STAT engineering design

I design Progressive Dies for a number of different Companies that serve a number of different Industries: Automotive, Appliance, Electronic, etc. Every Company has it's own Die design preferences including the choice of Springs they prefer to use in their Press Tools. Often the choice is dictated by the requirements of the Die, but just as often the choice is a matter of perceived cost effectiveness.

Nitrogen charged Gas springs are more expensive than conventional Die Springs by a factor of 8-10, and therefore many Companies prefer to use conventional springs. Gas Springs, however, end up being less expensive and more reliable than conventional springs for most Progressive Die applications. Let's look at the following :

A Progressive Die requires 4 Tons of pre-loaded Stripper pressure with a Stripper working stroke of 1\2"

USING 2" DIA. HEAVY DUTY DIE SPRINGS WITH 1/10" PRE-LOAD:

SPRING LENGTH REQUIRED: 3" (20% TOTAL DEFLECTION)
PRELOAD PER SPRING: 213.4 Lbs.
NUMBER OF SPRINGS REQ'D: 38 SPRINGS

USING 2" DIA. EXTRA HEAVY PRESSURE SPRINGS:

USING 2" DIA., 3/4 TON GAS SPRING WITH 3/4" STROKE:

EXTENDED LENGTH: 3.5"
PRESSURE EXTENDED (NO Preload req'd) 1820 Lbs.
NUMBER OF SPRINGS REQ'D: 5

Let's also presume for our example that when using a heavy duty spring (3" long), that you would need to machine a 1.0" deep Spring Pocket (and each spring also requires a spring-can for safety).

USING 2" DIA. X 3" LONG SPRINGS X 38 SPRINGS:

A rough cost difference analysis of using Gas Springs as opposed to conventional Springs would necessarily take into account the difference in machining time - as well as the difference in item costs.

Presuming that the Gas Spring will cost about 10 times the cost of a conventional spring (not including the additional cost of spring cages), the cost of the 5 Gas Springs would be about the cost of 50 conventional springs. This comes out to be about 24% more than the cost of conventional springs. BUT, when it comes to machining time, this difference more than likely reverses the cost-effective equation. In the first case the machining time for conventional spring pockets becomes 5.2 times greater than for Gas springs, and in the second case even more - about 7.7 times greater.

MORE ADVANTAGES OF NITROGEN GAS SPRINGS:

1. They do not require pre-load. As a matter of fact they recommend at least 1/32" clearance in the free state. This definitely makes in-Press service easier and less strain on all components involved.
2. In those cases where the springs need to be independently retained, as in the case of most Dies built for in-Press maintainance, Gas Springs require only two socket head cap screws as opposed to the extra hardware and machining required for conventional Springs.
3. Gas Springs provide a much more consistent pressure throughout the stroke, and the gas pressure charge can be regulated either by original charging pressure or by the use of multi-spring manifolds. This provides much more control over Die behavior, especially where longer spring deflection is required.
4. Catastrophic failure is almost non-existent with Gas Springs.