7 Failure Analysis and 6 Design Improvements of Cardan Shaft Assembly-Jiangsu Sitong Cardan Shaft Co.,Ltd DOCANDO INTERNATIONAL INDUSTRY CO.,LIMITED
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7 Failure Analysis and 6 Design Improvements of Cardan Shaft Assembly

Views:673     Author:Cardan Shaft Manufacturer     Publish Time: 2019-02-05      Origin:Sitong Cardan Shaft Co.,Ltd

The main function of universal joint is to transmit rotational torque between shafts on different axes. It is widely used in various industrial industries. Usually, needle bearing on universal joint cross shaft is not used for rotary motion during use. It is a motion of a rotary swing, and its load changes periodically. In this paper, finite element analysis and performance comparison of failure mode, failure reason, structural parameters and materials of WX0082 universal joint cross shaft for heavy-duty truck are comprehensively analyzed, and comprehensive performance of product is designed and improved.

 cardan shaft pic

7 Failure Mechanism Analysis

1. Oblique Imprint

We observed from failure piece that oblique embossing of cross shaft journal is serious, and inner wall of bearing ring also produces corresponding embossing, but depth is lighter than cross shaft. After analysis, for bearing mounted on universal joint assembly, when total circumferential clearance of bearing is large, needle roller is prone to skew, causing cross shaft journal to produce an embossing at an angle to axis. When embossing depth is deepened, needle can not rotate, thus increasing frictional resistance and accelerating extension of indentation. Appropriate total circumferential clearance should be controlled within the range of 0.1 to 0.4 mm, and total circumferential clearance given in JB/T3232 is not more than 0.5 mm.


2. Fatigue


In use state where transmission shaft swing angle is small, inner diameter of needle bearing on universal joint assembly and cross shaft journal will repeatedly oscillate within a small angle range. When radial clearance of cross shaft and bearing expands with wear, product load is concentrated, and fatigue indentation occurs prematurely in place where load is large, thereby developing a large-scale peeling. Especially at crosshead of cross shaft, due to influence of stress concentration, chamfer of loaded part is almost completely collapsed, and in severe cases, sintering occurs at a high temperature.


3. Oil-free Ablation


Failure to fill lubricating oil on time or bearing seal on the universal joint may cause oil loss in early stage of bearing and material of dust cover is not resistant to low temperature. The oil seal lip will produce aging cracks in cold winter weather, making grease early loss. In addition, universal joint assembly generates high temperature due to lack of oil during operation, causing bearing and shaft diameter to bite, causing needle to break and seal and dustproof cover to be damaged, and surface of ferrule has ablation marks. It is an abnormal failure form.


4. Needle Roller Failure


In failure sample of cardan shaft assembly, most of needle roller surfaces have pitting, pit and large area fatigue peeling, a few needle heads are broken, and a few needles have a large pit at about 1.5 mm from head of needle. And phenomenon of bruises, most of needle breakage is also broken from here, occurrence of this situation is mainly related to bearing structure size parameters and circumferential total gap design.


5. Gasket Failure


The shim is the first component to fail in gimbal assembly. Gasket failure is in the form of nylon gasket breakage and wear. We carefully analyzed the failed gasket samples and found that there were obvious cracks and debris on the periphery of the gasket. Obviously, periphery of gasket is where the stress is concentrated by gasket. Once gasket is damaged, axial force will be fully absorbed by remaining gasket, and force per unit area of gasket will increase. The film will be destroyed. Therefore, it is necessary to design a reasonable gasket shape and bearing structure. At the same time, since failure of gasket is accompanied by wear failure, gasket material must be selected from materials with high strength and wear resistance.


6. Cross Shaft Fracture


The break of cross shaft is generally caused under condition that user is seriously overloaded or under abnormal conditions. Although universal joint is designed to fully comply with standard safety factor of 2.5 times static joint strength of universal joint in JB/T8925-2008, proportion of faults occurring during three-package period of main engine factory still accounts for 2% to 5% of total faults. Although this situation is sudden, it is inevitable. Static torsional strength matching of universal joint must be more than three times the maximum output torque of engine to avoid occurrence of sudden failure caused by special overload.


7. Metallographic Examination Analysis


Physical and chemical analysis of failed parts, select characteristic position in typical parts of cross shaft, ferrule and needle roller, cut test piece, and take metallographic sample along longitudinal section, and metallographic structure level is qualified. In order to determine hardness of core of cross shaft, ferrule and needle roller, Vickers hardness gradient distribution was measured by microhardness tester, and hardness of parts was basically consistent with hardness distribution law. Take cross shaft of universal joint, outer ring test piece and several needle test pieces, heat them to 70~80 °C in 50% hydrochloric acid aqueous solution, heat for about 1h, check for hot pickling, and find that parts are burns in the use. This kind of burn phenomenon is judged to be due to poor sealing performance of universal joint and oil loss and poor grease performance, resulting in premature burn failure of various parts after joint of universal joint.

drive shaft pic  

6 Universal Joint Assembly Design Improvements

1. Improvement of Strength of Cross Shaft


Through finite element analysis of product, it is found that corner of journal shoulder is small, and it is formed by machining, roughness is poor, and stress is concentrated. We decided to increase rounded corner of shoulder to cross shaft and use CNC forming grinding. After improving designed cross shaft and using finite element analysis, stress value decreased from 234 MPa before improvement to 176 MPa after improvement, and stress value decreased by 24%. After static torsion strength test of  products before and after improvement, static torsional strength is increased from 28 000 Nm to more than 35 000 N m, increase is 25%, and improved safety factor of static torsion strength is increased from 2.5 times to more than 3 times. It can effectively prevent the cross shaft fracture caused by customer under overload conditions.


