Upon realizing that standard gantry cranes available on the market did not meet their specific conditions and requirements, Max Bögl Transport und Geräte GmbH & Co. KG decided to develop and build their own custom model. With the collaboration of Grunau & Schröder Maschinentechnik (GSM), a joint project was created and led to the development of the Travellift, a mobile gantry crane with a capacity of 65 tons. 
 

GSM, a division of Bauer Maschinen GmbH founded in 1993, is a service company specialized in mechanical engineering. Max Bögl Transport & Geräte GmbH & Co. KG, a division of Max Bögl group, is one of the five largest construction companies in Germany. In this collaboration, GSM was responsible for the entire design and engineering of the mobile gantry crane prototype.

In the early stages of the project, a feasibility study and pre-dimensioning of the crane was carried out. During pre-dimensioning, the specifications of the mechanical, electronic, and hydraulics systems were established. Based on this design, compliance to all applicable guidelines, standards and safety regulations were verified. In a second step, the machine safety and structural analysis of the bearing structure was done in close cooperation with external companies.

The complexity of the design and project management challenges were present throughout all phases of the project including the elaboration of the overall concept, procurement coordination, communication and coordination with external companies, and schedule control. During later project phases, all details of the hoisting gear, the hydraulic power unit and the chassis frame’s drive systems and controls were implemented. To do so, design software solutions like SolidWorks 2015, Automation Studio^TM and AUTOCAD were used.

Simulation of the winch control

During project planning of the winch control, safe holding of the winch was simulated under real operating conditions. Automation Studio^TM, a simulation software developed by Famic Technologies, was used to simulate the hydraulics and electrical systems.

In particular, the behaviour of the brake-lowering valves that have been flanged on the winches’ hydraulic motors was analyzed under the following operational states:

• Stop of the winches by withdrawing the control lever
• Stop by control outage = Emergency Stop Category 1, no electric energy, diesel engine still running
• Stop by outage of diesel engine = Emergency Stop Category 0

The most critical operation state “the Emergency Stop Category 0” will be explained in detail in the following section.

Description of the simulation modules

Figure 1 depicts the most relevant components for the simulation. All displayed components are simulated during the lifting process.

The drive unit with clutch transfers the power via a transfer gear box to the lower main pump and the upper charge pump. The fluid (red line with arrows indicating flow direction) flows through the proportional valve to the brake-lowering valve module and finally to the hydraulic motor. Inside the brake-lowering module, the brake piston is driven by the load pressure and the multiple disk brake is opened via an integrated brake-discharge unit (2/2 directional valve and pressure reduction valve).

The fluid in the hydraulic motor’s return line (blue line with arrows indicating flow direction) flows back via the brake piston towards the proportional valve. Thus, the load in the illustrated control mode remains hydraulically locked by the brake piston during regular operation.

Figure 1: Simulation of the winch control

In Figure 2, the stop of the diesel engine and the resulting blockage of the hydraulic pumps are simulated by disconnecting the clutch. In this case, the following situations can be observed:

• Because of the switched activated proportional valve, the load pressure at the brake-lowering valve remains high and is only slowly reduced through leakage (see measuring point MI-6).
• Due to the high pressure, the integrated brake-discharge unit is further charged with pressure and the multiple disk brake stays open. The load is further retained by the brake piston and held on the hydraulic oil column (see measuring point MI-8).
• However, by holding the load on the hydraulic oil column, a potential movement cannot be stopped completely. Due to a leakage inside the hydraulic motor, the winch is rotating at a speed that can’t be detected visually.
• Without the continuous feeding of hydraulic oil through the feeding pump, the hydraulic motor cannot absorb oil during the creeping rotation and starts to cavitate. This phenomenon can be observed at measuring point MI-9 with the constantly increasing negative pressure and drive speed at the driving wheel. When reaching less than one turn during cavitation, the motor cannot keep holding the load and slips.
• Furthermore, the integrated brake-discharge unit needs another 1-3 seconds to activate the multiple disk brakes with the proportional valve in neutral position (ABT connection). In the operational state, “engine stop – pilot valve still electrically operated” a time response of 6 seconds could be measured.

Figure 2: Engine stop without additional brake-discharge unit

Figure 3 shows a realistic comparison between the simulation with Automation Studio^TM and the real measurements at the plant:

• Blue line: load pressure to lift the precast element
• Turquoise line: absorption of the load by brake pistons with a tendency to slip (cavitation)
• Black line: delayed relief of control pressure on multiple disk brakes
• Green line: under-supply of feeding for the winch motors

Figure 3: Real-time measurement: Engine stop without additional brake-discharge unit

In order to eliminate critical situations during simulation, an additional brake discharge valve (3/2 directional valve) has been added between the integrated brake discharge valve, the brake-lowering valve unit, and the multiple disk brake. This integration can be seen in figure 4.

While disconnecting the engine clutch, the power declines immediately at the valve’s solenoid and the additional brake discharge valve opens the multiple disk brake directly to the tank. Consequently, the lifted load stands still immediately. A reaction time of 50 milliseconds is measured. The simulation illustrates that in spite of the feeding pump’s missing fluid volume (measuring point MI-11) there is no negative pressure at measuring point MI-9. Therefore, the load doesn’t slip and is held safely. The pressure held at measuring point MI-8 between the motor and brake piston resulting from the ascending lifted load is slowly reduced via the leakages before taking effect on the multiple disk brake. 

Figure 4: Engine stop with additional brake-discharge unit

Figure 5 shows again a realistic comparison with the real-time measurement. In the operational case “stop by outage of diesel engine”, the multiple disk brakes (black line) are immediately engaged while the load is safely held mechanically and no longer hydraulically held by the piston of the brake-lowering valve. The load and brake pressure is slowly reduced via the leakage. Hence, a safe operation of the winch can be ensured.

With Automation Studio^TM, the operational capability of the original design was verified through simulation. Thus, designing a defective set-off without an additional braking valve could be anticipated. Possible injuries to people and property damage could also be prevented.

Figure 5: Real-time measurement: Engine stop with additional brake-discharge unit