A vertical shaft impactor (VSI) is a type of impact crusher that breaks material by throwing it out from a rotor turning at high speed on a vertical axis. It is used at the final stage of the crushing circuit: in manufactured sand production, in improving the particle shape of concrete and asphalt aggregate (cubicising), and in producing fine product from hard, abrasive rocks.
At a time when natural sand resources are dwindling and dredging sand from riverbeds is restricted, the VSI has become a strategic machine for aggregate producers. In this guide you will find the VSI’s working principle, crushing modes, the settings that determine product quality on site and the maintenance points highlighted in manufacturers’ manuals.
How Does a VSI Work?
Material falls from the feed tube at the top of the crusher into the central rotor. The rotor accelerates the material by centrifugal force and throws it out through the discharge ports around it. Rotor tip speed is typically in the range of 45–75 m/s. The thrown rock breaks either by striking a bed of its own material formed around the rotor (rock shelf) or by striking a metal anvil ring. The crushed material falls by gravity onto the discharge conveyor below.
Rock-on-rock and rock-on-anvil
- Rock-on-rock: An autogenous lining of material (rock bed / rock shelf) builds up inside the rotor and around the crushing chamber. The rock breaks by striking this rock bed instead of metal. Wear costs are low and product shape is excellent; the reduction ratio, however, is relatively low. Preferred for abrasive materials and shaping.
- Rock-on-anvil (metal): The rock leaves the rotor and strikes the anvil ring directly. This gives more aggressive crushing, a higher reduction ratio and more sand production; the anvils and rotor tips wear faster. Suitable for sand production from low to moderately abrasive materials.
Cascade feeding
In some VSI designs, part of the feed is delivered outside the rotor as a “curtain of material” passing in front of the rotor ports. Rocks thrown from the rotor strike this curtain, giving additional crushing. Increasing the cascade ratio raises capacity and lowers power consumption and wear per tonne; however, the reduction ratio and sand production decrease. Cascade is a practical tool for adjusting the VSI’s product curve.
Main Components and Wear Parts
| Part | Function | Checkpoint |
|---|---|---|
| Feed tube and feed eye ring | Direct material to the centre of the rotor | Alignment; offset causes uneven wear in the rotor |
| Distributor plate | Distributes material at the base of the rotor | Wear, moving off-centre |
| Rotor tips and tip protectors | Tungsten carbide reinforced parts where material leaves the rotor | Rounding, breakage, replacement in opposite pairs |
| Rotor cavity wear plates and table liners | Protect the rotor body | Replace before the body is reached |
| Anvil ring or rock shelf | The crushing surface | Cracks, wear, exposure of the body |
| Bearing cartridge | Carries the main shaft and bearings | Oil level/flow, temperature, vibration |
| Lid seal | Prevents dust ingress | Sealing |
For details of rotor tip replacement, see VSI rotor tip replacement, and for all wear parts, our VSI crusher spare parts maintenance and replacement guide.
Settings That Determine Product Quality
- Rotor tip speed: The main way to adjust the reduction ratio. Higher speed gives a finer product and more sand; but wear increases rapidly (roughly with a power of the speed). Speed is adjusted with the pulley ratio or a frequency inverter.
- Feed size: The maximum size specified by the manufacturer for the rotor type must not be exceeded; large rocks cause blockages and imbalance in the rotor.
- Feed quantity and continuity: A VSI runs stably with constant, continuous feed. Feed should be controlled automatically to keep power consumption constant.
- Cascade ratio: Sets the balance between capacity and sand production.
- Moisture: Wet, clayey material sticks to the rock shelf and discharge; moisture control and, if necessary, a washing circuit are important in manufactured sand production.
The VSI in Manufactured Sand Production
Quality manufactured sand has three basic requirements: the right particle size distribution (grading), cubical particle shape and a controlled fines (filler / below 0.063 mm) content. The VSI is ideal for particle shape and grading; the fines content is controlled through circuit design:
- The VSI discharge is classified on a screen; oversize returns to the VSI in closed circuit.
