Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
USING METALLIC SPRINGS AS
DIE PRESSURE DEVICES
Carefully select and specify die pressure devices and systems considering the intended
service. Factors to help determine the correct spring choices are the required force,
allowable deflection, space limitations, stroking rates and production requirements.
Metallic die springs include the following types:
1. Helical round wire metal springs
2. Helical oval shaped wire metal springs
3. Dished metal washer springs known as belleville washers or springs
Most die springs develop their force when compressed. However, some die springs
develop force when stretched. These are extension springs.
When used within the manufacturer’s ratings, steel die springs can provide excellent
service life with little or no loss of force. When users experience spring breakage
problems, it is usually traceable to a misapplication of the spring.
Types of Metal Springs
Nearly all metal springs used in tool and die work are helical compression springs. These
have flat ends made by closing the last turn on each end. Often the end is ground flat,
especially in the higher force types in order to insure that the spring will set level on a flat
surface or counterbored hole into which it may be placed.
Some use is made of extension springs in tool and die work. A screen door spring is an
example of an extension spring. Extension springs have an eye or loop on each end to
permit attachment. Extension springs find use in many ways in tool and die as well as
fixture work.
Belleville spring washers are a special type of round slightly dished compression spring.
These have a shape like a flat washer except that it has a slightly dished contour.
Belleville washers find use where large forces are required through short travel distance.
Stacking belleville spring washers together can increase the force. Pressworking
applications include short stroke in die spring applications and light duty die clamps
applied by releasing hydraulic pressure. Belleville washers work best in static pressure
applications. Like any spring, belleville spring washers are subject to failure if cycled
repeatedly at or above their rated travel limit.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Other types of springs include spiral springs and flat leaf springs. These types find some
use in die applications. A typical use for a leaf or flat spring is to actuate a progressive
die positive type-starting stop.
Materials Used to Make Metal Springs
Plain carbon steel springs are the least costly and are suitable for low deflection
applications and/or light duty applications. If deflection is limited they may last for a
long time without failure.
Chrome vanadium alloy steel springs, while slightly higher-priced than those of carbon
steel, can last three or more times as long. When selecting die springs for a die design,
the best performance and reduced down time are ensured by use of chrome-vanadiumsteel springs, and derating the travel from the maximum deflection recommended by the
manufacturer to that recommended for long life.
Other spring materials include stainless steel and a variety of special alloys including
those developed for watch hairsprings and mainsprings. An example of a special spring
alloy is Elgiloy™ developed by the Elgin Watch Company. It is nonmagnetic, very
fatigue resistant and the spring force changes very little over a wide temperature range.
These materials are very useful for instrument springs and applications involving a
corrosive environment.
The best die spring steels require careful processing throughout each manufacturing step.
This careful processing may include, in part, the following good practices:
1. Vacuum degassing of the molten metal.
2. A continuous casting process carefully controlled to insure uniformity of the rod
used to form the spring wire.
3. Careful process control to draw or roll the wire to the desired shape and size
without atmospheric decarburization, contamination, or unwanted inclusions of
oxides or slag.
4. Use of the best winding and shaping practices to avoid stress concentration or
stress risers that may lead to crack formation, propagation, and early failure.
5. State of the art heat-treating practices to correctly harden the steel and draw it to
the correct spring temper.
6. Shot peening of the formed and heat-treated spring surface to leave the surface in
the most desirable state of residual compressive stress.
7. Presetting by compressing to a solid condition to increase set resistance and
fatigue life.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Compression Springs
Most metal springs used in die construction are compression springs. Compression
springs for strippers, pressure pads, and other spring operated die components can be
selected from the ratings given in terms of the amount of force per unit of travel. Obtain
this data from manufacturer's catalogs.
Round wire springs are suited very light duty pressure pad applications because of their
low load ratings. They are still a good choice for such applications as latch return springs
and for use in progressive die starting stops.
Compression die springs made from steel wire having an oval or a special trapezoidal
cross sectional area are designed for high forces together with long service. Winding
wire into a helical spring involves bending the metal. The inside of the helix goes into
compression while the outside stretches.
Wire with a trapezoidal cross section having smoothly rounded corners is favored by at
least one spring manufacturer because the small end of the trapezoidal shape is used to
form the inside of the spring helix. This cross sectional shape is easier to wind since the
small part of the trapezoidal cross section is easier to compress than the thicker edge of an
oval wire having an equal width and cross sectional area. The result is a spring having
less residual tensile stress on the outside of the helix and a more uniform cross sectional
area than would otherwise be the case with oval wire spring stock.
