Mechanical mechanisms are assemblies of components arranged to transmit or transform motion and force to perform a required function within a machine. They can convert rotary motion to linear movement, produce intermittent or oscillating motion, reverse direction, provide mechanical advantage, locate or clamp components, and generate required paths.
This section provides an overview of common mechanism types together with links to the detailed RoyMech references for ratchets, escapements, indexing, rocking and reciprocating mechanisms.
Pawl ratchets, sprag clutches and other arrangements for one-way motion, incremental movement and mechanical indexing.
Ratchet MechanismsInverse, true, runaway and tuned escapements for timing, intermittent motion and controlled release of stored mechanical energy.
Escapement MechanismsGeneva mechanisms, mutilated gears, cycloidal arrangements, star wheels and cam systems for intermittent rotary positioning.
Indexing MechanismsCrank-rocker linkages, rack and gear systems, quick-return linkages and cam-operated mechanisms producing angular oscillation.
Rocking MechanismsScotch yokes, slider-cranks, shaper mechanisms and Whitworth quick-return linkages for repeated linear motion.
Reciprocating MechanismsCam types, followers and motion relationships for mechanisms requiring specifically controlled displacement, rise, return and dwell.
Cam and Follower MechanismsThere is an enormous range of possible mechanical mechanisms. Selection is normally based on the function required, the available input motion and the desired output displacement, force, speed and timing.
The classifications below are functional and broadly follow the approach used in Theory of Machines and Mechanisms by Uicker, Pennock and Shigley. They provide a practical way of identifying candidate mechanisms rather than a rigid theoretical classification.
For mechanisms based principally on connected rigid links, see the separate RoyMech Linkages reference.
Traditional mechanisms generally provide a fixed geometric relationship between input and output. Once manufactured, a cam, linkage or gear mechanism will repeatedly produce substantially the same motion for a given input. This can provide a compact, reliable and predictable solution for high-cycle machinery.
Modern machinery can alternatively generate motion using servo motors, electric linear actuators, hydraulic or pneumatic systems and electronic control. These systems allow displacement, speed and timing to be altered without physically changing the mechanism.
The choice is therefore not simply between old and new technology. Dedicated mechanisms remain useful where a repeated mechanical relationship, simplicity, synchronisation or high cycle life is required, while controlled actuators are particularly useful where motion profiles or operating sequences must be changed.
| Function | Typical Mechanisms |
|---|---|
| Snap action | Toggle and bistable mechanisms |
| Linear actuation | Power screws, hydraulic and pneumatic cylinders |
| Fine adjustment | Screws, differential screws, gears and cams |
| Clamping | Screw, lever, toggle and cam clamps |
| Location | Jigs, fixtures and locating devices |
| One-way motion | Ratchets and one-way clutches |
| Controlled energy release | Escapements |
| Intermittent positioning | Indexing mechanisms |
| Angular oscillation | Rocking mechanisms |
| Linear reciprocation | Reciprocating mechanisms |
| Motion reversal | Gears, racks, linkages and directional drives |
| Rotational connection | Couplings and connectors |
| Sliding connection | Links, racks, gears, chains and fluid systems |
| Stop and dwell | Cams and indexing mechanisms |
| Curve generation | Four-bar and other linkages |
| Straight-line generation | Whitworth, Roberts and Chebyshev linkages |
| Tracing and scaling | Pantograph mechanisms |
Snap-action mechanisms include toggle and bistable arrangements that move rapidly between stable or limiting positions after passing through a critical geometric condition.
They are used in devices ranging from mechanical switches to clamps and latching systems.
Linear actuators convert supplied mechanical, electrical or fluid power into controlled straight-line movement.
A stationary screw and rotating nut can provide linear translation with high mechanical advantage. Applications include machine-tool traverses, jacks and valve operating mechanisms.
Alternatively, the nut may be fixed while the screw rotates and translates. This principle is widely used in power-screw actuators, jacks and valve drives.
For screw selection, torque, efficiency and strength calculations see the RoyMech Power Screws reference.
Single- and double-acting hydraulic and pneumatic cylinders provide direct linear actuation. Fluid pressure acting on the effective piston area generates the output force, while valves control direction and movement.
Hydraulic systems are particularly suited to high-force applications, while pneumatic systems are widely used for relatively simple and rapid machine actuation.
Fine-adjustment mechanisms produce a relatively small output movement from a larger or more convenient input movement. Common arrangements use screws, differential screws, gears or cams.
The mechanism shown below provides a small linear carriage movement from rotation of the input shaft. The original example gives approximately 0.5 mm of carriage movement per input revolution.
Clamping mechanisms apply and retain force to hold a component in position. Typical examples include toolmaker's clamps, G-clamps, screw clamps, toggle clamps and cam-operated clamps.
Two fundamental requirements are:
Toolmaker's Clamp |
G-Clamp |
Toggle Clamp |
Cam Clamp |
Horizontal In-Line Toggle Clamp |
Vertical Toggle Clamp |
Toggle clamps obtain high mechanical advantage as the linkage approaches its over-centre condition. Practical design should also consider stiffness, deflection, friction, wear and the consequences of accidental release.
Location devices position, align or centralise components before machining, assembly, inspection or another operation. Jigs and fixtures commonly combine locating features with separate clamping elements.
