Loading
Pins are subject to both vertical and horizontal loadings. The vertical
loading results from the weight of the conductor and its half-inch radial
coating of ice. The horizontal loading stems from the wind, from differential
tensions in adjacent conductor spans, from non tangent spans,
or from broken-wire conditions in which the tensions in the conductor
spans become unbalanced.
Under vertical load, the pin acts as a simple column, transmitting
its load to the cross arm at the
shoulders resting on the cross
arm. The stress is equal to the
load divided by the area under
pressure, the area of the shoulder
resting on the cross arm.
This component is usually not
large compared with the other
components acting on the pin
and is often neglected.
Under horizontal loadings,
the pin acts as a cantilever
beam, and the maximum stress
occurs at the point where the pin rests on the arm; refer to Figure 5-10. The
bending moment M is equal to the load P
multiplied by the distance of the conductor
above that point (h):
Double Pins
Double pins, one on each of the double arms, are used where the
strength of one pin is inadequate.
Pins in Lieu of Cross Arms
The advent of wye primary systems employing a common neutral
with the secondary (situated on the pole in the secondary position) allowed
single-phase primary conductors to be supported on a steel ridge
pin, as shown in Figure 5-11. The vertical loading on the pin, the weight
of the ice-coated conductor, is transmitted to the pole through the bolt
by which the pin is attached to the pole. Horizontal loadings, from both
wind and conductor tension, act on the pin as a cantilever beam, and the
same analysis of stresses in the pin, bolt, and pole applies here as with
cross arms.
In polyphase systems, the conductors
may be supported on pins attached
directly to the pole, eliminating the use
of cross arms (this method of support is
sometimes referred to as “armless construction”).
This not only makes for a
neater appearance, but, as indicated earlier,
improves electrical performance by
mitigating the voltage drop due to the reactance
of the line. (The arrangement and
spacing of the conductors accounts for the
lessened reactance. Also, this construction
employs bucket trucks or platforms for
easier access to the conductors.)
In this instance, the vertical loading
acts on the end of the pin, with the pin
acting as a cantilever beam; the bending
stress occurs at the pole, and the fiber
stresses on the pin are calculated in the
same manner described above for cross
arms and poles.
The horizontal loading
acts to create compressive stresses in the pin, which acts as a slender
column. Again, this component is small compared with the other components
and is often neglected. On the other side of the pole, however,
the horizontal forces act to pull the conductor away from the pole; while
there is ample strength for the tensile stress imposed on the pin and the
bolts with which it is attached to the pole, these forces do dictate that the
conductors be so attached to the insulators as to ensure the conductor
will not be separated from the insulator under the stresses imposed.
For higher voltages, additional space requirements may necessitate
longer pins or the installation of insulated conductors (or both). The
pins may be attached to the pole as indicated in Figure 5-12. Here, the
stresses on the pin from both the vertical and horizontal loads impose
bending moments about the bottom of the pin at the pole. The moments
and stresses on the pins and supporting bolts, indicated in Figure 5-12,
are computed in the same fashion as for the cases mentioned above.
For all applications of the pins, a minimum strength of 700 lb per
pin is usually specified in withstanding unbalanced tension in a conductor
supported by the pin.
Pins are subject to both vertical and horizontal loadings. The vertical
loading results from the weight of the conductor and its half-inch radial
coating of ice. The horizontal loading stems from the wind, from differential
tensions in adjacent conductor spans, from non tangent spans,
or from broken-wire conditions in which the tensions in the conductor
spans become unbalanced.
Under vertical load, the pin acts as a simple column, transmitting
its load to the cross arm at the
shoulders resting on the cross
arm. The stress is equal to the
load divided by the area under
pressure, the area of the shoulder
resting on the cross arm.
This component is usually not
large compared with the other
components acting on the pin
and is often neglected.
Under horizontal loadings,
the pin acts as a cantilever
beam, and the maximum stress
occurs at the point where the pin rests on the arm; refer to Figure 5-10. The
bending moment M is equal to the load P
multiplied by the distance of the conductor
above that point (h):
Double Pins
Double pins, one on each of the double arms, are used where the
strength of one pin is inadequate.
Pins in Lieu of Cross Arms
The advent of wye primary systems employing a common neutral
with the secondary (situated on the pole in the secondary position) allowed
single-phase primary conductors to be supported on a steel ridge
pin, as shown in Figure 5-11. The vertical loading on the pin, the weight
of the ice-coated conductor, is transmitted to the pole through the bolt
by which the pin is attached to the pole. Horizontal loadings, from both
wind and conductor tension, act on the pin as a cantilever beam, and the
same analysis of stresses in the pin, bolt, and pole applies here as with
cross arms.
In polyphase systems, the conductors
may be supported on pins attached
directly to the pole, eliminating the use
of cross arms (this method of support is
sometimes referred to as “armless construction”).
This not only makes for a
neater appearance, but, as indicated earlier,
improves electrical performance by
mitigating the voltage drop due to the reactance
of the line. (The arrangement and
spacing of the conductors accounts for the
lessened reactance. Also, this construction
employs bucket trucks or platforms for
easier access to the conductors.)
In this instance, the vertical loading
acts on the end of the pin, with the pin
acting as a cantilever beam; the bending
stress occurs at the pole, and the fiber
stresses on the pin are calculated in the
same manner described above for cross
arms and poles.
The horizontal loading
acts to create compressive stresses in the pin, which acts as a slender
column. Again, this component is small compared with the other components
and is often neglected. On the other side of the pole, however,
the horizontal forces act to pull the conductor away from the pole; while
there is ample strength for the tensile stress imposed on the pin and the
bolts with which it is attached to the pole, these forces do dictate that the
conductors be so attached to the insulators as to ensure the conductor
will not be separated from the insulator under the stresses imposed.
For higher voltages, additional space requirements may necessitate
longer pins or the installation of insulated conductors (or both). The
pins may be attached to the pole as indicated in Figure 5-12. Here, the
stresses on the pin from both the vertical and horizontal loads impose
bending moments about the bottom of the pin at the pole. The moments
and stresses on the pins and supporting bolts, indicated in Figure 5-12,
are computed in the same fashion as for the cases mentioned above.
For all applications of the pins, a minimum strength of 700 lb per
pin is usually specified in withstanding unbalanced tension in a conductor
supported by the pin.
