What goes up does, occasionally, come down

With container cranes becoming ever-bigger in order to accommodate the ever-wider container vessels, the holding and emergency rope braking systems must be ever-increasingly efficient. Guy A. Massey looks at what is happening within the industry with particularly reference to proving brake capabilities.

Rather unsurprisingly, container terminal operators are very reluctant to even admit that there has been a problem with the braking system on a container crane, far less to discuss it openly. Although the efficiency and reliability of brakes have greatly improved over the years, mainly as the result of the introduction of thruster (or calliper) disc brakes, there are still a number of occasions annually where a brake system fails and a container hits the ground with a sudden and undue force.

Without stating the obvious, most everyday, routine, braking (and short-term holding for that matter) on container cranes is done by using the drive motors. That is to say that the actual brakes are used almost solely for long term holding and emergencies; and it’s here that the first problem arises because what may well be quite adequate for holding purposes may be far from adequate for use in an emergency situation where a boom or hoist motor has failed and the equipment is free-falling out of control.

Bill Gentles from Drives International suggests “It must be clearly recognised that the main hoist service brakes must satisfy three important criteria:

they must be sufficiently robust to operate with 100% reliability

they must function as a holding brake [while also]

being equally capable of an emergency stop”.

Ian Bourhill of Hillmar Industries Ltd says “The primary criteria for brake selection is whether the application is absolutely static in nature or whether dynamic braking is required. This can be difficult at times as specifications can call for static holding duties, when the actual expectation is for the brake system to stop the load under emergency conditions”.

More and more of the container crane manufacturers are transferring the responsibility of brake selection from their in-house engineers to specialist brake suppliers such as Hillmar, Twiflex or Johnson Elevanja, although with the customer making the final choice of brake supplier. Colin MacCormack, sales manager for Johnson Elevanja says “.....most crane manufacturers leave the selection [of brakes] to specialists, such as ourselves and we, therefore, take the responsibility of the selection”.

Hillmar’s Ian Bourhill goes on to say “If the application is strictly static, then a safety factor is usually applied to the required torque to determine brake size, but if the application requires dynamic braking, then the following criteria must be considered:

  • rotational speed of hoist system at commencement of braking
  • potential and actual time delays due to the control system and brake calliper setting time before braking begins
  • total kinetic and potential load drop energy that must be dissipated to bring the load to a complete stop
  • the required stopping distance of the load under full speed/full load conditions. (It is of little use to have the load slowing down but still travelling at high speed when it hits the ground)
  • the maximum load capacity of hoist components such as gears, bearings sheaves and rope. (It is important to note that overcapacity braking could overstress these components producing failure and unsafe conditions)”.

Colin MacCormack emphasises “..... that it’s essential to consider the characteristics of each type of brake. The hoist motion of a container crane, for instance, requires a very quick acting brake to apply and release, therefore the actuator [the ‘heart’ of the brake along with the lining] used is most important”.

Nearly all modern container cranes now use a ‘fail-safe’ braking system whereby the brakes are of the ‘spring on, power off’ type. This means that power, usually a combination of electric and hydraulic, is used to release the brakes in a similar manner to a road-going truck, where the truck’s compressed air system (the power) is employed to release the brakes. However, this is completely opposite to a automobile where the brakes are spring off and powered, via the foot brake pedal, on.

Twiflex Ltd, maintain that ‘for any safety critical application, such as container cranes, the spring-applied, fail-safe, disc braking system is selected’.

The problem is that as cranes become bigger, not only taller but wider so as to cater for the predicted 18 wide container vessels, and have to work faster for longer periods of time while handling the ever-increasing box weights, the demands on the braking systems increase.

Traditionally, container crane brakes have been of the drum type whereby the brakes work, either internally or externally, on a drum that runs parallel to the output shaft, but in order to handle the increased demand, calliper disc brakes, where the brakes work by pressing on a disc running at right angles to the output shaft, are becoming the favoured option and have been for the last 10 - 15 years. However, the result is that the discs have become larger and heavier (the new Evergreen cranes in Los Angeles now have 1 metre diameter discs) and, once on the roll, these large diameter discs have enormous inertia, which in themselves require increased braking power.

Adam Zavery for Twiflex says “As cranes get bigger, and therefore line loads increase, it’s better to install four or six smaller [brake] units per disc, rather than one or two large units”.

Larry Wright, of McKay International Engineers, a mechanical engineering consulting firm in California, with special emphasis on container handling equipment, says “For the new high speed cranes, the braking requirements are beyond the available drum brakes. Certainly for the main hoist brakes on the current generation of dockside container cranes, the braking requirements can only be met with disc brakes”.

Hillmar’s Ian Bourhill reiterates this point by saying “At present, thruster disc brakes and emergency calliper brakes provide the best performance and safety at the lowest cost”. Colin MacCormack agrees “..... that the only type of brake that can be used on container cranes is a disc brake, mainly because of the inertia of the disc in comparison to the drum, but also because of the use of sintered linings that can cater for high emergency stopping speeds”.

The principle of any thruster brake is that the thruster comprises a hydraulic centrifugal pump, driven by a electric motor, which gives a ‘thrust’ on a piston to push apart the braking springs, thus releasing the brakes. As the brakes are ‘powered off’, any failure by a drive motor, such as a loss of electricity or over-speeding, or the operator hitting the ‘panic button’, will cause the springs to operate the brakes. The great advantages of thruster brakes is their reliability and ease of maintenance.