Compression Spring Ratings
Helical steel die springs are available in several load ratings or amounts of allowable
deflection expressed as a percentage of the uncompressed or free length and amount of
force developed per incremental unit of deflection. While the quality of steel used to
make springs is an important factor in their service life, the amount of allowable
deflection is mainly a function of the thickness of the round, oval or trapezoidal wire used
to form the spring.
However, springs made of thicker material have substantially lower allowable
percentages of total deflection for the same material stress levels. Cycling a spring by
repeatedly deflecting it at high stress values will eventually cause the spring to develop
fatigue cracks and eventual failure.
Greater wire size or thickness equals greater force developed per unit of deflection.
However, the operating stresses developed in the spring material increase with the
diameter or thickness of wire used. Therefore, springs wound from thick material
develop higher forces per unit of deflection than those made of thinner material.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
ISO Standard Spring Rating Color Coding
Spring Load Rating
ISO Color Code
Light Duty
Green
Medium Duty
Blue
Heavy Duty
Red
Extra Heavy Duty
Yellow
Table 1. International Standards Organization (ISO) standard color codes for die spring
load classes. These standards adopted by the North American Automotive Metric
Standards Group apply to springs made to metric standards.
An Unfortunate Source of Difficulty for the Diemaker and Designer
One would suppose that the ISO spring color-coding standard is an industry wide
standard for color-coding springs to identify their load or duty rating. Such a system
would seem highly logical in view of the adoption of this standard by both ISO and the
North American Automotive Metric Standards Group (NAAMS), which is a working
group of The Automotive Steel Partnership.
Unfortunately, a simple matter such as adopting the European and North American
International industry standard of uniform identification of the load rating of die springs
is not agreed upon by all manufacturers. Non-standard color-coding of springs includes
colors that do not correspond to the actual duty class so identified and even springs
having two-tone paint schemes. All of this can result in confusion and may even result in
a dangerous die condition should an incorrect duty class spring fail in an unexpected way
that might endanger personnel. To further complicate matters, Japan, although a metric
standard country, has a non-ISO standard for die springs.
From a diemakers point of view, there is enough difficulty in designing and maintaining
tooling without difficulty in identifying the duty class of a spring die to variant colorcoding schemes. Hopefully, the combined efforts of the North American based
automakers and ISO will force acceptance of common spring identification and rating
standard.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Figure 1. ISO standard spring rating color-coding. The Danly Die Set Division of
Connell Limited Partnership
Selecting Springs Step by Step 1
For a known spring diameter and length, you may refer directly to spring manufacturer’s
dimension tables to select springs with the desired total force or load capacity. However,
if the required diameter and length are not known you may use a proven step by step
spring selection process in order to determine the compression percentage, life
expectancy, and deflection Vs load from the manufacturer’s catalog data. There are seven
steps to the process.
Spring Considerations when Repairing Dies
When repairing dies that do not have enough spring force or are experiencing excessive
spring breakage this systematic process can help pinpoint the problem. In determining
the length of a spring, remember that higher spring forces require selecting larger
diameter and higher load class springs.
1
Based on the spring selection instructions in the Danly® Die Set Division of Connell Limited Partnership
Die Springs Catalog © 1996 and prior dates.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
For best economy and saving of space, choose Light and Medium Load springs or the
Heavy Load spring having a free length equal to six times the travel. If an Extra Heavy
Load spring is required, use a free length equal to eight times the travel.
If ratios lower than these are used because of height limitations, the number of springs
required will be substantially increased. In such cases, self contained nitrogen gas springs
should be considered an alternative.
A Seven Step Spring Selection Process
Step One
Estimate the level of production required of the die. This should determine the allowable
deflection. Short run dies where spring breakage is expected to occur may use deflections
such as the average or maximum deflection. Long run dies and tooling in constant
production should not be deflected more than the long life percentage.
Step Two
Determine compressed spring length “H” and operating travel “T” from the die print
layout. These dimensions may be measured if the die is open on the repair bench. These
dimensions are shown in Figure 2.
