Reciprocating compressor crank rocker driver

Crank rocker drive units with adjustable swing arm bearing

Crank-rocker drive units have a crank drive connected to the plunger rod by means of two links and a swing arm. The swing arm is also connected at the connecting point of the links, see figure 1. The stroke length is adjusted by changing the position of the pivot point of the swing arm relative to the axis of the linear movement of the plunger rod.

Figure 1: Adjustable crank rocker drive unit

The crank rocker drive unit contains many individual components with pivoting, highly loaded bearing points. Therefore, it is complex and relatively expensive to manufacture. Some stroke-adjustable metering pumps of earlier generations were equipped with crank rocker drive units.

Rocker arm drive units with pivot adjustment of the coupling

Rocker arm drive units also include a crank drive. However, it is not connected directly to the plunger rod but acts on the plunger rod via an intermediate rocker arm instead. This arrangement makes it possible to achieve stroke length adjustment for the plunger rod. This function is implemented making the supporting point of the rocker arm shiftable in order to transform the reciprocating movement with varying amplitudes. An adjustable rocker arm drive unit of this type is shown in figure 2.

Figure 2: Adjustable rocker arm drive unit

The rocker arm drive unit offers a harmonic movement and continuous adjustment and can be designed in a way so that the front dead center remains constant.

The disadvantages of this type of design result from the higher number of joints and highly loaded components resulting from the use of the rocker arm and its pivot point with sliding support. 

Earlier generations of stroke-adjustable metering pumps were often equipped with rocker arm drive mechanisms.

Cross crank drive units

In order to convert the circulating movement of an eccentric into reciprocating linear movement, two linear guides are arranged perpendicular to the rotating axis, as illustrated in figure 3. The arrangement looks like a cross. Designs with yoke and sliding block permit low-wear operation.

Figure 3: Cross crank drive unit

Cross crank drives are used to operate two pump heads with one drive unit, as illustrated in figure 4. Like straight crankshaft drive units, they can be equipped with stroke adjustment.

Figure 4: Example of a cross crank drive unit

Cross crank drive units are more elaborate in construction than straight crankshaft drive units, since they contain more parts. Some parts need a special design to protect them against wear. Therefore, they usually become cost-effective only when implemented in pumps with two pump heads. The use of a single drive unit for this purpose results in a compact pump that may also have a shared stroke adjustment for both pump heads.