Figure 1: Visual comparison . Steppers (Left) are dense and simple. Servos (Right) are longer and include a visible feedback encoder housing on the rear. The Million Dollar Question: "Which Motor Do I Need?" If you are designing a CNC machine, a packaging robot, or a conveyor system, you face the same dilemma every time: Stepper or Servo? Make the wrong choice, and you face two disasters: The Stepper Trap: Your machine "loses steps" (positional error) without knowing it, scrapping parts. The Servo Trap: You spend $5,000 on a system that could have been done for $500, blowing your budget. This guide bridges the gap between mechanical requirements and electrical reality. 1. The Stepper Motor: The "Digital Ratchet" Think of a Stepper Motor like a very strong, magnetic ratchet. It divides a full rotation into equal steps (typically 200 steps per revolution, or 1.8°). Pros: Incredible Holding Torque: Ste...
In my design work, I've frequently used indexing boxes from the Japanese company Sankyo Seisakusho. The model of the indexing box is SANDEX. It is a generic name for their cam units that combine a simple and ideal indexing mechanism: the Roller Gear Cam mechanism.
Sankyo's original cam technology consists of a precision-machined roller gear cam and a follower spider (turret) fitted with needle bearings. This combination creates a unit that has always enjoyed high sales and responds to industrial needs where new technologies are constantly evolving.
The Mechanics of the Roller Gear Cam
Unlike traditional Geneva mechanisms which suffer from high impact loads at the start and stop of motion, the SANDEX unit uses a continuous rib on the input shaft to drive the output turret.
The unique geometry allows for pre-loading: the tapered rib of the cam is in constant contact with two followers at any given time. This eliminates internal clearance, resulting in zero backlash and extremely high rigidity.
Types of Motion
The indexing units must accommodate three types of basic motion:
- Indexing Drives: Intermittent rotating motion (Start-Stop-Start). Used for assembly tables and packaging.
- Oscillating Drives: Oscillating rotating motion (Back-and-Forth). Ideal for pick-and-place arms.
- Roller Drives: Continuous rotating motion. Used for synchronized conveyors.
SANDEX units generally offer a higher number of indexes, flexible index angles, and a greater degree of design freedom than other indexing mechanisms. They are more rigid and have higher output values relative to the size of the unit. A broad lineup of models and sizes meets the needs of automated rotary machines from the world's smallest miniature indexes to the largest class of super-size indexes.
Parallel Shaft Models
In addition to the standard right-angle layout, one model with parallel input/output shafts is also available, using a parallel mechanism as an "ideal flat cam" (Para-Dex). This is optimum for conveyor drives with a large feed pitch or indexing drives with relatively long stopping intervals.
This is an example of indexing movement (intermittent rotation):
Selection Criteria for Mechanical Indexers
When selecting an indexer for your design, you must calculate the dynamic forces involved, not just the static load.
Key factors include:
- Moment of Inertia (J): The most critical factor. The torque required to start and stop the load is usually much higher than the torque required to keep it moving.
- Index Time vs. Dwell Time: How fast must it rotate (e.g., 0.5 seconds) and how long does it stay still? Aggressive timing curves (like Modified Sine) can smooth out the jerk.
- Number of Stations: Does the table need to stop 4, 6, 8, or 12 times per revolution?
- Accuracy Requirements: Roller gear cams typically offer arc-second accuracy, far superior to air cylinders or simple motors.
Useful Link: Official Sankyo Automation Website
Recommended Reading: Cam Design and Manufacturing Handbook
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