EXTENSION SPRINGS0 pages

factors affecting design

W
T
J

EXTENSION SPRINGS

SYMBOLS
W = Spring material width
T = Spring material thickness
L = Spring length (reference only)
J = Minimum extension for constant force
H = Off-set distance
D2 = Storage drum diameter
D1 = Outside diameter of spring when
fully wound on storage drum

D2
H

D2

SEE DETAILS OF
SPRING ENDS

D1
RECOMMENDED DISTANCE
BETWEEN DRUM FLANGES
EQUALS SPRING WIDTH (W+1) to 3mm

Fatigue Life

Stability

The fatigue life has a direct effect on the size of the spring and the
maximum load available in the least space. A comparison of various
chart values illustrates the effect fatigue requirements have on the
size of the spring. For any application the required number of cycles
or reversals, should be estimated for the life of the equipment or a
replacement schedule.

The free end of the extension
spring should be guided to
Held flat
prevent the spring from
wandering off the drum. The spring should be firmly
attached to the moveable member (see diagrams).

The fatigue life of a spring can be defined as either a full or partial
extraction and retraction of the spring i.e. the spring can be
exercised over its whole length and achieve the average life
expectation, however if it is exercised over any one section of the
spring repeatedly then that section can be expected to fatigue when
the total number of cycles approaches the life predicted for the
spring.

TYPES OF MOUNTING

Fatigue Life is not time dependant under normal operating
circumstances; it is solely dependent on the number of operations.
However pollutants even in low concentrations can have the effect of
shortening the fatigue life through corrosion or chemical attack,
please refer to the statements regarding uses and misuses of
Spiroflex springs in our brochure and web site and refer to Spiroflex
if in doubt.

or

Drum Mounting
The Spiroflex spring will be held on the drum by its inherent
gripping action. No other fixing is required, provided there is
sufficient spring material remaining on the drum when fully
extended. An exception to this is when the mechanism does not have
its own limiting stops, in which case we recommend the inner end
be secured to the drum by the holes provided (drum should be
stepped – see diagram).

Extension Spring used in conjunction with Idler Pulley
The Spiroflex spring can be used with an idler pulley provided it is
positioned to suit the natural curvature of the spring (see diagram).
Extension springs should not normally be bent back or reverse
wound, as this will permanently damage the spring.

CORRECT

INCORRECT

Cavity Mounting
A low cost mounting arrangement is the Spiroflex spring coil seated
in a cavity the obvious disadvantage is the friction between the
spring coil and the cavity surface. We would recommend the cavity to
be lined with a low friction material. Another method is to use the
free coil force against a moveable member.

Multiple Mounting
Various standard methods of multiple mounting are shown below.

Speed and Acceleration
There is almost no limit to the speed or acceleration to which the
spring will coil and uncoil. However, tests should be carried out to
satisfy yourself the spring will perform satisfactorily under your
conditions.

Springs can also be laminated to provide an increase in force or
fatigue life in a minimum space. Please apply to Spiroflex Sales
Department for further details.
To discuss your needs please call our Technical Sales Department
who will be pleased to help.
Issue No. 1 (Ext_spr_A/eng)