- Excess filler is removed with an air classifier in dry systems, or with a screw sand washer or hydrocyclones in wet systems.
- The target grading and filler content for concrete sand are set according to the relevant standards (such as EN 12620).
Maintenance: Why Is Discipline Critical in a VSI?
A VSI is a high-speed machine; a small imbalance or lubrication problem can quickly turn into serious damage. Points highlighted in manufacturers’ manuals:
Rotor and balance
- Rotor tips and wear plates must always be replaced in opposite pairs; replacing a single part upsets rotor balance.
- Rounded or chipped tips, or loss of tungsten carbide, reduce efficiency; tip replacement should not be delayed.
- Rotor cavity plates must be replaced before the body is exposed; repairing a worn rotor body is expensive.
Bearing cartridge and lubrication
- Oil level and circulation should be checked every shift; a low oil level is the main cause of overheating and friction.
- Water, metal particles or sludge in the oil indicate internal wear; regular oil analysis should be carried out.
- A rise in oil temperature means a friction or cooling problem.
Vibration
- The vibration switch tripping frequently is the earliest warning of rotor imbalance or a bearing problem; the switch must never be bypassed.
- Material building up inside the rotor (especially with wet material) creates imbalance; the inside of the rotor should be cleaned at stoppages.
Inspection frequency
Checks should be made at the start or end of a shift; in very abrasive applications, rotor tips and anvils should be checked every four hours.
Maintenance Schedule
| Interval | Check / task |
|---|---|
| Every shift | Oil level and flow, vibration, bearing temperature, feed tube alignment, lid seal |
| Every 4–8 hours (abrasive material) | Rotor tips, anvils/rock shelf, distributor plate |
| Weekly | Rotor cavity plates, feed eye ring, belt tension, cleaning inside the rotor |
| At the hours recommended by the manufacturer | Oil change and oil analysis, filter change |
| Yearly | Bearing cartridge overhaul, shaft and bearing clearances, body inspection |
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Increased vibration | Rotor imbalance, one-sided tip wear, build-up inside the rotor | Replace tips in pairs, clean the rotor, check balance |
| Sand yield has dropped | Rotor speed low, tips worn, cascade ratio too high | Check speed and tips, reduce cascade |
| Uneven wear inside the rotor | Feed tube not centred | Align the feed tube |
| High oil temperature | Low oil level, dirty oil, cooling problem | Check the oil and cooler, have an analysis done |
| Blockage at the discharge | Wet/clayey material, discharge belt inadequate | Control moisture, increase discharge capacity |
Frequently Asked Questions
Is sand production with a VSI economical?
With highly abrasive rocks, a VSI running in rock-on-rock mode produces sand economically with a low wear cost per tonne. The real factors determining economy are the choice of rotor speed, feed control and circuit design.
What size of material should be fed to a VSI?
Although it varies with rotor type, VSIs generally accept feed in the 0–40/50 mm range. The exact value is given in the machine’s technical data sheet.
How long do rotor tips last?
Life varies greatly with the abrasiveness of the material, rotor speed and tonnage. Ways to extend tip life are not choosing a speed higher than necessary, keeping the feed tube aligned and replacing tips in pairs on time.
Choosing the Rotor Type
| Rotor type | Characteristics | Suitable use |
|---|---|---|
| Enclosed rotor (with rock shelf) | An autogenous lining forms inside the rotor; low wear | Abrasive rock, shaping, rock-on-rock |
| Open rotor / table type | Accepts a larger feed size, works with metal surfaces | Low-abrasion material, high reduction ratio |
| Enclosed rotor + anvil ring | Rock-on-metal; aggressive crushing | Maximum sand production |
| Enclosed rotor + rock shelf (cavity) | Rock-on-rock; low wear | Sand and shaping from abrasive rock |
Rotor and crushing chamber are chosen according to the abrasiveness of the rock and the target product. Designs that can switch between modes by changing the crushing chamber on the same machine provide flexibility as product demand changes.
