A bolt is a fastener with a screw thread on its outer surface which, usually used together with a nut, joins two or more parts in a way that can be taken apart. A nut is the element with an internal thread that is fitted onto the bolt to clamp the joint. Although they look simple, bolts and nuts are critical parts that directly determine the safe operation of crusher liners, jaw plates, screen structures and conveyor frames. A single loose or broken bolt can lead to hours of downtime and serious damage.
In this guide we cover comprehensively the types of bolts and nuts, property classes, the concepts of tightening torque and preload, methods of preventing loosening, special bolts used in mining equipment (wear part bolts, lock bolts, liner bolts) and correct fitting practice.
How Does a Bolt Work?
When a bolt is tightened, it stretches slightly and behaves like a spring, pressing the joined parts together. This clamping force is called preload. In a well-designed joint, external loads are carried by friction between the parts and the preload; the bolt itself is not exposed directly to shear or alternating load. Insufficient preload leads to parts slipping, fatigue of the bolt under alternating load, and loosening. Correct tightening is therefore as important as the bolt itself.
Types of Bolt
| Bolt type | Characteristics | Typical use |
|---|---|---|
| Hexagon head bolt | The most common type; tightened with a spanner | General machine joints |
| Socket head cap screw | Cylindrical head; used in tight spaces | Machine parts, flanges |
| Countersunk bolt | The head sits flush with the surface | Liner and wear plate fixing |
| Stud (threaded both ends) | One end is screwed into the body | Housing and cover joints |
| Square head and T-head bolts | Do not turn in their slot | Crusher liners, slotted connections |
| Wear part bolts and lock bolts | Specific to crusher wear parts; the head seats in the part | Jaw plate, blow bar, concave and liner fixing |
| Anchor bolt | Embedded in concrete | Machine foundations |
| High-strength structural bolts | Designed for high preload | Steel structures, frames |
| Lockbolts (swaged fasteners) | Fitted by swaging a collar; do not loosen under vibration | Vibrating screen structures |
Bolt Property Classes
The property class marking on the head of metric steel bolts (e.g. 8.8, 10.9, 12.9) indicates the bolt’s strength:
- First number × 100 = approximate nominal tensile strength (MPa). For example, about 800 MPa for 8.8.
- Second number × 10 = ratio of yield strength to tensile strength (%). For example, for 8.8 the yield strength is about 80% of the tensile strength (≈640 MPa).
| Property class | Approx. tensile strength | Use |
|---|---|---|
| 4.6 / 5.6 | 400–500 MPa | Light loads, general purpose |
| 8.8 | 800 MPa | Most machine joints |
| 10.9 | 1000 MPa | High load, crusher and screen joints |
| 12.9 | 1200 MPa | Very high load; beware of hydrogen embrittlement risk |
Nuts are also marked with a property class (e.g. 8, 10, 12). The nut class must be compatible with the class of the bolt used; for example, a nut of at least class 10 should be used with a 10.9 bolt. Stainless bolts use material markings such as A2 and A4 and strength classes such as -70 and -80.
Types of Nut
| Nut type | Characteristics |
|---|---|
| Hexagon nut | Standard nut |
| Lock nut (jam nut) | A thin nut; prevents loosening together with the main nut |
| Flange nut | Wide bearing surface; spreads the load |
| Nylon insert lock nut | Resists loosening through friction of the insert ring |
| All-metal lock nut | Locks with deformed threads; suitable for high temperatures |
| Cap (acorn) nut | Covers the bolt end; protection and appearance |
| Wing nut | Tightened by hand |
| Weld nut | Welded to sheet metal |
| Castle nut | Secured with a split pin |
| Bearing lock nut and lock washer | Securing bearings and sleeves |
In cone crushers, special nuts such as the head nut secure the mantle; for these parts, see our main shaft and head assembly guide.
Tightening Torque and Preload
Only a small part of the torque applied when tightening a bolt turns into preload; most is used up by friction in the threads and under the head. Therefore:
- The same torque value creates very different preloads on oiled and dry threads.
- Torque tables are prepared for specific friction conditions; use the table suited to the lubrication condition.
- For critical joints (crusher liners, flanges), the torque values and tightening sequence given by the manufacturer should be followed.
- On large bolts, hydraulic tensioning or angle-controlled tightening gives more accurate preload.
- Torque wrenches should be calibrated periodically.
Wrong torque: too little leads to loosening and fatigue failure; too much leads to the bolt yielding or the threads stripping.
Methods of Preventing Loosening
In joints under vibration and shock loads, bolts can loosen over time. The main measures:
- Correct preload: The most effective method.
- Locking compound (liquid threadlocker): Hardens between the threads to prevent loosening. For details, see our locking compound guide.
