Here
is a 4600 Manitowoc Vicon (Variable Independent Converter), used in lift crane
service. With a different type boom and without the torque converter, they are
a preferred “dirt machine” because of their rugged machinery and single line
drum hoisting capacity. Also, they have large disc swing frictions well suited
for the wear and heat of “duty cycle” work as in clamshell operations requiring
constant rotating of the crane between the pick-up and depositing of the
material.
The typical clamshell bucket below shows the three lines that control the movement of the load from one location to another. The
tag line is under tension at all times from its take-up reel. It keeps the bucket
in line with the crane and reduces swinging somewhat.
The holding line does just that, and is attached to the top
of the bucket. Holding the bucket at a given elevation, then lowering the closing line the jaws open and unloads the material. After dumping the load the
closing line is hoisted to produce some slack in the holding line and shutting
the clamshell to swing back to the digging pile.
Then, the closing line is lowered opening the jaws of the
bucket which are counterweight to open automatically. Next, both lines are
lowered onto the material pile. Now the closing line is hoisted and the jaws
“close” picking up the material. The closing line picks up the full bucket load
keeping some slack in the holding line and both are raised in unison to the necessary
height and held there until it is swung over the deposit area there again; the
closing line is lowered opening the jaws of the bucket allowing the material to
dropping out.
Okay then, from all that you can see that a lot of skill is
necessary on the part of the operator. Also, there are a lot of variables that
affect the actual force on the crane. First, there are static weight and then
dynamic (motion) force. How quickly you start and stop influence these forces.
One other element to clamshell work is the sudden release of the load causing impact
loading.
Now, is the operator smooth or jerky? Does
he work within the rated capacity of the crane? Taking into account the shock
loading of bucket work, manufactures down grade their ratings by a minimum of
20%. What about the size (in cubic yards) of the bucket on the crane, correct
or larger? Is he beyond the maximum radius causing the crane become unstable? These cranes are tough, but they have their limits. Here we see
obvious signs of severe shock loading indicated by the pulling out of the rope
from the wedge sockets!
These cranes have their limits! After repeated mishandling
the upper works broke away from the lower pedestal. We rarely see this severe damage on a turntable with minimum
wear. Let’s review how it was being worked. You heard the old saying “being
road hard and put away wet”, well that applies here. The operator was jerky on the controls that shock load the crane. This was done on a regular basic and over some time. There were indicators to the operator and the crane owner that problems with the crane operations were damaging the crane.
And “the rest of the story” as Paul Harvey would say, is
this. The bolts pointed two in the photo to the right were replaced many times with
harder bolts should have been a tip off that something was wrong. This is the
connection between the crane’s upper works and the pedestal mounted on a dock,
so the crane is firmly held in place. This installation gives the operator a
false feeling of confidence then he would otherwise get from an overloading a
crane. Results of this “cowboy” operation is crane damage and the potential loss of load which could lead to death or injury of workers.
Did they learn from this or are they living in denial? What do you think?
