In a vertical shaft impactor (VSI), material thrown from the rotor strikes a surface around the crushing chamber and breaks. This surface is either an anvil ring made of metal segments or a rock shelf (rock box) formed from the material itself. Which one is used determines whether the crusher works in rock-on-metal or rock-on-rock mode, which affects everything from the reduction ratio to the wear cost.
In this guide we cover the differences between the anvil ring and the rock shelf, anvil wear, rotating and replacement steps and spare part selection. For general VSI operation, see our vertical shaft impactor guide.
Rock-on-Metal and Rock-on-Rock
| Feature | Anvil ring (rock-on-metal) | Rock shelf (rock-on-rock) |
|---|---|---|
| Reduction ratio | High | Lower |
| Sand production | High | Medium |
| Wear cost | Higher | Low |
| Particle shape | Good | Very good |
| Suitable material | Low to moderately abrasive | Abrasive rock |
Some crushers can switch between the two modes by changing the crushing chamber. The choice is made according to the target product, the abrasiveness of the material and the wear budget.
Construction of the Anvil Ring
- Anvil segments: Wear-resistant parts (usually high chrome or manganese) arranged around the ring.
- Anvil carrier ring: The supporting structure to which the segments are attached.
- Fixings: Keep the segments in place.
In many designs, the anvil segments are symmetrical; the worn face can be reused by turning it over or by rotating the segments around the ring.
Wear and Failure Signs
- Gouging and thinning on the anvil face,
- Cracked or broken segments,
- The carrier structure behind the anvil ring becoming exposed,
- Coarser product and lower sand yield,
- In rock shelf mode, the shelf breaking up or forming unevenly.
Inspection and Maintenance
| Check | Interval | Method |
|---|---|---|
| Anvil wear | Daily / start of every shift with abrasive material | Visual, with the lid open |
| Segment fixings | Weekly | Check for loosening |
| Carrier ring | Monthly | Wear and cracks |
| Rock shelf condition | Daily | Check formation and distribution |
Rotating and Replacing Anvil Segments
- The crusher is stopped, LOTO is applied and the lid opened.
- The anvil wear profile is examined and the most worn zones identified.
- Symmetrical segments are turned over or rotated around the ring to bring less-worn faces into the impact zone.
- Segments that have reached the end of their life are replaced as a set.
- Fixings are renewed and the carrier ring checked.
- The lid is closed and the result confirmed with a product sample.
Common Mistakes
- Missing the opportunity to rotate the anvils,
- Running until the carrier ring is exposed,
- Insisting on rock-on-metal mode with abrasive material,
- Running the rotor faster than necessary.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Anvil wears quickly | Abrasive material, high speed | Reduce speed, consider rock-on-rock mode |
| Segments breaking | Metal, oversized feed, brittle material | Metal separator, feed control |
| Sand yield has dropped | Anvils worn, tips worn | Check anvils and tips |
| Rock shelf breaking up | Wet material, unsuitable feed | Moisture and feed control |
An Example from the Field
At a granite plant, a VSI was running in rock-on-metal mode and the anvil segments were being changed every few days. Sand yield was higher than needed and the wear cost was very high. The crushing chamber was converted to rock-on-rock mode and rotor speed was reduced slightly. Sand yield stayed within the target range while the wear cost fell considerably.
Pre-Purchase Checklist
- Have the VSI model and crushing chamber type been identified?
- Are the number of anvil segments and the part number known?
- Are the fixings included in the order?
- Has a mode change (rock shelf) been considered?
Frequently Asked Questions
How often is the anvil ring replaced?
It depends on abrasiveness, rotor speed and capacity; life can be extended by rotating the segments.
Is rock-on-rock mode always more economical?
In terms of wear, usually yes; but the reduction ratio and sand production are lower. It should be chosen according to the target product.
Can anvil segments be of mixed materials?
Mixing different materials in the same ring is not recommended, as it can cause uneven wear.
Crushing Chamber Configurations
In VSIs, the crushing chamber is the structure around the rotor, and manufacturers offer it in different configurations. The configuration chosen determines product grading, particle shape and wear cost together.
| Configuration | Crushing mechanism | Typical goal |
|---|---|---|
| Enclosed rotor + anvil ring | Rock-on-metal | High sand production, high reduction ratio |
| Enclosed rotor + rock shelf | Rock-on-rock | Shaping, economical crushing of abrasive material |
| Open rotor + anvil | Rock-on-metal | Coarser feed, low-abrasion material |
| Cascade feed configurations | Interaction of material through and around the rotor | Higher capacity, lower wear |
Choosing the Anvil Material
- High chrome cast iron: High wear resistance; gives good results with controlled feed and material free of metal.
- Manganese steel: Impact resistant; preferred in applications with coarse feed and a risk of foreign material.
- Composite/ceramic reinforced segments: Extend life with very abrasive materials; require feed discipline.
Effect of Operating Conditions on Anvil Life
| Condition | Effect | Recommendation |
|---|---|---|
| High rotor tip speed | Anvil wear increases rapidly | Optimise speed for the target product |
| Worn rotor tips | Throw direction is disturbed, anvils wear unevenly | Replace tips on time |
| Off-centre feed | Uneven wear around the anvil ring | Feed alignment |
| Metal ingress | Segment breakage | Magnetic separator and metal detector |
Anvil Rotation Plan
Anvil segments wear in particular zones according to the angle at which material from the rotor strikes them. Moving the segments around the ring or turning them over at set intervals spreads the wear across the segment face. An example practice:
- At every rotor tip change, the anvil faces are photographed and the deepest wear point measured.
- When the wear depth reaches a certain level, the segments are turned.
- When the second face also reaches the limit, the segments are replaced as a set.
- The tonnage of each cycle is recorded to improve the next round of planning.
Maintenance Record Form
| Field | Record |
|---|---|
| Crushing chamber configuration | Anvil / rock shelf |
| Segment material | Chrome / manganese / composite |
| Turning and replacement dates | With tonnage |
| Measured wear depth | Deepest point |
| Product analysis | Sand yield, particle shape |
OEM or Aftermarket?
For anvil segments, dimensional fit, full seating on the carrier ring and equal weight between segments are important. When choosing aftermarket segments, the profile geometry, material analysis, hardness value and casting quality should be documented. An incompatible profile can change the behaviour of the crushing chamber and affect product quality.
More Questions
How many hours does a crushing chamber change take?
It depends on the crusher model and the team’s experience; at a planned shutdown, with the necessary parts and lifting equipment ready, it can be completed within a shift.
Is the anvil ring removed completely in rock shelf mode?
It depends on the design; in some crushers the rock box structure replaces the anvil ring, in others both are used together in different zones.
How does anvil wear affect the product?
A worn anvil changes the angle and distance of the impact surface, which can reduce crushing efficiency and make the product coarser.
Spare Parts and Service for the Anvil Ring
CSP Mühendislik supplies anvil segments, carrier rings and crushing chamber parts for vertical shaft impactors as part of its VSI crusher spare parts range, and provides service support for crushing chamber selection and mode changes. You can share your VSI model on WhatsApp.
