Knowledge

The spline part of the spline shaft exhibited a "deformation" phenomenon. After inquiry, the shaft showed a phenomenon of torsional neck contraction but had not yet fractured.

This phenomenon often encompasses multiple levels of issues.

1. "Twisting and Contraction" Specific Phenomenon Characteristics

The "twisting and contraction" phenomenon in group members' photos generally refers to the fact that during the process of transmitting torque, the shaft body or the spline part of the spline shaft has undergone a significant helical deformation, resembling a twisted rope. This situation is usually accompanied by the following characteristics:

1. The top of the spline teeth are worn, but no tooth breakage or fracture has occurred;

2. The root of the spline has undergone a slight twist, but there is no贯穿 crack；

3. The entire shaft has not broken， but has lost its precise fit or torsional stiffness；

4. This state usually occurs in situations where the transmission system is "overloaded but does not immediately fail".

This state is a "structural endangered" state. The shaft has entered the yield stage， but has not yet undergone brittle or fatigue fracture.

2. Twisting and Contraction is Not Equal to Fracture

Phenamenan	Occurrence  conditions	Material state	Failure mechanism
Contraction	Excessive  torque effect	The material hasentered the plastic stage(localyielding)	Deformation accumulates,but remains continuous.
Fracture	UItimate load or crack propagation	The material has exceeded its strength  limit.	Material integrity failure, crack instability expansion

The reason why "unbroken but shrinking" occurs is often due to:

1. The material is above the yield strength but has not reached the tensile strength limit;

2. Plastic flow has occurred in the stress concentration area;

3. The working condition is instantaneous or periodic overload, and it does not continue to cause fracture.

III. Why does this "just not broken" state occur?

1. The shaft material has good plasticity and toughness. Common alloy quenched and tempered steels (such as 42CrMo, 40CrNiMo) have a good balance between strength and plasticity, with a wide plastic zone, and up to 8-15% plastic deformation can occur before failure. Therefore, in short-term overload, the shaft will first "warn of deformation" and then "fail due to fracture".

2. The impact load does not persist.

If this shaft is subjected to a one-time overload impact (such as misoperation, sudden stop, sudden increase in load), but the duration is short, and the shaft body does not undergo a long period of fatigue cycles, it will not immediately fracture.

The wear of the spline meshing surface and the enlargement of the gap lead to concentrated load. The original design intention of the spline is to evenly distribute torque among multiple teeth, but long-term wear or poor processing accuracy can lead to "local single tooth being stressed". Once the tooth surface stress concentrates, deformation begins in this area through plastic torsion and gradually spreads. From the photo, the wear of the spline teeth is slight, and in this case, this reason can be excluded.

When the spline shaft shows "deformation but does not fracture", on the one hand, it indicates that the structure has certain "resilience" and "tolerance", avoiding sudden collapse; but on the other hand, this is also a sign that the system is moving towards progressive failure.