up motion.
In each of these cases, the PC would display the outputs of the four analog sensors and could be programmed to display the four travel paths as graphs. The PC also could be programmed to look for deviations from a linear travel path, report any anomalies and allow the stamper to note wobbling conditions.
Note: Employing the above-described procedures requires careful design, mounting and use of fixturing, while adhering to applicable safety practices and regulations. There are no short cuts when dealing with a power press in motion. MF
Metalforming Electronics
By George Keremedjiev
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One Voice Legislative Conference April 5-6, 2011 | Washington, D.C.
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