Steel hauling chains manufactured to accurate pitch
and uniform link dimensions are the key hauling cables for pulling
railway dry dock cradles. The enormous tensile strength of the
chains and ease of connection with special Crandall hauling shackles,
as strong or stronger than the chain itself, provide a suitable,
durable and economical cable far more satisfactory than the best
wire rope. These chains, fitted over alloy steel chain wheels
having a small pitch diameter, permit hailing very heavy loads
with relatively small hauling machines.
It is possible by caliper measurements to determine the strength
and corresponding safe live load that a chain can pull at any
stage of wear. This ability to measure the strength is a major
advantage that chain ahs over wire rope.
If properly lubricated with thin or medium-weight oil that can
flow into the grip of the links, a chain can have its useful life
doubled. Experience has shown that some steel chains are capable
of pulling capacity loads even after 30 years of continuous use.
It is particularly important to protect chain from abrasion caused
by sliding through sand, gravel or silt. Before the adoption of
metal chain slides, chains ran in wooden troughs; sand would become
embedded in the wood causing accelerated chain wear. The metal
slides cover only the track cross ties, allowing debris to drop
away from the chain. In some installations, it may be wise to
provide a means for washing the sand off the incoming chain with
A chain swivel with grease-packed ball bearings is used between
hauling and backing chains to permit a loaded chain to rotate
freely and prevent backing chain twist from being transmitted
to the hauling chain. In the absence of such a device, excessive
twist could cause a chain to jump its sprocket wheel.
Present-day roller systems consist of steel-angle frames with
welded-in malleable iron bushings to hold the roller pintles.
The holes are punched or drilled with extreme precision, resulting
in greatly reduced roller friction, elimination of roller flange
failure, reduction or elimination of cradle surging, and, most
important, reduction of the chance of derailment. The connector
used between frames allows proper self-alignment to take place
and makes it easy to connect and disconnect frames under water.
The reason for using rollers for railway dry docks of any significant
capacity is that a friction coefficient as low as 1-2 percent
can be achieved. Small railways of less than 150 tons' capacity
are usually provided with wheels rather than rollers. The necessity
of having to have divers retrieve slipped roller frames is thus
avoided, and steeper track gradients are possible. On the other
hand, wheels of the fixed-axle type have at least 2-1/2 times
the friction of rollers and so require heavier chain and more
It is important to maintain railway dry docks properly to avoid
derailment or structural failure due to excessive wear and deterioration
caused by corrosion, marine borer attack, and rot. Preventive
maintenance also extends the economic life of the equipment. It
is wise to have a regular program of maintenance and to have periodic
inspections by qualified dry dock personnel.
The practice of sandblasting steel vessels presents a serious
problem in railway dry dock maintenance. It has been largely overcome
by sheathing the cradle decks to prevent the sand from falling
to the ways below. It is still necessary, however, to remove sand
where it has accumulated along the side of the track and beyond
the fantail of the cradle.
Another design feature that is useful for flexible dry dock operation
also contributes to making maintenance more practical. A divided
cradle allows docking one large ship or two smaller craft. In
the latter case it is often possible to have one ship docked for
a long period on the forward section, while the aft section serves
for short-period hauls. For larger railways in locations having
small tides, the divided cradle solves the problem of cradle maintenance,
since it is a simple matter to float away the bow cradle and then
haul the after cradle clear of the water. Dividing the cradle
does require that hauling chains be secured to the forward end
of the outshore cradle and that the inshore cradle be held by
latches, but the cost of the extra chain and anchoring system
may well be worth while.
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