At the heart of a cone crusher is the main shaft and head assembly, which carries the mantle and converts the eccentric motion into crushing force. The main shaft works as a single group together with the head centre, mantle, head nut (lock nut), feed plate and, in some designs, a main shaft sleeve or bushing. The health of this group directly determines both the crusher’s capacity and the life of other expensive parts (eccentric, bushings, spherical bearing).
In this guide we bring together the function, design, failure signs, measurement and inspection methods, repair options, overhaul steps and spare part selection for this group, which is known on site by names such as “main shaft”, “head group” or “head assembly”. For the general working principle of cone crushers, see our cone crusher guide.
Function of the Main Shaft and Head Assembly
In a cone crusher, the head performs the circular oscillating (gyratory) motion created by the eccentric. The main shaft is the axis of this motion: its lower section turns inside the eccentric bushing (or together with the eccentric assembly), while the head is attached to the shaft with an interference fit or taper seat. The enormous forces generated during crushing are transmitted from the mantle to the head, from the head to the main shaft and from there to the bearings and frame.
- Carries the mantle: The mantle sits on the outer surface of the head centre and is secured with the head nut.
- Transmits crushing force: Distributes the reaction force from the rock to the bearings and frame.
- Forms the axis of motion: Together with the eccentric, ensures that the head oscillates along the correct path.
- Connects to the setting system: In crushers that raise and lower the main shaft hydraulically (hydroset type), the CSS is set directly by the position of the main shaft.
Parts of the Assembly
| Part | Function | Typical damage |
|---|---|---|
| Main shaft | Axis of motion, force transmission | Surface wear, scoring, fatigue cracks, runout |
| Head centre | Carries the mantle | Crushing of the mantle seat, cracks |
| Head nut / lock nut | Secures the mantle | Thread damage, loosening, wear |
| Main shaft sleeve / bushing | Sliding surface between shaft and eccentric | Wear, seizure |
| Head ball / spherical bearing surface | Carries axial load | Scoring, heating (see socket liner) |
| Main shaft step | Axial load and positioning in hydroset types | Wear, flaking |
| Burn ring / locking elements | Prevent the nut from loosening | Breakage, deformation |
For the spherical bearing and socket liner under the head, see our spherical bearing and socket liner guide; for the eccentric group, see our eccentric and eccentric bushing guide.
Signs of Failure and Damage
- Uncontrolled head spin: The head spinning too fast when idling can indicate spherical bearing or bushing problems.
- Knocking and metallic noise: A loose mantle, head nut or voids in the backing material.
- Rising oil temperature: Can indicate increased friction between the main shaft and bushing.
- Metal particles in the oil: Rising iron and bronze values in oil analysis.
- CSS instability: In hydroset types, the main shaft position cannot be held.
- Marks on the mantle seating surface: Crushing, burrs and corrosion seen during liner changes.
Inspection and Measurement
The main shaft and head are usually partly inspected during liner changes and completely removed and inspected during major overhauls.
| Check | When? | Method |
|---|---|---|
| Mantle seating surfaces | Every liner change | Visual after cleaning; check for crushing and burrs |
| Head nut threads | Every liner change | Visual and with a thread gauge |
| Main shaft surface and diameters | Major overhaul | Diameter and ovality measurement with a micrometer |
| Runout (misalignment) | Major overhaul | Dial indicator |
| Fatigue cracks | Major overhaul or when suspected | Penetrant or magnetic particle testing |
| Oil analysis | Every 250–500 hours | Tracking iron, bronze and silicon |
Repair Options
- Polishing and deburring: Used for light scoring and small crushing marks on the seating surface.
- Grinding to size: Worn shaft diameters can be ground if suitable undersize bushings can be used.
- Weld build-up and machining: Must be done with a suitable welding procedure, preheating and stress relieving; incorrect welding paves the way for fatigue cracks. Manufacturer approval is required in critical load-bearing areas.
- Thermal spray coating: Can be used on some surfaces to restore dimensions and improve wear resistance.
- Replacement: The main shaft or head must be replaced when there are cracks, large runout or dimensional limits are exceeded.
Main Shaft and Head Replacement / Overhaul Steps
- Planning: New or overhauled main shaft/head, seal and gasket sets, head nut, burn ring, backing material and lifting tools are prepared.
- Safe shutdown: The crusher, feeder, lubrication and hydraulic systems are locked out; hydraulic pressure is released.
- Removing the upper group: The feed hopper, bowl or upper frame is removed.
- Lifting the head group: The main shaft and head are lifted vertically together using the manufacturer’s lifting eye; the eccentric and bushings must not be damaged.
- Removing the mantle and nut: The head nut and burn ring are removed and the mantle is separated.
- Inspection: The shaft, head, seating surfaces and spherical bearing surface are measured and recorded.
