Why the Output Shaft Breaks: Cause Analysis

The output shaft of the auxiliary gearbox is a key component of the gearbox.

It bears a large torque during the operation of the vehicle, so it is required to have a high strength.

Technical requirements of the drawing: material 40Cr, medium frequency induction hardening at R angle and spline, hardening layer depth at R angle ≥ 5mm, hardening layer at spline 5~8mm from tooth bottom, surface hardness ≥ 55HRC;

The matrix shall be quenched and tempered, and the hardness shall be 235~265HBW.

At the beginning of 2019, the market reported that the output shaft of the auxiliary box broke a lot when the customer drove 30000 ~ 100000 km.

Through the analysis of the failed parts, the reasons for the fracture were as follows.

1) The R-angle structure of the output shaft is concave, which makes induction hardening more difficult, the quenching layer is shallow, and the quenching crack is sensitive.

There are induction hardening cracks at the R-angle in the actual detection.

2) The chamfer of the oil hole of the auxiliary box output shaft is 0.5mm × 45 °, due to the sharp angle effect, quenching cracking tends to be large, and quenching cracks also exist.

Related reading: Quenching Crack vs. Forging Crack vs. Grinding Crack

This article mainly analyzes the cause of the fracture of the output shaft of the auxiliary box from the principle, and formulates a series of improvement measures to solve the problem of the fracture of the output shaft of the auxiliary box.

1. Failure analysis

1.1 Failure detection analysis

The failure parts of the output shaft of the auxiliary box are shown in Fig. 1.

The appearance and fracture location of the failure parts are shown in Fig. 1a.

The red circle is the fracture location.

It can be seen that the output shaft of the auxiliary box is fractured from the R-angle position of the tool withdrawal groove.

The fracture morphology is shown in Fig. 1b.

The fracture is straight and used by the circumferential rotation of the shaft.

After the fracture, there are mutual wear marks at both ends, which conforms to the torsional fracture characteristics.

Why the Output Shaft Breaks: Cause Analysis 1

Fig. 1 Failure Parts of Output Shaft of Auxiliary Box

Nondestructive testing and metallographic analysis were carried out on the finished products produced and the failure parts fed back by the market.

See Table 1 for the test results.

It can be seen from the table that the induction quenching results of spline parts of finished products and market failure parts meet the technical requirements, the effective hardening depth of spline parts is ≥ 5mm, and the metallographic structure of the hardened layer is grade 4-5 acicular martensite.

However, the induction quenching results at the spline oil hole and R corner do not meet the technical requirements, as follows:

1) There are induction hardening cracks in the R-angle position and spline oil hole of the finished product.

2) The depth of induction hardening layer at corner R is shallow, or even there is no induction hardening layer depth, which is not more than 5mm as specified in the technical requirements.

Table 1 Magnetic Particle Testing and Metallographic Analysis Results of Finished Parts and Failure Parts

Samples

Detection location:

Metallographic examination

Nondestructive testing

Finished products in production

spline

Ds:6~8mm,4~5 gradeM

Spline oil hole crack (see Fig. 2a)

R angle

Ds:3.2mm,5 grade M(see Fig. 2b)

R angle crack

(see Figure 2c)

Market failure parts

spline

Ds:7~9mm,4~5 grade M

Spline oil hole crack

R angle

No hardened layer

(see 2d for the figure)

 

The above inspection results are consistent with the cracking characteristics of the failed part.

Because the induction hardening layer at the R corner of the output shaft of the auxiliary box is shallow, which does not meet the technical requirements, and there is an induction hardening crack at the R corner, the strength at the R corner is low.

During the operation of the vehicle, the R corner cannot withstand large torsional stress and finally cracks.

In addition, induction hardening cracks also exist at the spline oil hole, and some of the market failure parts also have the output shaft of the sub box broken here.

Why the Output Shaft Breaks: Cause Analysis 2
Why the Output Shaft Breaks: Cause Analysis 3

Fig. 2 Results of NDT and Metallographic Testing

1.2 Failure cause analysis

It can be seen from the failure detection results that there are two fracture risk points of the output shaft of the auxiliary box: one is the R angle position; the second is spline oil hole.

