ESDU 96002:2011

INTRODUCTION

Geneva mechanisms are used widely to generate intermittent
rotary motion from a continuously rotating input shaft. They have
many features of interest to designers of industrial machinery
including simplicity of design, cost effectiveness, precise control
of the output position during motion and dwell periods, a wide
range of feasible sizes, good reliability and long life.

The principal disadvantage is the lack of versatility. Once the
number of the dwells per revolution of the wheel is chosen
(typically between 3 and 18), the shape of the motion curves and
the ratio of motion period to dwell period are fixed. Finite
accelerations at the ends of the motion period will produce shock
loads. Although cam based indexing mechanisms can be designed with
better motion characteristics, these are usually more expensive and
in many applications a well designed Geneva mechanism is the most
cost-effective method for generating intermittent motion.

There are two basic designs of Geneva mechanism; the external
Geneva (see Sketch 2.1a) and the internal Geneva (see Sketch 2.1b).
Many variations on these two types have been suggested (see
References 6 and 14) and most represent attempts to modify the
motion characteristics for particular applications or to reduce
shock loads in mechanisms intended for high speed operation.
However, the basic designs have found wide use in many types of
instruments and in industrial machinery and it is with these basic
designs that this Item is concerned. It is useful to calculate
values of many quantities (geometric parameters, kinematic
characteristics, forces, torques, stresses etc.) at the design
stage so that service failure by known modes can be avoided.

This Data Item presents methods for calculating the important
indicators of performance of both internal and external Geneva
mechanisms. The analysis is time consuming and is best carried out
using the associated computer program (ESDUpac A9611 – see
Reference 11). The program determines values of wheel motion,
bearing forces, input torque, frame shaking force, contact stress
between the roller (or pin) and the slot, lubricant film thickness
(if applicable) and roller life (if applicable). In addition the
program finds limiting values of certain geometric parameters and
makes a number of checks on the dimensions of the mechanism. In
order to communicate with other computer aided design software, a
DXF file (from which the outline of the Geneva wheel can be
plotted) is generated (for external Genevas only). This facility is
described in greater detail in Section 5.2.4.

Correct implementation of the calculation procedures suggested
in this Item should result in a decrease in design time together
with improvements in machine reliability.