Page 23 - MetalForming July 2012
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attaches to the press slide and coun- terbalances the weight of the slide and upper die. Included is a pressure gauge (2) designed specifically for press-vibra- tion duty. An adjustable air regulator (3) on the press permits accurate adjust- ment to the correct setting. Bleeding the system occurs quickly by opening the blow-down valve (4).
Other components include:
• 5—Surge tank or tanks, built to ASME pressure vessel code
• 6—Accumulated water drain valve • 7—Tank safety pop-off valve
• 8—Check valve
• 9—Low air pressure switch
• 10—Shop air inlet.
Many presses have two cylinders for each slide (5). Double-action press- es have separate counterbalance sys- tems for each slide.
The check valve (8) prevents a sud- den loss of pressure from the counter- balance system in the event of an air- source failure. In addition, the pressure-actuated switch (9) will open the main motor run circuit should the pressure fall below a minimum value specified by the press manufacturer. This ensures that the press will not run without enough air to balance the slide alone.
Note: Never adjust the pressure switch below the required minimum setting specified by the manufacturer, or bypass the switch with a jumper wire. If the correct counterbalance pressure cannot be maintained, errat- ic press operation may result. Several likely causes include:
• Low shop air pressure;
• Excessive air leakage; or
• An accumulation of water in the
surge tank.
Documenting Pressure Settings
To manually adjust counterbalance systems, operators use a pressure reg- ulator to set the counterbalance pres- sure to a value that will correctly coun- terbalance the weight of the slide and its attachments (upper die, risers, par- allels, etc.). Doing so requires three key ingredients:
1) The diesetter must know the weight of the upper die, including any buildup.
2) He also must have an accurate table of pressure settings for the upper die weight. The metalformer should provide this information on a metal tag supplied with and attached to the press.
3) The pressure gauge must be accu- rate, and periodically tested.
Most presses include a chart that specifies the correct pressure setting for various upper die weights. If the infor- mation is missing, contact the press manufacturer. If this is not possible, determine the correct settings by per- forming the required tests or engi- neering calculations.
OSHA requires that the information regarding upper die weight be made available to the diesetter when neces- sary for proper air counterbalance- pressure adjustment. This will promote safe die handling and counterbalance setting. Stamping the upper, lower and total die weight on the press is the required method, unless the metal- former can show a foolproof alternative —such as written instructions, stating the upper die weight, being readily available to the diesetter.
Further, the total die weight must be known to avoid overloading die-han- dling equipment. OSHA regulations specifically state “stamped” rather than marked or painted, as there is always a danger that an incorrect figure may be repainted on the die.
Should the chart on the press listing air-pressure readings for different upper die weights be unreadable or missing, stampers should develop a new one. If a paper chart is used, keep a master copy in an office and hang a copy—placed in a protective plastic sleeve—near the counterbalance at the press.
Automatic Pressure Adjustment
Some newer and retrofitted presses designed for quick die change (QDC) feature automatic counterbalance adjustment, based on a computerized database of die-identification num-
bers. These systems can maintain the correct pressure under production con- ditions by compensating for expan- sion of the counterbalance air due to packing friction. In most cases, the metalformer must determine the cor- rect pressure and enter it into the data- base. Be sure to update the database when needed, rather than relying on manual adjustment after problems develop.
Press controls based on program- mable logic controllers, dedicated microprocessor-based controls, and industrial computers for setting press operating parameters can prove high- ly reliable. It is essential, though, that stampers correctly enter the data and carefully maintain all system functions.
QDC is a goal that all stampers should strive to achieve. Retrofitting to modern integrated press controllers having tonnage-monitoring capabili- ties together with automatic shut height and counterbalance adjustment can improve diesetting time, provide consistent setups and greatly improve press uptime.
Counterbalance lubrication must be supplied to the cylinder-piston packing and to the rod-gland packing. Dry piston packing on large presses may emit a characteristic sound best described as that of a cow mooing. If the problem is not promptly corrected, the packing will fail.
Lubrication is either supplied by a manual hand pump or metered auto- matically. In both cases, ensure that the correct lubricant is applied as need- ed. This must be part of a total pre- ventive-maintenance program. Prop- erly lubricated, the packing should provide decades of service. Should a stamper allow the rod-packing gland or piston packing to run dry, the packing will fail and the rod, piston and cylinder body will become scored—prepare for downtime and expensive repairs. MF
The author expresses gratitude to Jim Barrett, Link Systems, and Dan Fal- cone, Toledo Integrated Systems, for con- tributing to this article.
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