Cardan Joints: Improved Performance And Service Life-Jiangsu Sitong Cardan Shaft Co.,Ltd
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Cardan Joints: Improved Performance And Service Life

Views:23     Author:Cardan Shaft Manufacturer     Publish Time: 2012-06-08      Origin:Sitong Cardan Shaft Co.,Ltd

A Cardan joints also known as universal coupling, U joint, universal Joint, Hardy-Spicer joint, or Hooke‟s joint is a joint or coupling in a rigid rod that allows the rod to „bend‟ in any direction, and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90o to each other, connected by a cross shaft.

 

Currently, the Cardan joints suffers from one major problem: even when the input drive shaft rotates at a constant speed, the output drive shaft rotates at a variable speed, thus causing vibration and wear, Seriously affect the service life of the universal shaft. This article will propose some practical methods for improving their performance and service life.

 

Common causes of Cardan joints failure


Common causes of Cardan joints failure



That is followed by elaborating on the oscillatory behavior in the rotational speed and the torque that transmits through the intermediary shaft in the search for the causes of the failures in these popular bearings. With the same purpose in mind, we are made in order to dig into the causes of the failures in these joints and their bearing surfaces. With the aim of improving the performance and the life expectancy of these popular elements of the machinery, some practical recommendations are suggested.

 

Failure of the Cardan joints can end up with serious consequences and it can be very costly. It causes sudden disruptor in the supply of power between its source and the consumer device. Therefore, many studies are performed in order to recognize the nature of the corresponding forces and the failure within these mechanisms.

 

All such studies concentrated on the failure in the Cardan joints or its connecting rods, in a general sense. There is hardly any research report specifically concerned with the failure of the bearings in the universal joints. However, in practice, there have been many cases of failure associated with the malfunctioning Cardan joints bearings.

 

Failure of the Cardan joints can end up with serious consequences and it can be very costly. It causes sudden disruptor in the supply of power between its source and the consumer device. Therefore, many studies are performed in order to recognize the nature of the corresponding forces and the failure within these mechanisms.

 

All such studies concentrated on the failure in the universal joints or its connecting rods, in a general sense. There is hardly any research report specifically concerned with the failure of the bearings in the Cardan joints. However, in practice, there have been many cases of failure associated with the malfunctioning universal joint bearings.


The use of Cardan joints in automobiles



The use of Cardan joints in automobiles



The Cardan joints is amongst the most popular universal joints. It is widely used in the mechanical couplings and joints with the pre-condition that the input drive shaft and the output drive shaft are not aligned. It may also be desired to permit some angular deviations along the axis of rotation.

 

The Cardan joints comprises of the three main parts including the input drive shaft, the output drive shaft, and the cross like the piece. Two points of the cross piece connect to the input drive shaft and two other points connected to the output drive shaft. Connections are provided by the needle bearings. It is an important aspect of these bearings that while in action they never undergo complete cycles.

 

In other words, each of these bearings revolves only a few degrees around its axis before returning to its original position. Therefore, there is only a group of balls in these bearings that take the bearing load. On the other hand, even if the angular speed of the input drive shaft is constant, the angular speed of the output drive shaft oscillates. The size of such oscillation depends on the amount of the angular deviation of the output drive shaft.

 

These couplings are widely used in the automobile power transmission systems. However, they are prone to wear and malfunction and need to be replaced in comparatively short intervals of time. Naturally, it means that such parts have a limited life span.


Failure of The Cardan joints Bearings



At this point in research failure of the Cardan joints bearings, it is intended to investigate the causes for the deformation and damages endured by the bearing inner rings. Such an objective can be reached at by the proper examination of some failed Cardan joints bearings and also by careful study of the simulated results. It is also helpful to look back into the related documents that are already reported by the sophisticated bearing manufacturers. It is intended to find some remedies for overcoming such bearing failures.

 

This practically causes some parts of the inner and the outer ring surfaces to stay safe from the impact loading while some other parts repeatedly suffer from the impact. Since the contact surface of the needles with the inner ring surface is generally lower than its contact with the outer ring surface, deformations at the inner ring surface are more severe. The impacts act on the ring surface by impacting the needles. The impact loads land on certain parts of the bearing and the needles gradually ditch and mark their effects on the ring surfaces.

 

Practically and under the working conditions, the center of the Cardan joints has to endure bending and the tension that follows. The stiff and brittle materials are very weak against fatigue and tensile stresses. With the assumption that the balls and the outer ring of the Cardan joints bearing are stiff enough, one can expect that the major damages happen at the inner ring of this bearing.

 

As a summary, the sequence of the events that are presented in The following figure can be introduced as the main cause of the failure of the Cardan joints bearings.



