Function and variations
The driving gear of the compressor converts the rotating motion of the crank into the oscillating motion of the piston and also transmits work.
It usually works according to the kinematic principle of the connecting rod mechanism.
For single acting compressors usually the connecting rod is directly linked to the piston (plunger type piston – fig. a), whereas for double acting compressors linear reciprocating motion is achieved by linking the connecting rod to the cross head (crosshead design – fig.b).
On very small compressors (for example with household refrigerators) the kinematic principle of the cross slide ( fig. c ) will find application. It is a four member link with two linear guidances. Small fast running units can be designed as axial compressors ( fig. d ). Thereby several pistons are runnig parallel to the axis of the drive of the machine. The pistons are moved via a disc fitted at an angle to the drive shaft (wash plate).
For (combustion) engine driven compressors the ?free piston? execution is an interesting alternative (fig. g): The reciprocating action of the combustion engine is directly linked to the oscillating motion of the compressor. As the combustion engine is double acting motion control entirely is enabled by proper timing of the counteracting combustion processes in the two combustion chambers.
Kinematics and dynamics
The connecting rod drive results in an oscillating and nearly harmonic movement of the piston.
The piston velocity and acceleration can be calculated with sufficient accuracy by a Fourier series of 2nd order. The deviations from the harmonic motion increase with the rod ratio , that is the relation of crank radius to connecting rod length.
Primarily the drive will be loaded by the force F resulting from the gas pressure acting on the piston ( gasforce ). The resulting loads onto the individual components can be calculated according to the laws of statics:
On the guides of the crosshead acts the vertical force FN, on the connecting rod acts the force FS. On the crank bearing the connecting rod force can be split up into two orthogonal forces. The tangential force FT together with the radius of the crank results in the torque.
The gas forces within the machine are balanced . Exception: tipping moment ( as a result of the torque of the drive ).
Mass forces result as a reaction to the continuously necessary accelerations of the moving parts of the drive mechanism. These can be calculated according to Newtons law as the product of moving mass times their acceleration.
Contrary to the gas forces the mass forces act in full also to the outside and onto the foundations (also via their torques ). Therefore on multi cylinder machines an optimal balance of the mass forces by appropriate designs of crank and cylinder arrangement is an important design goal.
In order to be able to manufacture at low cost, compressor manufacturers mostly use well proven, standardized drive designs in various sizes.
In order to achieve smaller dimensions the compressor speed should be as high a possible.
However, considering the lifespan and efficiency the mean piston speed cmis not allowed to exceed certain limits. (The smallest values apply to non lubricated service and for highest pressures, the biggest for small plunger type compressors with intermittent service.)
The compressor speed is also limited by the maximum allowable working frequency fv zul of the self actuated compressor valves. The maximum allowable piston forces of a certain compressor drive series rise approximately with the square of the stroke.