Fracture analysis and prevention of high speed shaft of reducer

In the 2016 China reducer Technology Forum, Zhu Xiaolu, Professor of School of mechanical engineering, University of science and technology of Beijing (consultant of ward transmission (Tianjin) Co., Ltd.), pointed out in the report: in recent years, there are many failure cases of reducer in long-term use. The failure of some basic parts (gear, shaft and bearing) of the reducer is "frequently occurring", which is difficult to cure due to various reasons; while the failure of some basic parts may be difficult to explain clearly. Based on these "frequently occurring diseases" and "difficult and miscellaneous diseases", this paper puts forward why the high-speed shaft of hard tooth surface reducer often breaks? How to prevent and control? And give a detailed analysis and solutions.

The production experience shows that the high-speed shaft of hard tooth surface reducer is prone to fracture. For example, the high-speed shaft of a foreign reducer often breaks in two places

Fracture characteristics: 1) the fracture is fatigue fracture, the shaft is fatigue fracture.

2) Most of the fracture parts of the shaft are just at the edge of the interference fit between the coupling and the shaft.

3) Most of the earliest fatigue cracks occurred at the sharp corner or transition corner of the flat key keyway.

4) The fracture of the shaft is perpendicular to the axis of the shaft, which is basically a bending torsion fracture of high strength steel.

Under normal circumstances, the fatigue strength safety factor of the shaft in the design is very large, usually more than 3. It is impossible for the high-speed shaft to break, and after the actual inspection, the material heat treatment process of the shaft also meets the technical requirements.

After analysis, the following reasons are obtained

Reason 1: stress concentration in keyway. From the macroscopic observation of fracture, it can be seen that the earliest fatigue crack occurred at the sharp corner of the keyway of the flat key, so the weakening of the keyway to the shaft (the reduction of the cross-sectional area of the shaft and the stress concentration) is worthy of attention, especially the fillet r at the bottom of the keyway (as shown in the figure below), which has a great influence on the stress concentration.

When the shaft is in pure torsion, the effective stress concentration factor K & at the thread, keyway, cross hole and mating edge is as shown in the figure below. When the tensile strength rm of the shaft is 900MPa, the effective stress concentration factor of the keyway is 2, so the weakening of the keyway on the shaft is very large.

Reason 2: interference fit of coupling and shaft. In the previous figure, we can see that the fracture part of the shaft is just the edge of the coaxial interference fit of the coupling, and the interference fit has a great influence on the strength of the shaft. From the stress concentration factor diagram, it can be seen that the stress concentration factor of interference fit H7 / R6 can reach more than 2.2, and the stress concentration factor of interference fit H7 / M6 can not be less than 1.8. Therefore, the fracture of high-speed shaft at the edge of coupling and shaft interference fit can be understood.

The third reason is the installation and use of the reducer. A big problem in the design of hardened gear reducer is that the diameter of motor and reducer shaft does not match seriously, and the reducer shaft is much smaller than the motor shaft. Generally, the diameter of reducer shaft is about 3 / 4-1 / 2 of motor shaft, as shown in the figure below. If the coaxiality of motor shaft and reducer shaft is very poor, additional radial force fr will be generated on the coupling.

Due to the different shaft diameters, the stiffness of the two shafts is different (the stiffness is proportional to the diameter d ^ 3), and the additional radial force F generated by the coupling has different additional bending moment (stress) on the dangerous section of the two shafts.

Example: bending stress of dangerous section of shaft:

Motor shaft: σ 1 = FL1 / 0.1d1 ^ 3; reducer shaft: σ 2 = FL2 / 0.1d2 ^ 3;

When L1 ≈ L2, the ratio of the two stresses is:

If D2 = 1, D1 = 2, then σ 1 / σ 2 = 8, the stress difference is huge.

Reason 4: radial stiffness of shaft coupling. The so-called radial stiffness of the coupling refers to the radial force required for each unit of radial displacement Δ y generated by the two shafts of the coupling. The greater the radial stiffness is, the greater the radial force is, and the greater the adverse impact on the strength of the connecting shaft is. The radial stiffness of flexible couplings with non-metallic elastic elements, such as elastic sleeve cylindrical pin coupling, plum blossom coupling, tire type coupling, is smaller.

Reason 5: unbalance of rotating parts on shaft.

If the static balance or dynamic balance of the rotating parts is not good, the rotating parts will produce centrifugal force, increase the additional stress of the shaft, and affect the strength of the shaft. The figure above shows the configuration relationship between the coupling and the reducer. The mass of the coupling in the figure is a little eccentric.

Centrifugal force caused by eccentricity:

Q = π^2×n×m^2×r / 900

Where Q: centrifugal force (n) caused by eccentricity;

r: Eccentricity (mm);

n: Speed of shaft (R / min);

m: Mass of coupling (kg);

d: Diameter of shaft (mm)

Because the centrifugal force is directly proportional to the mass square of the rotating parts, the mass has a great influence on the centrifugal force.

In view of the above problems, we can take the following main measures to prevent high-speed shaft fracture:

(1) Strictly control the processing quality of keyway, especially the fillet r at the bottom of keyway, and take the maximum value according to the standard as far as possible; keyway without fillet cannot be used.

(2) The coupling and brake wheel installed on the high-speed shaft should undergo static balance or dynamic balance test to avoid excessive additional centrifugal force.

(3) The attached crops adhered to the coupling and brake wheel shall be removed in time.

(4) The coupling and brake wheel whose manufacturing quality does not meet the technical requirements shall not be used.

(5) The base surface of reducer and motor should be machined plane, the gasket should be flat, and it is better to have positioning measures.

(6) Regularly check whether the anchor bolts are loose or broken, so as to prevent the displacement of the motor or reducer after a period of operation and damage the adjusted coaxiality.

During installation, special person shall be responsible for adjusting and testing the different axial degrees of motor and reducer. It is possible to use a fast, simple and economical laser alignment device to detect the alignment of two axes.