Three published ceilings for electric linear actuators in heavy lifting sit at roughly 20 kN, 60 kN and 100 kN, and none of them is wrong.
Thomson Industries published the first in 2023, its own engineering manager the second in 2025, and TEA Hamburg the third in March 2026.
A duty landing between those figures is not an edge case. That band covers most of the heavy-lift range.
The load rating is the first number a specifier prices and the one that decides least. What settles a duty inside the band is cycle rate, the number of points that must stay level, and whether the load has to hold with the power removed.
What Caps Load and Stroke on Actuators and Screw Jacks
The screw inside the unit caps an electric actuator’s force, not the technology. Rod-style units are catalogued well above every published ceiling for the category:
- Tolomatic RSX, planetary roller screw: up to 100,000 lbf (445 kN)
- Power Jacks Rolaram, roller or ball screw: up to 400 kN dynamic load
- Power Jacks POWERAM, machine or ball screw: 10 kN to 200 kN
Published guidance describes a product class, not a physical limit: the 20 kN to 100 kN band is the commodity ball-screw and machine-screw rod actuator.
TEA Hamburg’s other figure, screw jacks as the cheaper choice from 150 kN, fails on availability. Actuators are catalogued far above it.
The same figure holds on economics. Reaching 400 kN through a roller screw is the costlier route to a force a mechanical jack already covers.
Screw type caps the jack side too. Duff-Norton’s ball screw jacks stop at 50 tons while machine screw models continue past 150, and worm screw hoists rated 1 to 100 ton cover the working middle of that range.
Stroke differs structurally, not numerically. TEA Hamburg catalogues actuator travel at 100 to 1,000 mm, while a jack’s screw is cut from bar stock.
The selection criteria have shifted underneath that ceiling. What a supplier should be asked is which screw at what duty and what stroke, not what maximum thrust.
Duty Cycle Limits on Screw Jacks and Electric Actuators
A screw jack’s published load rating already assumes intermittent duty, and the allowance falls as the load rises. Duff-Norton rates its 7515 and 7522 models at 33 percent duty at maximum capacity, 67 percent at three-quarters, and 100 percent at rated capacity.
Those percentages are thermal, set against a 100 °F rise above ambient not exceeding 200 °F.
The heat comes from the worm drive. Duff-Norton’s published efficiencies for machine screw actuators span 30.6 to 8.0 percent, the largest units at the bottom.
The self-locking ratios are least efficient of all, as low as 3.6 percent. When comparing lifecycle costs, that loss is the running cost of holding a load without power.
Actuator makers publish the same constraint as a duty percentage or a minutes-on ratio that also moves with load.
Lifting speed follows gear ratio and drive arrangement, spanning roughly 40 to 5,500 mm/min in Duff-Norton’s catalogue alone, with the heavy self-locking units slowest.
Duty cycle caps the share of the hour a jack runs, and speed caps the distance inside it. The jack concedes on both.
Synchronizing Several Lifting Points With Screw Jacks
Mechanically coupled screw jacks hold their relationship through the line shaft, so one motor drives an entire frame. Multi-point lifting systems on platforms, weir gates and jacking frames work this way.
That advantage stops in two published places. Duff-Norton caps the first worm shaft’s torque at 300 percent of its rated full load torque.
The spatial limit is published too. Ron Giovannone, an engineering manager at Thomson Industries, puts a typical frame at four to six jacks spaced 1.2 to 1.8 m apart, and warns of lag past that.
Point count, not a bigger jack, is what scales the frame: five coordinated jacks carry more than 1,990 kN.
Independently driven actuators win this axis on integration. Giovannone notes that one wire added to the power supply synchronizes multiple units, at the cost of a motor, encoder and sensor on each.
The two technologies converge at the top of the frame. Past either limit, Duff-Norton’s own answer is to motorize the jacks individually and synchronize them electronically.
Self-Locking Screw Jacks and Back-Driving Electric Actuators
Self-locking is a friction condition, not a brake, and the jack manufacturers say so themselves. Duff-Norton’s caution box states that standard ratio machine screw actuators are not always self locking and require motors with brakes.
The product literature adds a condition. A higher-ratio machine screw jack is load holding only as long as it is not subject to vibration, which otherwise makes the lifting screw creep down under load.
Where vibration is considerable, the remedy is a magnetic brake on the drive motor.
Ball screw jacks and high duty cycle actuators are self lowering outright and need brakes by design. The gap between ball screw and machine screw jacks is the same gap that makes a ball-screw electric actuator back-drive.
Adding a brake is not the whole cost. Duff-Norton notes most machine screw models need roughly half as much torque to lower a load as to raise it.
Any duty holding a person, a gate or a suspended load specifies that brake anyway.
When to Choose an Actuator and When to Choose a Screw Jack
The electric actuator wins duties that are light, fast, few-pointed and permitted to hold on a brake. The worm screw jack wins duties that are heavy, slow, multi-point, or required to hold with the power off.
| Choose an electric linear actuator when | Choose a worm screw jack when |
|---|---|
| Load per point under 100 kN | Load per point to 100 ton |
| Stroke fits a catalogue, 100 to 1,000 mm | Stroke cut to order, past 6 m |
| Travel must be fast or precisely positioned | Duty intermittent, travel near 40 mm/min |
| Points few, far apart, or independent | Points within 1.8 m, moving as one |
| A motor brake fits the design | Load must hold with power off, low vibration |
Hold your duty against both columns before shortlisting, then send the same numbers to a supplier. Every crossover here is per manufacturer and per duty.
If the duty lands in the jack column, screw jacks rated to 100 ton hold the load without power at 39 to 60 mm/min.
Which Duty Belongs to Which Technology
The worm screw jack owns the duty that is heavy, slow, multi-point and required to stay put. That means load per point above the commodity actuator class, travel cut to order, and points close enough to couple mechanically.
Tonnage is rarely the axis that decides it. The number of points that must stay level, and whether the load holds with the power removed, flip the answer outright.
Load and stroke only shift it gradually. A choice made on the load rating alone gets remade later, on the duty cycle, by whoever has to run the machine.