The basic design of the thruster disc brake has changed little over the years although there have been continuos improvements and refinements. One such refinement has been the introduction of the self-adjusting brake whereby the friction pads are kept at the optimum distance from the disc through the use of levers, a pawl and ratchet wheel. As the friction pad wears, a pawl, attached to an operating lever, engages the ratchet wheel, (which in turn is attached to the adjuster), and pulls it round thus adjusting the brakes by taking up any surplus movement or ‘play’.

Possible the greatest danger facing any braking system is ‘brake-fade’ which occurs when the brakes are applied for so long that the friction pads heats up to such a degree that the braking coefficient is lost and ‘brake-fade’ occurs. Brake-fad is a major problem for drivers of heavy-duty trucks in mountainous areas, thence the development of exhaust brakes and engine and gearbox retarders.

If a container crane drive motor fails, causing an emergency situation where the main boom or a container goes into free-fall, brake-fade could also occur. Although less likely with new brakes, what is happening is that over the crane’s operational years, oil, grease, dust and other contaminants become embedded in the friction pads reducing their coefficient, so when an emergency situation arises, and the brakes are required to perform to 100% efficiency, they fail.

The ban on asbestos lead to the introduction of non-asbestos linings, but to date, no non-asbestos drum brake lining has been introduced that can satisfactorily replace the asbestos lining and it’s generally accepted that the organic linings presently available have a 20% reduced capacity against previous asbestos brake linings.

Although non-asbestos linings have a lower friction coefficiency and suffer from a lower temperature limit, before brake-fade, than asbestos linings, Zavery says “.....experience has shown that by using correct bedding in methods and allowing an adequate swept area, it is possible to maintain both criteria”.

However non-asbestos materials, particularly metallic/ceramic friction materials are constantly improving and the introduction of the sintered linings, in the early 1980s, has greatly improved the situation. Ian Bourhill suggests that “Although more costly, [sintered linings] are capable of producing high friction coefficients at temperatures that would have destroyed the best of the old asbestos linings”.

According to Johnson Elevanja’s Colin MacCormack “It is quite common for the peripheral speed of a container hoist motion to reach 50 metres per second (180 kph) so ordinary organic linings would only see one emergency stop before the coefficient would disappear. However the introduction of sintered linings has overcome this problem and are now standard equipment”. He goes on to explain that sintered linings will endure very high temperatures, approximately 800°C, before the coefficient fades and the coefficient will be restored once the lining temperature has returned to normal. He concludes “The disadvantage is that it is essential to use a compatible steel disc and that the price is huge in relation to ordinary organic pads”. But can a price be put on safety? If an accident occured whereby a terminal worker was killed as the result of faulty or badly maintained brakes, the cost of the sintered linings would pale into insignificance compared to the resulting compensation claim.

As the majority of braking in container crane operations is done with the drive motors, and the wear on the static brakes is minimal, it is difficult to established conclusively that the brakes will work as required in an emergency.

Reputable specialist brake suppliers test their brakes and braking systems during the development stages, so one has to assume that the brakes are 100 per cent functional when brand new. Johnson-Elevanja state “Our brakes are tested in a million plus operations test. If a fault is found in the brake material, design or response specification; then the brake is altered and tests start again”. Hillmar conducts testing of its brakes and friction materials using a dynamic flywheel dynamometer.

But new brakes are not the problem; it is the brakes that are operational that cause the problems. As mentioned earlier, brake-fade occurs when contaminants become embedded in the friction pads but loss of braking efficiency can also occur when the pads have been heated to abnormal temperatures, such as might occur in an emergency situation.

Routine brake maintenance, such as the cleaning or replacement of friction pads will undoubtedly help but increasingly proof of testing is being demanded.

McKay International Engineers have been at the forefront of brake testing and Larry Wright says “We are very pleased with the testing now being performed by some of the brake manufacturers to justify the dynamic braking torques and thermal capability of their brakes and lining material.

He goes on to say “Based on dynamometer testing of the brakes, the brake manufacturers have a rational means of justifying the brake selection and giving assurances that the load will be stopped if the brake is properly maintained”. However some manufacturers are finding that the specified emergency stop is causing ‘glazing’ of the brake linings requiring the pads to be replaced, but the problem is that a crane driver may not report an emergency stop and will continue to work with less than efficient braking”.

He continues “If the brake manufacturers tests for these emergency stop conditions and can show no damage to the lining or disc, we can feel reasonable sure we have a good braking system”.

Regarding in-situ tests, Adam Zavery of Twiflex says “It’s possible to carry out brake testing once a crane is in-situ by releasing the brake in the ‘off’ position to check on any leaks and release pressure or by applying the brakes under load and by measuring the current drawn from the drive motor once the load starts to slip”.

Hillmar are increasing field testing of its brakes and says “Typically the tests are carried out at crane commissioning with full test load at full rated speed under E-stop (power-off) conditions. Tests are also done with an empty spreader at full speed for this condition which is usually twice the full-load speed. Tests results provided by the crane PLC data logging system are then compared to our own calculated values”.

In conclusion then, it is acceptable to assume that any braking system will be up to the job providing the manufacturers can justify their selection and that the brake are fully and regularly maintained but to be on the safe side remember the old adage ‘if you can’t stop it don’t start it’.