Closing the loop

One of the key issues at the moment is the drive to put cycle-based classification of the components and mechanisms of the crane at the heart of the design and specification process, as is already the case for the crane structure. This means replacing the previous approach where components and mechanisms were assessed on a time-based classification system.

“Looking at the supporting steel structure it is traditional that their classification is based on working cycles or stress cycles effected from the load and the frequencies of loads,” explains Eberhard Becker, a member of various standards committees and chair of the ISO technical committee 96, sub-committee 9 (bridges and gantry cranes), and chief senior engineer in global R&D at Terex Material Handling (Demag brand). He points to earlier standards such as the international ISO 4301-1: 1986(Ed2), the document FEM 1.001:1998, and the German national DIN 15018-1:1984 standard as having using the cycle based approach for decades.

But what is new is that standards covering components and mechanisms like hoists and their associated parts: the hook, the rope and the wheel and all other components of a crane are going to switch over to cycle-based classification.

This methodology has now been laid down in the new version of ISO4301-1:2016 (Ed 3). “By using the same classification principle for the crane as a whole and for the components and mechanism as a whole the combined selection of load spectrum factor and total number of crane working cycles, it is assumed to have a better understanding of the relationship between the mechanism and the supporting crane structure and this will lead to more accurate design for bridge and gantry cranes and their applications.”

In simple terms using a limit state design method for all of the components, not just the steel structure is required to predict the life of the crane more accurately, making the machinery safer.

“It is widely accepted that cycle-based classification of lifting mechanism, their components and parts, is more technically accurate and will lead to a more precise design, custom-tailored to the foreseen use than the historical time-based approach,” says Becker.

In terms of new standards, the ultimate aim is that the updated documents will present the opportunity for cranes to be more accurately classified in groups using the A-Class notation based on load cycles (total number working cycles) and a load spectrum rather than the previous M-Class. This had been the approach taken in the previous standard ISO 4301-1:1986 (Ed2) which looked at total duration of use, i.e the working life based on operating hours, and a load spectrum. “In the past for all mechanisms like hoist, traversing and travelling driving mechanism the utilization as a total duration of use in hours for the design life, combined with a load spectrum, had to be considered and selected. Now we must take additional parameters into account,” says Becker.

These additional parameters are used to calculate the component duties and they include the total number of component working cycles of the component during the design life; the relative frequencies of loads to be handled (load spectrum, state of loading); the average displacements; and the average number of accelerations per movement.

“As Professor Golder explains in his paper “As time goes by: classification of hoists, cranes and lifting equipment”, the cycle-based classification tells the user how often mechanisms like hoists, components or parts, can be used until they are worn. This must not automatically lead to a different design comparing with previous versions,” says Becker. However he does concede that depending on the use changes might be necessary. “It is theoretically possible, that a hoist with a very high hoisting speed will be stressed by more load cycles than estimated in the past. It is also possible that a hoist with a large hook path in the average process condition (average hoisting displacement) will be stressed by less load cycles than estimated in the past.”

The next important step then is to ensure that the standards reflect the changed approach. For Europe the requirements of the Machinery Directive provide a catalyst to getting the European standards up to date. From here then flow the updates on a discussion basis to the international ISO standards and the more local national standards. “It is important trying to unify the standards globally, to have as uniform rules as possible,” says Becker.

In Europe perhaps the most well known standard is EN 13001-1 General principles and requirements, which provides the cycle-based classification principles. Together with the two parts for load actions (EN 13001- 2) and limit states and proof of competence of steel structure (EN 13001-3) a cycle-based steel structure of a crane could be designed.

Parts 13001-3.2, 13001- 3.3 and 13001- 3.5 have been updated, covering limit states and proof of competence of wire ropes in reeving systems; wheel/ rail contacts and forged hooks.

Using these parts together with parts 13001-1 and 13001-2 these standards have now been changed from time to cycle-based classification. Pre-standards for sections covering the limit states and proof of competence of bearings (prEN 13001-3.4:2016) and limit states and proof of competence of machinery – hydraulic cylinders (prEN13001- 3.6:2016) and first drafts for the limit states and proof of competence of shafts are under preparation by working group WG2- Design general- of Technical Committee 147 (CEN/TC147 Crane - Safety).

