Now that I have reviewed all of the axle basics, let’s discuss more of the vehicle installation areas that are just as important. In order to have a great axle arrangement, we cannot overlook obvious items like the driveshaft system and proper installation as well as angle orientation of the driveshaft and axle. It is surprising how many vehicle vibrations at road speeds and shudder conditions during hard launches can be traced back to the driveshaft balancing and operating angles.
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The terms driveshaft, propshaft, and propeller shaft can be used inter-changeably. The driveshaft primarily transfers the torque and speed from the transmission output to the rear-axle input. It also must accommodate for length changes when the suspension goes through its arc of travel, as well as allow for angle changes across the connections at the transmission and axle. The drive-shaft needs to be strong enough so that it doesn’t turn into a pretzel when torque is applied to it. Auto- motive driveshafts are made out of hollow steel, aluminum, or composite tubing and can be constructed from different wall thicknesses.
Steel is the most common and inexpensive material, so I will concentrate on it for our examples. But the principles apply to aluminum and composites as well. In order to carry the torque, the shaft tube must be strong enough to resist the torque without yielding.
The speed at which the shaft begins to whirl or bow in the middle and resemble a jump rope is referred to as the critical speed. At the critical speed of the shaft, the shaft becomes unstable and typically vibrates so violently that it may even self-destruct. Therefore, the shaft must also be able to avoid torsional vibration and shaft whirl at the maximum shaft speed.
The critical speed can be calculated for the tube portion of the driveshaft. Here are the factors:
Nc = critical speed in revolutions per minute (rpm)
E = modulus of elasticity of the material in pounds per square inch (psi)
I = area moment of inertia based on the shaft geometry in inches to the fourth power (in4)
g = acceleration due to gravity in inches per second squared (in/s2)
W = total weight of the shaft in pounds (lb)
L = length of the shaft between the joint support in inches (in)
This assumes that the joints are in good condition and the mounting point at the transmission and rear axle are mounted properly. In addition, the rear-axle mounts can-not have worn out bushings and the motor and transmission mounts must be in good condition. Another overlooked area at times is the amount of shaft that is hanging out of the back of the transmission. You want to have enough to allow for the shaft stroke length during suspension movement, but not have the yoke hanging out too far during normal driving. If you have too much shaft hanging out, the yoke is poorly supported, and you will lose a fair amount of spline engagement, running the risk of twisting the spline.
If you are building a custom shaft or making changes to the suspension, you want to make certain that you have about 3⁄4 to 1 inch of the propshaft yoke at the transmission sticking out from the bottomed-out position. Make sure that this measurement is taken with the suspension at normal ride height. Either have the wheels on the ground or jack stands that are supporting the axle do not take this measurement with the rear wheels hanging free.
If we assume a hollow steel shaft with a density of 0.281 lb/in3 and modulus of elasticity of 29,000,000 psi, then the equation can be simplified to the following:
Nc =4,705,000 [√ (Do2 + Di2)] ÷ L2
Do = outside diameter in inches (in)
Di = inside diameter in inches (in) L = length of the shaft between the joint support in inches (in)
Let’s look at an example, using a 3-inch outside diameter, a wall thickness of 0.083 inches, and a length of 60 inches.
We need to calculate the inside diameter, which can be determined by subtracting twice the wall thickness from the outside diameter. In this case:
3 – (2 x 0.083) = 2.834 inches
Nc =4,705,000 [√ (32 + 2.8342)] ÷ 602= 5,394 rpm
We need to lower the speed for a 15 percent factor of safety, which is 5,394 x .85 = 4,585 rpm. Therefore, we want to limit the maximum vehicle speed so that the driveshaft does not exceed 4,585 rpm.
Now, if we go to a thicker wall tube, like 0.095 inch, we get a critical speed of 4,566 rpm. If we go even thicker with 0.109-inch wall material, we get 4,545 rpm. Wall thickness changes the critical speed significantly.
If we try a 4-inch-diameter tube, RPM increases significantly to over 6,000, with the 15 percent safety factor taken into consideration. This is for the tubular portion of the propshaft and does not take into consideration the end attachments and their rigidity. Therefore, a typical margin of safety of 15 percent is taken into consideration for most passenger cars. There is also an assumption that the shaft balance is within those reasonable guidelines. This may sound like trivial stuff, but when you’re pushing the limits of your car and its drivetrain, these are critical considerations. Many owners take their Mustang and defeat the top speed limiter to exceed the OEM design limits, and critical hardware can fail in extreme circumstances.
Although rarely seen, composite materials can be used to construct a propshaft. These materials can be wound carbon fiber or carbon fiber mat, and are often reinforced with Kevlar for high tensile strength. The main reason to use a composite material is to have a lighter, stiffer shaft, and therefore allow for a longer, single-piece shaft that still maintains a reasonable critical speed. Composite tubes can also be made to most common diameters and with a wide range of wall thicknesses.
Many different arrangements retain the end caps to the propshaft and journal cross. However, I am just going to cover the most common retention methods that you may encounter, such as staking, nylon injection, retaining rings, plastic, flanges/yokes, and straps/U-bolts.
The staking process is becoming more common on modern vehicles. The joints utilize a non-serviceable cap retention strategy. In this arrangement, during the assembly process of the joint caps to the yoke, the yoke and cap are deformed together to permanently hold the cap in place. The permanent deformation process is called staking.
Many GM universal joints are actually bonded to the yoke during the assembly process. The bonding process injects a plastic material to hold the cap in place to the yoke ears.
In another common retention scheme, either internal or external retaining rings hold the universal joint caps in place. The internal rings snap into a groove on the bearing cap outside diameter, just inside of the yoke ears, after the cap is installed.
The caps can also use external retaining rings, which can be either a traditional-style snap ring, or a pretzel-shaped ring. With this style, the retention groove is actually in the yoke ear instead of in the cup.
Once the rings are properly installed in the yoke, the bearing caps are trapped.
Whether the rings are internal or external, they make servicing and replacing the joints simpler. You just need to make certain the retention grooves are clean, and that the rings are properly seated. If not, the cups fly out of the yokes during high-speed operation from the centrifugal force.
A more traditional-looking variation to the pretzel-style is used for external retaining. A pair of needle nose pliers or the correct snap-ring pliers can be used to remove them.
Another type of retention is the encapsulated plastic, or adhesive bonding, which is utilized in most GM propshafts. This is actually a plastic material that holds the universal joint caps in position on the yoke, and it can be difficult to identify.. You need to look very closely for the telltale nub of plastic sticking out of the yoke.
In order to service this style joint, you need to apply heat to the yoke and wait for the injected nylon to melt. When it does, the liquefied nylon squirts out of the small holes in the yoke. Be sure to exercise care with this, and always wear safety glasses before breaking out your tools. You should first damage the seals on the caps with a screwdriver in order to vent the greased area. You don’t need to take care to preserve the seals because you are not going to reuse the joint anyway. The reason for this venting process is to make certain that pressure is not allowed to build up in the joint from the boiling grease. If this pressure were not allowed to vent properly, the joint cap could violently shoot out of the yoke. You want to evenly apply heat to the joint with a propane torch or similar. I like to use methylacetylenepropadiene (MAPP) gas as it is hotter than propane, but not as hot as oxy/acetylene.
Once the adhesive has melted it should flow out of the holes. With the interference of the plastic gone, the bearing caps can be pressed out. Usually it takes a little force to break them free, but once the cap begins moving relative to the yoke, they come out easily.
There are two methods to press the old bearing cups out of the yoke. The first is to use a special tool that resembles a large C-clamp. This tool allows you to apply a slow and even pressure to the cup to press out the joint.
The more common method is to use a bench vise and hammer. When using this method, be careful to not hammer excessively on the yoke. You just need to support one set of the universal caps across the jaws of the vise and hit the yoke on the other.
You need to be careful because you do not want the yoke to spread out or in during any part of this process. If the yoke spreads apart or collapses, you need to get it back into the correct shape, or you will be unable to properly install the new joint.
The plastic-style retention is a great time-saver for the manufacturer, but you may not be able to repeat this in your shop. The typical service-style joints use internal retaining rings, as discussed above, to hold the new caps in place. Before you install the new caps, take a few minutes to thoroughly clean the plastic remnants out of the yoke bores. Any small amounts of the plastic can interfere with the installation of the new bearing caps.
The service replacement joints may or may not have a grease fitting. Joints with the grease fitting usually have a zerk fitting screwed into a hole drilled into the journal cross. This hole can decrease the overall strength of the joint itself. Some joints get around this with the grease fitting in the end of one of the caps. A simpler way around this issue is to delete the grease fitting altogether.
In any case, the most common cause of premature universal joint failure is lack of lubrication or the wrong grease utilized (see grease discussion below). The grease that is in the joint when you first take it out of the box is there to keep the rollers in place for initial assembly from the manufacturer. It is not typically enough grease for the joint.
You need to add more grease before assembly.
You may also notice that most quality replacement joints have little thrust washers inside the caps. These washers help carry the loads of the joint, and reduce the articulation torque required across the joint.(Articulation torque is another term for the force required to swivel the joint back and forth while it is rotating.) The washers can be difficult to see because they are often black in color. You want to make certain that the joint caps are not forced excessively together from a bent-in yoke or the
wrong size retaining rings. You might be able to force the joint together, but the articulation torque will be too high and the thrust washers will wear out prematurely. The joint should swivel with mild resistance, but not require excessive force to swivel. The joint should not have any endplay once assembled.
To assemble the joint on the shaft yoke, you first need to position the journal cross in place on the yoke without the caps on. Once the cross is in place, you can work on getting the caps in position one at a time.
Next you need to press the caps into the shaft yokes and be careful not to have any of the rollers fall out of place and end up at the bottom of the cup. This can be difficult at times and may require a few attempts to get right. This is one of the steps where patience is very helpful. Take your time and don’t force the cups into place. This usually leads to damaged rollers and yokes.
Now I am assuming that the joints and tube are properly sized for the application. Certain racing rules, such as NHRA drag racing for cars running in 13.99 seconds or quicker with slicks, or 11.49 seconds or faster with street tires, require a driveshaft safety loop. The typical requirement is a 1/4-inch minimum thickness by 2-inch-wide steel retainer loop that provides 360 degrees of enclosure around the driveshaft. This loop needs to be installed within the first six inches past the front universal joint. in the event that the front joint was to fail, the safety loop contains the shaft to protect the driver and others at the track. You want to avoid the propshaft falling onto the track, and potentially wedging to the ground, and then folding under the car and coming through the floor, or lifting the car off the ground.
If the rear joint fails, the shaft would just pull out of the transmission and land on the track. Since the shaft is still spinning, hopefully it just comes to rest against the track wall, but it can become a dangerous projectile. A driveshaft safety loop is relatively inexpensive and simple to install. You can actually purchase universal kits that are pre-cut and bent for most applications.
Flanges and Yokes
At the ends of the driveshaft, there are yokes that are welded to the tube. The universal joints attach to the yokes, and then a flange on the other side. These attachments are different for each application. Let’s cover the most common that are out there.
The first is the half round. It is called this, because two of the joint caps are loose until the joint is installed in the vehicle. These are very common on most GM vehicles at the axle connection.
Straps and Bolts
Either straps and bolts or U-shaped bolts retain the caps. The strap style is more common and easier to assemble. The key here is to make certain that the straps are in good condition. Keep in mind that the straps are reacting to the bearing cap under torque, so you need to make certain that the strap is in full contact with the cap. Some times the straps wear and deform from normal use and should be replaced. Based on the design of this bolted arrangement, tightening the fasteners will not correct a worn or bent-up strap. You need to buy a replacement.
The U-shaped bolt-style retention for the half round is a little trickier to work with. You want to make certain that you do not over-tighten the nuts or you run the risk of deforming the bearing cap. I do not have a general torque-spec range for tightening these nuts because it depends so much on the condition of the thread, washers, and nuts. You also want to make certain that each nut is tightened evenly. When tightening these nuts, it is best to use a thread locking compound, and tighten the nuts until the lock washer has flattened out. Once the lock washer has flattened out, tighten just another 1/8 of a turn, and you are all set. Resist the temptation to overtorque them because the added pressure and potential deformation of the cage cause the rollers to wear out quickly.
The last type of retention is the flange yoke. This yoke is attached to the other two caps of the universal joint, and therefore is always part of the propshaft. It usually has a pilot feature on it to align with the axle flange, and has a bolted flange. They typically have four bolts.
The most common size of joints in most cars are the 1310, 1330, and 1350 series. There are even smaller and larger sizes available.
In the chart, notice that there are special combination joints listed. For example, a 1310 series cap on two opposite trunnions and a 1330 series set of caps on the other ends. This is often necessary when swapping a non-OEM rear axle in a vehicle sometimes the pinion yoke accepts a different size joint than the drive-shaft, and a combination joint is an easy way to adapt one to the other.
Even though the Beach Boys sang about them, there are no such things as good vibrations when referring to the driveshaft. There are three main sources that can cause vibration: criticalspeed, transverse, and torsional vibrations. If the source of the vibration is not found and corrected, it is just a matter of time before something fails. Think of the vibration as a warning that something is wrong with the propshaft.
Critical speed vibrations are created when the propshaft rotates at very high speeds. The mass of the shaft, length, diameter, and material determines the critical speed of a particular propshaft. As the prop-shaft speed approaches the critical speed, the shaft begins to bow and resemble a jump rope and it tends to vibrate excessively. The shaft usually fails in the middle of the tube. As the shaft length is increased, the critical speed or maximum safe speed to operate the shaft decreases as shown in the chart above.
