vertical impact shaft

Vertical Impact Shaft: An Overview

The vertical impact shaft, often referred to as a vertical shaft impactor (VSI) or vertical impact crusher, is a core machine in the aggregate and mineral processing industries. It utilizes a high-speed rotor to throw feed material against a crushing chamber comprised of anvils or rock shelves. The primary mechanism is "rock-on-rock" or "rock-on-steel" impact, which fractures materials along natural fissures, producing highly cubical end products. This article details its working principle, compares it with alternative crushing technologies, presents real-world applications, and addresses common operational questions.

Working Principle and Key Advantages
A VSI crusher operates by accelerating feed material through a centrally located feed tube onto a high-speed rotor. The rotor flings the particles outward at high velocity into a stationary crushing chamber (the "impact shelf" or "anvil ring"). The resulting violent impacts cause the rock to break. The adjustable speed of the rotor and the configuration of the crushing chamber allow precise control over product size and shape.

The primary advantages of this method are:

• Superior Product Shape: Produces the most cubical particles, essential for high-quality concrete aggregate and reducing asphalt binder demand.
• Fines Production & Sand Manufacturing: Highly efficient for producing manufactured sand from hard rock.
• Beneficiation: Can improve the quality of material by selectively fracturing weaker minerals.
• Versatility: Capable of handling abrasive materials (in rock-on-rock configurations) and medium to less abrasive materials (in rock-on-steel configurations).

Comparison with Horizontal Shaft Impactor (HSI) and Cone Crusher
While all are considered compression crushers, their applications differ significantly.

Feature	Vertical Shaft Impactor (VSI)	Horizontal Shaft Impactor (HSI)	Cone Crusher
Primary Mechanism	High-velocity impact (rock-on-rock/steel).	Moderate-velocity impact with hammers/blow bars on aprons.	Compression-shear between mantle and concave.
Product Shape	Excellent. Highly cubical.	Good. Moderately cubical with some fracturing.	Fair to Poor. More elongated and slabby particles.
Fines Production	Very High. Ideal for sand making.	High. Good for chip production.	Moderate to Low. Less controlled fines generation.
Abrasion Wear	Lower in rock-on-rock mode; higher in rock-on-steel. Can be self-limiting as material forms a protective lining ("autogenous wear").	High on blow bars and apron liners due to direct contact/grinding action.	High on manganese liners; wear affects gradation consistently.
Optimal Application	Tertiary/Quaternary crushing, sand manufacturing, shaping, glass/abrasive recycling.	Primary/Secondary crushing of soft to medium-hard materials, recycling (C&D, asphalt).	Secondary/Tertiary crushing for hard, abrasive materials where shape is less critical; producing base materials.
Energy Consumption	Generally higher due to very high rotor speeds.	Moderate.	Generally lower per ton for hard rock compression crushing.

Real-World Application Case Study: Manufactured Sand Production in Norway

A leading Norwegian aggregates producer faced challenges with natural sand availability and quality consistency for high-strength concrete production at their Fjord-based facility.

• Challenge: To produce consistent, high-quality manufactured sand meeting strict Norwegian Standard NS-EN 12620 specifications for concrete aggregate from local hard granite gneiss.
• Solution: Installation of a tertiary crushing circuit featuring a large-capacity VSI crusher in a "rock-on-rock" configuration after primary jaw and secondary cone crushing stages.
• Implementation: The VSI was equipped with a cascading feed system to ensure optimal rotor loading and an automated control system adjusting rotor speed based on feed rate and power draw.
• Result:
  • The produced sand achieved an exceptional particle shape with a cubicity index exceeding 95%, reducing water demand in concrete mixes by approximately 8%.
  • Precise gradation control allowed consistent production of multiple sand products from the same feed stock.
  • The use of locally sourced granite reduced reliance on imported natural sand, lowering logistical costs and environmental footprint.
  • Despite higher initial wear part costs compared to cone crushers, the overall cost per ton of saleable product was lower due to higher product value in the concrete market.

This case demonstrates the VSI's critical role in modern aggregate plants where product specification—particularly shape—is paramount.

---

Frequently Asked Questions (FAQ)

1. What is the main difference between "rock-on-rock" and "rock-on-steel" VSI configurations?
The difference lies in the crushing chamber design.

• Rock-on-Rock: Utilizes a stationary anvil ring or rock shelf onto which material thrown from the rotor impacts. A bed of previously crushed material forms against these surfaces; new feed then impacts this material bed ("autogenous" crushing). This minimizes wear on metal parts but requires more precise feeding control.
• Rock-on-Steel: Uses replaceable steel anvils within the chamber as direct impact targets against which material is thrown from the rotor via guide tubes or tables attached directly onto it . It offers greater flexibility in particle size adjustment but results in higher wear rates on metal components.

2 . Why does a VSI produce more cubical aggregate than other crushers?
The cubical shape results from its fundamental breakage mechanism: high-velocity impact along natural grain boundaries within the rock . Particles are shattered repeatedly until they can exit through adjustable cascading curtains surrounding the rotor . This multi-impact process rounds off sharp edges , whereas compression-based crushers like cones tend to produce more elongated fragments by squeezing rock along its strongest axis .

3 . How significant is maintenance downtime for VSIs compared to cone crushers?
Maintenance profiles differ . VSIs generally require more frequent inspections of wear parts like rotors , tips , anvils ,and feed tubes —especially when processing abrasive materials —but these changes can often be performed relatively quickly using modular components . Major overhauls may be less frequent than with cones . Cone crushers have longer intervals between liner changes but replacing mantles/concaves is typically a more time-consuming task requiring specialized lifting equipment .

4 . Can VSIs handle damp or sticky feed material?
They are sensitive to excessive moisture or clay content . Sticky material can build up inside the crushing chamber , clogging feed tubes , adhering to rotors ,and reducing throughput dramatically . Proper feed preparation —such as washing/scalping ahead of tertiary stage —is crucial . For applications involving moist fines , HSIs with their grate-cleaning systems are often more suitable .

5 . Is it possible to use VSIs for primary crushing applications?
Generally no ; they are designed as tertiary/quaternary machines receiving pre-crushed feedstock typically smaller than 50mm -75mm (<2"-3") depending upon model size used primarily because their principle relies upon accelerating individual particles effectively which cannot happen if large slabs block central feeding arrangement causing severe imbalance issues leading potentially catastrophic failure if attempted without proper engineering modifications made specifically toward such purpose known sometimes as 'primary' type units available from select manufacturers only under very controlled conditions usually involving softer non-abrasive feeds like limestone etcetera