CVs vs U joints
How can we make splines
what options do we have other than splines
what are the forces acting on the front vs rear axle
After conducting some research, the best option for axles would be the CV joints rather than a U joint. The one we have is a splined shaft that seems to be gotten from a Honda ATV.
Spline Size 18 teeth 0.780in
What are CV Joints?
Constant velocity joints
Essential to connecting the driveshaft to the transmission (inner joint) and to the wheels (outer)
Because of location, they are bound to be damaged and worn over time
Two types of joints
The cardinal (found in heavy trucks or 4wd systems)
Rzeppa (front wheel drive systems)
Function
Holds the driveshaft in place
Transfers power to the wheels from the transmission at constant speed
Must accommodate the up and down function of the suspension
When you hit the gas the joints ensure your wheels will turn
If the join has any problems, it can affect handling
Issues
Crack in the boot that protects the joints
Boot keeps out dirt and debris which wears the joints
If cracked, leads to loss of lubrication (happens more to the outer joints)
Can’t repair a joint, must be replaced
What are U Joints?
X shaped part that is made of steel with a bearing cap
Connects the driveshaft to the transmission to rotate
Mostly used in RWD system
Compensates for the up and down movement in cars otherwise would impact the driveshaft
Function
Enables the movement of ensuring uninterrupted power flow to the wheels
Use of u joints make up for the height un alignment between the transmission and rear axle
U-joints connect to yokes that also allow drive shafts to move fore and aft as vehicles go over bumps or dips in the road, which effectively shortens or lengthens the shaft.
Problems
Makes a clunk noise when worn and on gear
Cannot be serviced with additional lubrication
Vehicle vibration
CV Joint vs U Joint
U joints are inexpensive and easy to replace
Less movement available
Rougher ride
Break easily
CV joints allow for more wheel travel and smoother ride
Difficult to replace
Can cause other issues when/if broken
Don't break easily
CV joint are able to maintain velocity through a larger range of angle and also handle greater torque load
Splines are made using a CNC machine
Splines are ridges or teeth on a shaft that are used to mate with another component to transfer torque
Job of the splines of axles is they allow the differential to rotate the shaft to supply motion
The more splines you add, the size of the spline shrink
Higher spline counts gives the axle more surface to transfer the load of the torque from the differential.
Keyed Shafts
Shaft containing a key seat along with a corresponding key
Defined by two components
Shaft
Rotating element which has a key seat
Keyed Shafts vs Splines
Splines offer more uniform transform of torque and provide equal load of torque
Splined shaft experience longer fatigue life
Keyed shafts produce lower loads of torque
Keyed shafts many connected parts can be easily disjointed
Forces acting on the axles
How hard the axle pushes down on the wheel bore determines how much friction will oppose the spinning of the wheel
When car is stationary
total force of mass and acceleration of gravity
Each axle supports part of the body mass depending on how far it is from the center of mass.
Gravitational Force
acting on each axle
Centrifugal Force
Squashes the wheels between the track and car's axles
If it were the only force acting, the axles would touch the wheel bore at an angle at right angles (normal) to the car's foot print
On the ramp model, only the weight of the car body acts. In the worst case, the wheel has an infinitely large moment of inertia. This wheel would resist rolling and the axle would settle to the lowest point in the wheel bore, waiting for the wheel to begin rolling.
In the limit of no friction, the axle is content to stay there. When there is friction, it tries to get the axle stuck to the wheel bore and ride backward with it. However, the wheel doesn't want to go as fast as the axle which is driven downward by body weight. It is this push that normally overcomes axle friction.
If that is not enough push, the axle rides up backward on the wheel bore wall gaining potential energy until it overcomes the friction. This can only happen with a lot of friction.
With even more friction, the axle can "glue" itself to a spot on the wheel bore and travel all the way around the wheel, bouncing the car body up and down. This vibration is the worst enemy your car has. It means your axles are not sliding on the wheel bore, they're rolling on it!
