Rocking or oscillating mechanisms produce repeated angular movement of a link or shaft between two limiting positions rather than continuous rotation. The angular movement is normally less than one complete revolution and reverses direction during each operating cycle.
Rocking motion can be generated from continuous rotary input by several mechanical arrangements. Common methods include four-bar crank-rocker linkages, rack and gear systems, slotted-link quick-return mechanisms and cam-operated followers.
| Mechanism | Input | Output | Principal Characteristic |
|---|---|---|---|
| Rack and gear | Rotary crank | Oscillating gear or shaft | Converts reciprocating rack movement into angular oscillation |
| Crank-rocker linkage | Continuous rotation | Rocking link | Simple four-bar mechanism with motion determined by link geometry |
| Quick-return linkage | Continuous rotation | Oscillating link | Different times or input angles for forward and return strokes |
| Cam and follower | Continuous cam rotation | Rocking follower | Output motion can be tailored through the cam profile |
The mechanism illustrated below uses a rotating crank (b) connected to a toothed rack (c). The reciprocating movement of the rack drives gear (d) alternately in opposite directions, producing a rocking or oscillating rotary output.
The angular movement of the output gear depends on the rack displacement and the pitch radius of the gear. The crank and connecting geometry determine the variation of rack displacement and velocity through the operating cycle.
Rack and Gear Rocking Mechanism
A crank-rocker is a form of four-bar linkage. The fixed frame forms one link, while the remaining three moving links form the crank, coupler and rocker.
In the arrangement shown below, continuous rotation of crank (b) is transmitted through the connecting link and produces an oscillating movement of rocker (d).
The output rocking angle, velocity and acceleration depend on the link lengths and the locations of the fixed pivots. Suitable proportions are required if the input crank is to complete a full revolution while the output link remains a rocker.
Crank-Rocker Mechanism
Four-bar linkages provide considerable freedom to tailor the relationship between input rotation and output rocking motion. The geometry should therefore be selected for the required angular travel, transmission characteristics and clearance through the complete operating cycle.
A quick-return mechanism produces an oscillating output in which the forward and return movements occupy different portions of the input cycle.
In the arrangement below, crank (b) rotates continuously and acts through slider (c) to produce rocking motion of arm (d).
The input crank rotates through a larger angle during one output stroke than during the opposite stroke. At constant input speed, the stroke associated with the larger crank angle therefore takes longer, while the opposite stroke is completed more quickly.
Quick-Return Linkage
Quick-return mechanisms are particularly useful where useful work occurs during one part of the cycle and the return movement should occupy less time. The required time ratio is determined by the mechanism geometry.
A cam can also convert continuous rotary input into oscillating angular motion. In the simplified arrangement shown below, rotation of cam (c) acts through the contact at (b) and causes lever (d) to rock about its pivot.
Cam-Operated Rocker
The illustration shows the operating principle only. A practical mechanism requires a means of maintaining the follower in contact with the cam where continuous contact is required. Depending on the application, this may be provided by a spring, gravity, external loading or a positively constrained cam arrangement.
A major advantage of a cam-operated rocker is that the cam profile can be designed to provide a required displacement characteristic, including rise, return and dwell periods. Velocity and acceleration characteristics must also be considered when developing the cam profile.
| Requirement | Mechanism to Consider |
|---|---|
| Simple rotary-to-oscillating conversion using gears | Rack and gear mechanism |
| Continuous rotary input with simple rocking output | Crank-rocker four-bar linkage |
| Different forward and return stroke times | Quick-return linkage |
| Precisely tailored rocking displacement and dwell | Cam-operated rocker |
Selection should consider more than the required output angle. Important factors include: