Indexing mechanisms convert continuous or cyclic input motion into controlled intermittent output movement. The output advances through a defined angle or distance and then remains stationary during a dwell period before the next indexing movement.
Mechanical indexing is widely used where machinery requires repeatable positioning, dwell periods or sequenced movement. Common arrangements include Geneva mechanisms, mutilated gears, cycloidal mechanisms, star wheels and cam-based indexing systems.
The Geneva mechanism is one of the best-known methods of producing intermittent rotary motion. A driving pin enters a slot in the driven wheel and rotates it through a defined indexing angle. Between engagements the driven wheel remains stationary.
Three basic forms are shown below.
The external Geneva is the most common arrangement. The driver and driven members rotate about separate parallel axes, with the driving pin periodically entering a radial slot in the Geneva wheel.
In an internal Geneva mechanism the geometry is arranged so that the driver engages slots on an internally configured driven member. The resulting motion differs from the external arrangement but provides the same basic combination of indexing movement and dwell.
A spherical Geneva can be used where the input and output shafts are not parallel, including arrangements with approximately perpendicular shaft axes. The geometry is more complex and the mechanism is generally less convenient for substantial power transmission than conventional external or internal Genevas.
The driven member of a properly designed Geneva mechanism remains positively controlled during indexing and dwell, reducing the tendency for uncontrolled overtravel.
Impact can nevertheless be significant when the driving pin enters the slot. Slot geometry, pin geometry, manufacturing accuracy, operating speed and driven inertia therefore have an important influence on dynamic loading.
The pin should enter the slot with suitable geometry so that engagement occurs as smoothly as practicable. Some arrangements provide local resilience in the slot fingers to reduce impact, although this must not compromise the required strength of the driven member.
External and internal Geneva mechanisms have historically been used for both light and relatively heavy duties. They have also been used in mechanical counters and other high-cycle intermittent-motion equipment.
Geneva drives can be modified to obtain different motion characteristics. Examples include:
Gearing can also be arranged to produce intermittent rotary motion. A mutilated gear has teeth removed from part of its circumference so that the output gear is driven during one portion of the input revolution and remains stationary during the remainder.
The arrangement illustrated below incorporates holding surfaces that restrain the output during the dwell period.
A positive holding arrangement is important. If the driven gear is free to move during the dwell, the first teeth can meet tip-to-tip when engagement restarts. Because this contact occurs close to the line of centres, relatively modest input torque can then generate large local forces and damage the teeth.
Mutilated gears can also experience substantial impact and acceleration at higher operating speeds. Modifying the initial engaging teeth can reduce the severity of engagement, but the basic limitation remains.
For applications requiring higher indexing speeds or more controlled engagement, Geneva, star-wheel or cam indexing systems may offer more suitable motion characteristics.
Mutilated pinions have historically been used extensively in mechanical counters because they are compact, inexpensive and capable of repeatable indexing under instrument-type loads.
In the arrangement illustrated below, alternate teeth on part of the input pinion are removed so that the remaining teeth can restrain the pinion during the dwell period.
Cycloidal arrangements provide another means of obtaining intermittent output motion. Unlike a simple mutilated gear pair, the interacting members can remain engaged throughout the operating cycle.
The geometry can be selected to modify acceleration, deceleration, dwell and the relationship between input and output movement.
In the basic mechanism shown below, the location of the drive pin or roller relative to the planet-gear pitch circle is critical if the output crank is to come completely to rest. An incorrect location can result in the output merely slowing or even reversing instead of producing the intended dwell.
Related arrangements include hypocycloidal, epicycloidal and pericycloidal mechanisms. These can also be combined with Geneva systems where more complex output characteristics are required.
The mechanism below uses a driver moving within an internal gear. The input shaft rotates an eccentric while the inner member is restrained from freely rotating by a fixed pin.
The relative geometry of the eccentric, gear and restraint determines the angular movement of the output.
In the illustrated example, the original reference specifies approximately 36° of output indexing for each 360° input revolution, with the output remaining stationary through the remaining approximately 324° of input movement.
Because the gearing remains engaged, the arrangement can provide relatively smooth operation with low acceleration compared with more abrupt intermittent mechanisms.
A star wheel is another versatile form of intermittent-motion mechanism. In the arrangement illustrated below, pins on the driving member engage the driven star wheel, although suitably designed gear teeth can also be used.
Star-wheel mechanisms provide considerable freedom in selecting the amount of output movement. Unlike a simple Geneva mechanism, some star-wheel arrangements can produce more than one output revolution or different angular movements at successive indexing positions.
Appropriate tooth or engagement geometry can also be used to influence acceleration and deceleration. Internal arrangements are possible as well as external forms.
Cams provide substantial freedom in the design of intermittent motion because the cam profile can be shaped to control displacement, velocity, acceleration and dwell.
The scroll-shaped disc cam illustrated below indexes the output wheel through approximately 180° when the solenoid pulls the operating levers down and through a further 180° when the solenoid is released.
A face cam can also produce intermittent rotary movement. In the arrangement below, a reciprocating arm moves a pin or roller through a zig-zag groove in the face of the driven wheel.
The groove geometry controls the successive indexing movements. Impact can be kept relatively low if the reciprocating arm itself is driven with a suitable motion profile, and the positive groove engagement helps limit overtravel.
A cylindrical cam can be used to generate controlled intermittent rotary motion, with the cam groove defining the index and dwell portions of the cycle.
The original RoyMech reference records a commercial arrangement capable of approximately 1000 steps/min under moderate-to-high loading. This should be treated as an example of a particular mechanism rather than a general operating limit for cylindrical cam indexers.
The indexing principle below has been used for sequenced grab mechanisms. Each downward movement of the central shaft causes the surrounding internally slotted ring to rotate through a fixed angle.
Successive ring positions can therefore correspond alternately to open and locked conditions of the grab.
The following diagram represents the operating principle of a retractable ball-point pen mechanism. The actual mechanism is cylindrical; the diagram shows its cam track developed onto a flat surface so that the indexing action can be seen more clearly.
Each press of the operating member advances the mechanism to its next stable position, alternately extending and retracting the pen.
A major advantage of cam indexing is the ability to tailor the motion profile. The cam geometry can be designed to control:
This flexibility also means that cam geometry is application-specific. Small changes to the profile can significantly alter velocity, acceleration and dynamic loading, so the cam should be designed for the required motion rather than selected solely from its total indexing angle.
| Requirement | Mechanism to Consider | Important Characteristics |
|---|---|---|
| Simple fixed angular index with positive dwell | Geneva mechanism | Compact and positively controlled, but pin/slot engagement can create impact |
| Low-cost gear-based intermittent drive | Mutilated gear | Good repeatability but potentially severe tooth engagement at higher speeds |
| Continuous engagement with tailored intermittent motion | Cycloidal mechanism | Wide geometric flexibility and potentially smoother acceleration characteristics |
| Variable index angles or more complex indexing sequences | Star wheel | Flexible engagement geometry and output movement |
| Precisely tailored index, acceleration and dwell profile | Cam indexer | High motion-design flexibility but requires appropriate cam profile design |
| Perpendicular or intersecting input/output axes | Spherical Geneva or another spatial indexing mechanism | Useful geometry but generally more complex and application-specific |
Indexing mechanisms repeatedly accelerate, move and stop a driven component. Their design therefore involves more than simply selecting the required indexing angle.
| Consideration | Engineering Significance |
|---|---|
| Index angle | Determine the required movement during each operating cycle and the number of indexing positions. |
| Dwell | Establish how long the output must remain stationary between movements. |
| Input speed | Higher speed increases acceleration, engagement forces and the energy that must be controlled during each index. |
| Driven inertia | A high-inertia load can produce substantial dynamic torque during acceleration and deceleration. |
| Impact | Pin, tooth or follower engagement can generate local impact loads and wear. |
| Positioning accuracy | Clearance, manufacturing tolerance, wear, elastic deflection and overtravel affect the final indexed position. |
| Holding during dwell | The output may require positive mechanical restraint to prevent movement while the indexing mechanism is disengaged. |
| Acceleration profile | Cam, linkage and cycloidal arrangements can provide greater control over acceleration than simple abrupt-engagement systems. |
| Lubrication and wear | Repeated sliding, rolling or impact contact requires suitable materials, surface condition and lubrication. |
| Service life | Fatigue, contact stress, wear and repeated impact should be considered for the required number of operating cycles. |