Ball Screw Lead: The Trade Between Speed, Thrust and Resolution
"Can you send me a 1610 instead of a 1605? Same screw, just faster."
Same diameter, yes. Same screw, no. Doubling the lead doubles the travel per revolution, and it also halves the thrust you get from the same motor torque and halves your positioning resolution. Sometimes that is exactly the right trade. Sometimes it quietly breaks the axis.
Lead is the single specification that most changes how an axis behaves, and it is the one most often changed casually.
What Lead Actually Controls
Lead is the axial distance the nut travels per screw revolution. It sits in the middle of three relationships, and you cannot move one without moving the others.
Speed. Linear speed = motor rpm × lead. A 5 mm lead at 3000 rpm gives 15 m/min. A 10 mm lead at the same rpm gives 30 m/min. This is the relationship everyone knows and the reason people ask for coarser leads.
Thrust. Lead behaves like a gear ratio. Doubling the lead roughly halves the axial force available from a given motor torque. The mechanical advantage you lose does not come back.
Resolution. Linear resolution = lead ÷ encoder counts per revolution. With a 1000-count encoder, a 5 mm lead resolves 5 µm per count; a 10 mm lead resolves 10 µm. Your control system can interpolate, but it cannot create mechanical resolution that is not there.
You get to pick two of the three. There is no lead that is fast, strong, and fine.
The Vertical Axis Problem
This is where a casual lead change causes real damage.
A fine-lead screw has a shallow helix angle and is relatively resistant to back-driving - push on the nut and the screw does not easily spin. A coarse-lead screw has a steep helix angle and back-drives readily.
On a vertical axis, that difference is the difference between a load that stays put when power is removed and a load that descends under gravity.
Do not treat self-locking as a design feature to rely on, even with fine leads - friction varies with lubrication, temperature, wear, and load, and a screw that holds on the bench may not hold after six months of service. If a vertical axis must hold position without power, specify a motor brake. Treat any self-locking behaviour as a bonus, not a safety function.
When you increase lead on an existing vertical axis, check three things: motor torque against the new thrust requirement, holding brake rating, and what happens on power loss.
Lead and Critical Speed Are Independent
A common assumption: "coarse lead means the axis can go faster." Only partly.
Coarse lead means more travel per revolution, so you reach a given linear speed at lower rpm. That genuinely helps, because critical speed is a limit on rotational speed, and running slower in rpm keeps you further from it.
But the DN value - nominal diameter multiplied by rpm - and the nut's own speed rating are separate constraints set by the nut and return system design, not by lead. A coarse-lead nut does not automatically carry a higher DN rating than a fine-lead nut of the same diameter.
Check the manufacturer's published critical speed and DN limits for the specific model you are buying. Do not infer one from the other.
Choosing Lead From the Application
Work backwards from what the axis has to do.
Start with required linear speed and available motor rpm. Minimum lead = required speed ÷ usable rpm. This gives you a floor.
Check thrust at that lead. Take your worst-case axial load, add acceleration force and friction, and confirm the motor can deliver it through the chosen lead with margin. If not, you need more torque or a finer lead.
Check resolution. Divide the lead by your encoder resolution and compare against your positioning tolerance. Leave headroom - mechanical resolution is not the same as achievable accuracy, which also depends on screw accuracy grade, backlash, and stiffness.
Then check critical speed and DN at the rpm implied by your chosen lead.
If the numbers do not close, the answer is usually a larger diameter rather than a different lead. Diameter buys you critical speed, stiffness, and load capacity simultaneously.
Common Lead Choices in Practice
What we see most often, by application type:
- Lead = 5 mm on 16–25 mm shafts: general CNC, routers, engraving, most positioning work. The default for good reason.
- Lead = 10 mm: plasma and laser tables, axes where traverse dominates the cycle, larger gantries.
- Lead = 16–20 mm and above: pick-and-place, transfer, palletising, long-stroke handling.
- Lead = 1–2 mm on miniature shafts: optical stages, medical devices, fine positioning where the moves are short.
These are starting points, not rules. An unusual duty cycle beats a category every time.
If You Are Changing Lead on an Existing Machine
The screw is not a drop-in swap even when the diameter and shaft ends match.
Recheck: motor torque against the new thrust, drive tuning and gain settings, controller pitch/lead parameters, resolution against the positioning spec, and - on a vertical axis - the brake. A machine that ran correctly on a 1605 can behave badly on a 1610 with the same controller settings untouched.
It is a straightforward change when the numbers are checked first. It causes confusing faults when they are not.
Tell Us the Duty, Not Just the Part Number
When customers send a part number, we quote the part number. When they send the duty - stroke, speed, load, orientation, tolerance - we can often point out that a different lead or diameter reaches the target with a smaller motor or a cheaper screw.
That conversation costs one email and regularly saves more than the price difference on the screw.
Tianjin OuNaiDa Transmissions Technology Co., Ltd. manufactures ball screws, support units, linear guideways, gears, racks, sprockets, and couplings. Established 2008, with 20 years of manufacturing and 15 years of export experience, working to CE and ISO 9001:2015 standards.
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