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Pressure Sensors

BY MELISSA SCHUMANN

Monday, November 01, 2010
 
 Pressure sensors
To keep a stamping die properly lubricated, a pressure sensor can be used to monitor tank pressure and ensure that the proper amount of lubricant is dispensed. The lubricant in the tank or line exerts a force against the sensor’s measuring element, whose signals are converted into an electrical signal output for interpretation by the press-cell controller.
One of the most important yet least obvious s to improve die performance in stamping operations concerns lubrication. Proper lubrication helps offset the friction and high heat that occurs each time the die impacts a piece of sheetmetal. These forces not only cause equipment to wear over time, but they also may cause immediate damage to the tool or part. For example, slugs—the pieces of material extracted from the part during stamping—can remain on a part after stamping, obstructing the operation and causing subsequent stamped parts to be labeled as rejects.

Not Too Much, Not Too Little

Stampers rely on lubricant to help ensure the production of high-quality parts, and to protect and extend the life of the press and tooling. The effectiveness of the lubricant relies on ensuring the delivery of the proper amount of lubricant consistently, hit after hit.

Lubricant—typically stored in a tank next to the press line, and piped out through a nozzle—is applied either by spraying onto the sheetmetal or by misting directly onto the die; a valve controls the amount of lubricant delivered. To optimize results, stampers should consider monitoring and controlling the amount of lubricant applied with each press stroke—too much can lead to slug pulling, while too little will allow the detrimental effects of friction and heat to appear, such as tool galling, which interferes with the subsequent flow of lubricant from the nozzle.

Ultimately, the amount of lubricant dispensed requires ensuring the proper pressure in the tank, and for this task, a pressure sensor plumbed into the line can prove to be a valuable asset.

Static and Dynamic Pressure

Pressure sensors can be used to confirm two different pressure levels during the cycling of the lubricant system: static pressure and dynamic pressure. The sensors will provide valuable information to the press controller or to a press-line PLC that can then be used to either adjust lubrication-delivery parameters in-process, or shut down the press before die damage occurs.

Static pressure, which builds when the lubricant tank valve is in the closed position, typically depends on the amount of lubricant to be applied, as well as the lubricant’s viscosity. A sensor measures dynamic pressure when a change in the process occurs—when the valve opens. Here, the sensor tells the controller that the tank is correctly pressurized.

Dynamic pressure depends on the open position of the valve nozzle and the length of time the valve remains open when the lubricant is released—referred to as pressure drop. Here, the sensor tells the controller that the valve has opened a certain amount, allowing a predetermined amount of lubricant pressure drop to occur.

Each pressure sensor is plumbed into the pressurized tank or line. The lubricant exerts a force against the sensor’s measuring element—typically of stainless steel or ceramic. Ceramic proves to be very stable and consistent over time. If overpressure does occur, causing the sensor to reach its burst pressure, a ceramic sensor will simply fail rather than provide erroneous readings. This design ensures the integrity of the sensor’s results. Steel measuring elements, which typically have higher burst ratings than do ceramic elements, are more prone to zero and span offsets, which can lead to erroneous readings.

The measuring cell connects to the sensor’s internal electronics, which convert the measured pressure into an electrical signal output. The press control system, interpreting the output, can either convey the error state on an external display for the operator to see, or it can halt operations if an error occurs and prevent further rejects and equipment damage while allowing operators to quickly identify and address the problem.

Along with increased efficiency and extended die life, stampers can rely on sensors to monitor static and dynamic lubricant pressure to overcome several common setup errors. For example, pressure sensors can identify if a valve is not opening for the correct duration (or if it’s opening at all); if the wrong lubricant or lubricant mixture is being used; or if the amount of lubricant being applied is inadequate for the application.

Sensor Selection

Users can select pressure sensors that use either a dual switching output that provides simple on/off information, or an analog output for scalable results based on the continuous flow of information. Dual-setpoint sensors use one setpoint to monitor static pressure and the other to monitor dynamic pressure. Since analog sensors provide continuous output, one signal monitors both static and dynamic pressure.

For stamping-die applications, which locate the pressure sensor near the lubricant, metalformers typically specify a pressure sensor rated to IP67 to defend against lubricant ingress. An IP (ingress protection) number specifies the environmental protection capability of enclosures surrounding electronic equipment. The first number, “6,” identifies the enclosure as offering total protection from dust particles of any size; the second number, “7,” identifies the enclosure as protecting against the effect of immersion to a depth between 15 cm and 1 m. MF

Melissa Schumann is a product manager at Turck, Inc., Plymouth, MN; 763/553-7300, www.turck-usa.com.

 

See also: Turck, Inc.

Related Enterprise Zones: Sensing/Electronics


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