2. Gasket Design


The failure mode of gasket is crushing and wear, which will cause cross shaft to move axially and accelerate wear of universal joint. Therefore, strength and wear resistance of gasket should be enhanced first, and stress of gasket should be improved to avoid stress concentration. When axial force of cross shaft impacts gasket, surrounding of gasket is suspended by undercut of bearing, and periphery of gasket becomes a weak link for force to extend and expand. At the same time, contact point between outer end of gasket and undercut is also due to fact that it is not in planar contact, and stress concentration is also formed. In this way, stress concentration occurs under repeated impact load of cross shaft, thus causing damage around the gasket. As the pressure per unit area on gasket increases, entire gasket is broken. The improved gasket completely avoids above problems. At the same time, contact between the raceway undercut and needle is eliminated, so that the needle is evenly stressed, no stress concentration occurs, and the possibility of pits, bruises and breaks on the end face of needle is avoided. And effective contact length of needle roller is increased, so that life of universal joint is prolonged.


3. Roll-to-number Convexity Design Improvement


In roller bearing, contact fatigue pressure between roller and raceway is singularly distributed, resulting in early fatigue failure of bearing. Therefore, needle width of  needle roller is one of main measures to eliminate stress concentration of needle roller and improve service life of needle roller bearing. Considering that universal joint work is repeatedly oscillated under a large angle, due to alternating stress, needle roller may be skewed during rotation process, so that contact length between needle roller and journal is shortened, and processing technology and processing cost are taken into consideration. Influence (using circular arc full convex line forming structure design, manufacture of such needle roller can be formed by rolling collision method, high production efficiency and low cost). This improvement selects arc full-convex linear needle roller. It replaces original structural straight line needle roller to avoid the possibility of breakage during use.


4. Seal Structure Improvement


The design of automotive drive shaft is becoming more and more miniaturized, so the space for universal joint mounting seal is small. In addition, since universal joint is directly affected by muddy water during use, an interference contact seal must be used. Different from sealing in normal case, interference sealing contact surface is easy to be squeezed out due to reciprocating oscillating motion, and contact surface is prone to wear, so long-term good sealing performance is obtained. Therefore, wear of both the seal and contact surface should be minimized. The oil supply of seal contact surface should be sufficient, and sealing contact pressure should not be too large, so it is necessary to design a structure to supplement pre-tightening force. On the other hand, during use, sediment and water invade from upper mouth of bearing, so sealing lip should take into account upwardly inclined slope. In response to these factors, designer made following improvements to original seal structure:


A. Design Flexible Contact Seal

The hard-extruded contact seal of original sealing structure is changed into a flexible thrust inclined surface contact seal, which makes lubrication of contact surface lubricant easier, axial pre-tightening force is small, wear is small, and sealing performance changes little, therefore, flexible contact seal design is superior to squeeze contact and has a longer life.


B. Add Dust Lip

In order to prevent main sealing contact surface from being affected by muddy water or dry dust, it is necessary to add a dust lip, so two main sealing lips and one waterproof dust lip are selected as contact lips.


C. Design A Suitable Seal Preload

The main sealing surface is selected on cylindrical surface of cross shaft or bearing. Since cylindrical surface size error can be processed very small, interference margin deviation of sealing lip is reduced. In original structure, one of main seals is selected to be in surface contact at shoulder diameter of cross shaft, and sealing lip is in contact with chamfer of root of cross shaft, and roughness of mating surface generally matched with sealing lip is required to be 3.2 μm or less. If surface roughness value is poor, sealing lip will be aggravated and oil leakage will occur. The improvement is to adjust the sealing interference of one main sealing lip and another waterproof and dustproof lip. The interference is adjusted from 0.10~0.15mm on one side to 0.15~0.25mm, which greatly improves sealing effect.


5. Dust Cover Design Improvement

Dust cover is severely deformed under low temperature conditions. Since the winter temperature in Northeast China and Europe reaches -35~-20°C, low temperature brittleness temperature of nylon 66 material used in our country is about -20 °C, and cold resistance is poor. Therefore, nylon material is prone to change in performance at low temperature. The hard cover is hardened, cracked or even broken. The designer improved design using a steel skeleton vulcanized rubber vulcanized molded dust cover. Through trial production test, sealing effect is good, and sealing performance of universal joint under low temperature environment is solved.

6. Improvement Universal Joint Grease

The grease used in original structure universal joint is a lithium complex grease. This grease is based on general lithium base grease of No. 2 car to add some additives to improve extreme pressure performance and drop point of universal joint. The point is greater than 250 ° C, use temperature is -30 ~ 120 ° C, extreme pressure performance is up to 2 800N. Although performance of this grease is superior to that of No. 2 automobile general-purpose lithium base grease, environmental conditions of truck are not good enough to meet three-package mileage requirement. By searching for performance of various greases, it was found that a composite urea-based grease has a dropping point of more than 280 ° C, a use temperature of -30 to 200 ° C, and an extreme pressure performance of up to 9 800 N. Its high temperature resistance and extreme pressure performance are better than lithium complex grease. Through comparison test of typical product performance, wear life of composite urea grease is more than 70% longer than that of original lithium complex grease. To this end, we chose this grease for such universal joint instead of original lithium complex grease, and promoted it in universal joint series.

 transmission shaft pic 


Through comprehensive analysis of WX0082 universal joint failure parts, improved universal joint static torque strength increased by more than 25%. At the same time, through design of universal joint sealing structure, logarithmic convexity design of needle roller, improvement of bearing nylon gasket, dust cover and improvement of lubricating grease, wear life of universal joint is greatly improved. And increase service life of the universal joint from 20000km in 1 year to 50000km in 2 years. After more than one year of feedback from customers, failure rate has been reduced by more than 60%, good economic and social benefits have been achieved. This experience has been widely used in improvement of life analysis of other products.

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