Fig. 3a shows the structure of the R-angle position of the finished product in production.

It can be seen that the R-angle is an inner R0.5mm structure, which will have the following two effects on induction hardening.

1) The transition fillet at the bottom of the R corner of the inner R type structure is too small, and the machining stress at the bottom of the R corner depression is large, which will increase the sensitivity of induction hardening cracks.

2) The distance between the R angle depression of the inner R type structure and the inductor is relatively large.

Fig. 3b shows the induced current distribution in the case of induction heating at corner R.

Due to the proximity effect of induction heating, the farther away from the sensor, the smaller the induced current.

Therefore, for areas 1 to 4, the induced current is gradually reduced, and since the area 4 at the bottom of the R angle is the farthest from the inductor, the induced current at the bottom of the R angle is the minimum.

Under the same heating time, when areas 1 to 3 reach the quenching heating temperature as a whole, area 4 may not reach the quenching temperature completely.

At this time, water spray cooling, martensite transformation occurs in areas 1 to 3, and martensite transformation occurs or does not occur in areas 4.

As a result, the hardened layer depth of regions 1 to 3 is inconsistent with that of region 4, uneven deformation due to structural transformation occurs inside and outside the R angle, and region 4 is subjected to tensile stress due to structural transformation, which is also the concentration point of machining stress, and finally leads to quenching cracks in region 4 during quenching.

In addition, area 4 is the farthest from the inductor, which is the most difficult part for induction hardening, so the depth of hardening layer is not enough.

Fig. 3c shows the chamfered structure of the oil hole of the output shaft of the auxiliary tank currently produced.

The design size of the oil hole is 0.5mm × 45°.

This structure does not meet the design requirements of induction hardening for hole chamfer.

In order to better ensure the quality of induction hardening, it is required that the hole chamfer should be slightly greater than 1mm × 45°.

This is mainly because when chamfering is small, the temperature around the oil hole is too high due to the sharp angle effect of induction heating, which is easy to produce quenching cracks;

At the same time, because of the existence of the oil hole, the induced current is forced to bypass on both sides of the oil hole, and the induced current around the oil hole is uneven.

The eddy current density on both sides of the oil hole along the current direction is large, while the eddy current density on both sides of the oil hole perpendicular to the current direction is small, so the former becomes a high temperature area, and the latter becomes a low temperature area, as shown in Fig. 3d.

The heating temperature around the oil hole is uneven, the depth of current penetration is different, and the thickness of hardened layer after quenching is also different.

During induction quenching, organizational stress and thermal stress are generated, which is the fundamental reason for quenching cracks at the edge of the oil hole.

In addition, due to the special structure of the oil hole, the cooling of its edge is more intense than that of other parts during cooling, which further increases the sensitivity of the edge of the oil hole to produce quenching cracks.

Why the Output Shaft Breaks: Cause Analysis 4

Fig. 3 R angle and oil hole

2. Improvement measures

2.1 Structural optimization of R angle and oil hole chamfer

It can be seen from the above analysis that the root cause of the fracture of the output shaft of the auxiliary box is the unreasonable design of the R angle structure and the chamfer of the oil hole.

Therefore, the following improvement measures are specially formulated.

1) The improved transition fillet structure is R1.5mm, and the technical requirement for the depth of induction hardening layer at the optimized fillet is ≥ 3mm.

2) The chamfer structure of the improved oil hole is (1~1.5) mm × 45°.

Fig. 4a shows the optimized R angle structure.

During induction heat treatment, when the step root of the workpiece needs induction hardening, the step root needs to have a transition fillet, and the larger the fillet, the better.

This design has good processability:

① Reduce the stress concentration at the root of the step and reduce the cracking tendency during use.

② Reduce the difficulty of induction quenching, heat the step root evenly, and the step hardening layer is uniform and continuous, and the strength is significantly improved.