Another relevant standard which already use cycle-based classification by reference to EN 13001-1 for components of cranes is EN 13135:2013/prA1:2016 Cranes-Safety-Design-Requirements for equipment.

In terms of product standard, EN 15011+A1: 2014 Cranes - Bridge and gantry cranes already takes the cycle-based approach, as does EN 14985:2012 covering slewing jib cranes and EN 12999:2011 +A1:2012/prA2:2014 applicable for loader cranes.

For power-driven winches and hoists Becker is the convenor of the group WG17 of technical committee 147 (TC147) reviewing these current harmonised standards and prestandards have been published (FprEN14492-1:2016 and prEN14492-2:2016).

The industry is currently using the time-based standards for this equipment but Becker hopes the new standards will be used from 2017 onwards.

The European mobile crane standard EN 13001:2014 for mobile cranes is still a time-based classification but during the running review process there will be the move to cycle-based.

For tower cranes a pre-standard prEN14439:2014 has been developed taking the cycle-based approach.

In terms of international standards there has been less progress but the ball has certainly started rolling. “ISO 4301- 1:2016(Ed3) Cranes –Classification- General is just new in the market and for the first time we have a classification of the mechanism on a cycle basis. In parallel to this we have classifications for the product types of cranes in the ISO 4301 family given as parts like -2:2009, -3:1993, -4 1989 and -5:1991 that need to be revised. Sub committees have to decide but I don’t think that there is any doubt that they will make a reference to 4301- 1:2016(Ed3),” says Becker.

The overall guiding design principals for the ISO standards are led by the ISO subcommittee SC 10, Design principles and requirements of Technical Committee 96 (ISO/TC 96 Cranes), which has taken the new approach into account. The next step is for the subcommittees for mobile cranes, tower cranes, jib cranes and bridge and gantry cranes to revise their standards accordingly.

Some of the ISO standards related to design principals for load and load combinations such as ISO/DIS 8686-5: 2016-09 for bridge and gantry cranes have already adopted the cycle-based approach by referring to ISO 4301- 1:2016(Ed3).

There have also been a range of ISO standards published which reflect the cycle based approach by connection to ISO 4301-1. These include:

• ISO 20332-1: 2016(Ed2) Cranes proof of competence of steel structures

• ISO 17440:2014(Ed1) Cranes – General design – Limit states and proof of competence of forged hooks

• ISO 12482:2014(Ed1) Cranes – Condition monitoring Other ISO standards are awaiting of a revision to incorporate cycle based classification, especially for mechanism such as:

• ISO 16625: 2013(Ed1) Selection of wire ropes (under preparation)

• ISO 16881-1:2005(Ed1) Cranes – Design calculation for rail and wheels and associated trolley track supporting Structure (SC9)

• ISO 10972-1:1998(Ed1) Cranes -- Requirements for mechanisms – General

• ISO 10972-2:2009(Ed1) Cranes -- Requirements for mechanisms – Mobile cranes

• ISO 10972-4:2007(Ed1) Cranes -- Requirements for mechanisms – Jib cranes

• ISO 10972-5:2006(Ed1) Cranes -- Requirements for mechanisms – Bridge and gantry cranes

“It is important to achieve an integrated concept of cycle based classification for all crane mechanism in future in Europe and in global standardisation,” Becker points out.

Looking at the European standards, there are also other relevant standards which are yet to be reviewed to incorporate cycle based classification, especially for mechanisms:

• EN 13157:2004 +A1:2009 Cranes-Hand powered crane

• EN 13852-1: 2013 Cranes- Offshore cranes - Part 1: General-purpose offshore cranes

• EN 13852-2: 2004 Cranes- Offshore cranes - Part 2: Floating Cranes

In summary Becker says that the harmonisation of the standards remains a work in progress but that the direction of travel is clear.

“Beside the new classification method for mechanism on a cycle based method, ISO4301-1:2016 (Ed3) provides more detailed classification possibilities by providing additional load spectrums comparing to the previous versions,” he says.

“This will lead to more accurate classification and will give manufacturers the chance to optimize their designs related to the customer need and target application in a better way.