Figure 2. Combined formula diagram illustrating the factors needed to determine spring
selection in steps one through six. The Danly Die Set Division of Connell Limited
Partnership
Step Three
Determine free length “C” as follows: Decide which load classification from which the
spring should be selected. Normally, this involves choosing from Light, Medium, Heavy,
or Extra Heavy Load. Then choose the figure nearest the compressed length “H” required
by the die design from the appropriate charts supplied by the spring manufacturer. Take
note of the corresponding “C” dimension, which is the free length of the spring as shown
in Figure 2.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Step Four
Nearly all die springs used in pressure pad and cam return applications are precompressed
or preloaded in order to have useful force throughout their working stroke. The next step
is to estimate the total initial spring load “L” required for all springs when the springs are
preloaded or compressed “X” inches or millimeters. This is also illustrated in Figure 2.
Step Five
Determine “X” or initial compression by using equation 1 which is the following formula:
X = C - H -T
(equation 1)
Where:
X = Initial spring compression or preload
C = The relaxed or free length of the spring
H = The maximum compressed length of the spring during die operation
T = The operating travel when installed in the die
Figure 2 illustrates how the “X”: dimension or initial compression produces a calculable
force or load “L” which is determined from the spring manufacture’s data. Both the “H”
dimension which is the compressed length and the “T” dimension which is the operating
travel are subtracted from the “C” dimension which is the unloaded free length of the
spring.
Of course the “X” value or initial spring preload of the total number of springs must be
sufficient to provide adequate pressure for stock control upon initial pad or stripper
contact as the die closes. The same is true of stripping pressure as the die opens. A
safety factor to allow for expected punch metal pickup or galling is needed to insure
dependable operation before scheduled bench die maintenance.
Step Six
Inch Formula: Determine “R” (total rate for all springs in pounds per 1/10 inch) by
using equation 2 which is the following formula:
L
R = ———
10 x X
7
(equation 2)
Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Where:
R = Total rate for all springs in pounds per 1/10 of travel or deflection
L = Load when springs are compressed “X” inches.
X = Initial compression preload in inches.
Metric Formula: Unfortunately, there are at least two metric systems in use. European
and North American manufacturers generally adhere to a system for spring force based on
Newtons per millimeter. A Newton is equal to a force of 0.2248 pounds. In order to
determine the value “R” which is the total spring rate for all springs used under a pad or
other die application, a different formula than for inch units is used.
To determine the spring rate for all springs used in an application such as a spring pad
pressure or cam return forces in Newtons per millimeter use equation 3 which is the
following formula:
L
R = ———
X
(equation 3)
Where:
R = Total rate for all springs in Newtons per millimeter of travel or deflection
L = Load in Newtons for all springs
X = Initial compression preload measured in millimeter increments
Step Seven: Select the Correct Springs
First, the free length of the spring “C” illustrated in Figure 3 must comply with the length
determined in Step 3.
Next, divide “R”, determined in step six which is the total spring rate by the total number
of springs to be used in order to get the rate per individual spring. It is often not possible
to know this number with certainty since the spring diameter is not yet determined.
How and where springs can be placed around die details under die pressure pads and in
other limited die space applications must be determined. The required spring diameter
and allowable deflection depending on the duty class of spring needed are determining
factors in making the selection.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Once the number of springs and spring rate is determined, refer to the manufacturer’s
catalog to choose springs having the desired rate. If the number of springs is not known,
divide “R” from Step 6 by the rate of the spring you select for the correct number of
springs.
Spring Duty Class and Allowable Compression
Table 2 lists data for the maximum allowable deflection recommended for four different
ISO die spring classifications. This is based on spring design for long life, average life,
as well as maximum allowable deflection.
MAXIMUM ALLOWABLE SPRING DEFLECTION VS RELATIVE SPRING LIFE
ISO LIGHT LOAD
ISO MEDIUM LOAD
COLOR CODE GREEN
COLOR CODE BLUE
ALLOWABLE DEFLECTION
ALLOWABLE DEFLECTION
LONG
AVERAGE
MAXIMUM
LONG
AVERAGE
MAXIMUM
LIFE
LIFE
DEFLECTION
LIFE
LIFE
DEFLECTION
25%
30%
40%
25%
30%
37.5%
ISO HEAVY LOAD
ISO EXTRA HEAVY LOAD
COLOR CODE RED
COLOR CODE YELLOW
ALLOWABLE DEFLECTION
ALLOWABLE DEFLECTION
LONG
AVERAGE
MAXIMUM
LONG
AVERAGE
MAXIMUM
LIFE
LIFE
DEFLECTION
LIFE
LIFE
DEFLECTION
20%
25%
30%
17%
20%
25%
Table 2. Maximum allowable deflection recommended for four different ISO die spring
classifications based on design for long and average life as well as maximum allowable
deflection.