Rear CV Axle for Land Pride Treker 4200 4400 NT 2006 & 2008 "18 Splines"
https://cdn-assets.landpride.com/ari/attach/lp/public/specs/700-108s.pdf
wheel travel from 11 - 14
support bridge on inner front hub
aluminum blocks provided in lab
we'll need to make custom ones
something that will fit the number of splines
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Functional decomposition
Figure 1:CV joint functional block diagram
The main functions of the power transmission Joints are shown in figure 1 and are listed below:
To transmit power from the drive shaft to the wheel
To operate at various angle for power transmission
To provide constant velocity
Minimize vibration
Dirt protection
Improve steering ability
Function:
Power transmission:
To transmit power, a rigid shaft is needed. Shaft needs to withstand the high torque at high angular velocity to transmit power from the drive shaft to the wheel in front wheel drive.
High operating angles:
The constant velocity joints are generally used because of its ability to transmit power over high angles and it is achieved through balls fitted inside the cage. The cage with balls and inner race can move at various angles inside the housing. Thus, when the steering angle is high or car faces bumps on the road, the shaft in the CV joint adjusts to the angle. The maximum angle at which it can transmit power is 40 degrees.
Constant velocity:
CV joints are designed to operate without changing the speed that is input and output speed is always the same.
Minimize vibrations:
Since, the velocity of the shaft is constant and the joint can accommodate various angles, the vibration is minimal. Balls inside the cage adjust immediately to all reactions on the road and still supply constant velocity. To secure movement of the components relative to each other, clamp and Circlip is used that minimizes inter component vibrations. Thus, vibration is minimal. Greasing is done for smooth movement of components with minimal friction.
Dirt protection:
Dirt protection is very important to ensure smooth working of the CV joints. To prevent entry of the dirt, boot covers are used. Housing allows secured movement of the balls and cage. Housing also protects the balls and inner race from entry of dirty up to some extent.
Improving steering ability
Higher angles of operating and constant speed allows improved and smooth steering ability.
Concepts
Universal Joint
Figure 2:Universal joint (Saif M, 2021)
The universal joint is also termed as flexible coupling. This joint is usually used for power transmission in automobiles and trucks at different shaft angles. In automobiles, it usually transfers engine power to the differential from the transmission. The rear axle connected with the universal joint can move at up and down allowing more flexibility for power transmission of torque. It uses two Y shaped yokes that are connected by means of a cross spider. The four legs of the cross are called trunnions.
Advantages:
The main advantage of a universal joint is that it can transmit torque at different shaft angles.
It is easy to replace.
It is cheaper.
Disadvantages:
It offers a very small angle of 3degree between two U joints connected at the end of the transmission shaft.
The velocity varies continuously throughout each rotation. This variation in velocity causes continuous vibration in the system. Thus, it cannot offer constant speed.
Rzeppa Constant velocity joint
The constant velocity joints are the joints which can transmit power at constant velocity between output and input of the shaft. Constant velocity joints consist of a ball bearing with spherical cage and inner race in star shape that is fixed inside a housing. The spherical cage assembly with ball bearing slides inside the housing at different angles. CV joints can operate in angles upto 50 degrees. Front wheel drive vehicles usually have front wheels turning right and left while moving up and down, hence, this CV joint is mostly used for the front wheel drive. The boot provides protection from dirt. Lock ring and circlip along with clamp is used to secure the components. Since CV joints provide constant velocity, it reduces vibration significantly.
Figure 3:Rzeppa Constant velocity joint (CV joint) (ATV Camper, 2021)
Advantages:
It allows for a higher movement angle.
It improves the steerability of vehicles.
It provides a smoother ride.
Minimizes vibration in the system.
Can transmit higher torque load.
Disadvantages:
It is hard to replace.
It is costly.
