Page 23 - MetalForming-Feb-2018-issue

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A secondary wiper, as shown here, can be installed on a cylinder to help ward off fluids.
overstroke condition. An overstroked cylinder can be damaged in such a way as to cause the release of some com- ponents. Under pressure, these com- ponents can release at high speeds.”
Conscientious design not only pro- vides the tool, part and press protection inherent in nitrogen-die springs but also maximizes their use, leading to more room in a tool to fit needed processes.
“Gas springs are rated based on their force on contact,” Glynn explains. “The more that a spring compresses, the higher the pressure, and the higher the force.”
In other words, compressing a nitro- gen-gas-spring piston rod to nearly its maximum length can double the force.
“Die designers, if they consider where force is needed, whether at the end or at the beginning of a stroke, may be able to reduce the number of cylinders needed in a die,” he says. “Perhaps an application does not require full force until the bottom of the cylinder’s stroke, in which case the compressed force may be much greater. Performing these calculations can save money and provide more room in the die.”
Use Mounting Options
Whether using mounts supplied with nitrogen-gas springs, tapped holes at the bottoms of cylinders or some
other robust method, it’s best to affix springs to a tool.
“It’s a good practice that keeps the cylinders secure in the upper die shoe, or prevents a cylinder from vibrating in a pocket of a die,” Glynn says.
This helps ensure top pressure-pro- viding performance from a nitrogen- gas spring.
Provide Counteracting Force Upon Pad Release
“Sometimes a pressure pad can stick during a specific process, perhaps an extrusion,” says Glynn. “Never release a pad from sticking without any coun- teracting force against the cylinder. In other words, don’t press a cylinder down and let it release with no oppos- ing force.”
Such repeated actions can cause a cylinder to fail and a rod to eject.
“Again, at speeds surpassing 200 mph, the projectile can do serious damage,” he stresses.
Avoid Fluid Buildup
Dies often operate in a fluid-filled environment, with liquids applied to tooling and parts and other oils and fluids used in machine components. Deep-draw operations, for example, may require flooding the die with lubri- cant. Despite efforts to seal nitrogen- gas cylinders, over time fluid may seep into a cylinder.
“Fluids in the cylinder can wreak havoc on the wiper and the seal, and caustic lubricants can cause premature spring failure,” explains Glynn.
In addition, fluids seeping into a cylinder reduce cylinder volume, thus overpressurizing the cylinder. To help combat fluid in cylinders, Glynn sug- gests taking steps such as installing drain holes in spring pockets.
Some spring manufacturers, includ- ing Special Springs, offer products to help. For example, secondary wipers can be installed on a cylinder, as can covers that attach to a spring’s piston rod, to help ward off fluids. One safety feature, referred to as overpressure active safety, will release gas if a cylin- der becomes overpressurized. MF
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