- Assembly: The head is fitted on the shaft by the method described by the manufacturer (with heating if required); the new mantle is seated and the head nut is tightened according to the torque/heating instructions.
- Backing and installation: Backing material is poured behind the mantle (see liner backing compound; CSP Mühendislik supplies Aurox crusher backing compound), and the head group is lowered into place.
- Calibration and commissioning: The CSS is zeroed and load is applied gradually while monitoring oil flow and temperatures.
About the Head Nut (Lock Nut)
The head nut is the large threaded part that clamps the mantle firmly to the head. In many designs the nut is made to self-tighten in the crusher’s direction of rotation (left- or right-hand thread) and is additionally secured with a burn ring or welded locking elements. The nut threads should be checked at every liner change, and a nut with damaged threads must never be reused. A loose head nut lets the mantle move, breaks up the backing and damages the head centre.
Common Mistakes
- Running liners until they crack: A cracked mantle hammers the head seating surfaces.
- Pouring too little backing or pouring it cold: The mantle is left unsupported and the head centre takes impacts.
- Lifting the head group incorrectly: Lifting at an angle damages the eccentric bushing and shaft surface.
- Tightening the head nut without instructions: Insufficient tightening leads to loosening; excessive heating damages the material.
- Running with dirty oil: Causes scoring and seizure of the main shaft bushing.
- Unsuitable weld repair: Welding without preheating and stress relief initiates fatigue cracks.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Knocking noise | Loose mantle/head nut, voids in the backing | Stop; check the nut and backing |
| Head spins fast when idling | Spherical bearing/bushing friction too low or high, lubrication problem | Check oil flow, bushing and bearing clearances |
| Oil temperature rising | Bushing wear, dirty oil, overload | Oil analysis, clean the cooler, reduce load |
| Rising iron in oil | Shaft or gear wear | Plan an overhaul, take measurements |
| CSS cannot be held (hydroset) | Hydraulic leak, step wear | Check the hydraulic system and the step |
An Example from the Field
At an andesite quarry, knocking started a few days after a liner change on a cone crusher. When the head nut was checked, its tightness was found to be adequate; however, it turned out that the backing material had not reached the upper part of the mantle and had left a large void. The pour had been made on a cold morning onto unheated parts. When the mantle was removed, crushing marks had started on the head seating surface; the surface was lightly machined and the pour was repeated with temperature control. Thanks to early intervention, the head centre was saved.
Spare Part Selection
- Material and heat treatment: Main shafts are made from high-strength forged alloy steels; request a material certificate and an ultrasonic test report.
- Dimensions and tolerances: Diameters in contact with the bushing and head matter at the micron level.
- Head nut: The thread profile and pitch must match the existing head exactly.
- Set compatibility: The main shaft bushing, sleeve and seals should be assessed at the same revision.
CSP Mühendislik supplies main shafts, head centres, head nuts, burn rings, main shaft bushings and sleeves for different makes and models as part of its cone crusher spare parts range. We recommend assessing parts such as the eccentric bushing, socket liner and dust seal in the same overhaul.
Pre-Purchase Checklist
- Have the crusher make, model and serial number been identified?
- Do you have the dimensions and overhaul records of the existing shaft?
- Have the head nut and burn ring been considered together?
- Have a material certificate and inspection report been requested?
- Are the lifting tools and heating equipment needed for assembly ready?
Safety
- The head group is very heavy; lift it only with the manufacturer’s lifting tools and a crane of suitable capacity.
- Never stand under the load; plan the lift in advance.
- Take fire and chemical safety precautions during nut heating and backing work.
Frequently Asked Questions
How long does a main shaft last?
With correct operation a main shaft can be used for many years; its life is mainly determined by lubrication quality, timely liner changes and the frequency of overload events.
Can the main shaft be repaired by welding?
In some areas it is possible with a suitable procedure; but welding increases fatigue risk in highly stressed areas. The decision should be based on inspection results and the manufacturer’s recommendation.
Why does the head nut loosen?
Insufficient tightening, damaged locking elements, voids in the backing or poor mantle seating are the most common causes.
Are the main shaft bushing and the eccentric bushing the same thing?
It depends on the design. In some crushers the main shaft runs directly inside the eccentric bushing; in others there is a separate main shaft bushing or sleeve.
Is it safe to use an overhauled main shaft?
An overhaul that has passed dimensional, runout and crack inspection and is documented with a report can be used safely.
Spare Parts and Service for the Main Shaft and Head Assembly
CSP Mühendislik provides fast supply of main shafts, heads, head nuts and bushings through its cone crusher solutions and cone crusher spare parts range, and supports on-site head group removal, inspection and overhaul with our service team. Share your crusher model and requirement on WhatsApp for a quick quote.
