News
02 November 2011
Flywheel Catchers for conveyor belts at colliery
In January 2010, Hansen Transmissions SA (Pty) LTD, (HTSA), local subsidiary of the Belgium based Hansen Industrial Transmissions nv, received an order from Sandvik Materials Handling Division, the company contracted to supply conveyors to a local colliery for the supply of nineteen gearboxes to the mine.
The largest drives are the 1MW Variable Speed Drive (VSD) motor with a 225 kgm3 flywheel weighing 2.4 tons running at 1500 RPM, supported on oil lubricated SKF plumber blocks and bearings and a gearbox with a 31.5:1 ratio and 1640 AGMA rating. Some of them have brakes to allow for emergency stopping of the belt.
There are four of these drives mounted on two incline conveyors to convey the coal from the bottom of the mine to the surface as well as five additional overland conveyors with power ranges of 450kW to 1MW that have smaller size flywheels installed.
The coal mine’s contract specified flywheels on several of the conveyor belts so that the belts will continue to feed the coal even after power has been cut from the drive.
“This not only stops coal heaping up and spilling over the conveyors, but also stops coal chutes clogging up”, explains Hansen Transmissions SA (PTY) LTD, Willem Sullivan, and adds that the size and scale of the flywheels necessitated the need for catchers.
Sullivan discusses the project: “The size of the flywheels is determined by the inertia value that they will require to drive the belt. There were three sizes specified with inertia values of: 75 kgm3, 125 kgm3 and 225 kgm3. To achieve these inertia values, we had to consider the limitations of the design and adapt our design accordingly. One of the limitations was diameter due to the limit in the height of the drive components. We could go to a maximum of 900mm diameter which meant that to achieve the inertia value we had to modify the thickness of the flywheel. We also considered various different flywheel designs in order to improve the inertia-to-weight-to-cost ratio and a solid steel design was found to be the optimal solution”.
“Once we finalised our design, we came to the scary part. The flywheels weigh from 1200kg to 2400kg. The circumferential speed of the biggest flywheel is 250km/h at normal operating speed but can increase to as much as 305km/h. These factors started to raise concern regarding the safety of the flywheels should a catastrophic failure take place and the flywheels come loose. The largest flywheel takes 90 minutes to stop rotating if it is allowed to run down freely from 1500 RPM. Imagine a 2.4 ton flywheel coming adrift and running into objects at 250km/h for an extended period of time. That is when we decided that we required some form of “catcher”, which we engineered, designed and developed locally, to contain such a catastrophic failure”.
According to Sullivan, HTSA was concerned about one possible failure mode of the flywheel where the shaft will fail on one side of the flywheel causing it to be cracked by the motor. This effect will exert up to 200 tons of force on the catcher so it was decided to install steel blocks with vesconite pads inside the catcher within millimetres of the flywheel to limit the allowable movement of the flywheel.
“Even though all the components of the flywheel are designed for infinite life at peak load conditions, we know that there are still factors beyond our control that can cause a failure of the flywheel for example, accidental damage to the support shaft, bearing failure and drive misalignment. In the event of failure the catcher and pads will quite possibly be destroyed, depending on the failure mode, but the flywheel will be contained”.
The total design period of the flywheel and catcher took approximately three months. The last month was spent on finalising the design of the pads that fit inside the catcher to close up the distance from the flywheel to the guard to reduce the distance from the flywheel to the catcher. “Because this is something new, we have taken all the precautions to ensure that the catchers will do their job. We have done empirical calculations and Fine Element Analysis (FEA) to confirm that it will work and we are satisfied that it be up to the task in a once-off case”, says Sullivan.
Installation of the catchers was completed in November 2010. Sullivan says that it was a fairly complicated mounting procedure consisting of several steps which involved the removal of the flywheel, twice, and the installation of some of the pads when the flywheel was removed and then installation of the rest of the pads when the flywheel was back in position.
“We are very proud of this project because we have managed to come up with a new design that we could put in place to address a major safety concern that our customer wanted addressed. Safety is always the number one priority in all designs and all reasonable steps have to be taken to ensure a safe working environment for all applications”, concludes Sullivan.
About Hansen Industrial Transmissions:
Hansen Industrial Transmissions nv (HIT) is an established global industrial gearbox designer, manufacturer and supplier. The Company provides durable gear drives for a wide range of industrial applications throughout the world and has established a dedicated international service network.
In addition to its principal state-of-the-art manufacturing facility located in Flanders, Belgium, HIT has assembly, sales and service centres in the US, the UK, Brazil, China, South Africa and Australia. Strong in-house R&D operations maintain HIT’s technological leadership.
Since 4 March 2011 HIT is a 100% subsidiary of Sumitomo Heavy Industries Ltd. and HIT employs over 600 people worldwide.
