What are the reasons for the sudden fracture of the movable jaw plate?

I. Overview

The movable jaw plate (also known as the moving jaw plate) of the jaw crusher, as the core working component responsible for crushing tasks, is subjected to frequent impacts and compressions during operation. Under normal operating conditions, the main failure mode of the movable jaw plate is progressive wear, and its service life has a certain degree of predictability. However, in actual production, the phenomenon of sudden breakage of the movable jaw plate occurs from time to time, manifested as sudden abnormal noise during equipment operation, stop swinging of the moving jaw, and the flywheel continuing to rotate while the jaw plate has already broken. Such sudden failures not only cause equipment shutdown, but also require considerable time and cost for maintenance and replacement of parts. Thoroughly analyzing the causes of sudden fracture of the movable jaw plate is of great significance for taking targeted preventive measures and ensuring the safe and stable operation of the equipment.

2、 Defects in casting and heat treatment processes

（1） Casting defects

During the production and casting process of movable jaw plates, if the process control is not strict, defects such as dense pores, shrinkage porosity, shrinkage porosity, and slag inclusion may occur inside, which are common causes of early fracture of jaw plates. Specifically:

Pore defect: Insufficient gas discharge from the molten steel or insufficient mold permeability before pouring can form pores inside the jaw plate. The presence of pores reduces the actual load-bearing cross-section of the jaw plate. Under the action of crushing stress, the edge of the pores becomes the starting point of stress concentration. With prolonged use, microcracks originate from the edge of the pores and gradually propagate, ultimately leading to the overall fracture of the jaw plate.

Shrinkage and porosity: The jaw plate structure is complex, and the wall thickness distribution is uneven. If the pouring system design and riser setting are not reasonable, the thick parts will not be fully compensated during the solidification process, resulting in shrinkage or porosity defects, which also weaken the effective bearing capacity of the jaw plate.

Non metallic inclusions: Improper control of impurity content during the melting process (especially excessive sulfur and phosphorus content) can form non-metallic inclusions in the jaw plate matrix. These inclusions have significant differences in mechanical properties from the matrix metal and are prone to cracking along the inclusion boundaries under repeated impact.

（2） Improper heat treatment process

The high manganese steel jaw plate undergoes water toughening treatment (heating to 1050-1100 ℃ and rapid water quenching) to obtain a single austenitic structure, thereby possessing impact work hardening ability. If the heat treatment process parameters are not properly controlled, the following problems may occur:

Coarse grain size: Excessive pouring temperature or prolonged heating and insulation time can lead to abnormal growth of austenite grains. Coarse grains reduce the strength and toughness of the jaw plate, making it more prone to intergranular fracture under impact loads.

Carbide precipitation: Insufficient cooling rate or improper temperature control of the cooling medium can cause carbides to precipitate along grain boundaries, forming a network distribution that seriously reduces the impact toughness of the jaw plate, resulting in brittle fracture characteristics during use.

Organizational abnormalities: During the heat treatment process, abnormal microstructures such as Weishi tissue are formed, resulting in significant deterioration of the mechanical properties of the jaw plate and reduced fracture resistance.

（3） Improper material selection

The material of the movable jaw plate directly affects its impact resistance and fracture resistance. The mainstream choice is high manganese steel (such as ZGMn13, Mn13Cr2, etc.), which can quickly harden on the surface after impact and maintain toughness internally. If the jaw plate is made of ordinary carbon steel or materials with insufficient manganese content, its own strength and impact resistance cannot meet the requirements of the working conditions, and it is more prone to fracture after repeated impact collisions with the material. In addition, if the sulfur and phosphorus content in high manganese steel exceeds the standard, it will also cause thermal brittleness and affect the performance of the jaw plate.

3、 Foreign object entry and overload

（1） Non breakable materials enter the crushing chamber

The fracture of the movable jaw plate is directly related to the entry of unbreakable material (referred to as "iron") into the crushing chamber. When iron blocks (such as metal components left over from mines, tools left over from equipment maintenance, steel parts, etc.) or superhard materials with hardness exceeding the design range of the equipment enter the crushing chamber, the movable jaw plate and fixed jaw plate exert huge squeezing force on these objects, and the transient impact load borne by the jaw plate sharply increases, far exceeding the stress level under normal crushing conditions. Under such abnormal loads, the jaw plate may rupture instantly. Research data shows that when iron blocks with a diameter exceeding 50mm enter the crushing chamber, the elbow plate may break within 0.5 seconds, and the movable jaw plate also faces similar impact risks.

（2） Long term overload operation

The compressive strength of the materials processed by the jaw crusher should be within the allowable range of the equipment. If the hardness and particle size of materials arranged for a long time on the production line exceed the design capacity of the equipment, or if the actual working conditions are not fully considered when selecting equipment, resulting in the active jaw plate continuously working under overload, the stress level it bears exceeds the material fatigue limit for a long time, and the micro damage inside the jaw plate continues to accumulate. When the accumulated damage reaches the critical value, sudden fracture will occur. In a case of a certain quarry, after three months of use, the movable jaw of the new machine split into two parts from the middle, which was found to be related to the high hardness of the input material.

