CHAPTER V
CORDAGE AND KNOTS
The fibres used for the ropes and cords for scaffolding purposes are of jute and hemp. The strongest rope is made of the latter variety, and the best quality is termed the Manilla.
The cords are generally tarred for preservation, but this process has a bad effect upon the strength of the fibre, more especially if the tar is impure. The process of tarring is as follows:—
The fibres before they are formed into strands are passed through a bath of hot tar. Immediately afterwards, and while still hot, the material is squeezed through nippers, by which means any surplus tar is removed.
The ropes, as used in scaffolding, are known as shroud laid or three strand.
A shroud-laid rope consists of three strands wound round a core, each containing a sufficient number of fibres to make an equal thickness. A three-strand rope is similar, but has no core.
The following gives the breaking weights of scaffold cords and falls according to circumference[2]:—
| Size | Cwts. | qrs. | lbs. | ||||
|---|---|---|---|---|---|---|---|
| Tarred hemp scaffold | 2 inches in circumference | 20 | 0 | 0 | |||
| Cords | 21⁄4 inches in circumference | 25 | 0 | 0 | |||
| Tarred jute scaffold | 2 inches in circumference | 14 | 0 | 0 | |||
| Cords | 21⁄4 inches in circumference | 18 | 0 | 0 | |||
| White Manilla scaffold | 2 inches in circumference | 32 | 0 | 0 | |||
| Cord | 21⁄4 inches in circumference | 39 | 0 | 0 | |||
| 21⁄2 inches in circumference | 44 | 0 | 0 | ||||
| White Manilla rope for | 3 inches in circumference | 70 | 0 | 0 | |||
| pulleys and block ropes | 31⁄2 inches in circumference | 98 | 0 | 0 | |||
| 4 inches in circumference | 126 | 0 | 0 | ||||
| 41⁄2 inches in circumference | 158 | 0 | 0 | ||||
It should be noted that all these weights are the actual breaking strains of the ropes tested to destruction. In practice, one sixth of the breaking weights only should be allowed, in order to leave a sufficient margin for safety.
The strength of a rope is the combined strength of each separate yarn. Therefore, if the fibres are not carefully twisted together, so that each bears an equal strain, the rope is unsafe. Care should be taken when choosing a rope that the strands are closely, evenly and smoothly laid.