Super large low speed diesel engine frame processing

Dalian Marine Diesel Engine Co., Ltd. Gao Bo

With the rapid development of manufacturing technology and the growing demand in the market, the operating speed of low-speed diesel engines is gradually approaching the most efficient propulsion speed of the propeller. As a result, their design is shifting towards lower speeds, lighter weight, and more compact structures. To achieve this, the stroke-to-bore ratio of these engines has been continuously increased. For example, the newly developed G model by Mann and the X model by Wärtsilä have a stroke-to-bore ratio of 4.65 and a minimum speed of 66 r/min. This evolution presents new challenges, especially in the machining of large structural components, where frame processing is particularly sensitive to changes in stroke length.

1. Challenges in Rack Machining

(1) The rack is a massive box-type component with dimensions ranging from 15 to 22 meters in length, 5.65 meters in width, and 5 meters in height. Due to its size, significant deformation can occur during flipping (see Figure 1).

Figure 1: Top View of the Rack

(2) The entire frame is constructed by welding multiple steel plates together. This results in numerous weld beads that cross over each other, creating high internal stresses. After machining, this can lead to stress release and deformation. (3) The accuracy requirements for the frame’s guide slides are extremely high, making it one of the most challenging aspects of the machining process. With the increased stroke of the main engine, the frame becomes taller, and the guide plate becomes longer. This creates issues related to reduced rigidity and machining vibrations caused by the overhang of the machine ram. (4) The large openings at the bottom of both ends of the frame reduce its rigidity. During machining, this can cause chatter and tool chipping, leading to poor surface finish and excessive tool wear.

2. Process Solutions for Rack Machining Challenges

To address these difficulties, the following solutions can be implemented: (1) Since the rack tends to deform during the turning process, it is essential to complete the most critical surfaces—such as the guide slide—after the final station. This involves flipping the part 180° to ensure the most important surface is finished in the correct orientation. (2) If possible, after completing the welding of the rack, it should be subjected to stress relief annealing in a heat treatment furnace. In cases where this is not feasible, the workpiece should be roughly machined and left to age for a period, allowing internal stresses to dissipate before final machining. (3) For the machining of the rack guide, the CNC gantry milling machine must meet specific hardware requirements. First, the table’s length and width must be sufficient, with a minimum width of 6 meters. Second, when the machine beam is raised, the height from the work surface should be at least 7 meters (greater than the pad height + workpiece height + milling head length). Finally, the ram travel should ideally reach 3.5 meters, ensuring that the sum of the ram stroke and the milling head length exceeds the machining height of the frame guide by at least 5 meters (see Figure 2).

These strategies help mitigate the challenges associated with machining large, complex parts like the rack, ensuring higher precision and better overall quality. By addressing deformation, internal stress, and vibration issues, manufacturers can improve efficiency and reliability in the production of low-speed diesel engines.

For more detailed information, please download the full article or refer to Metalworking (Cold Processing), Issue 23, 2013:

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