Transactions

556 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941
solid. While the relationship need not and cannot be mathematically
constant, the higher test stresses should not be assigned
to the springs of lower solid stress.
The higher solid stresses are produced by removal of permanent
set, i.e., by cold-working the springs; and this cold-working
has a marked effect on the endurance. The ideal condition for
the tests would be a group of springs with graded solid stresses,
produced without cold-working; but this is obviously impossible.
The logical alternative is to maintain a relation between the test
stress and the solid stress, and so between the test stress and the
cold-working effect.
The individual test stresses having been assigned, the test loads
are calculated therefrom, by the formula
T A B L E 3
C A L C U L A T IO N F O R P R O B A B L E S-N C U R V E ; G R O U P E
Each spring is then compressed under its test load in a testing
machine, and the height accurately measured with the micrometer
height gage. The springs are then ready for the fatigue test.
C o n d u c t o f F a t ig u e T e s t
The machine used for the fatigue test should be of sturdy and
rigid construction, so that the cyclic deflection impressed on the
springs will be accurate and uniform. Speed should not be so
high, nor the stroke so great, as to cause impact stresses. Geared
presses of the type suitable for this work usually run about 40 to
70 strokes per min, and the higher speed is not excessive for the
purpose. A stroke of not more than twice the spring deflection is
believed to be satisfactory. If possible, the machine should be
equipped with a screw adjustm ent in the spring support, so that
the rig can be accurately adjusted to give the desired deflection to
the spring.
To set up a spring for the test, the machine is turned over until
the ram is at the lowest point of its stroke, and the screw adjustment
manipulated until the distance between ram and spring support
corresponds to the “height under test load,” described in a
previous section. The ram is then raised, the spring mounted on
the support, which should be shouldered to position the spring
centrally under the ram, the machine again turned over to its
lower center, and the compressed height of the spring checked.
The revolution counter is then set to zero or the initial reading
recorded, and the test run started. The run on each spring should
be as nearly continuous as is practicable. The machine on which
recent Committee tests have been conducted is equipped with a
photocell, which shuts down the machine when the spring breaks,
so that practically no attention is required during the test run.
Almost any helical spring, in the course of a fatigue test, will
suffer a slight change in rate, particularly during the first few
thousand cycles of stress. I t is therefore recommended that the
spring be removed from the fatigue machine and the height under
test load remeasured, say, after 5000 cycles, and again after 10,000
cycles, and finally after 25,000 cycles. At each of these stages, the
machine should be reset to the new height reading; which, on the
first recheck and usually on the second as well, will be a few
thousandths under the previous figure.
The test is then continued until the spring fails, or has run
1,500,000 to 2,000,000 cycles. Usually no good purpose is served
by any longer run than this; and with proper assignment of
stresses, no more than one or two springs in the group need be
run more than 1,000,000 cycles.
A n a l y s is a n d R e p o r t o f T e s t s
The primary object of the test is to establish a complete
“stress-endurance curve” for the material represented. Such a
curve, in addition to its scientific interest, is of considerable prac­
F i q . 3 F a t i g u e T e s t s a t W r i g h t F i e l d o f H o t - W o u n d H e l ic a l
C o m p r e s s i o n S p r i n g ; S e r i e s E , P l a i n - C a r b o n E l e c t r ic
tical value to the spring industry. In many (perhaps most)
spring applications, the design stress exceeds the endurance limit.
This is often permissible because the failure of the spring does not
constitute a catastrophe; therefore the consumer is interested in
longest spring life per dollar of cost, rather than unlimited life
regardless of cost. Therefore, the comparative life at stresses
above the endurance limit becomes very important.
Hence, the first step in analyzing the test results is to plot them
graphically. I t is customary to use a semilogarithmic crosssection
chart for this purpose, plotting stresses as ordinates
(Cartesian scale), and cycles to failure as abscissas (logarithmic
scale). If the tests have been carefully planned and carried
out, the results should give an indication of a smooth curve, which
approaches a horizontal (stress) asymptote. The latter can