Inboard Plunging Tripod
This type of joint is usually used to connect the transaxle to the drive shaft. It accommodates drive shafts movement in upward or downward direction when the vehicle drives over the bump. It has a unique feature to allow the change in length of axle shaft for accommodating suspension travel movements. Therefore, because of this unique feature of being able to plunge in and out, it is also called an inboard plunging tripod. This design uses needle bearings which are mounted on a 3legged spider. The rollers are arranged at 120 degree angles with respect to each other. These rollers can slide the tulip housing axially. Roller bearings can ride over the trunnions. As it can be seen in figure 4, the shaft has a spline to slide inside the tulip assembly.
Advantages:
It allows the shaft to operate at different angles.
It allows plunging movement of the shaft.
Cheaper to produce.
Transmits uniform speed for change in angle.
Disadvantages:
Can operate efficiently over limited angles.
Figure 4: Inboard plunging tripod (Andreas A. Polycarpou, 2005)
Figure 5: Tripod CV joint parts (Andreas A. Polycarpou, 2005)
Double offset CV Joint
Its similar in construction to the Rzeppa CV joints. It has only the difference of high axial stroke movement. As it can be seen that the housing also has grooves to allow for sliding of the cage and inner race. Thus, it allows for both angle and axial movement of the joint.
Figure 6:Double offset CV joint (Larry Carley , 2021)
Advantages:
Allows for both axial and angular movement.
Allows transmission of high torque.
Disadvantages:
Limited protection from entry of dirt and foreign elements, hence, that can cause dry joints and failure.
Governing principles
The following are the important equations that influence the constant velocity joints:
Maximum torque=Power Input to CVAngular velocity of the input shaft
Since, the input shaft is subjected to both radial and transverse load when the vehicle moves over the bump, this exerts loads at varying angles in the constant velocity joint.
Therefore, ball bearing needs to be designed and tested for its ability to withstand both radial and thrust loads.
Power transmitted from engine shaft = Power transmitted out to the wheel (Approximated)
Input shaft needs to be designed to transmit maximum torque and power from the engine using following equation:
TJ=GθL=R
Where,
T = torque transmitted
J = polar moment of inertia
G = modulus of rigidity
ϴ = Angle of twist
L = shaft length
R = shaft radius
τ = shear stress in shaft
If the angle of twist found for the shaft from the above equation is within permissible limit and the shear stress does not exceed the permissible shear stress limit, then the shaft is considered to be safe.
Evaluation
Comparison of Constant velocity over universal joint
The major drawback of the universal joint is that U-joint have continuous change in velocity in every single rotation. This causes a huge speed variation and fluctuation in the system. These fluctuations cause vibrations. Vibrations result in increased rate of wear and tear of the power transmission system. This also results in a vibrational ride.
Second major limitation of Universal joints is that with increase in angle between the U joints, the variation range of speed also increases. Thus, higher the operating angle, higher is the speed variation and higher vibration of the transmission component is observed.
Constant velocity joints overcome the above problem. By its geometrical design, it always splits the operating angle into half which means the driven and driving shaft are always operating at the same speed. Hence, it does not matter what angle is made by the joint, the input and output shaft angle is always equal. Therefore, no vibrations are observed. This increases the life of the transmission components. CV joints can operate at angles twice of U joints and therefore, offer higher stability in steering over road bumps. CV joints can also adjust its shaft length along with handling angles. Therefore, a constant velocity joint is considered to be the best option over a universal joint.
Refinement and Selection
It was found from the above discussed material that the Rzeppa CV joint is the best option for ATV because it can adjust according to different angles and provide constant velocity. The only major issue occurs in this joint is when the boot fails and the dirt particles enter. This leads to dry running of balls and internal race of the bearing resulting in heavy wear of the components. To increase the life of components, the bearing ball can be designed to be self-lubricating. Replacement is hard, therefore, it is desired to design a better mounting position so that it's easy to install and replace this joint.