POWER TRANSMISSION REFERENCE GUIDE
Using torque limiters
to protect servo-driven machinery
S
ervomotor-driven machinery abounds in today’s automated applications. These designs typically require high speeds for top throughput, dynamic operation for nimble positioning, and (in many cases) quick reversals for indexing. So it’s key to have some means to: Protect the servomotor from damage by sudden load changes, over-torque conditions, or shock due to machine issues. The latter can include errors in programming, mechanical or feedback malfunctions, faulty braking, and machine-operator mistakes. Protect the power-transmission assembly (including gearing and couplings as well as ballscrew, belt, or other drive components) from the behavior of its own inertia, motor-rotor inertia, and (reflected) load inertia — and even leverage system inertia — in atypical situations.
Torque-limiter selection requires calculation of torques to be experienced by the axis on which the limiter will operate — from the axis end back to the servomotor. Electronic protection sometimes fails to protect mechanical components in between. Shown here are Series T ball and detent torque limiters from DieQua Corp. Their low inertia makes them suitable for servomotor-driven designs (and those incorporating ballscrews) needing quick acceleration.
That’s because the processes that servo-driven machinery execute usually involve either expensive workpieces or high volumes … mission-critical operations with equipment that is itself costly. Such processes are intolerant of unexpected downtime from shock and jam-induced failures of subcomponents. Overload protection includes often-complementary electronic and mechanical means. Consider how servosystems can include electronic fusing, current limiting, and other modes of electronic overload protection to address crashes and sudden axis stops. Such protection uses controls (such as PLCs or servo drives) and feedback to track the axis state — monitoring and controlling motor-winding temperatures, current and voltage, as well as output force or torque and position in some cases. Where applicable, these modes of electronic overload protection also complement the use of powerful low-inertia servomotors to boost overall performance and efficiency. Then when there are issues with any monitored parameters (especially those that indicate total axis jams) motion controls trigger preset corrections with motor-drive shutdowns, brake engagement, or other commands to trigger a motor-based response.
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But one limitation of electronic torque limiting is that it’s not always quick enough to prevent machine or workpiece (or process) damage. That’s especially true for addressing inertial effects on dynamic motion axes; the masses of the mechanical components exert behavior that electronic corrections generally don’t resolve. In other words, the reliance of electronically programmed torque limiting on motor operation to correct problems leaves no immediate mode of addressing the action of components’ rotating masses (inertia) downstream. In contrast, mechanical torque limiters provide nearinstantaneous response (breaking free in just 1 msec in some cases) to issues. When installed at the right location in the drivetrain, these disconnect system inertia from the locked portion of the axis and let the drivetrain coast to a stop. That helps prevent damage in the first dozen or so milliseconds of machine-axis crashes or over-torqued conditions, when the most damage typically occurs. Torque limiters used this way are also called torque-overload motioncontroltips.com | designworldonline.com
5/14/19 1:20 PM
