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CLOOS couplings connect two flanged half-couplings to their respective shafts using standard parallel keys, and then bolt the two half-couplings together into a single unit to transmit motion and torque. This type of coupling has two main structural forms: (1) alignment of the two shafts is achieved by bolts in reamed holes, with torque transmitted through compression and shear on the bolt shanks; (2) alignment is achieved by the fit between a shoulder on one half-coupling and a groove on the other. The bolts connecting the two half-couplings can be standard bolts of grade A and grade B, with torque transmitted by the frictional moment at the mating surfaces of the two half-couplings.

CLOOS coupling introduction

It consists of two hubs and a thin-walled metal tube, connected by either welding or bonding. Although many other materials are available, stainless steel and nickel are the most commonly used materials for the metal tube. Nickel tubes are produced by electrodeposition. This method first involves machining a solid mandrel into a corrugated shape, electroplating nickel onto the mandrel, and then chemically dissolving the mandrel to obtain the nickel bellows. This method allows control over the precision of the bellows wall thickness and can achieve thinner walls than those produced by other methods. These thin-walled bellows give the coupling high sensitivity and rapid response, making it ideal for applications in extremely small and precise instruments.

CLOOS coupling structure composition

The input end of the coupling features a clamping hub structure, which generates preload via clamping screws to rigidly connect the power input shaft to the clamping hub. The output end features a taper-lock structure, where tightening screws preload a cone, causing the locking sleeve to expand and fit with the inner bore of the power output shaft, forming a single unit. The center features a bellows structure, with both ends of the bellows bonded and rigidly connected to the input and output ends.

CLOOS coupling troubleshooting

From the coupling structure, it is known that power is transmitted from the coupling to the power output shaft (gearbox shaft) via a taper-lock fit, and to the power input shaft via a clamping hub. Power is transmitted between the clamping end and the taper-lock end of the coupling through the bellows, which is bonded to both the taper-lock end and the clamping hub. The fault location is at the clamping hub and the bellows. Two pre-deformation slots are machined radially into the clamping hub, with a slot width of approximately 2mm. On the side with the clamping screw, the slot is through in the diametrical direction but not fully open in the depth direction at the junction with the bellows, featuring a beveled cross-section. The other slot is not open radially, but its longitudinal section at the junction with the bellows is also not fully open, featuring a beveled cross-section. During clamping, the clamping screws are tightened to the specified value using a torque wrench, eliminating the clearance between the bore and the shaft through deformation of the clamping hub to achieve clamping. As seen from the cross-sectional view, due to the presence of the two pre-deformation slots, the cross-section at this location is weaker, and deformation occurs mainly at these slots. The cross-sectional area of the pre-deformation slot on the side with the clamping screw is the smallest, making this section the weakest. Permanent V-shaped deformation marks at its end indicate that the stress in this area during clamping is significant, exceeding the yield limit of the super-hard aluminum. According to the stress distribution diagram, once the bevel gear box and the main gearbox are assembled and clamped, the stress value at the bonded surface of the pre-deformation slot on the clamping screw side is 164Mpa, with a deformation magnitude of 0.027mm. During launcher operation, the coupling is subjected to alternating composite stresses, resulting in even greater stress and deformation at this location. If minor defects such as voids, inclusions, or flaws exist at the bonded surface during manufacturing, micro-cracks will first form at defective areas within the region of maximum stress on the coupling's bonded surface when the alternating stress exceeds the bond strength, creating a fatigue source. Due to stress concentration at the crack tip, the crack gradually propagates with an increase in the number of stress cycles and prolonged exposure to constant stress. As the crack spreads, the bonded surface is progressively weakened, leading to fatigue failure when the crack reaches a certain size. The coupling functioned normally during initial testing, but as usage time increased, the cracks gradually propagated, causing the coupling to fail even under low loads. The presence of stress at the bonded joint is a potential hidden cause of failure. To avoid generating high stress in the clamping hub during clamping, annular grooves are generally cut into the clamping hub for stress relief.

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