- Lock nuts and jam nuts.
- Wedge-locking washers: Prevent rotation through the wedge effect of the washer pair.
- Lockbolts: Common in vibrating screen structures.
- Safety wire, split pins: In special applications.
Note: Conventional spring washers are known to have limited effect in preventing loosening in highly preloaded joints; more effective methods should be preferred for critical joints.
Critical Bolts in Mining Equipment
| Equipment | Critical joint | Point to watch |
|---|---|---|
| Jaw crusher | Jaw plate wedge bolts, cheek plate bolts | Check and renew at every jaw change |
| Impact crusher | Apron liner bolts, housing liner bolts | Countersunk or special-head bolts; protection against wear |
| Cone crusher | Frame flange bolts, arm guard bolts, head nut | Manufacturer’s torque values |
| Vibrating screen | Side plate and cross member joints, motor/exciter connections | Lockbolts, periodic checks |
| Conveyor | Frame, idler stand, pulley bearing bolts | Loosening under vibration |
| Mill | Liner bolts | Sealing and breakage |
Crusher liner bolts are usually not reused at a liner change; worn and stretched bolts should be renewed together with the new liner. For how wear parts are fixed, see our impact aprons and housing liners guides.
Corrosion and Coatings
In wet and corrosive environments, bolts can rust until they cannot be removed. Galvanising (hot-dip or electro), zinc flake and similar coatings reduce corrosion. However, with high-strength bolts (especially 12.9), some coating processes can carry a risk of hydrogen embrittlement; coating selection should therefore be made with care. Using a suitable assembly paste during fitting makes later removal easier.
Common Mistakes
- Using bolts of unknown property class.
- Using a low-class nut with a high-strength bolt.
- “Estimating” tightness without a torque wrench.
- Reusing stretched or damaged bolts.
- Welding the bolt head to the structure (makes removal impossible and can damage the material).
- Neglecting periodic checks.
Frequently Asked Questions
What is the difference between a bolt and a screw?
In general usage, a bolt is a fastener used with a nut, while a screw is screwed directly into the material or a threaded hole. Technical standards give more detailed definitions.
What does an 8.8 bolt mean?
A bolt with a tensile strength of about 800 MPa and a yield strength of about 80% of that.
Does a bolt torque table apply in every case?
No. Torque tables are prepared for specific friction conditions; when lubrication, coating and surface condition change, so do the values. Manufacturer values should be followed for critical joints.
What types of nut are there?
The main types are hexagon, jam, flange, nylon insert, all-metal lock, cap, wing, weld and castle nuts.
Can crusher liner bolts be reused?
This is generally not recommended; liner bolts should be renewed at every change because of wear and stretching.
Information Needed to Order Bolts and Nuts
- Thread type and size: Metric (e.g. M20), fine or coarse thread, pitch.
- Length: From under the head to the tip; partially or fully threaded.
- Head type: Hexagon, socket, countersunk, square, special head.
- Property class: 8.8, 10.9, 12.9 etc.; nut class.
- Surface finish: Black, galvanised, zinc flake etc.
- Standard: The relevant product standard or drawing.
- Quantity and application.
Wear part and lock bolts specific to crusher wear parts are produced with a head geometry that fits the part’s slot; for these bolts, the crusher model and part number should be specified.
Types of Washer
| Washer | Function |
|---|---|
| Plain washer | Spreads the load, protects the surface |
| Hardened structural washer | Prevents surface crushing in highly preloaded joints |
| Wedge-locking washer pair | Prevents loosening under vibration |
| Disc spring (Belleville washer) | Compensates for loss of preload |
| Tab (lock) washer | Secures the nut with a bent tab |
Glossary
- Preload: The force with which a tightened bolt presses the parts together.
- Property class: The strength class of a bolt (e.g. 10.9).
- Thread pitch: The distance between two thread crests.
- Hydrogen embrittlement: A risk of brittle fracture that can arise in high-strength steels through some coating processes.
- Jam nut: A second nut that locks the main nut.
More Questions
What are wear part bolts and lock bolts?
Special bolts used to secure crusher wear parts (jaw plates, blow bars, liners), designed so that the head seats in the part’s slot.
How is a rusted bolt removed?
Apply a penetrating oil and, if necessary, local heat; apply force gradually so the bolt does not break. Using a suitable assembly paste when fitting prevents this problem.
Support from CSP Mühendislik
CSP Mühendislik supports the supply of wear part and lock bolts specific to crusher wear parts, liner bolts, screen fasteners and special nuts. See our crusher spare parts page for crusher parts and our vibrating screen spare parts page for screens, and our spare parts page for all products.
