Rodless actuators: belt drive vs ball screw

In short:Choose a belt-driven rodless actuator for long strokes and high speed (Tolomatic quotes up to about 5 m/s and strokes beyond 5 m), and a ball screw for thrust, stiffness and accuracy on strokes up to about 1–3 m and speeds up to about 1.5 m/s; then size the guidance for the cantilevered payload's moments, not just its weight.

The short answer

Rodless actuators carry the load on a carriage that runs along the body, so the stroke is the full working length and the actuator guides the load itself. The drive inside is either a toothed belt or a screw. Belt for distance and speed; screw for force, stiffness and accuracy. Most selections are settled by four numbers: stroke, peak speed, thrust and position tolerance.

Belt vs ball screw: the comparison table

Figures below are Tolomatic’s published comparison for its own rodless ranges, converted to metric. They are a fair guide to the technologies in general.

Sources: Tolomatic 9900-9210; Tolomatic blog.

Other makers’ belt axes sit in the same region. Festo’s ELGD-TB-KF-120 toothed belt axis, for example, lists repetition accuracy of ±0.04 mm, 3 m/s maximum speed and 50 m/s² maximum acceleration (Festo datasheet).

Speed and stroke: why long screws get slow

A screw’s top speed is not only a nut or bearing limit. On long strokes it is critical speed: the rotational speed at which the unsupported screw reaches its first bending resonance and whips. For a solid steel screw it can be estimated from beam theory:

n_c ≈ (60 λ² / 2π L²) × (d_r / 4) × √(E / ρ)

where L is the unsupported length (m), d_r the root diameter (m), E/ρ for steel gives √(E/ρ) ≈ 5,100 m/s, and λ is the end fixity factor. Linear Motion Tips lists λ = 1.875 (fixed–free), 3.142 (supported–supported), 3.927 (fixed–supported) and 4.730 (fixed–fixed), and recommends running below 80% of critical speed (Linear Motion Tips). Critical speed falls with the square of length, so doubling the stroke cuts it to a quarter.

A belt has no equivalent limit. Its speed is set by pulley speed, belt rating and the guide’s maximum velocity. Belt travel per revolution is large (60–170 mm per revolution on Tolomatic’s MXB), so 2 m/s needs only around 700–2,000 rpm at the motor or gearbox.

Tolomatic’s ten-tip guide adds that heavy loads on belt drives are hard to stop: the regenerated energy on deceleration has to go somewhere, so check the drive’s braking capacity (Tolomatic 9900-9201).

Accuracy, repeatability and stiffness

Accuracy is how close the carriage gets to the commanded position along the stroke. A rolled ball screw has a defined lead error per 300 mm. A belt’s effective pitch varies with tension and material, so its lead accuracy is about three times worse. Both can be improved with error mapping in the controller or a linear encoder on the carriage.

Repeatability is how closely the carriage returns to the same point. Belt repeatability is good enough for most pick-and-place (±0.05 mm in Tolomatic’s figures), but Tolomatic notes it can drift as the belt stretches, so tension must be maintained.

Stiffness is where the technologies differ most:

• A ball screw is a stiff steel load path; a preloaded nut removes backlash (Tolomatic’s B3S and MXE offer low-backlash nuts, quoted from 0.05 mm).
• A belt behaves like a spring whose rate falls as the free belt length increases. Stiffness changes along the stroke and is usually lowest at the end furthest from the drive pulley. Expect longer settling after hard decelerations, and expect resonance if the servo is tuned aggressively.

If the process applies force at the carriage (pressing, drilling, inserting), choose a screw. If the carriage only carries a payload between two points, a belt is usually fine.

Guidance: what carries the load

The drive provides thrust; the guide carries the payload and its moments. Rodless actuators are offered with different guides:

• Plain or solid bearings running on the body: simple, tolerant of dirt, modest load and moment capacity. Tolomatic’s MXB-S uses self-cleaning solid bearings for dusty sites.
• Recirculating ball profiled rails: higher load, stiffness and moment capacity, and higher speed. MXB-P and MXE-P use profiled rail; B3S and B3W have recirculating ball bearings integral to the extrusion.
• Unguided (MXB-U): the actuator drives an external guide. Fastest option, but your guide carries everything.

Tolomatic’s B3 family quotes up to 35.6 kN of load capacity with carrier options, which shows how much capacity guidance can add independently of the drive’s thrust.

Cantilever and moment loads: how to size them

The commonest rodless sizing error is checking payload weight against the “max load” headline and ignoring moments. Tolomatic’s guide is explicit: off-centre loads create pitch (My), roll (Mx) and yaw (Mz) moments, published moment ratings are usually maxima for one moment alone, and acceleration at the ends of the stroke adds dynamic moments (Tolomatic 9900-9201).

Calculate for each axis of offset:

• Static moment = m × g × offset (horizontal offset from carriage centre)
• Dynamic moment = m × a × height (height of the centre of mass above the carriage, in the direction of travel)

Then check each against its rating, and check them in combination. A common catalogue method is to sum the ratios of each applied load to its rating (force and each moment) and keep the total at or below 1. Use the exact method in the catalogue of the actuator you are buying; some include life factors.

If moments are too high, options are: a larger profile, an auxiliary second carriage (Tolomatic says this doubles load capacity on the BCS), a dual or mirrored carriage, or an external guide with the actuator only pushing.

