What Is a Clamp Coupling?

The corrugated pipe coupling uses a corrugated thin-walled pipe (corrugated pipe) to directly weld or bond the two halves of the coupling to transmit motion. This coupling has a simple structure, small dimensions, convenient processing and installation, and high transmission accuracy. It is mainly used in small-power precision machinery and control mechanisms that require compact structures and high transmission accuracy.

Corrugated tube coupling consists of two hubs and a thin-walled metal tube.

The input end of the coupling structure is a clamping hoop structure, and a pre-tightening force is generated by a clamping screw, and the power input shaft and the clamping hoop are fixedly integrated into one body. The output end is an expansion structure. The cone is pre-tightened by an expansion screw to expand the expansion sleeve, cooperate with the inner hole of the power output shaft, and connect as a whole. The middle is a bellows structure, and the two ends of the bellows are glued to the input and output ends.

1.Corrugated pipe coupling has no gap and high sensitivity

2.High flexibility, can better protect equipment

3.Can absorb vibrations, and has strong ability to compensate radial, angular and axial deviations

4.Resistant to oil and corrosion

5. Torsion rigidity, clockwise and counterclockwise rotation characteristics are exactly the same

6.Commonly used in CNC machine tools, indexing discs and other industrial machinery

The slipping failure of the bellows coupling is usually caused by a failure of the clamping hoop, the expansion sleeve or the glue joint of the bellows. The failure cause analysis is as follows:

a) The input end of the coupling is faulty: the clamping screw is damaged or loose, the clamping hoop structure is damaged or the tightening thread on the clamping hoop is damaged, so that the clamping hoop cannot properly clamp the power input shaft;

b) Coupling bellows failure: the bellows structure is damaged, and the glued surface of the bellows and the clamping hoop or the expansion end debonds the bellows to transfer power normally;

c) Coupling output failure: the expansion screw is damaged or loose, the structure of the expansion sleeve is damaged or the expansion thread on the expansion sleeve is damaged, so that the expansion sleeve cannot properly tighten the power output shaft ^[2]

From the structure of the coupling, it can be known that the coupling and the power output shaft (reducer shaft) transmit power through the expansion sleeve and the power input shaft is clamped to transmit power through the clamping hoop. The clamping end of the coupling and the expansion The tight end transmits power through the corrugated tube, and the corrugated tube is connected to the expanded end and the clamping hoop by gluing. The fault is in the clamp hoop and bellows.

There are two pre-deformed grooves in the radial processing of the clamping hoop, the width of the groove is about 2mm, the diameter of the end with the clamping screw is a through groove, the depth direction is not open with the corrugated pipe, and the cross section is a bevel; the other The groove is not opened in the radial direction, but the longitudinal connection between the groove and the corrugated pipe is not opened, and the cross section is an inclined surface.

When clamping, use a torque wrench to tighten the clamping screw according to the specified value, and eliminate the hole and shaft clearance through the deformation of the clamping hoop to achieve the clamping purpose. It can be seen from the cross-sectional view that when clamping, due to the existence of two pre-deformed grooves, the cross section is weaker, and the deformation is mainly at the two pre-deformed grooves. The cross-sectional area of the pre-deformed groove on the side with the clamping screw is the smallest, and the cross-section is the weakest at this end. There is a permanent deformation mark in the figure at the end, indicating that the stress in the clamping state is greater than the yield limit of super-hard aluminum.

According to the stress distribution diagram, as long as the bevel gear box and the main reducer are assembled and clamped, the stress value at the pre-deformed groove on the side of the clamping screw is 164Mpa, and the deformation amplitude is 0.027mm. When the launcher is running, the coupling is subjected to the effect of alternating composite stress, where the stress and deformation will be greater.

If there are trace defects such as cavities, inclusions, and defects at the bonding surface during manufacturing, when the alternating stress exceeds the bonding strength, microcracks will first be generated in the defect area of the coupling bonding surface with the highest stress Form a source of fatigue. Due to the stress concentration at the crack tip, with the increase of the number of stress cycles and the extension of the constant stress time, the crack gradually expanded. As the cracks spread, the bonding surface is gradually weakened. When the cracks reach a certain size, fatigue failure occurs. The coupling can work normally in the preliminary test. With the use of time, the crack gradually spreads. Even if the load is small, the coupling will fail.

The existence of stress at the joint is a potential hidden danger of failure. In order to avoid large stress generated during clamping of the coupling clamping hoop, an annular groove is generally performed on the clamping hoop for stress relief.

According to the mechanism analysis, the fault was reproduced. Take the same type of coupling used on the equipment and use a torque wrench to simulate loading on the test tool. During the test, an accelerated fatigue test is performed by increasing the torque. When torque is applied, starting from 40N.m, it is gradually loaded to the maximum tolerable torque, and each torque point is loaded at least three times. When loading near the rated torque, you hear two slight sounds inside the coupling, check the coupling for damage and deformation. Continue to load to rated torque, and no abnormal situation occurs. After standing for 4 days, the load test was repeated. When loading starts from 40N.m, the clamp hoop and the corrugated pipe adhesive surface will rotate relative to each other. The fault point is consistent with the position of the faulty coupling.

The abnormal sound during loading is the sound of microcrack spreading and bursting where the stress of the bonding defect is the largest. After failure there, the stress is redistributed to reach a new equilibrium. Even if the load is continued, the crack will not spread immediately in a large area and the coupling will fail. After the applied torque load is unloaded, the clamping torque still exists. Clamping stresses and environmental stresses still cause cracks to slowly propagate. Over time, when it reaches a certain level, it can be destroyed even when a small torque is applied. Through the above tests, the coupling failure caused by the small load under normal use conditions was reproduced. ^[3]

An annular cutting groove is formed on the coupling clamp hoop, which changes the stress state of the structure of the clamp hoop, and greatly reduces the stress value at the adhesive surface of the clamp hoop. After the improvement, the maximum stress value of the adhesive surface of the clamping hoop is reduced to 30Mpa, which is 134Mpa less than the maximum stress of the original structure.

The improved coupling was simulated loading on the test fixture and subjected to accelerated fatigue tests, and the coupling worked normally. After the equipment was replaced with a new structure coupling, after a long period of operation, the coupling worked normally and the equipment worked normally.

The elastic element of the bellows coupling is a precision metal bellows, which is divided into two types: flexible series and rigid series. The flexible series has greater flexibility and is often used in encoders, sensors and other small occasions. The rigid series is mainly used in servos. Motors, CNC machine tools (machining centers), etc. The origin of corrugated pipe couplings mainly includes: Europe, the United States, Japan, Taiwan, China ^[1] .