Page 26 - MetalForming February 2017

Die Springs
Gas Springs:
Heed the End Force
When it comes to specifying gas springs, Glynn notes that most are rated for a maximum speed of 1.8 m/sec., and, depending on press stroke, gas springs can run at press rates to 200 strokes/min.—depending, of course, on the amount of stroke.
“With just a shorter stroke, less than 12 mm,” he says, “you can run with gas springs at faster press speeds. But, with more than an inch of stroke, press speed can be limited to around 100 strokes/min.—the limiting factor being the heat caused from the friction and pressure rise in the springs. When selecting the right gas spring for the application, the stamper must carefully consider the force profile needed for the entire spectrum of forming, pierc- ing and stripping requirements.
“Also important is understanding where you need the force,” Glynn con- tinues. “Gas springs provide force on contact, and the force can increase as
much as 50 percent as the piston rod compresses. If the stripping or forming force is calculated for the end of the stroke, then the die designer may be able to use fewer springs and reduce die cost, since the end-of-stroke force can be significantly greater than the initial contact force.”
While most gas-spring manufactur- ers offer a variety of sizes in terms of force, diameter and overall length, the industry, Glynn says, is settling in on ISO and VDI (The Association of Ger- man Engineer) standard gas springs. Most major gas-spring manufacturers comply with these standards, which allows the springs to be interchange- able and helping stampers reduce downtime and cost.
Glynn warns, however, that “when repairing gas springs, use the repair kit specified by the manufacturer— these are not interchangeable. And, as a general rule, I recommend that shops only repair gas springs with a body diameter larger than 50 mm.”
Sharing Other Best Practices
Glynn stresses that die designers should specify gas springs so that only a maximum of 90 percent of rated spring stroke is used. “The less stroke the better,” he says, “to reduce friction and pressure and prolong spring life.”
Also sound practice: Ensure that the spring sits on a flat surface, with the rod perpendicular to the striking sur- face or pad. This is critical to avoid side loading.
“To optimize spring life,” Glynn adds, “attach springs using a mount or the tapped holes in the bottom of the springs, so they’re not loose in the die. If the springs aren’t fastened, metal contaminants or debris can wedge their way underneath the springs, which would then create a misaligned or angled spring and lead to side loading.”
If stampers do elect to just drop the springs into the pockets machined into the die, Glynn suggests that they design the pockets with clearance
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24 MetalForming/February 2017
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