A roll crusher is a crusher type that breaks material by compressing it as it passes between one or two cylinders (rolls) turning towards each other. With its low fines generation, controlled product size and simple construction, it is preferred for soft to medium-hard materials such as coal, limestone, clay, salt and gypsum, and especially in applications where fines are unwanted.
In this guide we cover the working principle of roll crushers, their types, the factors that determine reduction ratio and capacity, gap setting, roll surface maintenance, the difference from high pressure grinding rolls (HPGR) and spare parts planning. For a general comparison of crusher types see our crushers page, and for hammer crushers used in similar applications see our hammer crusher guide.
Working Principle
In a double roll crusher, the two rolls turn at the same speed towards each other. Material arriving from the feed opening is “nipped” by the roll surfaces and drawn into the narrow gap between the two rolls. As it passes through this gap, the material is broken by compressive force. The top size of the product is determined directly by the gap between the two rolls.
For material to be nipped by the rolls, there must be a suitable ratio between particle size and roll diameter. If the friction between roll surface and particle and the “nip angle” are not sufficient, the particle bounces on top of the roll and is not crushed. Feed size is therefore limited relative to roll diameter in roll crushers; larger particles need either larger-diameter rolls or a toothed/segmented surface.
Reduction Ratio
Roll crushers have a lower reduction ratio than other crushers, generally considered to be around 3:1 to 4:1. Where greater reduction is needed, staged crushing (two or three roll crushers in series) or a different crusher type is considered. On the other hand, product size is very well controlled and fines generation is limited, which is an important advantage for coal and some industrial minerals.
Types of Roll Crusher
| Type | Construction | Typical Use |
|---|---|---|
| Single roll | One roll facing a fixed breaker plate | Primary crushing of coal, clay and soft, sticky materials |
| Double roll (smooth) | Two smooth rolls | Fine, controlled crushing of medium-hard material |
| Double roll (toothed/segmented) | Toothed or replaceable segmented surface | Coarser feed, coal, salt, limestone |
| Triple/quadruple roll | Staged pairs of rolls | Greater reduction in a single machine |
| Sizer | Two low-speed shafts with large teeth | Primary crushing of coal and soft ores, low fines generation |
The Difference Between a Roll Crusher and HPGR
High pressure grinding rolls (HPGR) look similar to a roll crusher but work on a different principle. In a roll crusher, particles are broken individually between the rolls. In an HPGR, the rolls compress a bed of material at very high pressure and breakage occurs as particles are crushed against each other (inter-particle comminution). HPGR is used mainly for energy efficiency ahead of grinding in hard ores and makes grinding easier by creating micro-cracks in the particles. For its place in mineral processing circuits see our mineral processing guide.
Selection Criteria
- Material hardness and abrasiveness: Roll crushers are efficient on soft to medium-hard materials. On very hard and siliceous rocks the roll surface wears quickly. We explain the effect of rock properties on crusher selection in our rock crushability guide.
- Moisture and stickiness: With clayey, wet materials, single roll or sizer machines with scrapers give better results.
- Feed size: Roll diameter and surface profile are selected according to feed size.
- Product size and fines content: If fines must be kept low, a roll crusher is a strong option.
- Capacity: Roll width, speed and gap determine capacity.
Gap Setting and Safety System
The gap between the rolls is usually adjusted by moving one of the rolls on sliding bearings. The movable roll is pressed towards the fixed roll by a spring pack or hydraulic cylinders. When an uncrushable object arrives (iron, a tree root, very hard stone), the movable roll retracts to let it pass and then returns to its original position.
- Spring preload or hydraulic pressure should be kept at the manufacturer’s values. Low preload lets the gap open during operation and coarsens the product; high preload weakens the protective function.
- The gap should be measured and reset periodically as the roll surfaces wear.
- The gap must be equal at both ends; otherwise the roll surface wears to a taper.
- A magnetic separator and metal detector installed ahead of the crusher protects the rolls by preventing frequent operation of the safety system.
Roll Surface Maintenance
The roll surface is the most important wear item on a roll crusher. As the surface wears, the nip angle deteriorates, the gap opens and control of product size is lost.
| Surface Type | Maintenance Method | Note |
|---|---|---|
| Replaceable shell | Shell replacement | Manganese or alloy steel shells are common |
| Segmented surface | Replacement of worn segments | Faster, part-by-part replacement |
| Toothed surface | Tooth replacement or hardfacing | The tooth profile must be maintained |
| Smooth surface | Surface renewal by hardfacing, grinding | Surface trueness should be checked after welding |
For even wear, feed should be distributed evenly across the roll width. Feed piling up in the middle of the roll leads to wear in the central zone known as “waisting”. Using a vibrating feeder or distribution chute reduces this problem; for feeder selection see our feeder types guide.
Mechanical Maintenance Checklist
- Bearings: Follow the greasing interval and monitor temperature and vibration. For bearing selection and failure analysis see our bearing guide.
- Drive: Check belt tension and pulley alignment, and on gear drives, tooth contact and oil level. Our gearbox maintenance guide covers gearbox drives.
- Scrapers: With sticky material, keep roll scrapers adjusted.
- Dust sealing: Check housing seals and covers.
