On the reverse angle issue of reciprocating compressors
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Previously, there was no such concept as a reverse angle; I became very confused after learning about it recently. In a reciprocating compressor, the piston rod and all the driving components are repeatedly subjected to compressive and tensile forces. These forces cause the crosshead pin to be pressed against one side of the connecting rod cap, creating a gap on the other side, which allows lubricant to flow in and lubricate as well as cool most of the crosshead pin and the connecting rod cap on that side. If the crosshead pin is subjected to forces in only one direction, always pressing against one side of the connecting rod cap, then that side will never have a gap, and thus will not receive lubrication or cooling. As a result, the crosshead, crosshead pin, and connecting rod cap can reach high temperatures and get damaged within just a few minutes of operation. Therefore, the direction of force on the piston rod must be changed so that the bearing surfaces on both sides of the rod’s small end are lubricated and cooled in turn; this is what is meant by \"reversing the load\". The load reversal must be maintained for a certain period of time to allow the lubricating oil to reach the lubricating oil as well as the crosshead pins and rod journal bearings. This time is expressed in terms of crankshaft angle and is called the “reverse angle”. Standard API618 specifies that the reverse angle must not be less than 15°. ” This is an interpretation of the reciprocal angle by a expert on the forum. I have the following questions: 1. Shouldn’t the reverse angle and the forward angle each be 180° in theory? Shouldn’t the time it takes for the piston to move forward or backward be the same for one full rotation of the crankshaft? Why is the minimum limit for the reverse angle so low? 2. Why is it said that in the case of a reciprocating compressor operating at low load (50%) or no load (idling), \"a low reverse angle increases the risk of burning out the small-end bearing\"? 3. Besides serving as a basis for \"ensuring the lubrication of the pin and the small connecting rod bearing,\" does the reverse angle have any other functions (only discussing reciprocating machines)? 4. For reciprocating compressors with stepless air volume control, are there any special considerations regarding the reverse angle?
1. The reverse angle and the forward angle are not necessarily both 180°, as they actually depend on the design of the machine and the way power is transmitted. Although theoretically, one full rotation of the crankshaft results in the piston moving back and forth for half the time each, in practice it is necessary to ensure that there is sufficient time and space at the end of the piston’s stroke for lubricant to enter and cool the components. Therefore, the reverse angle is set to at least 15° to ensure adequate lubrication during pressure reversal. 2. In the case of a reciprocating compressor operating at low load or no load (idling), the pressure inside the machine is low, which results in lower pressure between the crosshead pin and the rod bearing shells. If the reverse angle is low, this may lead to insufficient lubrication, thereby increasing the risk of wear or damage. 3. The reverse angle is mainly used to ensure lubrication and cooling during reciprocating motion. In addition, it also plays an important role in power transmission and preventing excessive mechanical wear. Through proper reverse angle design, unnecessary friction and wear in components such as pistons, connecting rods, and crossheads can be reduced, thereby enhancing the reliability of the machine and extending its maintenance interval. 4. For reciprocating compressors with stepless air volume control, the adjustment of the reverse angle is particularly important. As the compression load changes, the reverse angle may need to be adjusted accordingly to ensure good lubrication and cooling under various operating conditions. It is recommended to consult the manufacturer’s guidelines in detail and carry out appropriate adjustments to ensure the machine operates stably under all load conditions. .
First, it is necessary to determine whether the lubrication surface indeed loses its lubricating properties when pressure is applied as you said. The oil film on a surface under stress does not disappear just because the pressure increases; it remains in place. Once this is understood, there’s no need to worry about other things. For reference.
Thank you so much for your answer! I still have a few questions: 1. How should I understand \"Answer 2\", since the reverse angle is definitely low when a reciprocating compressor is operating at low load or at no load (idling)? If so, why? My understanding is that at low loads or even no load, the piston end is not under any force; therefore, the clearance between the pin and the journal bearing should be sufficient. So why is it necessary to consider the issue of the reverse angle? 2. What does it mean to adjust the reverse angle accordingly under stepless air volume control? By calculating the opening and closing times of the intake and exhaust valves, the stroke of the compressor piston can be changed, which in turn alters the angle of the reverse angle. For example, during the compression process, if the exhaust valve is forced to close, then although it is still within the compression phase, the force acting on it is directed toward the crankshaft side; as a result, the reverse angle becomes larger than it actually should be. By adjusting the stroke at which force acts on the piston, the reverse angle can be altered indirectly. Is this understanding correct? 3. Could you give an example of a situation where the reverse angle is less than 180°? I’m not clear about what it means when the reverse angle is small.
Is the definition of the opposite angle that I quoted therefore incorrect? How should this be understood?
Since your argument is about bearing damage, if it’s not that topic then I didn’t understand; my apologies.
There are papers available online that you can check out; for example, Cooper’s software (for Ajax) includes functions for calculating direction angles as well as graphics and text. Understand inertial force and gas force, as well as the resultant force ； Combine it with the direction of motion as well. It’s quite intuitive to understand the concept of the reverse angle