Choosing a concrete vibrator comes down to three numbers: shaft diameter vs. the narrowest rebar spacing, frequency band matched to mix workability, and amplitude (typically 0.5-3.0 mm) scaled to the section thickness being consolidated.
Internal pendulum vibrators in the 30-50 mm shaft class running 11,000-12,000 vpm (roughly 183-200 Hz) cover the majority of wall, column, and beam pours 100-300 mm thick; external form-type units in the 3-15 kW bracket drop to 6,000-9,000 vpm for >500 mm mass sections where a poker cannot reach the form face [S2][S3].
Internal vs. External vs. Surface: Three Operating Classes
Concrete vibrators split into three functional classes, and the right pick depends on section thickness and rebar density rather than marketing claims [S2]. Internal (immersion/pendulum) vibrators carry an eccentric shaft housed inside a steel poker head, usually 25-70 mm diameter, driven through a flexible shaft by a 0.75-2.5 kW motor at 2,800-3,000 rpm input speed; the head itself multiplies that to roughly 11,000-12,000 vpm at the eccentric mass. External (form) vibrators clamp to the formwork face in 3-15 kW ratings and run 6,000-9,000 vpm, with a 220 V single-phase or 380 V three-phase supply typical of Chinese OEM offerings. Surface (screed) vibrators ride on the slab top, 0.5-1.5 kW, and only suit <150 mm flatwork. For a full taxonomy of concrete vibrator types and their drive architectures, the in-house reference is the starting point before sourcing.
Frequency is a hard selection knob, not a preference: high-frequency internal heads (12,000 vpm) consolidate low-slump stiff mixes and high-strength concrete where air voids must be driven out fast, while 6,000-9,000 vpm external units handle fluid mixes in thick walls without segregating the aggregate. Centrifugal force scales with the square of the eccentric mass radius times the shaft speed, which is why a larger-diameter head at lower vpm can match a smaller head at higher vpm on consolidation energy but with very different concrete flow behaviour.
Match Shaft Diameter to Section Width and Rebar Spacing
The first rule on a job card is: poker diameter must be at least 1/3 of the narrowest section dimension and small enough to clear the rebar grid by 25-50 mm, otherwise the head will lock against the cage and leave voids [S3]. In practice this means 30-40 mm shafts for 100-150 mm walls, 50-60 mm shafts for 200-300 mm columns, and 70 mm+ shafts only for >500 mm mass pours. Rebar spacing below 75 mm typically forces a 25-30 mm head regardless of section width, accepting longer pour time in exchange for complete consolidation between bars.
Insertion pitch (the spacing between successive immersion points) should be 1.5x the head diameter, so a 50 mm poker on a 300 mm column means roughly 75 mm between insertions, with each insertion held 5-15 seconds until air ceases to escape and the surface goes glassy. Pull the head slowly at 50-80 mm/s so the cavity refills rather than trapping a void at the tip.
Power, Frequency, and Amplitude: The Three Levers
Three levers interact: motor input power (kW), eccentric frequency (vpm), and amplitude (mm). For a 50 mm high-frequency internal head, expect 1.5-2.5 kW input, 11,000-12,000 vpm, and 0.5-1.5 mm amplitude at the head tip. For a 150 mm external form vibrator on a precast battery mould, expect 2.2-3.0 kW per clamp, 6,000 vpm, and 1.5-3.0 mm amplitude. Under-powering any of these three is the most common cause of bug-holes, honeycombing, and the segregation that shows up as laitance bleed at the top of a wall lift. [S1]
Power frequency on the eccentric itself is 280-300 Hz for high-frequency internal heads, which is why even a 50 Hz mains supply can drive these through a frequency converter in the motor housing; OEM specs in this class name 220 V single-phase / 380 V three-phase at 50 Hz input with the head running its own 200 Hz class output [S2]. Builders of large slipform walls frequently site a portable electric motor inverter on the deck to dial frequency up or down as the mix design shifts through the day.
When a Vibrator is the Wrong Tool
Concrete vibrators are NOT for self-compacting concrete (SCC) with slump-flow >550 mm, since the mix is already fluid and mechanical vibration risks segregating the aggregate and breaking the thixotropic paste structure. They are also a poor fit for mixes with large aggregate above 40 mm in narrow forms, where the stone arches against the poker and refuses to rearrange. In both cases the correct decision is to revisit the concrete admixture package and the mix design, not to buy a more powerful head. [S2]
Limitations show up in failure modes that any site engineer can read: a head that leaves a crater when withdrawn means the mix is too dry or the immersion time too long; bleeding at the surface means over-vibration; bug-holes on the form face mean amplitude is too low for the form geometry and an external clamp should be added. Where fibre reinforcement is in the mix, vibration requirements change again; the concrete fiber reference covers the trade-off between vibration energy and fibre orientation.
Safety, Standards, and Operator Discipline
Electric immersion vibrators in Chinese safety guidance require a residual-current device (RCD / leakage protector) on the motor supply, a reliable protective earth, and the operator wearing insulating boots and gloves [S3]. The flexible shaft must be inspected for kinks before each shift because a partially seized shaft destroys the head's bearings within hours. For shaft-driven units, the shaft length is usually 4-6 m on portable frames and 9-12 m on trolley units, and exceeding the rated length drops the head below its minimum vpm because shaft whip absorbs energy.
There is no single international standard that pins a vibration frequency for a given slump, but the operator-side rules (RCD, earthing, PPE) and the maintenance interval for the flexible shaft (typically 200-300 operating hours before replacement) are the levers a buyer can verify against the OEM manual before sign-off.
Sourcing Levers in the 2026 China Market
Chinese OEM offerings cluster in three bands: spring-return and non-spring-return internal heads (1.5-2.5 kW, 50 mm class) from YangFine and peers, eccentric high-frequency heads for precast and dry-mix applications, and flexible-shaft pump combinations bundling the drive motor and hose [S2]. Lead time on stock 50 mm internal heads with 6 m shaft is typically 7-15 days ex-works Zhejiang, with custom lengths at 30-45 days. Trolley-mounted twin-head units for slab pours add a concrete batching plant tier of integration that some buyers run as a single procurement line.
Two signals worth tracking in the next 90 days: any shift in 50-70 mm high-frequency head pricing from YangFine and comparable Zhejiang makers, and the publication cycle of GB/T 8529 rotary vibrator revisions that govern eccentricity tolerance. Read alongside our concrete groove cutter reference, the selection logic for cutting after curing mirrors the same shaft-diameter-vs-thickness reasoning used to pick the immersion head.