In cases where high initial compression is required, high-pressure nitrogen cylinders or
hydraulic pressure systems may be required. Both nitrogen and hydraulic die pressure
systems have the advantage of providing high forces upon the initiation of travel. On
other words, an initial compression, which uses up available spring travel, is not needed.
The required pad and cam return forces should be carefully calculated during the die
design process. Springs are often the best choice from a cost and reliability standpoint.
However, if extremely high forces are required, nitrogen and hydraulic systems should be
specified.
Mixing or Replacing Springs with Self Contained Nitrogen Cylinders
In the event that the die as designed and built fails to have enough pad or cam return
force, self contained nitrogen cylinders are available that are size for size compatible with
many popular die springs. Replacing some or all of the die springs with self contained
nitrogen cylinders can serve to increase both the initial contact force and total system
force.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Replacing springs with self-contained nitrogen cylinders is not a simple substitution
process. In many cases, provision must be made for a hard wear surface for the nitrogen
cylinder rod bearing surface. This can involve substantial modification of the die,
especially if the cylinder rod end is in line with a pilot hole used to counterbore a spring
pocket. Depending on die geometry, the hole may need to be fitted with a hardened insert
or a wear surface of air hardening weld overlayment provided.
Spring Mounting and Care
The locating and mounting of springs in pockets around pilots or bolts, in tubes, or by
other methods are determined by space available, service requirements and
malfunctioning through slug interference, misalignment or other causes.
Springs must be well supported in a hole, over a rod or stripper bottom guide them
adequately under stress. Lack of support can result in crushing, twisting, binding, or
surface wear. 2
When springs are set in holes, the bottom of the hole should have a flat bottom to provide
a flat seat and eliminate the possibility of crushing or deforming the ends. The edges of
the holes should have a small chamfer to prevent interference with the movement of
springs.
However, if the unguided length of the spring is greater than the diameter, a center guide
rod should be used. The guide rod also serves to retain the broken pieces should the
spring fail.
Tubular steel spring cages or cans placed around the spring as illustrated in Figure 3 are a
good alternative for a rod to guide the spring. The can or cage serves several important
purposes. It helps prevent the entry of dirt and debris into the spring pocket. Dirt caused
by flaking of protective material coatings is especially a problem with dies used to work
galvanized steel.
Another important function of placing spring cages or cans around springs in bored
pockets is to retain any broken pieces of failed springs. This is especially important if
there is a possibility of a spring fragment flying and causing personal injury. Another
consideration in retaining pieces is to prevent them from causing interference in pads that
bottom out.
Good quality spring cans have a hard surface treatment for wear resistance. They are
available in a variety of outside diameters and lengths. The hole (H) is sized to
accommodate a shaft or rod if desired.
2
D. Smith, Die Design Handbook, Section 22, Die Sets and Components, © the Society of Manufacturing
Engineers, Dearborn, Michigan, 1990.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Figure 3. A spring cage or can used to keep debris out of spring pockets and contain any
broken spring pieces in the event of spring failure. The Danly Die Set Division of
Connell Limited Partnership
Analysis of Metal Spring Failures
Properly selected and used, metal die springs provide long trouble free service. If die
springs fail frequently, then there is a reason and normally an alternative is available to
reduce or eliminate the failure rate.
Misconceptions Concerning Metal Spring Failures
In conducting both public and in-plant training, the writer asks the class attendees if they
are having problems with die springs breaking. The answers range from hardly ever does
a spring break to broken springs are a serious downtime, cost and safety concern.
The next logical question to ask is how many attendees have problems with the valve
springs in their automobile engines breaking. With the exception of a very few persons
who have exceeded the mechanical endurance of valve springs in racing engines, the
usual answer is virtually no one has a problem with automotive valve springs breaking.
Die engineering is solidly based on mechanical engineering principles. Any mechanical
failure has a cause and in most cases a straightforward solution. The automotive valve
spring comparison leads to a sensible conclusion. Since both valve springs and die
springs are made of similar high quality steel, then die springs that fail must be
excessively stressed.
