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Electromechanical vs servo-hydraulic universal testing machines: which to choose
The drive decides what a UTM can do well: ball screws give precise, clean, low-force testing; hydraulics give economical high force. Here is how they compare and a simple rule for picking one.
Definition: An electromechanical UTM moves its crosshead with a servo motor and ball screws, while a hydraulic UTM moves a piston with oil pressure from a pump; "servo-hydraulic" means the oil flow is closed-loop controlled by a servo or proportional valve against the measured load, position or strain.
1. How the two drives work
In an electromechanical (EM) machine an AC servo motor drives one or two precision ball screws through a gearbox or belt; the moving crosshead is a nut on those screws. Speed is set by motor rpm, so the drive is inherently a displacement-controlled device with a resolution of a micrometre or better. Force is measured by a strain-gauge load cell in series with the specimen.
In a hydraulic machine a pump pressurises oil into a cylinder; the piston carries the lower or upper crosshead. In a conventional "electronic" hydraulic UTM the operator or a controller sets a flow-control valve and the load is read by a pressure transducer or load cell. In a true servo-hydraulic machine a servo valve modulates the flow several hundred times a second so that the machine follows a commanded load, position or strain rate exactly — the same technology used in fatigue rigs.
2. Comparison
| Criterion | Electromechanical (ball screw) | Hydraulic / servo-hydraulic |
|---|---|---|
| Typical capacity | 0.5 kN to 300 kN (600 kN possible) | 100 kN to 3000 kN and above |
| Cost per kN | Higher above ~300 kN | Lower at 400 kN and above |
| Speed range | 0.001 to 500 mm/min or wider; constant across the load range | Roughly 0.1 to 100 mm/min; slower at high load on non-servo designs |
| Low-force accuracy | Excellent — class 0.5 / 1 down to 1 % of the load cell | Good with a load cell; pressure-based systems lose accuracy below 10 % of range |
| Control modes | Position, load and strain control as standard | Load control native; true position/strain control only with a servo valve |
| Stroke | Long (1000 mm+ frames available) | Limited by the cylinder (typically 200–300 mm) |
| Cyclic / fatigue | Low-frequency only | Servo-hydraulic is the standard for fatigue |
| Maintenance | Lubrication of screws, belts; no fluids | Oil, filters, seals, pump; periodic overhaul |
| Noise, cleanliness | Quiet, clean — suitable for laboratories | Pump noise; oil handling |
| Power supply | Single-phase up to ~50 kN; three-phase above | Three-phase for the power pack |
| Typical work | Plastics, rubber, wire, textiles, sheet metal, machined metal specimens, R&D | Rebar, structural steel, fasteners, castings, forgings, concrete, brick, heavy compression |
3. When an electromechanical machine wins
- Capacity up to 100–300 kN. Below 300 kN the frame and drive of an EM machine cost about the same as a hydraulic one and deliver far better speed control and low-load accuracy.
- Strain-rate controlled tests. ISO 6892-1 method A, plastics to ISO 527 / ASTM D638, elastomers and films all need slow, constant, repeatable speeds — 0.5 to 50 mm/min held to ±1 %. A ball screw does this natively.
- Multi-material laboratories. Test-method changeover is a fixture swap and a software method; there is no hydraulic warm-up and no oil to keep clean.
- Long-stroke tests. Elastomers, films and textiles that stretch by several hundred per cent need crosshead travel that a hydraulic ram cannot provide.
- Environment. Quiet operation, no oil, and single-phase supply make EM the choice for college laboratories, quality-control rooms and clean production areas.
4. When a hydraulic machine wins
- Capacity 400 kN and above. Rebar of 25–40 mm, structural plate, chain, anchors, forgings and concrete are simply too big for a ball-screw drive at a sensible price.
- Rugged production testing. Hydraulic wedge grips clamp full-section rebar and rough bars automatically; the frame tolerates the shock of a 600 kN fracture day after day.
- Compression and transverse tests on masonry, concrete and timber. High force at modest precision is where hydraulics is most economical.
- Fatigue and dynamic tests. Only servo-hydraulic systems deliver cyclic loading at 1–50 Hz with load control.
5. "Electronic hydraulic" versus "servo-hydraulic" — read the specification
Many Indian hydraulic UTMs in the 100–1000 kN class are electronic hydraulic machines: a strain-gauge load cell or pressure transducer feeds a digital controller or PC that records load, displacement and extensometer strain, computes results and prints the report, while the loading rate is set by a motorised flow valve. This is fully adequate for IS 1786 rebar, IS 1608 / ASTM E8 steel coupons, bend, compression and shear tests, and it is what most steel plants and testing laboratories buy. A servo-hydraulic machine adds a servo valve and closed-loop rate control — necessary for strain-controlled tests, fatigue and research, but at two to three times the price. Ask the supplier which one they are quoting, what the load-cell class is (ISO 7500-1 class 1 is the minimum for tensile testing), and how the speed is controlled.
6. A simple decision rule
- Maximum force ≤ 100 kN → electromechanical, unless you mainly test rebar.
- 100–300 kN, mostly machined coupons, plastics or wire → electromechanical.
- 100–300 kN, mostly rebar, bolts, castings → hydraulic (cheaper grips, faster throughput).
- ≥ 400 kN → hydraulic; add servo control only if you need strain control or fatigue.
- Both needs → a 10–50 kN EM machine plus a 400–1000 kN hydraulic one costs less than a single machine that compromises on both.
Frequently asked questions
Is an electromechanical UTM more accurate than a hydraulic one?
Force accuracy depends on the load cell and its verification to ISO 7500-1, not on the drive; both types reach class 1 or better. The EM drive is more accurate in speed and position, and it keeps its accuracy at small loads, which matters for thin, soft or small specimens.
Can a hydraulic UTM test plastics or wire?
It can, but a 100 kN hydraulic machine testing a 200 N plastic specimen uses 0.2 % of its range, where neither the load reading nor the speed is reliable. Use a 5–25 kN electromechanical machine for such work.
What is the life of a ball-screw machine?
With lubrication at the recommended interval, precision ball screws last for many years of daily testing. Wear shows up as backlash in position readings; force accuracy is unaffected because the load cell measures the specimen directly.
Do I need a computer with the machine?
Not necessarily. Touch-screen or panel controllers compute yield, UTS and elongation and print reports on their own; a PC adds curve overlays, database storage, batch reports and export to LIMS. Choose according to how many results you must archive.
Which standards govern the machine itself?
Force verification: ISO 7500-1, ASTM E4, IS 1828 Part 1. Extensometers: ISO 9513, ASTM E83. The test methods (ISO 6892-1, ASTM E8/E8M, IS 1608, IS 1786) then state the class of machine required — usually class 1.
Which ETS machine
- Electro-mechanical UTMs (ETS-TNE series)Twin-column ball-screw machines of 10, 25, 50 and 100 kN with AC servo drive, plus 500 kg bench models for plastics, wire and small specimens.
- UTE 40 hydraulic UTM, 400 kNElectronic hydraulic machine for rebar to 25 mm, structural coupons and fasteners; also 100, 200, 600 and 1000 kN sizes.
- UTE 100 hydraulic UTM, 1000 kNFor 40 mm rebar, heavy forgings and compression of masonry and concrete specimens.
- All universal testing machinesCompare capacities, grips, extensometers and software across the range.
Need help choosing? Tell our engineers what you test — material, size, standard and volume — and we will recommend a machine, scales and test blocks from our range.
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