（3） Abnormal transmission of crushing force

The elbow plate (also known as the thrust plate) is an important safety protection component in jaw crushers. Under normal operating conditions, when an unbreakable object enters the crushing chamber and causes overload, the elbow plate should break first, thereby cutting off power transmission and protecting the main components such as the moving jaw and connecting rod from damage. If the equipment uses elbow plates or elbow pads that do not meet the design requirements (such as self purchased and modified accessories), their strength and toughness parameters do not match the host, and the elbow plates fail to self break protection according to the design requirements during strong impact, the huge impact load is directly transmitted to the moving jaw, causing cracking of the moving jaw.

4、 Installation and fixing issues

（1） Improper installation of foundation and rack

When installing the jaw crusher, if the foundation is not stable or the levelness of the machine body does not meet the standard, the frame will experience jumping and vibration during operation. The jumping frame constantly impacts the movable jaw plate, accelerating the contact and wear between the jaw plate and surrounding components. Over time, this will indirectly affect the service life of the jaw plate, exacerbate fatigue damage, and increase the risk of fracture. The tilt of the fuselage causes uneven stress on the jaw plate, with one side bearing significantly greater loads than the other. Under biased load conditions, the jaw plate is more prone to fracture from stress concentration areas.

（2） Loose fixing bolts

The movable jaw plate is fixed to the movable jaw body by multiple bolts. When the equipment is in operation, the material comes into direct contact with the jaw plate, generating continuous impact and vibration. The fixing bolts connecting the movable jaw plate are prone to loosening or falling off. After the bolt loosens, the position of the movable jaw plate deviates, and the crushing force that should have been evenly transmitted over the entire installation surface becomes concentrated in a local area, deteriorating the stress state of the jaw plate. More seriously, the loose jaw plate may sway during operation, and its bottom may collide with the edge guard plate inside the frame. The impact stress generated by the collision repeatedly acts on the jaw plate, forming cracks at the collision site and gradually expanding, ultimately leading to jaw plate fracture.

（3） The discharge port is too small

The size of the discharge port is an important parameter that affects the working state of the jaw crusher. When the discharge port is adjusted to a value below the specified lower limit, the material discharge resistance increases, and the crushed material cannot be discharged in time, resulting in the lower part of the crushing chamber being filled and compacted with material. In this state, the bottom of the movable jaw plate may collide with the bottom of the fixed jaw plate during the swinging process, and the hard contact between the two jaw plate bottoms may generate an impact force exceeding the material strength limit, directly causing the jaw plate to fracture.

5、 Operational and operational issues

（1） Unreasonable feeding

Feeding is an important link that affects the operation status of jaw crushers. If the feeding position is biased towards one end of the movable jaw plate, or the angle of the discharge hopper is designed too steep, the material will directly impact the head of the moving jaw under the action of gravity, and the impact pressure borne by the moving jaw will exceed the load under normal crushing conditions. Under repeated impacts, cracks and fractures are likely to occur from the impact point. In addition, if the particle size distribution of the feed is too concentrated or the feed amount fluctuates, it can also lead to unstable stress state of the jaw plate and increase the risk of fatigue fracture.

（2） The spindle speed is too fast

The eccentric shaft speed determines the oscillation frequency of the movable jaw plate. Some operators increase the spindle speed in pursuit of increased production, exceeding the rated speed of the equipment design. When the speed is too fast, the swinging frequency of the movable jaw plate increases, and the crushed material cannot be completely discharged from the discharge port in time, resulting in blockage in the crushing chamber. When blockage occurs, the retention time of materials in the crushing chamber is prolonged, and the number and amplitude of compressions borne by the jaw plate exceed the normal range. Prolonged exposure to this can lead to fatigue fracture of the jaw plate.

（3） Tension failure of tension rod spring

The tension rod spring is a key component that maintains the normal working state of the jaw crusher, ensuring that the movable jaw plate moves according to the designed trajectory through its tension. After a period of operation, the tension of the tension rod spring may decrease or even completely fail due to fatigue. If not replaced in a timely manner, the movement trajectory and force state of the movable jaw plate will change, causing abnormal vibration and impact when it is crushed too heavily. Long term effects may accelerate the fatigue damage of the jaw plate until it fractures.

（4） Lack of continuous and uniform feeding and reasonable operation intervals

The feeding is discontinuous and uneven, causing the crushing chamber to be sometimes empty and sometimes full, and the stress state of the jaw plate to alternate, forming a fatigue loading unloading cycle, which accelerates the fatigue damage of the material. At the same time, the movable jaw plate works under continuous high impact conditions, causing changes in grain structure and work hardening layer, which poses hidden dangers for fracture.

6、 The interrelationships between various factors

There are intersecting and overlapping effects among the various reasons for the sudden fracture of the movable jaw plate:

Casting defects result in insufficient inherent strength of the jaw plate, and even normal crushing loads may exceed its bearing capacity, leading to fracture shortly after being put into use.

When unqualified elbow plates are used and foreign objects are present in the feed, the elbow plates fail to provide safety protection, and the overload caused by foreign objects directly impacts the moving jaw. The combination of these two factors significantly increases the risk of fracture.

Once unstable basic installation and loose fixing bolts occur simultaneously, the displacement amplitude of the moving jaw inside the frame increases, and collisions with the edge guard plate become more frequent, exacerbating the tendency for jaw plate fracture.

Therefore, when analyzing and diagnosing the fracture fault of the active jaw plate, a comprehensive investigation should be conducted from multiple aspects, rather than examining a single possible cause in isolation.