When replacing the bowl wall, the locking bolt cannot reach the torque value no matter how much it is tightened, and it frequently breaks. How should it be handled?

During the replacement and installation process of the cone crusher bowl wall, the locking bolt (usually a U-shaped bolt) cannot reach the specified torque value no matter how it is tightened, and frequent fractures occur during the tightening process or shortly after operation, which is a rather tricky on-site fault. For such problems, the first reaction of many maintenance personnel is' poor bolt quality ', but based on the experience of repairing multiple devices, the problem often goes beyond the bolts themselves. It is necessary to conduct systematic investigation and disposal from multiple dimensions such as thread pair condition, contact surface fitting status, pre tightening force control method, and installation process.

1、 Multi dimensional analysis of fault causes

1. Thread pair failure (slipping): the most common direct cause

When there is always a feeling of 'no force added' during the tightening process, the bolt cannot reach the specified torque value, and thread slippage is the first troubleshooting direction. The main manifestation of thread slippage is that the torque cannot be applied during assembly, and after removing the bolt, it can be observed that all or part of the thread is worn flat. Its causes include:

Residual old filling material or foreign objects in the threaded hole: When replacing the rolling bowl wall, if the old zinc alloy, epoxy resin residue or dust in the adjusting sleeve threaded hole is not thoroughly cleaned, foreign objects entering the threaded pair will cause a reduction in effective engagement length, insufficient thread contact area, and thread slippage during tightening.

The thread itself is worn or deformed: after repeated disassembly and assembly of the equipment, the threaded hole on the adjusting ring may be worn, deformed or partially damaged. When the number of threads engaged decreases or the internal and external threads do not come into contact within the pitch diameter, the load-bearing capacity of the connecting pair will significantly decrease.

Decarburization or insufficient hardness of threads: If decarburization occurs on the surface of threads due to improper heat treatment, the surface hardness will decrease, and the thread profile is easily ground flat when tightened.

When tightening, do not forcefully tighten the hole: If the axis of the bolt does not coincide with the axis of the threaded hole during assembly, forcibly tightening will cause thread shear damage.

2. Contact surface fitting defects: causing bolts to bear additional loads

The working principle of high-strength bolt connections is to rely on pre tightening force to generate sufficient friction between the connecting parts to resist external loads, rather than relying on the "hardness" of the bolts themselves to withstand shear forces. When the connecting surface has the following defects, the bolt will bear additional shear or bending loads:

Insufficient contact area: If the contact surface between the rolling bowl wall and the adjustment sleeve is too small, the inherent friction force is insufficient, and there will be slight slippage during operation, with shear force directly applied to the bolt. In this working condition, the bolt is actually "supporting the structure", and the root cause of bolt fracture is not the bolt itself, but the design or machining accuracy of the connection surface that does not meet the standards.

There is a gap or non fitting on the contact surface: the deviation of the machining accuracy of the adjustment sleeve, deformation caused by multiple disassembly and assembly, or the presence of old filler residue on the joint surface may make the contact surface between the rolling bowl wall and the adjustment sleeve appear to be in contact, but in reality it is not "real". When the equipment is running, the combination of impact load and vibration causes the rolling bowl wall to bounce up and down, and the bolts are repeatedly subjected to impact tension, forming typical fatigue fracture characteristics.

Improper fit clearance between U-bolt and bowl wall ear seat: If there is excessive clearance between U-bolt and bowl wall ear seat, or if the ear seat itself has worn and deformed, the bolt will not be able to effectively press the bowl wall. Even if tightened to the specified torque, the clamping force will be difficult to transmit to the bowl wall body.

3. Improper control of pre tightening force: dual risks of over tightening and under tightening

Insufficient pre tightening force: If the operator does not use a torque wrench and only "feels" tightening based on experience, the actual pre tightening force often does not meet the design requirements. When the pre tightening force is insufficient, the connecting surface is not compacted, and the bolt bears additional alternating loads during operation, accelerating fatigue fracture.

