Formerly it was thought that the driver contact material in the
grooves of grip drums brake must be of such a soft material or more than the
same cable conductor. It was a mistake.
These brakes material "soft" after a period of use, had
some scuffs especially in the first two rounds. Basically drums were
aluminum with machined grooves in the same drum or steel structure with
interchangeable sections aluminum material coated channels "soft".
Origin of the first problem
In these materials the lead wire pressure on the material
"soft" channels produces an anchoring effect that prevents the driver
free rotation. So the driver can not download the torque builds up when
moving from one groove to another brake drums and must endure until the brake
output.
As a result, unable to relax the torque the driver, stress the
cable is evidenced by the phenomenon known as "bird cage"
(birdcaging).
Origin of the second problem
The wear occur in the first two channels are depletable diameters,
causing an incompatibility peripheral speed thereof.
It is obvious that if the peripheral speeds are not equal in all
channels of drums, the cable suffers and tows or landslides occur in the brake
drums. To avoid the diameters of all channels of the two drums of a brake
must be exactly the same.
To correct these problems , manufacturers have studied and tested different materials
to get the most suitable for the grooves of the brakes "hard" material.
Even since 1985, they are made of steel with chemical and thermal treatment in
the grooves with excellent results.
Do not forget that the pressure between the wire and throat
elemental brake must be less than the pressure between two elementary wires of
adjacent layers of the conductor. In other words, the coefficient of
friction of the outer layer of the cable driver brake groove must be less than
the friction between two adjacent layers of the conductor.
The scheme presented here can help you understand what we have
said. See drawing of the layout and think of a very normal situation, 80
kilograms of pressure at the inlet of the driver to brake channel 5 and 3,000
kilograms of pressure per channel 1 output driver brake coated material
channels "soft ".Channel 5 supports a negligible pressure and channel
1 a very strong pressure tends to produce a flattening of the material
"soft" and ramming the veins of the cable into the material
"soft" producing a cable anchor driver that prevents free rotation.
In 1990, Dr. Engineer Giancarlo Gamba, an authority on these
issues, and denounced the mistake of using softer than the same driver as
contact material in the grooves materials.
You can send comments or photos on "bird cages" and
other manifestations of stress cable passing through the brake lines. The
aim is to improve this entry through the experiences of more observers.
Here is quotation of bircaging from Standard IEEE 524
ReplyDelete"As the conductor enters the bullwheel groove, the pressure of contact tends to push the loose
outer strands back toward the reel where the looseness accumulates, leading to the condition
commonly known as birdcaging, see Figure 28. If this condition is not controlled, the strands can
become damaged to the extent that the damaged length of conductor must be removed. This problem
can be remedied by allowing enough distance between the reel and tensioner to permit the strand
looseness to distribute along the intervening length of conductor and simultaneously maintaining
enough back tension on the reel to stretch the core and inner strands to sufficiently tighten the outer
strands. It is recommended that the back tension or braking tension of the conductor reel not exceed
4.5 kN since drawing down of the conductor into the lower layers on the reel may cause surface
damage. For smaller diameters and wooden reels, the back tension should be considerably less."
As you can see there is one number of tension force, better say braking tension and it is 4.5 KN,
but on the bullwheel there is much higher tension force using during stringing. I am really curious about the
differnce between braking tension on the reels and tension on the bullwheel. What is the proper ratio between the two,
or what tension force is proper to use on the bullwhhel? I know it depends on the diameter of conductor and distance
betwwen the structures.
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