Changzhou Cranes Mechanical Electrical Engineering Co., Ltd.
Mobile no. 0086-18001505166
Add: No.8-20 Shunyuan road, Xinchen industrial park, Xuejia town, Xinbei district, Changzhou city, Jiangsu province, P.R.China.
Grab crane modernization
After the modernization of this grab slewing dockside crane, it makes the port now disposing of an up-to-date crane with optimized grab control, and benefits from reduced costs thanks to minimized maintenance.
The grab slewing crane manufactured in 1990, is primarily used for the loading of scrap metal. In 2005 it was taken out of operation due to technical defects. Our retrofit partners in the field of crane modernization, was contracted to develop and realize a completely new drive concept. The solution was based on VFX variable speed drives and a unique grab control system. The results are optimized cycle times and minimized mechanical stress.
As the general contractor, they were responsible for engineering, delivery of the motors, construction of the cabinet equipment, commissioning and assembly. The crane operators also participated during commissioning, an ideal opportunity to introduce them to the new technology under realistic conditions, who can now solve minor malfunctions themselves. Since the cabinet equipment had been tested before delivery, only a week was required for installation and verification of all signals. The commissioning of all drives, including a load test and the acceptance performed by an authorized inspector, took three days. This was possible due to the fact that the grab control is a standard system, which can be utilized for all four-rope grab cranes. This is also an important prerequisite for reliable solutions and easy servicing.
Included in the delivery was the complete cabinet equipment for the engine room, VFX variable speed drives and grab control system, brake resistors, standard asynchronous motors as a replacement for the old slip-ring motors, disassembly respectively assembly, wiring of the new motors, commissioning and finally acceptance by an authorized inspector. Existing components incorporated in the new concept were crane disconnection switches, long travel motors, slip-ring system, illumination and climate control in the engine room, the cabin and the complete mechanical system including the brakes, the magnet system and the wiring. The four-rope grab is controlled by the electronic grab control. It coordinates the two drives controlling the hold and close motors. For the crane operator, the crane works as if there were only a single hoisting device with the additional functions of "open" and "close". Setting the nominal grab positions only has to be performed once after a change of a rope or grab unit.
Only four signals are required for the grab operation: hoist, lower, open and close. Joystick with the corresponding contacts delivers the signals. The individual functions can be combined in any order. An integrated position control ensures that the two hoisting devices move correctly towards each other at the correct rate at all times. To minimize cycle times and reduce stress on ropes, the grab can be closed or opened while being hoisted and lowered.
Furthermore, a slightly opened grab can be hoisted and lowered. If the order for hoisting is given during closing, the grab is first closed and is then immediately hoisted. The hoisting process can also be started at a predetermined angle before the grab has been fully closed. The system minimizes the jerking motion that occurs during tightening, thereby removing the strain from the mechanical parts.
In order to keep the grab securely closed, the torque of the close motor must always be slightly higher than the torque of the hold motor. The synchronization ensures the correct division of the load torque when the grab is hoisted or lowered in its closed condition. This makes it possible to select smaller motors and variable speed drives. When picking up material, the grab digs itself in utilizing its own weight. The hold ropes should not raise it while doing this, however should at the same time be sufficiently taut to avoid any slack rope on the drum. The grab control puts the hold motion into a state of suspension during closing. The ropes are held with a defined torque and are slowly rolled up.
The application was configured using combined crane control system and operator panel PP41 that is built into the crane cabinet. The operator can adjust the grab settings as well as monitor control signals and current values. Parameters are set during commissioning of the system and a password is required to prevent accidental or unauthorized modification. An auto tune function senses the system and automatically sets the required values. A PC is not required and the system is easy to install and use.
The project was concluded in a very short time against a reasonable cost, and the modernization will be cost-efficient in the long run. The customer now benefits from an up-to-date crane, which will be serviceable for another 15-20 years. Maintenance will be reduced considerably thanks to the minimizing of mechanical stress on components. Above all, the replacement of brake pads is a thing of the past as the old operating brakes are now only used as holding brakes and when conducting emergency stop tests.