CF SPRINGS

CF spring is an integrated part of the brush holder for DC-Motors. The quality and tension of the spring is very important for good carbon brush performance.

  • Engineered to give optimal performance for your equipment
  • Designed improve brush life and lower maintenance costs
  • A strong force and uniform Tension CF springs

How to maintain optimum contact pressure throughout the life of the brush is one of the major problems facing designers of brushgear for electrical machines

A spring that applies the same force, independent of extension, would seem to provide a simple answer, and when such springs became readily available, brushgear was developed to take advantage of this unusual characteristic.

Trials carried out on a variety of machines showed that the use of the constant force (C/F) spring was a sound, practical approach to the problem of constant brush pressure – but there was little factual information available on such crucial matters as the life expectancy of constant force springs in brush holders, their ability to withstand the severe environments in which brushgear is often required to work and their effect on brush behavior generally.

Testing proved that this type of spring does fulfill the constant pressure requirement and, furthermore, that it introduces other advantages in terms of improved brush stability, better commutation (particularly with worn collectors), mechanical simplicity and compact brush holder design.

Successful brushgear design involves much more than an appreciation of brush holders. The brush itself is an integral part of brushgear; therefore, joint brush holder and brush development is essential. From a combined brush holder and brush

development project, and from a detailed study of C/F spring behavior, a basic design of brushgear has emerged, combining simplicity with robustness and compactness.

Fulmer C/F spring brushgear

C/F spring brushgear designs are available for many types of machine: d.c. motors and generators, variable speed a.c. commutator motors, induction motor and turbo-alternator sliprings, Rail Traction, and Auxiliary machines.

Full details of the standard ranges of Fulmer C/F spring brushgear are available from Fulmer Company.

The constant force (C/F) spring

When a conventional helical or
clock-spring is extended or deflected, the
whole length of the spring is stressed and
the deflection is
proportional to the force applied.
Thus, the maximum deflection is
limited if permanent deformation is to be
avoided.
The extension of a constant force spring,
however, is not proportional to the applied

force and, therefore, the spring is not limit-
ed to only small extensions.

The typical C/F spring characteristics of
fig. 2 show that the force is constant over
a wide range of spring extensions. This
unusual feature, which appears to disobey
Hooke’s Law, is achieved in the following
way:*
Flat spring stock (stainless steel for
brushgear applications) is processed to
produce a curvature of constant radius at
all points along its length. As a result, the
strip, when relaxed, forms itself into a tight
coil of
pre-determined diameter. When the
spring is deflected as in fig. 3, there is (i)
relaxed material remaining on the coil, (ii)
straightened material, Zone A, which is
fully stressed and merely serves to transmit
a force and (iii) material which is changing

from its natural curvature to the
straightened form, Zone B.
neither the straight section nor the fully
relaxed coil can contribute to the restoring
force – this is derived solely from Zone B.
Thus, as the spring is extended, the stress
is
non-cumulative, giving the constant force
characteristic shown in fig. 2.
The spring material used is capable of
withstanding very high stresses
without permanent deformation; hence,

large forces can be obtained with surpris-
ingly small springs.

Practical spring assemblies

In general, the essential mechanical
requirements of a practical C/F spring
assembly are a means of securing the free

end of the spring and a flat surface to sup-
port the straight section. However, to

arrive at an effective design, many other
factors need consideration; for instance,
ease of removing brushes for replacement
and servicing,
compactness and simplicity and ease of
manufacture.
The value of the C/F spring in brushgear
applications is
considerably enhanced by the fact that the
spring can be arranged to press directly
onto the brush, so

providing a low inertia system with excel-
lent brush stability and the

minimum of moving parts (pivots and pres-
sure fingers are completely

eliminated).
The designs are the “sugar-tong” and the
‘V’ Clip, standard versions of which are
shown figs. 4 and 5.
The sugar-tong assembly consists of a ‘U’
shaped stainless steel clip to which the
C/F spring is attaches by riveting; small
lugs pressed out in one side of the clip
serve to locate the assembly in the brush

holder. This design of unit is most fre-

quently used in brush holders constructed

either from extruded brass sections or from
sheet metal pressings. By squeezing the

clip between finger and thumb, the assem-
bly is easily pushed into the holder.

Withdrawal from the holder is equally sim-
ple.

The ‘V’ Clip assembly is adopted where
demand permits the
economical use of a die cast brush holder

or where the geometry of the holder pre-
vents the use of the

sugar-tong. The ‘V’ Clip unit is designed to
fit into a separate
compartment in the brushgear,
external to the brush-box itself.
The standard spring and clip
assembly designs are readily
detachable from the brush-box for easy
maintenance but, where
detachable components are not
desirable on machines, semi-captive clip

designs can be manufactured as an option-
al extra.