Page 27 - MetalForming February 2017

based on the manufacturer’s recom- mendations, to minimize the chance of contamination. And, in applications requiring a lot of die lube, use springs with protective rod covers or wipers to guard against contamination, “a leading cause of spring failure,” he says.
Connecting Gas Springs to Control, Adjust Force
Hosed or linked nitrogen systems provide the most flexibility for metal- forming. “Gas springs can be linked together,” Glynn explains, “with a series of high-pressure hoses, or via a 1-in. manifold plate. This system is the cleanest and, I believe, the most reliable.”
To take this a step further, designers can add pressure switches (electronic pressure monitors) to the system, Glynn says, allowing the stamper to set parameters based on the pressure profile that optimizes part quality. And, should pressure fall below a certain level, due to a leak in the system, it signals the press to stop.
A great example of the benefits of connecting gas springs via a manifold setup, says Osborne: in-die tapping, where considerable stripper travel is needed to locate and control the part during the tapping operation.
“In some cases,” Osborne notes, “the tapping unit is mounted in the upper die, within the stripper. It is critical that those gas springs be linked togeth- er to ensure uniform balance in the stripper pad. Otherwise, should one or two gas springs go flat in one corner, the stripper won’t hold down firmly in that corner, which can wreak havoc with the tapping unit and lead to bro- ken taps and cause damage to the unit. It’s mandatory that stampers use a manifold when using nitrogen for in- die tapping operations.”
A Critical Note About Safety
Lastly, Glynn notes a relatively new focus on safe use of gas springs, which came to light in November 2015 when VDI added new safety guidelines to its VDI 3003 Standard, “Nitrogen Gas
Springs in Large Stamping and Forming Dies.” These guidelines arose from the fact that gas springs, under high pres- sure require that safety be top of mind.
“Because these springs contain a lot of pressure,” Glynn says, “which can range from 2000 to more than 4000 psi during compression, they’re noth- ing to fool around with. Because of this, we and other manufacturers have focused on developing new safety fea- tures, following the VDI guidelines. These fall into three categories: pro- tection from over-stroke and over- pressure, and protection from return stroke without counterforce.”
Here’s the language contained in the VDI standard:
• Return stroke of nitrogen-gas spring without counterforce
Do not use gas springs in such a way that the piston rod is released freely from its compressed position. This can be caused by a jammed pres- sure pad or cam. As a result, the nitro- gen gas spring exceeds the permitted
speed during the return stroke, causing the piston rod to slam unchecked onto bushing. This can damage the nitro- gen-gas spring or cause it to fail (high- speed return of the rod). To avoid this, use only nitrogen-gas springs that are designed to vent gas, thereby depres- surizing the spring.
• Over-stroke
Over-stroke occurs when the piston rod is pushed deeper into the gas- spring body than the nominal stroke length. This can damage the gas spring. Use only gas springs that are designed to vent the gas completely to atmos- phere in a controlled manner in the event of over-stroke.
• Over-pressure
Gas springs can break if the pressure inside rises due to fluid penetration or incorrect charging above the permitted limit. If fluids are used, it is advisable to ensure that gas springs have over- pressure protection. When the over- pressure-protection system is triggered, the gas is vented safely. MF
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