|
|
| 1.3. CRITICAL POINTS IN ROPE CHOICE |
Rope compositions are chosen according to work conditions, load to be subjected to and possibility of life threat.
General information in choice of rope is given in Table-4. Following points must be taken into consideration in choice of rope:
1.3.1 Wire Tensile Strength
The following points are changed according to the wire tensile strength:
• Breaking load,
• Resistance agaist smashing and crashing,
• Flexibility,
• Bending Fatigue Resistance
As it is seen in Table-4, while wire resistances in range of 160-200 kg/mm2 are used almost all wires the resistance groups under and above these values are preferred for only special conditions.
1.3.2. Rope Compositions
• Resistance against abrasion and shock and
• Bending fatigue resistance
are changed according to rope compositions. This must be also kept in mind while taking condition of use into account.
As it will be seen in Table-4, while the ropes of 6x7(1+6) ve 6x10 FS(3+7) with thick external wires show great strength against abrasions and smashes the ropes in the group of 6x37 having many thin wires show great strength against bending fatigue. These features are balanced in 6x9 filler and 6x19 ropes. For this reason these ropes are commonly used in the speeds of middle degree as heave and work ropes in hard conditions.
Table 4. Factors in choice of ropes
1.3.3 Rope safety coefficient, metallic cross section area
Aim of safety coefficient is to define a satisfactory ratio between the amount of various forces effecting rope and breaking load. In defining the ratio following points must be taken into consideration:
a. Rope’s own weight “static weight”,
b. Instant loading weights “dynamic weight”,
c. Force changing in the moment of acceleration and deceleration, “accelerated weight”,
d. Stresses in bendings “rope efficiency ratio”,
e. Stress changes that happen in vibrations and winding,
f. The tipe of carried material (human, material etc.)
g. Conditions of use,
h. Difficulties while defining rope life.
i. Unforseenable changes in respect with usage errors.
Metallic cross sectional area may be used in defining elastic elongations and calculations of rope breaking load if there are no ready tables. Breaking load is defined multiplying it with metallic area breaking load range average. For example, in the range of 160-185 kg/mm2 average resistance is 172,5 kg/mm2. Calculated Rope Breaking Load P is formulated in following way:
P=Metallic cross sectional area(mm2) x Wire resistance(kgxmm2) x Coefficient rope loss
1.3.4. Rope angle- Lifting Capacity and the points that must be taking consideration in use:
1. In lifts with a sling making a certain angle lifting capacity of rope is effected.
This amount is given in Table-5.
Table-5. Relation between lifting angle and rope lifting capacity
2. Ropes must be used in a way that they brush the sharp corners in order to prevent wire cuts.
3. Ropes with fiber cores must not be used in carriage of molten metals in high temperatures.
4. The important points such as rope diameter, number of broken wires, oiling etc. Must be always checked during usage and be recorded.
5. Ropes used in pair must be attached, assessed and detached together.
|
|
|
 |
 |
|
