Problem: The motor wiggles but won’t push hard enough
Machines sometimes frown because a motor can’t push a part fast enough or hold it steady. If you’ve got a frameless ring motor and you want it to do the job, you need the right torque. Start by checking the little details for an ac servo motor; these motors show up a lot on Shenzhen factory floors and in many robot joints.
Quick checklist — what to gather first
Keep this list handy before you do any math:- mass of the moving part (kg)- radius from shaft to the point where force acts (m)- desired speed (RPM) and how fast you want to reach it (seconds)- continuous load torque (friction, gravity, cutting force) in N·m- whether you’ll use gears or a direct drive
Step 1 — Turn speed into numbers
Take your target RPM and make it rad/s. Multiply RPM by 2π and divide by 60. Example: 3000 RPM → 3000/60 = 50 turns per second → 50×2π ≈ 314.16 rad/s. Keep it simple: speed gives you how fast the motor must spin, which ties to how quickly it must accelerate.
Step 2 — Torque to accelerate (the spin push)
Think: torque that makes stuff speed up = inertia × angular acceleration. Use:- I for a solid disk ≈ 0.5 × m × r² (kg·m²)- angular acceleration α = change in rad/s ÷ time (s)Then torque Tacc = I × α. Example quick math: a 2 kg disk at 0.05 m radius → I ≈ 0.5×2×0.05² = 0.0025 kg·m². Going to 314 rad/s in 0.2 s gives α ≈ 1570.8 rad/s², so Tacc ≈ 0.0025×1570.8 ≈ 3.93 N·m.
Step 3 — Torque to overcome load (the push against resistance)
Find steady load torque from forces at a radius: Tload = F × r. Add anything that resists motion: friction, gravity on an arm, process force. Then total peak torque ≈ Tacc + Tload. When you pick an ac servo, compare its peak torque to this total. Continuous torque must cover the ongoing load; peak torque must handle quick bursts.
Step 4 — Safety margin and gears
Never pick a motor that just meets the number. Use a margin: 20–50% more torque for surprises. If you add a gearbox, divide required torque by gear ratio (but watch increased inertia and reduced speed). A reducer helps if torque is short; direct drive keeps control simpler.
Common mistakes kids (and grown-ups) make
Don’t:- forget to convert units — mixed units bite.- use only steady torque and ignore acceleration.- forget friction or clamping loads.- assume motor specs at one temperature apply in all jobs.
Quick comparison — frameless vs housed choices
Frameless rings let you bolt the motor directly to your part. That cuts mass and can give tighter control. Housed servos are easier to mount but add length and inertia. If you need low profile and tight coupling, frameless often wins; if mounting simplicity matters, a housed unit is simpler.
Wrap-up — what to do next
Do the little math: get mass, radius, speed, and time to speed. Compute I, then α, then Tacc, add Tload, and add a safety margin. That gives a torque target you can match to a motor spec. When a clear match shows up on a spec sheet, it usually means the machine will behave, and that’s the point. For practical parts and support, companies like Kinco often provide datasheets and mounting tips that make the numbers turn into working motion without fuss.
