I have a question regarding a point that is not clear in the manual for the oil-free vertical vacuum pump
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After major repair and reassembly, when the piston is at the top and bottom dead centers, the flat surfaces at both ends of the piston should be roughly level with the upper and lower flat surfaces of the cylinder (within 1.5–2.5 mm). What does \"more or less the same\" mean in this context? And what does the range of 1.5–2.5 mm within the parentheses refer to?
It refers to the condition where the upward and downward clearances of the piston within the cylinder are equal. Otherwise, as the temperature rises, cylinder collision accidents may occur.
There is another sentence afterward: Check the clearance at the top and bottom dead centers; the clearance at the top dead center should be about 1 mm larger than that at the bottom dead center. Does this conflict with the requirement that the upper and lower gaps be equal? Is it necessary for the upper gap to be larger than the lower gap? What is the sum of the upper and lower gaps?
The top dead center must be 1 mm larger than the bottom dead center to be correct, because once the piston compressor starts operating, the piston rod expands due to heat, which reduces the clearance at the top dead center. If the clearances at the top and bottom dead centers are equal, it is easy for the piston to hit the cylinder walls. The algorithm for the total clearance is the length of the cylinder after the cylinder head is installed, minus the total length of the piston.