Is the thread sticking when removing the nut due to a problem with the working condition adaptation or improper operation?

Is the thread sticking when removing the nut due to a problem with the working condition adaptation or improper operation?

1、 The core mechanism of thread sticking: force imbalance and surface damage

The essence of nut thread sticking during disassembly is that the frictional resistance of the thread engagement surface exceeds the disassembly driving force, or local stress concentration leads to thread tooth deformation and biting. Its causes can be divided into two categories: one is the "innate adaptation defect" that does not match the selection, working conditions, and nut characteristics, and the other is the "postnatal operation error" during installation and disassembly. The two often affect each other, but can be distinguished by characteristic differences.

2、 Improper adaptation of working conditions: the "innate cause" of jamming

The problem of adapting to working conditions arises from the incompatibility between nut selection, material, structure, and actual working conditions. The jamming often manifests as "repetitive jamming" or "jamming during initial disassembly", including:

Thread specification and accuracy do not match

The thread type and tolerance grade of the disassembly sleeve and nut are not adapted to the overload standard, for example, in the overload scenario, ordinary metric threads (ISO 965-3) are mistakenly used instead of trapezoidal threads (ISO 2903), or the internal thread tolerance of the nut (such as 6H) and the external thread tolerance of the disassembly sleeve (such as 7e) have a too small fit clearance, resulting in interference between the thread crest and root during meshing. Especially when there is vibration in the shaft system, insufficient clearance can accelerate thread wear and cause jamming.

The material does not match the strength grade

When disassembling under heavy load, the material strength of the nut is lower than the working condition requirements. If ordinary carbon steel (Q235) is used instead of high-strength alloy structural steel (42CrMo), the threaded teeth will undergo plastic deformation under high torque, resulting in poor fit of the teeth; Or the material may have insufficient corrosion resistance, resulting in rusting of the threaded surface under humid and dusty conditions, forming oxide skin and increasing meshing resistance.

Poor compatibility between structural design and working conditions

The nut is not equipped with a chip removal groove or lubrication channel, and the iron chips and impurities generated during heavy-duty disassembly cannot be discharged, which accumulate in the thread gap and form "grinding particles", exacerbating jamming; Or the height of the nut does not match the length of the disassembly sleeve thread (such as the nut height exceeding 1.2 times the thread length), resulting in uneven thread engagement and local stress concentration.

Temperature and medium adaptation failure

Not using high-temperature resistant alloy nuts under high temperature conditions (>120 ℃) resulted in a decrease in material strength and thread deformation; If the nut is not galvanized, passivated, or chrome plated in a corrosive environment (such as humidity or chemical media), pitting and rusting may occur on the surface of the thread, resulting in "sticking points" during engagement.

3、 Improper operation: the "postnatal dominant factor" of jamming

Improper operation is a more common cause of jamming in heavy-duty disassembly, often manifested as "accidental jamming" or "sudden jamming after a certain step", directly related to the operation process and tool use:

Insufficient cleaning and lubrication of threaded surfaces

Failure to thoroughly clean the threaded surface before disassembly results in residual solidified grease, iron filings, or impurities from installation, which can form a "grinding medium" during meshing, causing scratches and biting on the threaded surface; Or, without using specialized disassembly grease, only ordinary lubricating oil (such as engine oil) is used, which has poor high-temperature stability. When disassembling, the lubricating film ruptures, and the friction coefficient suddenly increases (μ=0.15-0.20 without lubrication, and μ can reach 0.3 or more when lubrication is insufficient), causing jamming.

Improper application of disassembly force

The multi-stage pressurization method was not used, but high torque/high pressure was applied instantaneously, resulting in the thread being subjected to instantaneous force exceeding the yield strength and causing plastic deformation; Poor fit between the hydraulic nut and the end face of the disassembly sleeve (flatness deviation>0.01mm), uneven disassembly force, concentrated force on one side of the thread, and jamming after shear deformation of the tooth root.

Tool usage and positioning deviation

The centerline of the hydraulic nut cylinder does not coincide with the axis of the retraction sleeve, resulting in axial thrust deviation and uneven force on the threaded engagement surface; Or the precision of dismantling tools (such as wrenches, hydraulic pumps) is insufficient, pressure/torque control is out of control, and instantaneous overload causes thread bite.

Potential hazards left over from early installation

Excessive tightening of the nut during installation resulted in residual stress on the threaded teeth, which caused deformation due to stress release during disassembly; During installation, the threaded surface may be scratched or bumped, forming burrs. When disassembling, burrs may be embedded into the mating surface, hindering rotation.

4、 Analysis method for the causes of stagnation: rapid localization through features

Check the timing of jamming: In the early stage of disassembly (short-term after disassembly or installation), jamming occurs, often due to compatibility issues with working conditions (such as thread accuracy and material mismatch); After multiple disassembly, if there is sticking, it is more likely to be accumulated damage caused by improper operation (such as insufficient lubrication, accumulation of impurities).

Check the stuck state: Uniform stuck (with high resistance throughout the entire process and no visible graphics points) is mostly due to insufficient lubrication or too small fitting clearance; Local sticking points (sudden increase in resistance at a certain turning position) are often caused by thread damage, accumulation of impurities, or local deformation due to biased loading.

Observing the form of thread damage: uniform wear and corrosion on the thread surface, which is a problem of adapting to the working conditions (due to insufficient material corrosion resistance and lubrication channel design); Local deformation, biting, and burrs of threaded teeth are caused by improper operation (overload, off load, inadequate cleaning).

Check repeatability: Nuts of the same specification may get stuck multiple times under the same working conditions, indicating a defect in adapting to the working conditions; Only a few nuts or single disassembly stuck, indicating improper operation.

5、 Targeted solutions and preventive measures

Optimization of working condition adaptation: Select trapezoidal threaded nuts according to the specifications of the disassembly sleeve to ensure tolerance fit (such as nut 5H/disassembly sleeve 7e); Under heavy load conditions, 42CrMo quenched and tempered nuts are selected, and special materials and surface treatments are used for high-temperature/corrosive environments; Priority should be given to structural designs with chip removal grooves and lubrication channels.

Operating procedure specification: Before disassembly, clean the threads with kerosene to remove impurities and rust, and apply molybdenum based special disassembly grease; Adopting the process of "preloading step-by-step boosting holding pressure" to avoid instantaneous overload; Ensure that the hydraulic nut is in contact with the end face of the disassembly sleeve, and calibrate the coaxiality with a dial gauge.

Emergency handling of jamming: Stop operation immediately in case of jamming and prohibit hard twisting; Repeatedly pressurize and depressurize with low-pressure hydraulic oil to loosen impurities or stress; If it is stuck due to rust, rust remover (such as penetrating oil) can be injected and soaked for 30 minutes before attempting to disassemble; If the thread has been deformed, the nut and disassembly sleeve need to be replaced to avoid secondary damage.