- Spring pack / hydraulics: Check spring length, broken springs, hydraulic leaks and accumulator pressure.
Common Faults
| Symptom | Possible Cause | Remedy |
|---|---|---|
| Product too coarse | Roll wear, gap opening, low spring preload | Measure and reset the gap, renew the surface |
| Low capacity | Material bouncing, poor nip | Check feed size, surface profile |
| Roll surface wearing to a taper | Unequal gap or feed | Correct the setting, feed distribution |
| Bearing overheating | Lack of lubrication, contamination, misalignment | Greasing, sealing, alignment |
| Frequent safety release | Uncrushable material, overfeeding | Metal separation, feed control |
Field Example
Suppose that in a coal preparation plant the proportion of oversize in the product of a double roll crusher increases over time. Inspection shows that the roll surface has worn more in the central zone and the gap has opened in the middle. The root cause is the feed belt dropping material onto the centre of the rolls. When a distribution plate is added to the feed chute and the roll surface is renewed by hardfacing, control of product size is restored and the interval to the next renewal is extended. This example shows that maintenance on a roll crusher is not only about changing parts but also about getting the feed arrangement right.
Spare Parts Planning
- At least one set of roll shells or segment sets should be held in stock according to the wear rate.
- Bearings and bearing seals, spring packs or hydraulic seal kits are critical spares.
- If hardfacing is carried out, a stock of suitable electrodes or flux-cored wire should be kept.
- Belt and pulley sets should be held as spares according to the drive type.
You can find the spare parts your crushers need on our crusher spare parts and general spare parts pages.
Frequently Asked Questions
What materials is a roll crusher suitable for?
It is suitable for soft to medium-hard materials such as coal, limestone, clay, salt, gypsum and coke. On very hard and abrasive rocks the wear cost rises.
Does a roll crusher produce fines?
It produces fewer fines than other crushers, which is an important advantage in applications where fines are unwanted.
How often should the roll gap be checked?
Depending on abrasiveness, it can be checked weekly or every shift; product size analysis shows changes in the gap early.
Is an HPGR the same as a roll crusher?
No. An HPGR compresses a bed of material at very high pressure for inter-particle comminution; a roll crusher breaks particles individually.
Can the roll surface be renewed by welding?
Yes; the surface can be renewed with a suitable hardfacing material. After welding, the trueness of the surface and the equality of diameter at both ends should be checked.
Roll Crusher Spare Parts List
| Part | Function | Check |
|---|---|---|
| Roll shell | Crushing surface | Diameter measurement, surface profile |
| Segments or teeth | Replaceable crushing surface | Tooth height, breakage |
| Roll shaft | Carries the shell, transmits torque | Cracks, bearing surfaces |
| Bearings and housings | Support the shaft | Temperature, vibration, grease |
| Spring pack or hydraulic cylinders | Gap protection | Spring length, leaks, pressure |
| Adjustment shims and mechanism | Gap setting | Wear, seizure |
| Belts, pulleys or gears | Drive | Tension, tooth contact |
| Scraper blades | Clean the roll surface | Wear and adjustment |
| Housing liners and seals | Protection and sealing | Wear, dust leakage |
We can supply shells, segments and mechanical parts for roll crushers to dimensions. Send your parts requirements via our crusher spare parts page or our WhatsApp line. For hopper and feeder parts serving roll crushers, see our hopper spare parts and feeder spare parts pages.
Maintenance Record Form
| Field | Record |
|---|---|
| Roll diameters (both ends and centre) | Weekly measurement |
| Gap (at both ends) | Weekly or according to product analysis |
| Spring preload / hydraulic pressure | Monthly |
| Bearing temperatures | Daily |
| Hardfacing / shell replacement | Date, tonnage, material |
| Safety release events | Number and cause |
Cost and Life Analysis
The most significant operating cost of a roll crusher is renewing the roll surface. Renewal by hardfacing offers lower parts cost than shell replacement, but welding labour and downtime must be taken into account. Replaceable segmented designs shorten downtime. The comparison should be made on total cost per tonne (parts + labour + downtime). Our wear part materials guide helps with wear material selection.
Glossary
- Nip angle: The angle that allows a particle to be nipped between two rolls; depends on roll diameter and particle size.
- Gap: The narrowest distance between the two rolls; determines the top size of the product.
- Sizer: A low-speed, two-shaft sizing crusher with large teeth.
- HPGR: High pressure grinding rolls; compress a bed of material for inter-particle comminution.
- Hardfacing: Depositing wear-resistant weld metal on a worn surface.
More Questions
Do the two rolls of a roll crusher turn at different speeds?
In some designs a slight speed differential is used to create a shearing effect and improve cleaning with sticky material; in most standard designs the speeds are equal.
How is a roll shell replaced?
The roll assembly is removed from its bearings, the old shell is cut or pressed off, the new shell is fitted by a suitable method (shrink fit or keyed connection) and the bearings are realigned.
Can a roll crusher be used to produce sand?
It can be used on some soft materials, but VSI crushers are generally preferred for producing manufactured sand from hard rock.
CSP Mühendislik Support
CSP Mühendislik provides engineering and service, maintenance and repair support for roll crusher selection, roll surface renewal, and bearing and drive maintenance. You can browse our crushing and screening equipment on our products page.
