German Kral coupling Kral model Kral price

Kral couplings contain an elastic compound with pre-compressed rubber, providing additional strength and extending service life. This coupling accommodates all types of misalignment. The hub material is high-strength aluminum alloy, which is both lightweight and corrosion-resistant. The rubber component is primarily used for vibration damping, ensuring smooth and quiet power transmission, thereby protecting the drive source and driven machinery. Installation is simple; since the rubber part is a split insert, it can be installed by insertion after shaft alignment. Elastic couplings use parallel or spiral slit systems to accommodate various misalignments and transmit torque precisely. Elastic couplings typically offer good performance and price advantages, making them the preferred choice in many practical applications of stepper and servo systems. The one-piece design enables elastic couplings to achieve zero-backlash torque transmission and maintenance-free operation. Elastic couplings mainly consist of the following two basic series: spiral slit type and parallel slit type.

Kral coupling introduction

Consists of two hubs and a thin-walled metal tube, connected together by welding or bonding. Although many other materials are available, stainless steel and nickel are the most commonly used metals for the tube. Nickel tubes are produced via electroforming. 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 controls the precision of the bellows wall thickness and allows for 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.

Kral coupling structure composition

The input end of the coupling structure features a clamping collar design, where clamping screws generate preload to rigidly connect the power input shaft with the clamping collar. The output end features a taper-lock design, where tightening screws preload a cone, causing the locking sleeve to expand and fit into the inner bore of the power output shaft, forming a unified connection. The center features a bellows structure, with both ends of the bellows adhesively bonded to the input and output ends.

Kral coupling troubleshooting

Based on the coupling structure, the coupling transmits power to the power output shaft (gearbox shaft) via a taper-lock sleeve expansion, and to the power input shaft via clamping collar compression. The coupling's clamping end and taper-lock end transmit power through the bellows, which are adhesively bonded to the taper-lock end and the clamping collar. The fault location is at the clamping collar and the bellows. Two pre-deformation slots are machined radially into the clamping collar, 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 junction with the bellows is also not fully open, featuring a beveled cross-section. During clamping, a torque wrench is used to tighten the clamping screws to the specified value, eliminating the clearance between the bore and the shaft through the deformation of the clamping collar 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 pre-deformation slot on the side with the clamping screw has the smallest cross-sectional area and is the weakest point. 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 gearbox and main gearbox are assembled and clamped, the stress value at the adhesive interface 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 blemishes exist at the adhesive interface during manufacturing, micro-cracks will first form at the defective spots within the area of maximum stress on the coupling's adhesive interface when the alternating stress exceeds the bonding strength, creating a fatigue source. Due to stress concentration at the crack tip, the crack gradually propagates as the number of stress cycles increases and the duration of constant stress extends. As the crack spreads, the adhesive interface is progressively weakened. When the crack reaches a certain size, fatigue failure occurs. The coupling functioned normally during initial testing, but as usage time extended, the cracks gradually propagated, leading to coupling failure even under low loads. The presence of stress at the adhesive joint is a potential hidden cause of failure. To avoid generating high stress when the coupling clamping collar is tightened, annular grooves are generally cut into the clamping collar for stress relief.

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