The plant superintendent for an automotive gasket supplier was perplexed as he stood beside the indexer service technician. Before them was a $350,000 tableau of precision die sets, fixtures, and structural steel frame interlaced with hydraulic piping and electrical conduit. In the center was a $9,000 cam indexer, thought to be perfect for the application: generous torque capacity, predicted bearing life into the next ice age, and accuracy exceeding the specifications.

When new, these rotary dial machines achieved station accuracy of ± 0.004 in., but now three were off by almost twice that amount. One machine was consistently off-station by more than 0.250 in. — at each of its eight work stations! Though the error on three machines wasn’t compromising part quality yet, the fourth could no longer produce good parts. So it sat idle, with zero output and zero revenue.

The technician examined the indexer for run-out, shaft end play, and backlash. The assembly appeared to be in good working order. Finally, after several hours, he found the answer beneath a layer of grease and dust: there were no positioning dowels between the indexer and machine base. The indexer was held in place only by four bolts with enough hole clearance to cause trouble. It no longer lined up with scribe lines on the base, having shifted within the bolt hole clearances.

Correcting the problem was easy. Properly aligned and held in position by tight dowels, the indexer would now provide many years of reliable service.

This story shows the importance of going beyond ordinary machine design practices when applying cam indexing drives. The repeated reversing loads inherent in these drives invite a gaggle of gremlins not found in continuous motion systems.

As a designer, stopping these gremlins means you must carefully select the drive components, evaluate any external loads on the indexer, and consider the operational effects of emergency stops and jogging.

Be sure to install gear reducers, motors, and torque limiting clutches so they will perform as expected under the repeated reversing loads. Finally, look beyond the machine acceptance tests and anticipate the eventual use — and abuse — of the indexer on the plant floor.

Accommodating external loading

Cam indexing drives have static and dynamic torque ratings, plus bearing load ratings. Unfortunately, suppliers usually publish only the predicted life for the cam follower bearings at rated cam speed. But, be aware of other hidden loads and ratings when selecting a cam indexer.

For example, an unwary designer might select an indexer that has excellent predicted bearing life based on a slow index time. However, the unit may be unable to withstand the forces of drill heads pushing down on the work stations, which are located at a distance from the dial center. In this case, a gremlin surfaces as a moment load on the dial that exceeds the rating of the output shaft support bearings, Figure 1.

Manufacturing processes such as rivet forming, stamping, pressing, die cutting, drilling, reaming, sawing, and some types of welding, produce moment and thrust loads and, to some extent, radial loads, depending on the orientation of indexer to peripheral equipment. Identify these external loads and work with vendors to select an indexer that satisfies both transfer time and loading requirements.

If a load exceeds the cam indexer rating, add backup structures to isolate the indexer from the load. For axial (thrust) or moment loading, back up the dial with anvils or support blocks that clear the bottom surface of the dial by 0.005 to 0.008 in. (depending on dial flatness) when indexing. When a load is applied during dwell, the dial flexes and bottoms out against the anvil, easing the load transmitted to indexer output shaft bearings.

Where an external load is applied tangent to the dial at a distance from its center, Figure 2, the resultant force on the cam follower is F_r = F_tan × (WR/PR), where F_tan is the tangent force, WR is the work station radius, and PR is the cam follower pitch radius.

Tangential loading generates a shear force on the cam follower stud and a compression force on the needle bearings in the follower head. Extreme force can cause stud failure, cam deformation, or reduced indexer life. To counter tangential forces, either clamp or shot-pin the dial so the clamp or pin takes the load. Either method greatly reduces or eliminates the force on the indexer.

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