When installing the crosshead, the positions of the force-bearing surfaces on the front and rear crossheads must not be reversed
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In clause 4.6.5 of the \"SH∕T 3544 Specifications for the Construction and Acceptance of Opposed Reciprocating Compression Units in the Petrochemical Industry\", it is stated that when installing the crossheads, the positions of the force-bearing surfaces on the upper and lower sides of the crossheads must not be changed. Does this direction refer to the up-and-down orientation of the crossheads? What are the consequences if it is installed upside down?
The upper and lower parts of the crosshead refer to the slip surfaces covered with babbitt; these are the contact surfaces where the crosshead moves. If they are installed in the wrong order, it will lead to poor contact conditions, overheating during movement, and thus a reduced lifespan for the slip surfaces as a whole. Without monitoring of the piston rod’s downward movement, severe equipment failures can occur due to excessive wear.
The crosshead sliders cannot be installed in reverse up or down, mainly due to the issue of stress on the upper and lower sliders. In some cases, it is the upper sliding shoe that bears the force, while in other cases it is the lower sliding shoe. So it must not be installed backwards.
There are no special requirements for the installation of the crosshead slider, but requirements do apply to the crosshead itself. When the upper slider is under stress, the center line of the crosshead will shift downward relative to the center line of the slide; the amount of this downward shift is generally equal to the gap between the crosshead slider and the slide； The sliding shoe is under force, with the centerline of the crosshead coinciding with the centerline of the slide.
The direction of the forces acting on the crosshead, either upward or downward, is determined by the rotation pattern of the compressor. When the crankshaft rotates downward, the force-bearing surface on the crosshead does work; when the crankshaft rotates upward, it is the force-bearing surface on the crosshead that does the work. Shims are placed between the crosshead and the slider to adjust the center height of the crosshead as well as the clearance between it. If the top and bottom parts are reversed, it will change the center height of the crosshead, causing the crosshead and the slide to be out of alignment.
The direction has changed, the center has shifted – what consequences should there be?
If the upper and lower slides are swapped, due to the different levels of wear that occur during their operation, it is necessary to measure the gap in the slide tracks; thereafter, grinding or the use of adjustment shims is required to meet the technical specifications for maintenance.