Reciprocating mechanisms produce repeated linear movement between two limiting positions. Reciprocating motion can be generated directly by pneumatic, hydraulic or electric linear actuators, or mechanically from continuous rotary motion using cranks, cams, gears and linkages.
Mechanical reciprocating mechanisms remain useful where a simple geometric relationship between rotary input and linear output is required. Common arrangements include the Scotch yoke, slider-crank and several forms of quick-return mechanism.
A Scotch yoke converts rotary motion directly into reciprocating linear motion. In the arrangement below, rotation of crank (b) causes its pin to slide within the slot of yoke (d), moving the yoke backwards and forwards.
For a crank rotating at constant angular velocity, the displacement of the yoke follows simple harmonic motion. If the crank radius is r, the total linear stroke is:
where S is the total stroke and r is the crank radius.
Scotch Yoke Mechanism
The Scotch yoke has fewer principal links than a conventional slider-crank, but sliding contact between the crank pin and slot can produce friction and wear. Lubrication, bearing pressure and the resulting side forces should therefore be considered in practical designs.
A slider-crank mechanism converts continuous crank rotation into reciprocating movement of a slider. In an offset slider-crank, the line of slider motion does not pass through the crankshaft axis.
The offset changes the crank angles corresponding to the two ends of the slider stroke. Consequently, at constant crank speed, the two strokes can take different amounts of time.
Offset Slider-Crank Mechanism
With appropriate geometry and direction of rotation, the mechanism can be arranged so that the working stroke occupies a larger crank angle and therefore takes longer than the return stroke. This produces a quick-return action.
For zero offset, the dead-centre positions of an ordinary slider-crank are separated by 180° of crank rotation, giving equal times for the two strokes when the crank rotates at constant speed.
Quick return is useful in machinery where useful work occurs primarily during one stroke and reducing the time occupied by the unloaded return movement increases machine productivity.
Mechanical shaping machines historically used quick-return mechanisms to produce the reciprocating motion of the cutting ram. The cutting stroke is relatively slow while the non-cutting return stroke is completed more rapidly.
The arrangement illustrated below is a linkage developed to obtain this unequal timing from a continuously rotating input.
Shaper Quick-Return Mechanism
The quick-return effect results from the input crank rotating through unequal angles during the forward and return portions of the output stroke. With constant input angular velocity, the stroke corresponding to the smaller crank angle is completed in less time.
The Whitworth quick-return mechanism is another linkage for converting continuous rotary input into reciprocating output with unequal forward and return stroke times.
Its geometry differs from the other shaper arrangements and therefore produces different displacement, velocity and acceleration characteristics through the operating cycle.
Whitworth Quick-Return Mechanism
As with other quick-return systems, the mechanism is useful where one stroke performs the principal work and the opposite stroke should be completed more rapidly.
For a mechanism driven at constant angular velocity, the time occupied by each stroke is proportional to the angle through which the driving crank rotates during that stroke. The quick-return ratio can therefore be expressed as:
where:
For a conventional quick-return application, Q is greater than 1 because the working stroke takes longer than the return stroke.
| Mechanism | Input | Output Characteristic | Principal Feature |
|---|---|---|---|
| Scotch yoke | Continuous rotation | Reciprocating linear motion | Simple harmonic displacement at constant input angular velocity |
| Conventional slider-crank | Continuous rotation | Reciprocating linear motion | Widely used general rotary-to-linear mechanism |
| Offset slider-crank | Continuous rotation | Reciprocating motion with potentially unequal stroke times | Offset provides timing asymmetry |
| Shaper mechanism | Continuous rotation | Slow working stroke and faster return | Developed for machine-tool quick-return duty |
| Whitworth mechanism | Continuous rotation | Reciprocating quick-return motion | Compact linkage giving unequal forward and return times |
Selection and design of a reciprocating mechanism should consider the complete motion cycle rather than stroke length alone. Important factors include:
| Requirement | Approach to Consider |
|---|---|
| Fixed repeated relationship between rotary input and linear output | Mechanical crank, cam or linkage |
| Simple harmonic reciprocating displacement | Scotch yoke |
| General rotary-to-reciprocating conversion | Slider-crank |
| Faster unloaded return stroke | Quick-return mechanism |
| Programmable stroke, position or speed | Electric linear actuator or servo system |
| High linear force from fluid power | Hydraulic or pneumatic cylinder, according to the application |