Why getting a stalled actuator moving in the first place can demand noticeably more torque than keeping it moving once it's already going.
A motor spec sheet often gives you a single, clean number for continuous torque and calls it a day. But a system sitting perfectly still and a system already gliding at speed are two different mechanical situations, with two different amounts of friction to overcome. Sizing a motor for only the second one — the easier one — is one of the more common and more consequential mistakes in actuator selection, because it can produce a motor that runs beautifully once moving and simply never starts.
Breakaway torque (also called static friction torque) is the torque required to overcome static friction and get a stalled, motionless system moving from a complete standstill. Every load-bearing interface in the system — bearing surfaces, gear teeth, seal contact, any surface pressed against another — is sitting in static friction while at rest, and static friction is generally higher than the friction that shows up once relative motion has already begun. Running torque(also called dynamic or kinetic friction torque) is the torque required to maintain steady-state motion once the system is already moving, when those same surfaces have transitioned to the lower-friction kinetic regime. The result: the torque needed to start a stalled actuator moving is typically greater than the torque needed to keep it moving at a steady speed once it's already in motion.
Every contact surface in a mechanical system — bearing races, gear tooth flanks, seal lips, slide interfaces — has two friction values: a static coefficient that applies while the surfaces are motionless relative to each other, and a kinetic (dynamic) coefficient that applies once they're sliding. Static is essentially always the higher of the two, because at rest, microscopic surface asperities have had time to settle and interlock more thoroughly than they can while already sliding past each other. The instant a motor supplies enough torque to overcome that static friction, the interface transitions to kinetic friction, the resisting torque drops, and — unless the motor's output also drops or the load accelerates uncontrolled — the system suddenly has torque to spare. That's the spike-then-drop shape on the torque-vs-time curve: not a control glitch, just physics changing regimes the moment motion begins. Systems that have been sitting stationary for a while can see this effect amplified further by "stiction" — extra adhesion from squeezed-out lubricant film that hasn't yet re-established itself, on top of ordinary static friction.
False, and incomplete in a way that can leave a system unable to start at all. It's true that once a system is moving, the motor only needs to supply the lower running (kinetic-friction) torque to sustain that motion. But the torque required to initially start a stalled, motionless system moving — breakaway torque, which has to overcome the higher static friction present at rest — is typically greater than that steady-state running torque. A motor or actuator sized only to the lower running-torque figure can genuinely fail to ever get the system moving from a standstill in the first place, no matter how well it would perform once already in motion. This is exactly why proper motor and actuator sizing calls out and separately verifies both a breakaway torque spec and a continuous running torque spec, rather than assuming one number covers both starting and running conditions — and it matters most for systems that start and stop frequently, or that sit stationary for extended periods where static friction (and sometimes added stiction from resettled lubricant) is most pronounced.
Explains why the torque needed to start a stalled motor or actuator moving from rest — breakaway torque, overcoming static friction — is typically higher than the torque needed to sustain motion once the system is already moving — running torque, overcoming lower kinetic friction — and why sizing a motor to only the lower running-torque figure is a common, real mistake that can leave a system unable to start at all.
A single nameplate or datasheet torque figure is easy to treat as the whole story: pick a motor whose rated torque meets the application's steady-state requirement, and assume starting will take care of itself. In reality, a system at rest and a system already in motion present two different friction conditions to the motor, and the torque required to transition between them — breakaway torque — can be substantially higher than the torque required to sustain motion afterward. Two applications can share an identical steady-state running-torque number while requiring very different breakaway margins, depending on the friction characteristics of their bearings, gear meshes, and seals.
Breakaway torque — also called static friction torque — is the torque required to overcome static friction and initiate motion from a complete standstill. Every load-bearing contact surface in a stationary mechanical system is held in place partly by static friction, which is generally higher than the friction that surface exhibits once it's already sliding.
Running torque — also called dynamic or kinetic friction torque — is the torque required to maintain steady-state motion once relative motion between those same surfaces has already begun and friction has transitioned to the lower kinetic regime. A motor's continuous running-torque requirement, once already moving at operating speed, is typically less than the peak torque it needed to supply momentarily just to break the system free from rest.
A motor or actuator sized only for its steady-state running-torque requirement — assuming the system is already in motion — can genuinely fail to start the system moving at all from a standstill, if the actual breakaway torque requirement exceeds what that motor can deliver. This is a real, common, and consequential sizing mistake, not a theoretical edge case. Motors and actuators must be sized with adequate margin to overcome the higher breakaway/static-friction torque requirement, not just the lower steady-state running torque — particularly for systems that start and stop frequently, or that have been sitting stationary for a while, where static friction effects (and sometimes additional stiction from resettled lubricant film) can be even more pronounced. This is exactly why many actuator and motor selection processes specifically call out and separately verify both a breakaway torque spec and a continuous running torque spec, rather than assuming a single torque number covers both starting and running conditions.
Because static friction is generally higher than kinetic (dynamic) friction for the same pair of contacting surfaces. At rest, microscopic surface asperities settle and interlock more thoroughly than they can while already sliding, so more torque is needed to shear that interlocking and initiate motion than to sustain motion once it has begun.
It varies widely by application, bearing type, lubrication, and how long the system has been sitting still, so there is no single universal ratio — some well-lubricated rolling-element bearing systems see only a modest breakaway premium, while systems with plain bearings, heavy preload, or extended dwell time can see a much larger spike. This is exactly why breakaway torque should be measured or estimated for the specific system rather than assumed from the running-torque figure alone.
Stiction is extra static-friction-like resistance beyond ordinary static friction, caused by lubricant film that has been squeezed out from between contact surfaces during a long dwell at rest and hasn't yet re-established a full separating film. It adds to the breakaway torque a motor must overcome, and tends to be most pronounced after long stationary periods — one more reason breakaway torque, not just running torque, needs real sizing margin.
Yes — even a single start still has to get past the static-friction threshold before motion begins at all. Breakaway torque matters most for systems that start and stop frequently or sit idle between starts, but any system starting from a dead stop, even just once, has to supply it at that moment.
Determine or estimate the breakaway torque requirement separately from the running-torque requirement — through measurement, manufacturer bearing/seal friction data, or established estimation methods — and confirm the motor's peak or short-term torque capability (not just its continuous rated torque) comfortably exceeds that breakaway figure, in addition to confirming its continuous rating covers the steady-state running torque.
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