Where the face of the warehouse is sufficiently close to the water to permit of the crane rope plumbing the hatches without requiring a jib of excessive radius, it is a very convenient plan to place the whole crane on the warehouse roof.
Dock-side jib cranes for working general cargo are almost always made portable, in order to enable them to be placed in correct position in regard to the hatchways of the vessels which they serve.
The jib is usually inclined, so as to enable the travel to be performed by gravity in one direction, and the object of the transporter mechanism is to ensure that pulling in or slacking out the lifting rope shall perform the cycle of operations in the following order: - Supposing the load is ready to be lifted out of a vessel on to a quay, the pull of the lifting rope raises the load, the travelling jenny being meanwhile locked in position.
When the loads are heavy the above mechanisms are supplemented by systems of purchase blocks suspended from the jib or the traveller crab; and in barrel cranes trains of rotating gearing are interposed between the motor, or manual handle, and the barrel (fig.
This class is the ordinary jib crane.
For dock-side jib cranes the use of electric power is making rapid strides.
The derrick crane introduces a problem for which many solutions have been sought, that of preventing the load from being lifted or lowered when the jib is pivoted up or down to alter the radius.
The limit of speed of lift of hand cranes has already been mentioned; for steam jib cranes average practice is represented by the.
II represents a portable steam jib crane; it contains the same elements as the fixed crane (fig.
Load is suspended from the crane jib; this jib is attached at the lower end to the side frames, and the upper end is supported by tierods, connected to the framework, the whole revolving together.