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Cixi Jieheng's analysis on the sliding teeth of fasteners

2020-07-27

Fastener assembly must meet the requirements of assembly firmness. The most important point is that the fasteners cannot slip. The article analyzes and deals with the screw slipping problem caused by several situations, and provides a reference experience for subsequent vehicle development.
 
 
 
In the process of automobile production and assembly, the bolt-nut matching is the most common matching method. In the actual production process, the phenomenon of sliding teeth often occurs. People often attribute this site to assembly skills. In fact, through engineering analysis at some slip-tooth sites, it will be found that the actual engineering is unreasonable, which will also lead to a certain probability of slipping problems.
 
 
 
 
 
Nut
 
 
 
The special-shaped nut (Figure 1) of a certain vehicle model cannot be tightened to the target torque. According to PFMEA (process failure mode analysis) analysis of bolt slipping will cause the function to be unusable, the severity is 10, the risk priority is 1, and the slippage rate must be reduced to 0.01%.
 
 
The slippage situation can be analyzed from several aspects:
 
 
 
(1) Tool rationality.
 
 
 
(2) The design rationality of the opponent's bolt diameter.
 
 
 
(3) Reasonable design torque.
 
 
 
1.1 Whether the selection of tools is reasonable
 
 
 
The tool calibration torque deviation is less than 5% of the set value, and the tool is qualified; the tool Cmk=1.55>1.33, the tool stability is good. From the tightening curve analysis, see Figure 3:
 
 
 
The ascending curvature of the bolt from 0 to 20Nm is faster, and other bolts mainly have spring washers; the final tightening step is the same as the other bolts, and there is no overshoot.
 
 
 
To sum up: the stability of the bolt tightening tool is good, and it does not exceed the set value of 5%; the electric gun program is set reasonably, and the final tightening step has an obvious tightening process without overshoot. The bolt cannot reach the target torque, the slippage is not caused by tool fluctuation or the tightening process is unreasonable.
 
 
 
Conclusion: The selection and design of tools are reasonable, not the cause of slippage.
 
 
 
1.2 Is the design of the opponent's bolt diameter reasonable?
 
According to the process requirements, the grade of the hexagonal bolts is 8.8, the torque is 50±15N·m, but from the cross-sectional view, as shown in Figure 4, it can be seen that the thread diameter of the upper special-shaped nut φ9.4mm is more obvious than the thread diameter of the lower bolt φ11.1mm Small; under the same material/bolt grade, a bolt with a smaller diameter can bear less torque.
 
 
 
Conclusion: The small bolt diameter is the main reason.
 
 
 
1.3 Is the nut tightening torque design reasonable?
 
The diameter of the special-shaped nut is smaller than the bolt diameter in other positions, but the same torque is designed. For this reason, the team members tested the torque load of the nut. The supplier initially used the pointer wrench for the test, as shown in Figure 5. It is concluded that the nut can withstand a torque above A+20N·m, and the theory is OK.
 
 
 
However, after further analysis, the torque applied by the pointer wrench to the fastener is a slow process and the speed is low, which is different from the torque applied by the electric gun.
 
 
 
In this regard, the team members tracked the nut installation on the production line, and there are still 5% of the fasteners slipping. In this regard, the production line conducts a torque reduction test for the installation point of the special-shaped nut. After assembling with the stun gun with the set value of A-10N·m, none of the nuts slipped, and the road test did not fall off.
 
 
 
Conclusion: The design torque is too large.
 
 
 
Solution selection: Based on the above analysis, it is concluded that the main reason for nut slippage is that the bolt diameter is small and the design torque is too large. Considering the cost and development cycle, the reason 2 is that the small bolt diameter changes the design involves the change of the part shape, from the main reason 3 tightening torque Started from above, verified qualified in small batches and used for production.
 
 
 
 
 
Self-tapping screw
 
 
 
The sliding teeth of a certain type of fastened self-tapping screws have a sliding tooth rate of as high as 5%, which increases the repair time of the workshop; and the vehicle automatically falls off during the road test, which is easy to cause abnormal noise.
 
 
 
No torque is released in the design, the workshop uses a BN·m fixed-torque electric gun to tighten it based on experience.
 
 
 
The slippage situation can be analyzed from several aspects:
 
 
 
(1) Whether the tool is reasonable.
 
 
 
(2) Whether the torque setting of the tool is reasonable.
 
