Front Cover May - WorldCargo News Online

CARGO HANDLING
WorldCargo
news
Putting a stop to braking problems
As previously reported (World Cargo News,
March 2003, p1), a driver was killed in
Surabaya in February when his crane was
blown down the quay and collapsed. This
is not an isolated event; while compiling
this article anecdotal evidence of over 12
cases of cranes set in motion in ”wind
events” in the past year came to light.
The term “microburst” as mentioned
in the Surabaya case has become something
of a catch-all and, indeed, one US
consultant, Tom Simmons, doubts if this
incident was a microburst event at all
given the reported wind speeds (see his
letter, p2). Are preventable accidents caused
by wind gusts that are common at ports
being blamed on unstoppable forces?
Microbursts are localised outbursts of
winds created by downdraft pressures
rather than normal frontal weather patterns.
Sudden winds caused by microbursts
can reach over 200 kph (55.5 m/
sec), severe enough to set parked and
clamped cranes in motion. However, not
all crane runaways are caused by such severe
winds and questions must be asked
about brake selection and brake failure.
In addition, questions arise about operating
procedures and practices and
whether they are observed. But they have
to be dealt with by terminal operators,
their insurance companies and the victims
of accidents and their families.
The incident at Surabaya again raises
the question of the type of brake that is
most appropriate for gantry travel. After
acquiring Sime the Stromag group now
covers electromagnetic motor-mounted
brakes and thruster discs. Stromag’s John
Brierley says the choice between the two
“is personal preference...some people like
the robust construction of the disc brake
but this requires separate protection. Others
prefer motor-mounted brakes which
are totally protected from the environment...”
Both brakes are suitable for long
travel but, adds Brierley, “what is required
is sensible sizing of the brakes, both torque
and energy wise,” taking into account “all
conditions in the E-stop with crane travelling
at full speed, with maximum load
and maximum wind and any gradient.”
Over the years a vocal critic of electromagnetic
motor-mounted brakes has
been Ican of Japan. Ican recommends a
long travel system of dynamic wheel
brakes on the non-driving wheels and
thruster discs on the driving wheel motor
shaft. Technical director Rob Harrison
says while all braking systems meet the
necessary specification at the time they
are fitted, one of the key issues is maintenance
and thruster disc brakes are much
more easily inspected and adjusted.
Crane brakes are back in the spotlight
after “runaway” incidents in Vancouver,
BC, and Surabaya but there is little consensus
on how to prevent them
possible in an emergency situation.
The wear properties of any brake
should be properly considered when the
braking system is specified. Adequatelysized
brakes, says Brierley, can compensate
for some maladjustment and lack of
maintenance. Furthermore it is becoming
more common to request a “proving
switch for monitoring brake operation
and an additional switch for wear detection.
This data can be fed through the
crane PLC for maintenance purposes.”
Fitting thruster discs and dynamic
wheel brakes, says Harrison, is a “belts and
braces” approach that offers maximum
security, but is expensive. He argues that
the extra price of thruster disc brakes
compared with electromagnetic, motormounted
brakes is tiny in relation to the
crane, but crane OEMs are under pressure
to minimise component costs, so
price is often a key factor.
Conservative approach
This is not always the case and some
OEMs recommend more expensive solutions.
Pat O’Leary from Liebherr, for
example, says the company stopped using
motor-mounted brakes some 20 years
ago and prefers to install an “independent
brake on the gearbox shaft which is
conservatively rated. This practice ensures
that full braking is available even when a
motor is removed.” For storm brakes
Liebherr’s preferred option is an independent
rail head brake that O’Leary says
“acts as an efficient back up for the primary
braking system.”
In many cases, however, the choice of
braking system is not the crane manufac-
Maintaining the force
An electromagnetic, motor-mounted
brake requires the air gap between the
coil body and armature plate to be set to
the correct size. The gap is checked either
by removing the cover or inserting a
feeler gauge after removing inspection
bolts. Adjusting the air gap requires removing
the cover and turning the adjusting
bolts in equal amounts to maintain a
uniform gap. With a thruster brake the
air gap can be inspected, adjusted and
measured at one point and, therefore, it is
easier to maintain correctly over time.
Another problem is monitoring wear
of the friction lining group. On an
electromagnetic disc brake, wear is more
critical to performance as, unlike thruster
brakes, there is no automatic wear compensation.
If the brakes are not checked
properly, they can deteriorate to a level
where maximum performance is not
Dynamic wheel brake from Ican (IWB series)
May 2003 37