Excessive pre tightening force: If the applied torque exceeds the yield limit of the bolt material, the bolt will undergo permanent plastic elongation deformation or even fracture directly. The standard pre tightening force for high-strength bolts is usually set at 70% to 75% of the material's yield strength, exceeding this threshold will result in overload fracture. It is worth noting that the accuracy error of tightening tools (such as the error of pneumatic torque wrenches, which can reach 10% to 40%) can cause the actual applied torque to exceed the specified torque. When the material yield limit is exceeded, the bolt will fail.

Torque coefficient fluctuation: The lubrication status, cleanliness, and surface treatment process of the thread surface can affect the torque coefficient. A low friction coefficient can result in excessive preload force under the same torque, which may lead to bolt yielding; Excessive friction coefficient can reduce torque conversion efficiency and result in insufficient actual preload force.

4. Improper installation method: lack of step-by-step tightening and sequence control

The fastening bolts for rolling bowl walls usually require multiple bolts (such as 4 M27 bolts) to work together. If the alternating and symmetrical tightening method is not used, it will cause the rolling bowl wall to deflect and some bolts to bear excessive additional bending moments. The correct approach is to divide the initial tightening and final tightening into two stages, alternating the tightening in diagonal order to ensure even adhesion of the rolling bowl wall and balanced force distribution of each bolt.

5. Bolt quality defects

Although 'poor bolt quality' is not the real cause of every fracture, quality defects do objectively exist. Common quality issues include:

Chemical composition deviation: The content of carbon, sulfur, phosphorus and other elements does not meet the standard requirements, resulting in insufficient strength or toughness;

Defects in heat treatment process: Improper quenching temperature leads to coarse grains or insufficient hardness, and insufficient tempering results in residual stress not being eliminated;

Hydrogen embrittlement: If high-strength bolts are treated with electroplating surface without timely hydrogen removal, hydrogen atoms will diffuse to the grain boundaries, causing brittle fracture, and the fracture surface will exhibit a "sugar like" characteristic;

Unqualified size: The bolt size does not match the design drawing, or the thread accuracy does not meet the matching requirements.

6. Overload fracture caused by operating conditions

During the operation of the equipment, if too many iron blocks or viscous materials enter, it will cause overload of the crusher, and the impact load borne by the locking bolt will exceed the design limit, resulting in bolt fracture. In addition, after installing or replacing the rolling mill wall, the fastening condition should be checked and the U-bolts should be tightened again after working for 6-8 hours; If this step is ignored, the initial looseness will quickly evolve into bolt fracture.

2、 Systematic processing flow

For the above faults, it is recommended to follow the following steps for troubleshooting and disposal:

Step 1: Shutdown inspection and fault location

Check the state of bolt fracture: observe the morphology of the fracture surface. If there is obvious necking and plastic deformation elongation on the fracture surface, it indicates overload fracture (excessive torque); If the fracture surface is flat, without obvious necking, and shows fatigue characteristics, it is mostly caused by insufficient pre tightening force or contact surface defects.

Check the condition of the threaded hole: Use a thread gauge or new bolt for trial tightening to confirm if there are any loose threads, wear, or foreign objects blocking the threaded hole.

Check the fit of the contact surface: Remove the rolling bowl wall, check whether there are high points, pits or deformations on the contact surface with the adjustment sleeve, and use a feeler gauge to detect the contact gap.

Step 2: Select a solution based on the type of fault

Scenario 1: Thread hole slipping or thread damage

If the threaded hole has slipped or the thread is damaged, simply replacing the bolt cannot solve the problem. The following repair solutions can be adopted:

Thread repair sleeve (steel wire thread sleeve) scheme: Install the thread repair sleeve in the original threaded hole to rebuild the high-strength threaded structure. This method reconstructs the thread within the original hole position, redistributing the force and helping to restore or even improve the connection strength. Compared to directly using thicker bolts or enlarging the aperture, this solution will not weaken the local structural rigidity of the adjustment sleeve, nor will it damage the original assembly benchmark.

