Sizing a pick-and-place gantry
Most pick-and-place requests arrive as a target — 120 picks a minute over 500 mm — and most of them are physically impossible as stated. Here is the math that decides, and the catalog hardware that fits once the numbers are honest.
The cycle-time equation nobody skips
A pick-and-place cycle is two full moves (out and back) plus the Z dips. With a trapezoidal profile and a 0.25 s acceleration ramp, the time for one move is t = accel + travel ÷ speed. A 500 mm move at 500 mm/s is 1.25 s; the round trip is 2.5 s; add four Z moves of 100 mm at 100 mm/s (1.25 s each) and the cycle is 7.5 s — 8 picks per minute. To reach 120 picks per minute the whole cycle must fit in 0.5 s, which at 500 mm travel implies speeds above 2 m/s on every axis with near-zero settle. That is a different class of machine. The engine runs this arithmetic on every request and says so.
What the catalog can do
| Family | Motor frame | Payload (typical, 2× safety) | Max travel per axis | Speed cap |
|---|---|---|---|---|
| SBA45 belt | NEMA 17 | to ~3.5 kg with 3:1 gearbox | 500 mm | 500 mm/s stepper · 1,000 mm/s servo |
| SBA60 belt | NEMA 23 | to ~10 kg | 1,000 mm | 850 mm/s stepper · 1,500 mm/s servo |
| SBA90 belt | NEMA 34 | to 25 kg stepper · 50 kg servo | 1,275 mm | 1,200 mm/s stepper · 2,500 mm/s servo |
| LSA45 / LSA60 lead-screw (Z) | NEMA 17 / 23 | 7 kg / 15 kg vertical | 50–500 mm | lead-dependent |
The engine picks the smallest family whose payload-at-speed envelope covers the request, keeps the family consistent across axes, applies a 2× load safety factor as a hard gate, and sizes the motor to at least 25% torque margin at the commanded speed. Inertia mismatch above 10:1 triggers a computed gearbox remedy rather than a warning.
Stepper or servo?
For a 2.5 kg payload over 500 × 500 mm at 8 cycles a minute, both work: the stepper SBA45 with a 3:1 gearbox carries roughly 165% torque margin; the servo version carries similar margin with far more speed headroom (its catalog cap is 1,000 mm/s versus 500) and costs about $150 more per X-axis. Throughput at spec is identical. Servo wins when you expect the spec to grow; stepper wins on price when it won't. The engine shows both columns side by side.
Gantry topologies
Dual-X gantries (XX′Y, XX′YZ) carry the Y bridge on two synchronized X actuators; the engine sizes each X motor for half the moving mass, including the Y and Z actuators themselves, using catalog mass formulas. Elevated variants add I-beam supports. Single-rail XY and XYZ systems are lighter and cheaper when the Y span is short.
Size it yourself in about a minute
Type the task — payload, travel, speed — and the Engineering Decision Engine returns a validated, priced 3D system with the torque margins, inertia check, and cycle-time math shown. Free to design and quote.
Frequently asked
Can I get 120 picks per minute over 500 mm?
Not with a single gantry at that stroke. The cycle must fit in 0.5 s, which implies more than 2 m/s on every axis with no settle time. Realistic options are shorter strokes, parallel stations, or a delta/SCARA for the fast part. The engine will quantify the gap for your exact numbers.
What safety factor does the engine use?
A 2× load safety factor on product ratings is a hard gate; the motor must also show at least 25% torque margin at the commanded speed. Both are printed in the Engineering Assumptions panel of every build.
Does payload include the gripper?
Yes — enter the total moving mass on the Z axis: gripper, tooling, and part. The engine adds the actuators' own masses for the X and Y sizing automatically.