Figure 4b shows the optimized oil hole chamfer design, and the chamfer size is increased to (1~1.5) mm × 45 °, under the same heating condition, the larger the oil hole chamfer is, the greater the current density at the edge of the oil hole will be, and the tendency of the oil hole edge to crack due to overheating will be reduced.

Why the Output Shaft Breaks: Cause Analysis 5

Fig. 4 structural optimization

2.2 Induction hardening process optimization

Because the quenching area of the output shaft of the auxiliary box is large and the power supply is small, the scanning quenching method is used to complete the quenching of the auxiliary box.

The biggest advantage of scanning quenching is that it can use equipment with smaller capacity to handle large workpieces.

The scanning quenching method places the workpiece in or near the inductor, so that the inductor and the workpiece move relatively.

The inductor connects high-frequency or medium-frequency current to inductively heat the workpiece to the quenching temperature.

At the same time, the inductor or water jet ejects quenching cooling medium, so that the part of the workpiece that has reached the quenching temperature can be quenched.

Until the quenching area of the workpiece is all quenched, the inductor current is cut off first, and then stop the injection of quenching cooling medium.

Fig. 5 shows the optimized design of the effective circle of the inductor.

The effective circle of the inductor is a whole circle structure.

The effective circle rotates at a certain angle (generally 45 °) to meet the heating of the plane and the R angle at the variable section.

The effective ring is equipped with“ Π”. 

The slot of the magnetic conductor is inclined to the R angle area.

By using the slot effect of the magnetic conductor, the medium frequency current of the effective coil is expelled to the R angle area, so that the heating of the R angle area is strengthened.

The gap between the front end of the effective coil and the R angle is 3~5mm, and the R angle can be heated rapidly.

Generally, the quenching temperature can be reached within 10s, so that a very ideal hardening layer distribution can be obtained.

Considering that when heating the R angle, in order to obtain sufficient hardening layer depth at the R angle, the inductor needs to stay in the R angle area for heating for a period of time, and at this time, the adjacent splines above the R angle are also being heated.

In order to prevent the hardening layer depth of the splines here from being too deep, resulting in “bulging” of the hardening layer at the transition between the R angle and the axial spline, the proximity effect of induction heating is used, that is, when designing the inductor, the surface of the heating spline and the spline axis form an included angle of 7.5 °.

The closer to the R angle area, the smaller the spacing, so the induced current distribution in the R angle area and its adjacent areas are shown in the black shadow in Fig. 5.

When the R corner area is heated, the inductor moves upward to heat and quench the spline area, finally obtaining a uniform and continuous hardening layer, which improves the overall strength of the output shaft.

Why the Output Shaft Breaks: Cause Analysis 6

Fig. 5 Design of effective circle

3. Conclusion

Through the above failure cause analysis, the improvement measures for the fracture of the output shaft of the auxiliary box mainly include three aspects:

First, optimize the structure of the transition fillet, improve the structure of the transition fillet to r1.5mm for the outer fillet, and require the technical requirement of induction hardening layer depth at the fillet to be ≥ 3mm;

Second, optimize the chamfer size of the oil hole at the spline, and improve the chamfer structure of the oil hole to (1 ~ 1.5) mm × 45°;

Third, optimize the structure of the effective ring of the inductor, and adopt the scanning induction hardening method to make the hardening layer of the fillet and spline continuous and uniform.

After the implementation of the above improvement measures, the output shaft of the auxiliary box was detected and tracked, and obvious results were achieved.

1) The R-angle strength of the output shaft of the sub tank has been significantly improved, and the oil hole chamfer and the R-angle area no longer have induction hardening cracks.

2) The fillet and hardening layer of the output shaft of the auxiliary box are continuous and uniform, the hardening layer at R angle is 4 ~ 6mm deep, and the hardening layer at spline part is 5 ~ 8mm deep.

The metallographic structure of the hardening layer is 4 ~ 6 grade acicular martensite, and the surface hardness is 56 ~ 59HRC, which can meet the technical requirements.

3) After delivery and loading, the output shaft of the sub box has not cracked, which greatly reduces the risk of market claims and improves product quality and customer satisfaction.

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