Failure of The Cardan joints Bearings




Propositions for Improving the Life Expectancy of the Cardan joints Bearings



1.The use of the intermediate spring and damper to reduce the size of the impact loads



It is already stated in the previous chapter of this article that the regular impact loading is the most effective cause for the deformation of the inner and the outer ring surfaces in the Cardan joints. As a first remedy, it is possible to remove the impact nature of the loading by installing a torsion spring and a damper in the middle of the Cardan shaft. this set up has considerably reduced damage to the Cardan joints mechanism.



2.Reducing the load on the bearings that are viable to damage by increasing the torque arm




Reducing the load on the bearings that are viable to damage by increasing the torque arm


It was stated that the force applied to the bearing on the intermediate shaft is higher than the load on the bearings on the input or the output shafts. This is caused by the variations in the torque arm during the revolving of the shaft. It causes early damage to the Cardan joints bearings. The inner rings of the four bearings on the ends of the joint cross piece are manufactured as one piece.

 

Consequently, when the time comes one has to replace the cross piece altogether. This happens while the bearings that connect to the outer shaft are not exposed to serious damages to the same extent as the bearings that connect to the intermediate shaft.

 

Therefore, one may slightly increase the length of the arm of the Cardan joints that connects to the bearing on the intermediate shaft. This elongates the Cardan joints life span by allowing the four bearings to reach to their damaged status within the same time span.

 

It needs to be reminded that the difference in the length of the arms of the Cardan joints depends on the operating angle of the joint. If the Cardan joints arms are out of proportion, this can increase the possibility of failure at the base. It is necessary to make sure that in the four bearings on the joint endure the same amount of maximum loading. 


3.Increasing the degrees of freedom in order to remove the regularity in the impact loading



Increasing the degrees of freedom in order to remove the regularity in the impact loading


regularity in impact loading is amongst the most important factors in the deformation of the Cardan joints bearing inner rings. If one could remove the regularity in the impact loading, this certainly spreads the deformation throughout the ring surfaces. It can prevent the emergence of the wavy appearance in the inner ring surface.

 

In order to omit the regularity in the impact loading of the inner rings by the needle bearings, the idea of adding one degree of freedom to the bearing is proposed. It can be achieved by adding a second bearing to the system. In such a bearing, the inner ring can revolve freely. This causes all the balls to enter into the angular zones of the forces that act upon them. Under such conditions, all the balls will share the loads and endure the same amount of damage.

 

Moreover, at the time of the impact, the contact point of the balls with the inner surface of the bearing will be randomly selected. Through such an idea, the surface of the inner ring will deform uniformly and the wavy appearance will not be formed. The idea of added roller bearings naturally increases the size and the price of the Cardan joints. However, it can be very useful for special cases when the Cardan joints is hardly accessible or very costly to be replaced.



4.Installing rigid rings over Cardan joints arms to act as the inner rings

Installing rigid rings over Cardan joints arms to act as the inner rings


Commonly, the Cardan joints and its‟ bearing inner rings are manufactured as one piece. This restricts the rigidity of the inner rings. The brittle materials are very weak against tensile fatigue loads. The Cardan joints has a bending load in its middle parts. This generates tension in its upper layers.

 

Therefore, a proper recommendation can be the use of hardened metal bushes to act as the inner rings that sit on the four ends of the Cradan joint. This improves the performance of the inner rings without the need to alter the material content of the Cardan joints.

 

The Cardan joints may appear as a simple component within a machine. It is no longer in use for many modern-day automobiles. However, almost all semi-heavy and heavy trucks and automobiles with an engine power of more than 250 Hp have rear driving wheels and have to be equipped with the Cardan joints. On the other hand, Cradan joint is fairly cheap and very easy to be replaced. This can be performed during routine maintenance services.

 

But, it needs to be reminded that automobiles are not the only machinery that is equipped with Cardan joints. Other industries also use the Cardan joints in order to shift the direction or the alignment of the rotating shafts. Moreover, a damaged Cardan joints causes rigorous vibrations to its output shaft. Such vibrations can severely harm other components of the power transmission mechanisms. This certainly emphasizes the importance of the Cardan joints and the parameters that can increase its performance and elongate its lifetime.

 

Therefore, some remedies are proposed, such proposals can increase the performance and improve the life expectancy of the Cardan joints. These include;

- The use of the intermediate spring and damper to reduce the size of the impact loads

- Reducing the load on the bearings that are viable to damage by increasing the torque arm

- Increasing the degrees of freedom in order to remove the regularity in the impact load

- Installing rigid rings over Cardan joints arms to act as the inner rings

 

With the aim of reducing damages to the Cardan joints and its bearings, one may choose a solution that is practically possible. It is also possible to combine some of the proposed methods to improve the quality of power transmission and to achieve a higher life expectancy for the bearings.


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