Industrial machinery health assessment is part of condition monitoring activities that is in practice for long time. Both the product and process related industries follow this as a predictive methodology to keep the operational uptime of the equipment at its optimum. International standards such as ISO 20816 govern the vibration limits based on equipment type, its mounting conditions and power rating.
However, there are some specific equipment in an industrial environment that due to their reach, operations and other constraints are difficult to bring under the conventional condition monitoring program. Of many such equipment, EOT cranes used in large steel industry is discussed here.
The EOT cranes are a special variety of mechanical equipment that have a combination of moving sub-systems along with axis drive motors, lifting motors and associated gear-box kinematics; further, there are long travel and cross travel platforms with the entire crane girder moving on rail bogies.
Typical double girder EOT crane
Since the crane operations are both intermittent and continuous by the nature of their usage, the dynamic loads and the vibration response of crane as a whole is subject to many varying factors. The span of the crane, its inherent mass and structural stiffness, loads carried by the crane hook, long and cross travel accelerations and braking events are all the contributing factors that results in vibrations of varying magnitudes and frequencies.
As the traditional condition monitoring programs consider steady and relatively constant operating conditions, this approach yields less reliable vibration responses and their correlation to the root cause/s in the equipment.
To comprehend the data for improved condition monitoring, in one such case for an EOT crane, an in-depth investigation was carried out on a hot steel mill crane. This crane is in service for over 50 years nearly on a 24 x 7 basis and needs to be evaluated for its present state of health, endurance and continued usability.
The challenge began with finding the design data of the crane including the specifications of the motors, gearbox and other critical sub-systems. There were a minimum set of handmade drawings from which basic assumptions of the prime movers and other drive components.
From finding the speeds of the drive motors to establishing the gear ratios to arrive at the final long travel wheel rotation speed, it took a good effort of reverse engineering; once the design parameters were established, the vibration assessment plan from many operationally critical locations were identified and a detailed measurement campaign was initiated.
Extensive vibration data mapping was conducted at all the 4 main motors of the long travel, their connected gearbox and bogie structures. Multiple loads, loading positions combined with long travel braking incidences were considered gathering variety of vibration data from crane.
In addition, structural response to determine natural frequencies of the girders and the supporting ground columns were conducted to check if there are any influencing factors on the operational response of the crane.
Data processing and its interpretation revealed some very interesting vibration response patterns, both in time domain and frequency spectrum. From uneven breaking forces to differential inertial resistance from the bogie units, from un-synchronised starting events of drive motors to gear engagement faults, all showed up in the vibration signatures to assess and quantify the overall performance of the crane.
I began this article with the pretext of vibration based health monitoring as a predictive maintenance tool for industrial machinery; the take-away from the present study revealed that a non-standard equipment and operations like a large EOT crane and similar equipment requires specific studies to determine their dynamic responses for varying operational conditions; based on the outcome, monitoring procedure & derivatives can be established for continued health monitoring, either by traditional methods or using IoT based sensors and tools.
Reciprocating machinery are one other category of equipment that require honed methods and tools to both assess their behaviour and to arrive at vibration severity levels and patterns that reflect their condition and health; I will bring up an article on this in the coming issue.