T.A. van Vianen
Analysis of luffing drive concepts.
Report 2004.TL.6879, Transport Engineering and Logistics.
For smaller terminals the use of mobile slewing cranes for handling
general cargo, containers and even bulk becomes increasingly popular.
These cranes consist of a jib, which is the boom of the crane. Basically
there are two types: with or without luffing. A fixed boom has usually a
base that can travel along rails. To avoid frequently traveling, the
slewing crane can be equipped with a luffing boom. The purpose of this
type of cranes is to achieve a horizontal load line during luffing. The
horizontal motion of the load requires minimum luffing power, independent
of the load, and makes the task for the operator less complex and safer
because of the predictable behavior of the load.
In the past, cranes with luffing booms have been built with ingenious
luffing systems based on mechanical gears or clever rope reeving systems.
These level luffing systems are investigated and can be ordered in three
groups concerning the movement of the top sheave; circular-, horizontal-
or other curved path.
The five most common level luffing systems are analyzed according the
basic principles and working advantages. Safety is a very important aspect
because off the operation by an operator and the requirements of the
"Machinery Directives". The level luffing system must provide safety
by itself to prevent tipping over of the entire crane by a failing luffing
drive. This can be achieved by lifting the load at both ends of the
outreach. The Top Liss system of Gottwald does not fulfill this criterion.
This system is driven by a hydraulic cylinder and if this cylinder fails,
the boom drops forward and with not enough counter-mass the crane tips
over. To achieve a safe situation a second hydraulic cylinder must be
positioned. Another option is to use the better level luffing system:
Most of recently built mobile cranes make use of a hydraulic cylinder for
luffing the boom. This is a practical and cost-effective construction.
However in praxis wear of the frequently used hydraulics causes
disturbances and additional maintenance, increasing the costs of the
operation. An electro-mechanical alternative is simpler, cheaper and
probably more reliable. The best alternatives, crank and boom gear, are
investigated and checked to some criteria. The boom gear, boom directly
driven in boom pivot by gear drive, gets the highest value in the
technical evaluation. If the weight of the booms is not balanced
completely the torque becomes complete positive, which is desirable.
The final recommendations are that the Double Boom system satisfies the
requirements of the Machine Directive more than the Top Liss system. To
achieve a safe Top Liss use at least a second cylinder. Make besides the
showed technical- also an economical investigation of the several level
luffing systems. An alternative for the luffing hydraulic cylinder is the
electro-mechanical boom gear configuration. And finally investigate the
balancing method for the double boom to achieve a complete positive shape
of the boom torque.
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