Mechanism is a system of rigid elements arranged and connected to transmit motion in a predetermined fashion. Indexing mechanisms generally converts a rotating or oscillatory motion to a series of step movements of the output link or shaft.  In machine tools the cutting tool has to be indexed in the tool turret after each operation. Also in production machines the product has to be indexed from station to station and need to be stopped if any operation is being performed in the station. Such motions can be accomplished by indexing mechanisms. Indexing mechanisms are also useful for machine tool feeds. There are several methods used to index but important types are ratchet and pawl, r ack and pinion, Geneva mechanism and cam drive.

1.  Ratchet and pawl mechanism

Fig. 4.5.1 Ratchet and pawl mechanism

A ratchet is a device that allows linear or rotary motion in only one direction. Figure 4.5.1 shows a schematic of the same. It is used in rotary machines to index air operated indexing tables. Ratchets consist of a gearwheel and a pivoting spring loaded pawl that engages the teeth. The teeth or the pawl, are at an angle so that when the teeth are moving in one direction the pawl slides in between the teeth. The spring forces the pawl back into the depression between the next teeth. The ratchet and pawl are not mechanically interlocked hence easy to set up. The table may over travel if the table is heavy when they are disengaged. Maintenance of this system is easy.

2.  Rack and pinion mechanism

Fig. 4.5.2 Rack and pinion mechanism

A rack and pinion gear arrangement usually converts rotary motion from a pinion to linear motion of a rack. But in indexing mechanism the reverse case holds true. The device uses a piston to drive the rack, which causes the pinion gear and attached indexing table to rotate (Fig. 4.5.2). A clutch is used to provide rotation in the desired direction. This mechanism is simple but is not considered suitable for high-speed operation.