The location system should constrain the required degrees of freedom without introducing unnecessary or conflicting constraints.
Ratchets permit or control movement in one direction while preventing or resisting reverse movement. They can also produce incremental movement from oscillating input.
RoyMech covers pawl ratchets, sprag clutches, spring-blade, cam-driven, friction and magnetic arrangements on the dedicated reference page.
Escapements control intermittent movement or the release of stored mechanical energy. Horological escapements regulate mechanical clocks, while related principles can be applied to counters, timers and machine indexing systems.
Indexing mechanisms produce a sequence of controlled movements separated by dwell periods. Typical arrangements include Geneva drives, mutilated gears, cycloidal mechanisms, star wheels and cam indexers.
Rocking mechanisms produce repeated angular movement between limiting positions rather than continuous rotation. Examples include crank-rocker linkages, rack-and-gear systems and cam-operated rockers.
Rocking and Oscillating Mechanisms
Reciprocating mechanisms produce repeated straight-line movement. Mechanical examples include Scotch yokes, slider-cranks and quick-return mechanisms.
Direction reversal can be obtained mechanically using gears, racks or linkages, or by reversing the direction of an electric or fluid-power actuator.
Two Racks and Pinion |
Two-Pinion Reversal |
Linkage Reversal |
Hydraulic and pneumatic systems can reverse direction using suitable directional-control valves. Mechanical drive systems may use clutches or alternative gear paths to select between opposite directions.
Where reversal occurs while components are moving, the inertia of the driven system and the resulting transient loads must be considered. Direction reversal should not be treated purely as a kinematic problem.
Rotary motion can be transmitted between coaxial, parallel, intersecting or other shaft arrangements using couplings, gears, belts, chains and specialised link mechanisms.
A conventional shaft coupling primarily connects nominally coaxial shafts. Selection depends on transmitted torque, rotational speed, required stiffness and the angular, parallel or axial misalignment that must be accommodated.
Gears can transfer rotation between a variety of shaft arrangements while also changing speed, torque and direction. Belt drives permit greater shaft separation and can accommodate arrangements that would be inconvenient for direct gearing. Chain drives provide positive engagement without the normal slip associated with friction belt drives.
Two geometrical coupling arrangements are illustrated below.
Universal Coupling |
Reuleaux Coupling |
A single universal joint operating at an angle does not generally transmit uniform output angular velocity when the input rotates uniformly. Where constant-velocity transmission is important, the complete coupling geometry must therefore be considered.
Sliding connectors transfer movement between translating members, often where the two sliders move in different directions or along different axes.
Possible arrangements include:
Sliding Linkage Connection |
Rack and Pinion Sliding Connection |
Some machine operations require an output component to move, stop for part of the operating cycle, and then move again.
Cams provide one method because a cam profile can incorporate defined rise, dwell and return regions. Indexing mechanisms can similarly provide movement followed by a stationary dwell.
An engine valve train is a familiar example of a cam-controlled system in which valve displacement varies through the operating cycle and includes periods during which the valve remains closed.
See the Cam and Follower Mechanisms and Indexing Mechanisms references.
Four-bar linkages can generate a very wide range of coupler-point paths by changing the link lengths, fixed-pivot positions and the location of the selected point on the coupler.
The moving pivots connected directly to fixed pivots necessarily follow circular paths. Points on the coupler link can, however, follow much more complex curves.
Modern servo-controlled systems can generate programmable paths, but a dedicated mechanical linkage can remain attractive where a fixed path must be repeated mechanically over many cycles.
Where a true straight-line constraint is required, a physical linear guide or slideway is normally the direct solution. Historically, however, numerous linkages were developed to approximate straight-line motion without a slideway.
Examples include the Whitworth, Roberts and Chebyshev straight-line mechanisms illustrated below.
Whitworth Straight-Line Mechanism |
Roberts Straight-Line Mechanism |
Chebyshev Straight-Line Mechanism |
These mechanisms approximate a straight line over a selected portion of their motion. The quality and usable length of the approximation depend on the linkage proportions.
A pantograph is a linkage used to reproduce the movement of one point at a different scale.
In the arrangement illustrated below, movement of point P is reproduced at point P'. The linkage uses parallelogram geometry so that corresponding points maintain the required geometric relationship as the mechanism moves.
Depending on the relative pivot and tracing-point positions, a pantograph can enlarge or reduce the reproduced movement.
| If You Need... | Consider... |
|---|---|
| One-way movement | Ratchet or one-way clutch |
| Intermittent rotary positioning | Geneva, star wheel, cam indexer or other indexing mechanism |
| Controlled release of stored mechanical energy | Escapement |
| Repeated linear movement | Slider-crank, Scotch yoke, cam or linear actuator |
| Repeated angular oscillation | Crank-rocker, rack and gear or cam-operated rocker |
| Unequal forward and return times | Quick-return mechanism |
| Precisely defined displacement and dwell | Cam mechanism |
| High mechanical advantage for linear movement | Power screw or fluid-power actuator |
| High clamping force with short operating movement | Toggle, screw or cam clamp |
| Fixed geometric path generation | Linkage |
| Programmable motion | Servo, electric, hydraulic or pneumatic controlled actuator |
For a fuller selection process, including comparisons between similar mechanisms, see the Mechanism Help and Selection Guide.