 
 
(3) Whether the design of self-tapping screws is reasonable.
 
 
 
(4) Whether the design of the sheet metal hole of the counterpart is reasonable.
 
 
 
2.1 Whether the selection of tools is reasonable
 
For Phillips screws, the Bosch stun guns currently used in the workshop usually have 400r/min, 600r/min, 900r/min, 1100r/min. Generally, in order to ensure assembly efficiency and assembly quality, the hexagon head bolts are assembled with high-speed electric guns. For the gun, the fastener of the cross gun head is easy to jump, and the low-speed stun gun is used. According to on-site inspection, the Phillips screw assembly station uses a 400r/min electric gun, which conforms to the principle of electric gun selection.
 
 
 
2.2 Is the tool torque set reasonable?
 
The investigation found that the Phillips screw at the fixed point of the retractor has no torque requirement. Based on experience, the setting value of 2.5N·m was selected on site. In order to verify whether the torque is reasonable, the team members conducted a sheet metal experiment for the torque setting, as shown in Figure 6, and the results are shown in Table 1.
 
 
 
It can be seen that the screw will not slip between the set value of 1N·m~1.6N·m. The risk of slippage gradually increases at 2.0N·m~3N·m, and 2.5N·m will cause higher Slippage rate.
 
 
 
Conclusion: The main reason is that the torque setting of the stun gun is too high.
 
 
 
2.3 Is the design of self-tapping screws reasonable?
 
Since self-tapping screws are parts that come with the assembly, they are determined by the supplier, and the OEM cannot monitor the materials and parameters of the fasteners. The screws provided by the two suppliers are quite different. One is a screw with a washer and the other is a screw without a washer, as shown in Figure 7.
 
 
 
Theoretical studies have found that the spring washers of some screws are misaligned at the opening, which will cause the self-tapping screw to be driven into the sheet metal at a certain inclination, and the sheet metal hole will be reamed, and the fastening will fail (sliding teeth).
 
 
 
In order to further verify the impact of the misaligned spring washer, the team members used normal screws and the misaligned spring washer to install them with different torques. The results are as follows, as shown in Table 2.
 
 
 
Tests have proved that screws with misplaced spring washers will slip even if the torque is between 0.8N·m and 2N·m
 
 
 
 
 
Conclusion: The misalignment of the spring washer is the main cause.
 
 
 
2.4 Is the opponent's sheet metal hole design reasonable?
 
The sheet metal hole for self-tapping screws is a φ3.6mm smooth hole. In order to verify whether the current sheet metal hole has a problem, the team members made a sheet metal hole with a diameter of φ3.6mm for testing, as shown in Figure 8.
 
 
 
The test proves that the φ3.6mm sheet metal hole can be effectively tightened under the torque within 2N·m without slipping teeth. The data is shown in Table 3.
 
 
 
Table 3 Breaking torque of φ3.6mm sheet metal hole
 
 
 
 
 
Conclusion: Unreasonable design of sheet metal holes is a non-essential cause.
 
 
 
Summary and plan selection: The main reason for self-tapping screws slipping is that the torque setting is too high and the spring washers are misplaced. The team decided to choose all suppliers without spring washer screws, and the self-tapping screws are 1.3±0.3N·m. Bulk verification is valid.
 
 
 
bolt
 
 
 
After the trial production stage of the A model, it was found that the frequency of bolt slippage was very high. As shown in Figure 9, the slippage rate of the similar installation points of other models on the same production line was very low, which increased the repair time of the workshop, and the quality of the tapping installation was difficult to guarantee. Standard analysis.
 
 
 
After comparing the existing model and the A model, we found that:
 
 
 
(1) The selection and setting of tools for mass-produced models and A models are the same, and the reasons for tool and torque design can be eliminated.
 
 
 
(2) Is the bolt structure the main reason:
 
 
 
The bolt ends of mass-produced models are chamfered, which can act as a guide. Model A has no chamfers, as shown in Figure 10.
 
 
 
According to the technical analysis of the company’s fastening experts: the difficulty of installing the inclined plane is far more difficult than the plane. In order to improve the hole rate of the inclined plane bolt, the guide bolt must be used. The bolt selection of the A model does not take into account the installation surface is inclined. .
Company name:
Cixi Jieheng Electric Appliance Co., Ltd.
Company phone:
+86-057-63203576
Address:
Tiantan Industrial Zone, Zhouxiang Town, Cixi City, Zhejiang Province
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