Motion System Design

Indexer overload protection

Jams or sudden stops during an indexer's moving periods can subject its drivetrain to damaging torques. A few setups prevent damage in such situations.

In engineering, indexing is the movement from an initial position to a new position, starting and ending in rest. Mechanical cam-driven indexers are motion-control devices that use physical parts to convert constant rotary motion into this intermittent, dynamically controlled indexing. Beverage cappers, machine-tool changers, and medical-product stampers are just a few example applications. Electronic cams are also available, but mechanical cams frequently offer superior performance and accuracy in a simpler, more robust design.

These cam-driven indexers must be started and stopped only during their rest or dwell periods, and never when they're turning through a dynamic indexing position. Otherwise, the system can be exposed to uncontrolled overload.

The problem defined

Overload at a cam-driven output is:


Msd = Dynamic overload, Nm

J = Total mass inertia, kg × m2

ω = Angular speed, degrees per sec

ta = Transmission stop time, sec

Sudden stops or unexpected jams during index periods can subject machinery drivetrains to these damaging torques.

One solution:Input-side protection

Installing a current or torque limiter at an indexer's power input can mitigate some of the damage caused by overloads. There are three basic setups. The first is to connect a common amperage-draw controller to the motor. These detect overload situations and stop the machinery drivetrain when spikes do occur. Amperage-draw controllers provide a minimum level of protection, as they do not prevent immediate crash effects; they only reduce the propagation of damage.

A more sophisticated approach is to include a motor controller or inverter with a current/torque limit function in the indexing machinery. With this setup, the controller stops the machinery gradually when required: Stop time is adjusted to create a deceleration ramp, within acceptable safety limits, during emergencies. This substantially reduces dynamic overload, as well as its negative effects on both indexer mechanisms and the transmission drivetrain. That said, this setup does not reduce the immediate effects of crashes, either; it only eliminates further damage in such situations.

The most effective input-side overload protection is a mechanical torque limiter. These are installed on the power-transmission drivetrain and protect the system in most index-period jam situations. One variety, friction-plate torque limiters, stay mechanically coupled and transmit power up to a certain point; once their limit is exceeded, the plates slip on each other, and input power is no longer transferred. Shear-pin limiters are another option; they disengage completely when rated torque is exceeded. Specifying reducers with built-in torque limiters can sometimes reduce initial machinery costs.

One caveat: Torque limiters require recalibration after the system is subjected to shocks or overload situations, because torque repeatability is compromised after such events.

Even so, troubleshooting and recalibrating torque limiters is relatively simple. The most common symptom of compromised calibration is slipping; this usually causes audible noise or tooling-plate vibration when the indexer goes into its dwell position — because here, the mass is no longer under the cam's deceleration ramp control.

Output-sideoverload prevention

Synchronous torque limiters provide maximum indexer protection by detecting overloads upstream. They also provide great reset positioning accuracy, torque repeatability, and rigid backlash-free connection of driven members. Synchronous torque limiters fully protect against jams during index periods, and overloads generated by working stations when the indexer is in a dwell period.

Synchronous torque limiters consist of five basic parts: a torque adjustment ring, a spring preloading ring (that can activate a sensor to produce an alarm), phase rollers, a rotating support flange, and an assembly ring. When dynamic or additional torque exceeds the system's set limit, the rotating flange is released and partially rotates. At the same time, the rollers lift the ring, and alarm sensors detect that the torque limiter has been activated.

Some cam manufacturers include synchronous torque limiters on their mechanisms. One type is designed for and installed on indexer output shafts; another variety is installed on output flanges.

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