The Relationship Between Rotor Tip Speed and Product
Rotor tip speed (m/s) = π × rotor diameter (m) × speed (rpm) / 60. For example, if a 0.9 m diameter rotor turns at 1,500 rpm, the tip speed is about 70 m/s. General trends:
- As tip speed increases, the proportion of 0–4 mm sand and filler increases and the product becomes finer.
- As speed increases, the wear rate rises markedly; tip and anvil life shortens.
- Low speed may be enough for shaping (rounding corners); if the goal is only to improve particle shape, high speed is not needed.
In practice, the most economical setting is the lowest speed that meets the target sand yield. On systems with a frequency inverter, speed can even be changed shift by shift according to product demand.
The Place of the VSI in the Sand Circuit
The VSI is usually at the tertiary or quaternary stage, taking 0–32/40 mm material from cone crushers as feed. A typical sand circuit looks like this:
- The cone crusher discharge is classified on a screen; material in the range suitable for sand is sent to the VSI feed bin.
- The VSI is fed continuously and full with a level-controlled feeder.
- The VSI discharge is split on the sand screen into 0–4 mm and oversize; the oversize returns to the VSI or is sold as chippings.
- The filler content of the 0–4 mm sand is controlled with an air classifier or washing.
For the whole circuit and quality criteria, see our manufactured sand production guide.
Spare Parts Stock
- Always in stock: A full set of rotor tips and tip protectors, distributor plate, feed tube and feed eye ring, rotor cavity wear plates, a set of anvils (in rock-on-anvil mode).
- Critical spares: A complete bearing cartridge (allows a quick swap), belt set, oil filters and seals.
- Planned: Rotor body, crushing chamber liners, lid seals.
Keeping a spare bearing cartridge reduces downtime after a bearing failure from days to hours; the failed cartridge is then overhauled in the workshop.
An Example from the Field: The Cause Behind the Vibration Alarm
At a basalt quarry, a VSI was stopping on vibration alarms several times a day. The first suspect was the bearing cartridge. The investigation found that the rotor tips were being replaced one at a time rather than in pairs, and that wet material was building up on one side inside the rotor. The tips were renewed in opposite pairs, the feed tube was centred and cleaning the inside of the rotor at every stoppage was added to the procedure. The alarms disappeared, and it was confirmed that there was nothing wrong with the bearing cartridge. In a VSI, the most common cause of vibration is imbalance, and it usually comes from maintenance practice.
Pre-Purchase Checklist
- Target product: sand, shaping or fine chippings?
- Are the abrasiveness and compressive strength of the rock known?
- Have the feed size and feed control (bin, level) been planned?
- Will filler control be dry or wet? Is there a water source?
- Is there a way to adjust rotor speed (inverter/pulley) and use cascade?
- Have lid opening, rotor removal and lifting equipment been planned for maintenance?
More Questions
Can a VSI work with wet material?
It can work up to a certain moisture content; but as moisture increases, the risk of sticking inside the rotor and at the discharge increases. Clayey, wet materials need pre-washing or pre-screening.
How can energy consumption be reduced in a VSI?
Avoiding a rotor speed higher than necessary, using cascade feeding in a suitable proportion and feeding the VSI only with material of a size that really needs crushing reduce energy consumption.
When should the anvil ring be turned?
In many designs the anvils are symmetrical and can be turned over to use the worn face a second time. Turning should be done before the anvil’s holding area is damaged.
VSI Solutions from CSP Mühendislik
CSP Mühendislik offers vertical shaft impactor solutions for sand production and aggregate shaping, and supplies VSI crusher spare parts such as rotor tips, anvil rings, distributor plates and feed tubes. Contact us to design your sand circuit or improve the efficiency of your existing VSI.