Long strokes: support and deflection

Long actuators must be supported along their length. Tolomatic’s guide says to use enough support points in the right places, mount to a straight surface, and allow for dead length and the motor in the overall envelope. Festo gives dynamic deflection under a moving load for its belt axis of 0.05% of axis length, maximum 0.5 mm (Festo datasheet): a figure worth asking for on any long, self-supporting span.

Worked example: 2.6 m transfer axis, belt or screw?

Requirement. Horizontal transfer, stroke 2,600 mm. Payload 15 kg with its centre of mass 150 mm to the side of the carriage centre and 100 mm above it. Move the full stroke in 2.0 s including accelerations of 0.3 s each, 20 times a minute. Process force: none. Position tolerance at each end ±0.2 mm.

Speed. With 0.3 s ramps: 2.6 = v × (2.0 − 0.3) → v ≈ 1.53 m/s, a ≈ 5.1 m/s².

Screw check. A 25 mm ball screw with about 21 mm root diameter, unsupported length about 2.8 m, fixed–supported (λ = 3.927):

n_c ≈ (60 × 15.4 / (6.28 × 7.84)) × (0.021 / 4) × 5,100 ≈ 18.8 × 26.8 ≈ 500 rpm.

At 80%, about 400 rpm. Even with a 32 mm lead that is about 0.21 m/s, a seventh of what is needed. Screw supports or a rotating-nut design would raise it but not by a factor of seven. Screw is out.

Belt check. At 1.53 m/s and, say, 120 mm per revolution, the pulley turns 765 rpm: easy. Thrust = 15 × 5.1 + friction ≈ 77 N + 30 N (assumed carriage drag) ≈ 110 N, well inside belt ratings. Repeatability of ±0.05 mm meets ±0.2 mm.

Moments.

• Roll from the side offset: 15 × 9.81 × 0.15 ≈ 22 Nm (static, always present)
• Pitch from acceleration: 15 × 5.1 × 0.10 ≈ 7.7 Nm (at each end of stroke)
• Yaw from acceleration with the side offset: 15 × 5.1 × 0.15 ≈ 11.5 Nm

These act together during the ramps, so check the combined ratio. A profiled-rail carriage, such as the MXB-P, is the natural choice; if the combined ratio is above 1, add an auxiliary carriage.

Other points. At 20 moves a minute the belt’s 100% duty rating is useful. Ask for the deflection over the chosen support spacing and put supports at the recommended pitch.

Result. A profiled-rail belt-driven rodless actuator, sized on combined moments, with a servo and low-ratio gearbox. If the process later needs ±0.02 mm at an intermediate station, add a linear encoder rather than changing to a screw.

Products in our range to consider

• Tolomatic MXB: belt-drive rodless in six sizes, unguided, solid bearing or profiled rail, up to 5,080 mm/s and 10.5 m stroke.
• Tolomatic B3W: wide steel-cord belt rodless for strokes to 14.6 m and 5 m/s with integral bearing guidance.
• Tolomatic MXE: screw-drive rodless in six sizes, solid bearing or profiled rail, up to 19.1 kN thrust.
• Tolomatic B3S: screw-drive rodless with integral recirculating ball guidance, loads up to 35.6 kN with carrier options.
• Tolomatic BCS: economical rodless screw actuator with stainless band seal, strokes to 3 m.
• LB belt-drive linear module: steel-belt module with integral guide rail, 2,000 mm/s and strokes to 3,050 mm.
• LE ballscrew linear module: ball screw module with built-in guide, repeatability ±0.02 mm, leads 5–32 mm.
• BLM built-in linear module: compact C7 ballscrew module, repeatability ±0.01 mm, for 100–400 W servos.
• Tolomatic TRS: twin profile rail screw stage for heavy, high-moment loads up to 1,960 kg.

What else the build needs

• Motor and, for belts, usually a low-ratio gearbox to match inertia: see servo and stepper motors, how to select a planetary gearbox.
• Drive with enough braking capacity for decelerating heavy belt axes: see servo drives.
• Brake on any vertical or inclined axis, belt or ball screw.
• Feedback: motor encoder, plus a linear encoder if absolute accuracy matters along a belt axis. See encoders.
• Cable management: drag chain sized for the stroke and speed.
• Controller: see motion controllers.
• For rod-style actuators and full thrust sizing, see how to size a linear actuator and ball screw vs roller screw.

Things that catch engineers out

• Checking payload weight, not moments. The offset and the acceleration set the guide size.
• Using single-moment ratings for combined loads. Ratings usually assume one moment acting alone.
• Long screws at speed. Critical speed falls with length squared; a 2 m+ screw axis is usually slow.
• Belt axes in vertical service without a brake. Belts always back-drive.
• Tuning a belt axis like a screw axis. Lower, position-dependent stiffness means lower usable gains and longer settling.
• Belt tension drift. Repeatability changes as the belt stretches; plan for re-tensioning.
• Forgetting dead length. Overall length is stroke plus carriage plus end caps plus motor.
• IP ratings on rodless bodies are modest. Slotted bodies with band seals typically sit around IP44; washdown needs a different design.

Checklist

Send these to our engineers, or enter them in the spec builder:

• Stroke and overall envelope, including motor position (inline or reverse-parallel)
• Orientation: horizontal, vertical or inclined
• Payload mass and its centre of mass offsets (side, height, along the stroke)
• Move profile: distance, time, acceleration, dwell, moves per minute
• Process forces at the carriage, if any
• Accuracy and repeatability required at each position
• Support points available along the length
• Environment: dust, coolant, washdown, temperature
• Motor and drive preference, or ask us to supply
• Required life in km or cycles

For the wider family, see our linear actuators overview.