How do we deal with stripped screw during the assembly process?

The failure mode of fastener design is generally bolt fracture,  bolt and nut are not allowed to slip. Therefore, if the bolt matching counterpart is a nut, the bearing capacity of the nut is generally higher than that of the bolt. For example, the bolt of grade 10.9 needs to match at least the nut of grade 10, and the bolt of grade 8.8 needs to match at least the nut of grade 8.

If the hand piece is threaded on the part, the screwing depth of the part is also required. The following table is the minimum tightening depth of steel bolts. As can be seen from the table below, when the hand piece is made of aluminum alloy, the screwing depth is generally required to be between 2D and 2.7D. The purpose of this design is that the failure form after tightening overload is bolt fracture, rather than sliding teeth of nuts and bolts.

In the actual production process, the stripped screw of bolts and nuts are inevitable, and the rework workload caused by stripped screw is also large.

The following are the effects of various factors on the stripped screw of bolts and nuts in production, for reference only.

Thread lead bearing material Mechanical character Bolts with strength grade ≤10.9
Minimum tightening depth in mm
Minimum tensile strength (Rm)
Minimum brinell hardness (BHN)
Keywords steel, carburizing and hardening, quenching and tempering treatment 1000MPa (315BHN) 0.8d
Keywords steel, quenching and tempering treatment, cold work hardening 800MPa (250BHN) 1.0d
Steel, low alloy and non-alloy 400MPa (120BHN) 1.3d
Malleable (cast) iron 550MPa (180BHN) 0.9d
Nodular cast iron 500MPa (170BHN) 1.0d
Nodular cast iron 400MPa (140BHN) 1.3d
Gray pig iron 250MPa (210BHN) 1.3d
Aluminum and magnesium casting alloy 80BHN 2.0d(Strength grade8.8)
2.5d(Strength grade10.9)
60BHN 2.7d
(Strength grade≤8.8)
Pinchbeck alloy 350MPa 1.3d
Sintered metal 510MPa 1.0d
Remarks: The minimum engagement length corresponds to the thread length of the support. In general, the minimum meshing length of fine thread must be increased by 20%.

01 Electrophoretic paint film is too thick

In order to improve the corrosion resistance of parts, a layer of electrophoretic paint will be coated on the surface of parts, and it is inevitable that electrophoretic paint will adhere to the threaded holes. Then, if the thickness of electrophoretic paint is thick, will it affect the stripped screw of fasteners?

The test results are shown in the following figure. When the electrophoretic paint is thick, it will generally lead to jamming when tightening, and will not cause screw stripping and serious damage. When the bolt is screwed out, the internal thread and the bolt will occupy paint powder.

Luo Junjun believes that the paint is generally attached to the workpiece by negative electrophoresis. Due to the process characteristics, the thickness of the paint after electrophoresis is not too thick, and generally 60-80μm is already on the electrophoresis line. Therefore, it is generally not too serious to tighten the thread, or even lead to stripping screw.

02 Welding slag

This situation is common in welding nuts, which will inevitably lead to residual welding slag in the thread. See the following figure for the specific test results. Because of the existence of welding slag, the screw has a strong sense of jamming when it is screwed in. After the bolt is taken out, the front part of the bolt has broken teeth, and the thread position where the nut has residual welding slag also has obvious thread loss.

Whether this state will cause serious thread slip depends on the size and number of welding slag particles. When the number is large and the particles are large, the thread will have serious stripped screw.

In view of this situation, it is suggested that measures should be taken to prevent welding slag, such as adding a protective cover or coating anti-welding slag glue to prevent welding slag residue.

03 Loss of pressure

When the welding current is large and other factors, stamping will cause screw tooth deformation. In the process of bolt tightening, the thread teeth are not aligned, which leads to the damage and sliding of the stripped screw.

The state of thread deformation should be avoided as much as possible. If the thread deformation is found, the production should be stopped, and the appropriate technology should be explored to ensure that the thread teeth cannot be deformed.

04 Off center

If the welding nut is eccentric in the welding process, the test results are shown in the following figure, which will lead to smooth screwing in the first half and obvious jamming in the second half. If it is forced to break in, it will lead to screw thread damage and stripping.

The problem of welding positioning of welding nut should be optimized to ensure that nut welding will not be eccentric.

05 Oblique beating

When the screw has just been screwed in, it is found to be inclined. Take it out immediately, straighten it and continue screwing in. The front part of the screw is damaged, the middle part of the screw has traces of thread alignment, and the tail is intact.

06 Experience and summary

Residual paint is easy to cause the screw to jam when screwing in, which will not cause the screw to strip and serious damage.

When the welding nut with welding slag is screwed in, the welding slag will be eaten into the thread with the head of the screw until the welding slag falls into the thread, which is easy to damage the thread of the screw and cause the screw to fail to drive and strip.

The screw thread of welding nut with pressure loss is deformed, which is easy to cause the screw thread to be misaligned when the screw is screwed in, resulting in the screw thread damage and stripping.

Eccentric welding nut is easy to cause serious jamming when the screw is driven in half. If it is driven by force, it is easy to cause screw thread damage and strip.

The oblique driving of the screw directly leads to the complete damage of the nut and screw thread.

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