Ratchet mechanisms permit or transmit motion in one direction while restricting motion in the opposite direction. They are widely used for one-way drives, indexing, anti-reverse functions and intermittent mechanical motion.
Ratchet action can be produced by a toothed wheel and pawl, by wedging elements such as sprags, or by other mechanical, frictional or electromagnetic arrangements. The appropriate design depends on torque, speed, indexing accuracy, impact loading, wear, allowable backlash and the required operating duty.
There are many forms of ratchet mechanism, and their design often requires a mechanism suited specifically to the required motion.
A conventional pawl ratchet uses an asymmetric toothed wheel and a pawl. Rotation in the permitted direction causes the pawl to pass over successive teeth. Reverse rotation brings the pawl into engagement with a tooth and prevents further movement.
Ratchets can also form part of an intermittent drive. An oscillating input can advance a ratchet wheel by one or more teeth during each operating stroke while a holding pawl prevents the wheel moving backwards during the return stroke.
Pawl Ratchet |
Sprag Clutch |
The pawl ratchet is a simple arrangement consisting of a toothed wheel and an engaging pawl. The tooth and pawl geometry permits movement in one direction while mechanically preventing movement in the other.
A sprag clutch performs a similar one-way transmission function without a conventional toothed ratchet wheel. Sprags are positioned between inner and outer races. Relative rotation in one direction allows the sprags to move without locking the races; relative rotation in the opposite direction causes them to wedge between the races and transmit torque.
The sprag clutch is therefore more accurately described as a one-way or overrunning clutch rather than a conventional pawl-and-ratchet mechanism.
The pawl ratchet is simple, versatile and inexpensive and can be adapted to a wide range of sizes and operating loads.
Its action may be relatively harsh or comparatively smooth depending on tooth geometry, pawl design, manufacturing accuracy, operating speed and the method used to drive the pawl.
Where a spring or solenoid drives the pawl, the pawl may strike the ratchet wheel with appreciable kinetic energy. With a lightly loaded wheel this can cause impact, vibration or overtravel beyond the intended indexing position. A detent, brake or suitable secondary locating feature can be used where more accurate positioning is required.
The arrangement illustrated below incorporates a holding pawl. This prevents the ratchet wheel from reversing while the driving pawl returns to engage the next tooth.
The ratchet illustrated below uses flat spring elements as the driving and holding pawls. For suitable materials, heat treatment and light loading, this provides a compact arrangement with relatively few components.
The original RoyMech reference notes that mechanisms of this type have been used at indexing rates of several operations per second and, in suitable precision arrangements, with indexing accuracy approaching approximately one-tenth of a degree. These figures should be regarded as indicative of particular designs rather than general performance limits for spring-blade ratchets.
Applications have included spring winding mechanisms, printed-circuit commutators and timing cams.
A cam can be used to control the movement of the driving pawl. By shaping the cam motion appropriately, the velocity of the pawl as it engages the ratchet wheel can be reduced, decreasing impact and the tendency for the wheel to overtravel.
The arrangement illustrated below provides another variation of cam-driven ratchet operation. The mechanism indexes the ratchet wheel clockwise for either direction of rotation of the input shaft.
Small versions of this type of mechanism have been used in counters, but the same operating principle can be applied at larger scales. Permissible torque, power and indexing rate depend on the strength, inertia, impact loading, wear and dynamic behaviour of the particular design.
Intermittent one-way motion does not necessarily require a toothed ratchet wheel. An oscillating member can drive an output through a one-way clutch, producing an indexing action during one part of the input cycle and free movement during the return stroke.
The arrangement illustrated below uses a reciprocating linkage to drive the output shaft through a one-way spring clutch. Other overrunning or one-way clutch arrangements can be used on the same principle.
The arrangement below illustrates an electromagnetically operated ratchet. When the solenoid is energised, the soft magnetic pole piece engages a ferromagnetic tooth on the wheel and advances the wheel.
When the solenoid is de-energised, a compression spring returns the pole piece towards the next tooth while a permanent magnet restrains the wheel. This type of arrangement can be useful where the driven wheel must be located within a sealed enclosure and direct mechanical penetration is undesirable.
The simplicity of a ratchet mechanism can conceal significant local forces and dynamic effects. A practical design should consider both the required motion and the loads produced during engagement.
| Design Consideration | Engineering Significance |
|---|---|
| Direction of motion | Determine whether the mechanism must simply prevent reversal or must actively produce intermittent one-way movement. |
| Torque and load | The ratchet wheel, pawl, pivots, races and supporting structure must withstand the transmitted load and any engagement forces. |
| Tooth geometry | Tooth pitch and profile influence indexing increment, engagement, strength, backlash and release behaviour. |
| Impact | Pawl engagement can produce substantial local dynamic loads, particularly at high operating speeds or where the driven member has significant inertia. |
| Overtravel | Lightly loaded indexing mechanisms may require a brake, detent or positive location feature to prevent movement beyond the required position. |
| Holding pawl | An indexing mechanism may require a separate holding pawl to prevent reverse motion while the driving pawl resets. |
| Wear and lubrication | Repeated engagement can wear tooth and pawl surfaces. Material selection, surface condition and lubrication should suit the operating duty. |
| Noise | Repeated tooth engagement can generate significant noise. Spring loading, geometry and operating speed affect the severity. |
| Backlash and accuracy | Clearance necessary for reliable engagement can limit positioning accuracy. Precision indexing may require a separate locating system. |
| Failure behaviour | For load-holding or safety-related applications, consider the consequences of pawl, tooth, spring, bearing or actuator failure. |
| Requirement | Arrangement to Consider |
|---|---|
| Simple mechanical anti-reverse function | Pawl and toothed ratchet |
| Intermittent indexing from an oscillating input | Drive pawl and holding pawl ratchet |
| Compact light-duty indexing | Spring-blade ratchet |
| Reduced engagement velocity and controlled indexing motion | Cam-driven ratchet |
| Smooth overrunning without discrete tooth increments | Sprag or other one-way clutch |
| Reciprocating input driving one-way rotary output | One-way clutch indexing arrangement |
| Actuation through or within a sealed arrangement | Magnetic or other remotely actuated mechanism |