Expanding the hole and tapping with enlarged bolts: Under the premise of adjusting the size of the sleeve structure, expand the original threaded hole and drill a larger specification thread, and replace it with high-strength bolts of the corresponding specification. However, it should be noted that this method will weaken the local structural rigidity of the adjustment sleeve and may damage the original assembly benchmark, making it suitable for equipment with significant structural redundancy.

Re machining threads after welding: In cases where the threaded hole is severely damaged, the threaded hole can be re drilled and tapped after welding. However, it should be noted that welding may affect the adjustment of the base material. After repair, necessary heat treatment should be carried out to eliminate stress.

Scenario 2: Bolt fracture but intact threaded hole

If the threaded hole is in good condition, but the bolt frequently breaks during tightening or after operation, the following aspects should be focused on:

Check and repair the contact surface:

Thoroughly clean the contact surface between the bowl wall and the adjustment sleeve, removing all old filler residue, rust, and foreign objects;

If there are high points or unevenness on the contact surface, scraping or machining repair should be carried out to ensure the fit;

If necessary, rubber pads or fiberglass pads should be added between the contact surfaces to compensate for minor unevenness, but it should be noted that the thickness of the pads should not affect the fitting accuracy.

Standardize the tightening process:

Use calibrated high-precision torque wrenches and do not estimate torque based on "hand feel";

Adopting a step-by-step operation of "initial tightening - final tightening", tightening alternately in diagonal order to ensure even fit of the rolling bowl wall;

The final tightening torque should refer to the value specified by the equipment manufacturer, usually around 70% of the bolt's guaranteed load.

Replace qualified bolts:

Select high-strength bolts that meet the design specifications (strength grade, dimensional accuracy);

Before installing new bolts, check the cleanliness of the threaded surface to avoid oil stains and rust affecting the consistency of the friction coefficient;

The same batch of bolts should use products from the same manufacturer and production batch number to ensure the stability of the torque coefficient.

Scenario 3: The bolt experiences both tooth slippage and fracture simultaneously

If both tooth slippage and fracture occur simultaneously, a dual inspection of the quality of the adjustment sleeve threaded hole and bolt should be carried out, following the order of "repairing the threaded hole first, and then replacing the qualified bolt".

Step 3: Verification and subsequent maintenance after installation

Verification after initial tightening: After completing the installation of the rolling bowl wall and bolt tightening, a no-load test run should be conducted to observe for any abnormal vibration or noise.

Secondary tightening: After installing or replacing the rolling bowl wall, the equipment should be stopped and checked for bolt tightening after 6-8 hours of operation, and tightened again according to the specified torque.

Regular inspection: During the normal operation of the equipment, a regular inspection system should be established to check the bolt tightening status and threaded hole condition once a month, and any looseness should be promptly dealt with.

3、 Preventive measures

To fundamentally reduce the occurrence of such failures, it is recommended to implement the following measures in equipment maintenance management:

Specific measures for prevention dimension

When purchasing high-strength bolts for spare parts management, suppliers are required to provide material reports and prioritize the use of bolts that have undergone sufficient dehydrogenation treatment; Maintain batch consistency for bolts of the same specification

Regularly calibrate torque wrenches for tool management (recommended every six months) and establish a ledger for the use of torque tools; Prohibition of using uncalibrated tools for final twisting operations

Personnel training provides specialized training on high-strength bolt tightening process for maintenance personnel, clarifying torque parameters and tightening sequence; Operators need to master the method of checking threaded holes

Develop standardized operation instructions for the replacement of mill bowl walls in accordance with process specifications, specifying the cleaning standards for contact surfaces, inspection methods for threaded holes, and tightening parameters

Install metal detection and iron removal devices in the crusher feeding system for operation monitoring, reducing the entry of non crushable materials into the crushing chamber; Avoid excessive wear of the bowl wall before replacing it