A source of confusion is the tendency of a few manufacture’s of both die springs and die
nitrogen cylinders to use negative comparisons of competing products in their advertising
literature. In the writer’s opinion, there is a best application for each type of die pressure
system.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
Table 2 illustrates how die springs are rated for maximum deflections based on the
required life expectancy. If die springs are deflected beyond the long life rating, it is
assumed that spring failure is likely to occur and failure should be expected.
While automobile designs vary, engines operate at approximately 2,000 revolutions per
minute at highway cruising speeds. Therefore, a four-cycle engine would undergo 1,000
spring compressions per valve during each minute of operation. Thus, automotive die
springs routinely withstand well over 100,000,000 compression cycles during the
conservatively rated nominal life of the engine.
High-speed pressworking is accomplished at speeds of from 300 to over 2,000 strokes per
minute (SPM). A typical speed for an electrical connector die is 1,200 SPM. Such dies
will complete over a million hits in a typical 16-hour two-shift operation. In such costly
precision tooling, spring breakage could result in catastrophic damage.
Winding Springs in House
Many tool and die makers are taught how to wind springs as part of their apprenticeship
training. The usual material for springs made in the toolroom is music spring wire. This
material is also commonly known as piano wire, although the term music spring wire is
the correct term for the commercial product.
Winding spring wire onto an arbor in a lathe is one way to make springs in the toolroom.
A hazard to be aware of is that a finger or other body part may become entangled in a
loop of the wire as it is fed into the lathe. Should this occur, serious injury such as an
amputation can result.
Commercial spring winding machines are used in a few large toolrooms that need a
variety of special springs on short notice. This in house ability is especially handy if
prototype or jig and fixture work requires the development of special springs. Some
spring winders are hand cranked and can be operated by a single individual. This greatly
reduces the possibility of injury. Appropriate safety equipment such as approved safety
glasses should always be worn to avoid injury when working with springs.
In general, the use of springs that are catalog items will help insure that the spring will
meet the engineering specifications of the manufacturer. However, for prototype and
instrument work, the knowledge and equipment to wind a special spring quickly is a
valuable skill.
Appropriate Use of Metal Springs
The use of coiled metal die springs is one of the most widespread die pressure system
applications. Readily available engineering data predicts that metal fatigue will not cause
failure problems if springs are carefully manufactured and not over deflected. In general,
designing total deflections including initial compression for preloading below the
manufacturer’s recommendations for long life will result in long trouble free die pressure
service.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
There is an obvious need for North American manufactures to adopt the ISO color code
designations as their internal standards for duty class, which is also the standard of the
North American Automotive Metric Standards Group. Maintaining tooling to meet low
inventory reliability requirements makes standardized procedures a necessity. If your
company adheres to ISO spring standards, any tooling built in non-ISO countries should
be built to ISO standards to avoid maintainability problems.
Designing dies with metal spring deflections greater than those specified for long life is
advised only for tooling designed with redundant springs and an absolutely fool proof
means to contain broken springs and spring attachments such as cam return rods. This is
advised to avoid the potential for personal injury and unscheduled downtime. In general,
spring deflections greater than those specified for long life are apt to fail in service. This
is almost a certainty if the maximum deflection rating is chosen.
Common Errors That Result in Coiled Die Spring Failure
Most spring failures result from excessive deflections, which cause stress-cracking
leading to rapid failure. A partial listing of bad shop practices includes:
1. Replacement of springs with a higher load class resulting in deflections in excess
of the die design criteria leading to stress cracking failures.
2. Using the wrong load class spring due to a color-coding error.
3. Failure to specify that spring suppliers follow the widely accepted ISO standard
color coding system—include this requirement in your die construction standards
and contractually insist that all vendors follow your standards.
4. Neglecting to specify that tooling construction sources use ISO standard springs
and that the deflections are specified for the desired life expectancy—if there is
doubt, require that a copy of the spring suppliers invoice be supplied.
5. Shortening die springs with abrasive cutoff wheels or cutting torches—this does
not provide a flat surface on the end of the spring resulting in lateral bowing.
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Using Metallic Springs as Die Pressure Devices D07.doc © 1990-2006 rev October 12, 2006
David Alkire Smith, 530 Hollywood Drive, Monroe, Michigan 48162-2943
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