38 n MECHANICAL June 2023 www.drivesncontrols.com Quarry skip upgrade reduces the risk of catastrophic failures Newry-based TH Moore (Contracts) supplies quarry materials, aggregates and bituminous products. It was having problems with an outdated skip drive that ran on a set of inclined rails to move products at a quarry that it operates. The motor-driven skip moves on the rails to sit below a mixer, or above one of three bin positions – one used for loading into a lorry, and the other two for storage bins. The system was used intermittently and had no set running hours. It was plagued by electrical problems, and breakdowns that led to safety concerns. Because the system was being used for only two to three months each year, the faults were hard to identify and rectify. One of the main problems was excessive brake wear on the skip motor. If an operator selected the first bin and pressed start, the skip would first drive down to the mixer. When it triggered a limit switch, the brake was slammed on and the motor disengaged, resulting in a rapid halt. Once the deposit had been completed, the system re-engaged the motor and accelerated rapidly to the bin position. TH Moore brought in the industrial drive specialist Technidrive to design and commission a new skip drive and control system. Technidrive worked with the motor and drives manufacturer WEG, which provided a 22kW four-pole cast-iron brake motor with a 1024-pulse encoder for the project. WEG also recommended using its CFW700 VSD which has a built-in high-torque brake control. This has been combined with a bespoke program running on the drive’s on-board soft-PLC to almost eliminate brake wear and to reduce motor maintenance significantly. Instead of the previous array of pushbuttons and controls on a busy panel, the new system uses a 10-inch Hitachi touchscreen HMI. An automatic slowdown function now allows the VSD to stop the motor before the brake is applied. The brake is used only as a holding mechanism, and the system is no longer driving against an engaged brake. In addition, virtual positions have been programmed for each location, allowing slow ramp-ups using the VSD. Before the skip arrives at its final position, the system gives it time to slow down. This allows the skip to stop from a slower speed in the same position every time, further reducing mechanical wear-and-tear. Another issue with the old system was that it used an unreliable, obsolete PLC. If a digital input or card developed a fault, replacements were not readily available. The original system used a series of limit switches linked to a PLC acting as a safety interlock. The new PLC system reads the encoder and combines this with a live skip position function, to tell the operator exactly where the skip is at any point. This results in easier and safer control between the four positions, which is shown on the touchscreen display. The system can also be monitored from a tablet or mobile phone, allowing any problems to be diagnosed remotely and rapidly. If the encoder fails, the operator has full manual control. For example, if the encoder slips on the motor shaft while the skip is moving upwards, the encoder will not recognise this. A belt-and-braces approach prevents the skip from driving over the end. To increase safety further, protection was built in to prevent the skip motor from freewheeling. Safety was the primary concern when developing the new control system, giving TH Moore as much control and reliability as possible. This new system has reduced the number of failure points from eight to one. “The project was completed in April 2021 and ever since, our plant has been operating at full efficiency,” reports Mark Poots, quarry manager at TH Moore’s parent company, GibsonBros. “We used to be marred by electrical problems, but Technidrive provided a cost-effective way of upgrading the obsolete control system to an industry-leading alternative. This has not only reduced downtime but, if a component failure does occur, replacements are more readily available. No one wants to see a skip rocket across the quarry, and our new system is the safest way of operating this system.” n If a quarry skip carrying more than a tonne of material was to hurtle off its rails at high speed, the consequences could be disastrous. But this was the risk faced by an Ulster quarry operator before it installed a new drive system with an enhanced brake control. TH Moore’s skip system now operates under VSD control, improving safety, reducing wear on the mechanical components, and making any repairs easier.
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