Finding the cause of broken piston rods in reciprocating machines
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The new piston rod broke after being used for just over a month; the break occurred at the thread where the piston rod connects to the crosshead, and the picture shows that broken area. The vertical vibration of the piston rod is excessive; it is difficult to adjust due to foundation settlement, pipeline stress, and tight deadlines. This is probably the main reason; at what level of deviation does the piston rod’s movement exceed before it breaks?
Installation issues: the lock was not secured properly, leading to uneven stress distribution in certain areas, which in turn caused repeated impacts. As for \"piston rod play,\" it cannot be absolute; it must remain within the clearance range specified by the installation requirements.
The location of the fracture at the thread where the piston rod is connected to the crosshead explains the cause of the breakage: Setting aside factors such as the material of the piston rod, the manufacturing process, and any installation errors, the direct cause of the fracture in the piston rod of a reciprocating compressor is torsional stress. The main cause of the additional torsional load is excessive tightness at the contact point between the piston rod and the nut. This tight contact increases the compressive force exerted on the piston, resulting in deformation of both the nut and the piston rod as well as a loosening of the nut’s position. The additional torsional load generated in this situation can easily lead to the fracture of the piston rod.
This post was last edited by Shutong on 2020-7-14 09:29. Regarding the allowable range for the play of the piston rod, the original standard HGJ206 of the Ministry of Chemical Industry, the Code for Installation and Acceptance of Chemical Machinery issued in 1992, specifies that the total play of the piston rod’s axis shall not exceed the tolerance value specified in Grade 9 of the standards on ‘Tolerances for Shape and Position’ (with a tolerance value of approximately 0.07 mm). Assuming a deviation of 0–0.07 mm occurs during the process, I think this difference is acceptable for radial play; however, for angular error, it seems a bit too large – beyond this value, fracture is inevitable. I’m providing this old standard for reference; although it has been discontinued, it still holds great value as a guide for the installation of reciprocating compressors
Based on my experience in assessing and dealing with issues related to piston rod vibration: if the axial level of the cylinder is consistent with that of the slide rail, then when there is vibration in the piston rod, it is usually due to a misalignment between the center of the cylinder and that of the slide rail. The solution in this case is to grind the protruding and recessed surfaces on the flanges connecting the slide rail to the cylinder in order to adjust the alignment. If, on the other hand, there is a difference in the axial levels of the cylinder and the slide rail, and vibration occurs in the piston rod, it is generally due to an angular misalignment between them. The solution here is to grind the flanges connecting the slide rail to the cylinder in order to correct this angular misalignment.
This post was last edited by Shutong on 2020-7-14 08:53. A 4M-type compressor at a nitrogen fertilizer factory in Qipanjing, Ordos, Inner Mongolia, has experienced frequent breaks in the connection between the piston rod and the crosshead; within no more than three months, either the piston rod or the crosshead would break. Faced with this situation, the factory owner was at a loss as to what to do. In 2010, at the request of a friend, I assisted with the diagnosis. There were no suitable testing tools available on site; instead, the difference in the movement of the piston rod was measured in order to determine the approximate deviation of the cylinder block. The piston stroke was 320 mm, and the dial indicator indicated a deviation of approximately 0.10 mm. The piston end was lower. According to the specifications, this deviation far exceeds the normal limit of 0.07 mm. If nothing is done to address this issue, it is quite likely that problems will arise with the crosshead and piston rod of the machine.
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