Why are the thicknesses of the shims on the crosshead slider of reciprocating machines different at the top and bottom?

Put it this way: there is a gap between the crosshead and the slide. If they were two circles, the crosshead would be in contact with the stressed parts of the slide, and the centerline of the crosshead would not align with that of the slide. As a result, the thickness of the shims on either side of the crosshead would differ in order to ensure that the centerlines are aligned.

I understand a bit now, but there’s still one question: why does one end of the horizontally opposed pair of crossheads have more shims on the top side and fewer on the bottom, while the other end has more shims on the bottom side and fewer on the top?:dizzy:

When the compressor is in operation, the slide rail is under stress on one side and on the other side; the gap is also above or below on those respective sides, which ensures concentricity during operation

The shims placed above and below the crosshead slider of the reciprocating machine indicate that the clearance in the crosshead slider has increased, necessitating the use of shims to compensate for it.

This is determined by the degree of concentricity with the cylinder, and adjustments are made according to the actual situation!

The function of the gasket is to ensure the installation clearance, with the aim of achieving concentricity

The force-bearing surfaces of the compressor crosshead vary; in some cylinders, the force is applied on the upper side, while in opposite cylinders it is applied on the lower side. The gaskets on these force-bearing surfaces are slightly thicker to account for wear and tear

This post was last edited by Liu Tao 2008 on 2015-10-14 at 11:16. While ensuring the concentricity of the piston rod, it is possible to adjust the shims above and below the crosshead; the amount of shim used isn’t fixed – it depends on the amount of scraping and grinding required. Nowadays, crossheads can be adjusted using shims. One factor to consider is the concentricity of the piston rod, as well as the clearance in the guide rails. For crossheads that move along the lower guide rail, those with a diameter of 450 MM will not rise more than 0.1 MM, which is similar to the movement of the piston rod within the cylinder. In such cases, it’s generally not necessary to account for any additional clearance in the crosshead guide rail. As for crossheads that move along the upper guide rail, there isn’t any lack of clearance in that area either; during operation, only the lower guide rail moves slightly away from the guide rail, with the crosshead being supported by an oil film. The same applies to crossheads on the upper guide rail – it’s just that the direction of the force acting on them changes. Therefore, once the levelness and concentricity of the cylinder are ensured, as long as the clearance in the crosshead guide rail, the center height of the piston rod, the piston rod’s fluctuation values, and the circumferential clearance of the support rings are all taken into account, there are usually no major problems. Additionally, wear on the crosshead guide rail during operation is very minimal. From my observations, as long as the scraping is done properly, traces of the scraping tool can still be seen even after 3 years of use

How to determine whether the load is applied below or above the crosshead