Hydromobile was recently commissioned by Galt Equipment to design and supply a hydraulic system for a cherry picker. The Galt Cherry Picker was developed specifically for maintenance and recovery work on overhead railway power lines.
The most important requirement was that the mechanical design should eliminate the need for an electronic stability control system. The high frequency vibrations inherent in rail travel can cause failures in hardware and Galt wanted to avoid the use of such a system because reliability was a priority. Technical support was also an issue. Units would be scattered all over the country and technical staff with PLC knowledge are generally found only in the main centres. Galt did not want to send technicians from Gauteng into outlying areas to carry out support work.
This was achieved by 'pulling' the main beam up over a constant radius using a chain. This meant that the pressure only needed to be monitored in the pull cylinders in order to decide if the unit was stable or not. To do this a very simple large displacement pressure switch was developed which triggered a proximity switch when the load limit was reached.
Other cherry pickers on the market use a cylinder to push the beam up. Since the moment the arm changes as the beam goes up and down, both the pressure and the angle have to be monitored. Calculations then have to be performed using a PLC to decide if the unit is stable or not.
The unit was also designed to double up as a 3 ton.m crane. This feature is especially useful for planting poles along railways. Normal truck-mounted cranes can only be used in accessible areas. Otherwise, for instance in mountainous areas, the job has to be done with manpower and ropes.
To reduce the overall mass of the unit, a moving counterweight was used. If it was moved to work on the left side of the truck the counter weight moved to the opposite side and vice versa. This significantly reduced the counterweight mass required compared with fixed cranes.
Hydromobile provided all the hydraulic, electronic and proportional controls. A 100% manual system was also provided in case the electronic system failed. Special proportional control valves were imported from the Italian company Aron because they were able to handle the control complexity required by the application while also having the simplicity required by Galt.
The system had electronic joystick control and was powered by an independent electronic box through which the ramping, gains and dither on the valves were adjusted. The power supply and intelligence were provided in a modular form so that in the event of the electronic control giving problems the unit could be replaced without having to use expensive labour.
The unit was deployed in the Middelburg area, and feedback from Spoornet has been very positive. Galt expects to manufacture 50 more units for Spoornet alone. There are also worldwide marketing possibilities in any country where electric trains are used.
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