Transverse vibrations occur at a frequency of once per revolution. The primary cause is a driveshaft that is out of balance. This type of vibration source is the easiest to correct; you just need to have the shaft assembly rebalanced by a reputable shop.
Notice that the flange yoke on many axles have signs of material being welded on or removed in order to balance the yoke. You should mark the propshaft flange relative to the axle yoke before your remove it. This way you can be certain that you re-installed it in the same location.
Torsional vibrations occur at a frequency that is twice per shaft revolution, and as a consequence, this is one of the biggest inherent problems with universal joints. They do not have a smooth angular motion when articulated at an angle. Universal joints have a non-uniform velocity because the joint is operated at an angle. As the angle across the joint increases the sinusoidal magnitude, or the overall distance between the peaks of the speed ratio, increases.
Another way to illustrate this is to simply hold the two ends of the shaft on either side of a universal joint and rotate the shaft while keeping the centerline of the shafts in place as they are attached to the vehicle. You quickly see that the journal cross is rotating in a circular path around its centerline while the caps are rotating or pivoting on the trunnions. You should also notice that the journal cross is articulating back and forth twice per revolution.
When the angles across the joints are excessive (typically beyond 5 degrees), this causes vibration. This can result from having the joints out of phase with respect to one another. By phasing, I am referring to the relationship of the end yokes on the tube as compared to one another.(The graph is for a single joint.) Typical automotive propshafts have a joint on either end. Therefore, we would have a separate angular motion graph per joint. If you properly align the sinusoidal motion to the angle between the operating planes, you can effectively cancel out most of the speed variation across the entire propshaft. This alignment or stagger of the yokes on the tube is known as phasing.
The good news is that most prop-shafts used in passengers cars are a single piece, and the yokes are welded to the tube. Therefore, the propshafts are always correctly phased and the installer cannot incorrectly index or phase them. This propshaft arrangement only becomes an issue if you are having a custom propshaft built or having yours lengthened or shortened. But keep in mind that you always want to have the yokes (and therefore joints) phased, such that as one joint is accelerating, the other is decelerating and therefore you cancel out the non-uniform velocity inherent in the design.
The true joint angle, also called the compound angle, occurs when the joint operates at angles that are in two planes as opposed to one. Typically, there is an operating angle in the vertical plane and another in the horizontal plane. Imagine the driveshaft is pointing downward in the vehicle and is not going exactly front to back but has a slight angle from left to right. Basically, the shaft is pointed downward and slightly across the vehicle diagonally. In this situation, the total angle, or true joint angle, is more than just the measured downward angle.
The true joint angle can be approximated by simply taking the square root of the sum of the squares or:
TJA = sqrt (A2 + B2)
Where: A and B are the two operating angles in degrees of the different planes across the joint.
The minimum continuous true operating angle across the U-joint should be at least 0.5 degrees, so proper rolling and lubrication distribution throughout the rollers is achieved. A maximum of 3 degrees should not be exceeded; this is to ensure vibration-free performance. You also want to balance the angles at the front and rear of the propshaft and get them within 1 degree of one another to cancel out torsional vibrations. Note that these conditions cannot always be met but serve as an ideal guideline when setting up the overall powertrain geometry.
The engine and transmission are typically designed to slope down-ward from front to back. In other words, the engine’s harmonic damper is higher than the transmission output shaft. You can easily approximate the powertrain angle by measuring off the crankshaft harmonic damper. Typically, this angle is about 4 or 5 degrees. Under acceleration, you want the angles in the universal joints to be as close to 0 as possible. Typical stock suspensions allow the axle pinion to rotate upward between 3 and 4 degrees. Therefore, you need to point the pin-ion upward around 1 to 2 degrees. So when the vehicle accelerates, the pinion rotates upward around 3 degrees, and therefore the engine/transmission angle matches.
Now, this all assumes that your suspension is adjustable and you can change the pinion angle. If your suspension is totally stock, then there is no adjustment mechanism available to you. Also, if your suspension is stock, you probably don’t need to make any changes. Check the angles first, and then decide whether you need to invest in suspension components so the pinion angle can be adjusted. These are especially handy if you have lowered or raised your vehicle from the stock ride height. If the pinion angle is off during hard acceleration, you may notice a shudder or vibration during hard acceleration only.
The most common failure mode of a universal joint is from improper lubrication, such as lack of grease or the use of the wrong grease. You can-not use standard old grease used to lubricate your chassis ball joints and tie rods. You want to always use good-quality EP lithium-based grease that meets the National Lubrication Grease Institute (N.L.G.I.) Grade 2 specifications. This may sound like an elaborate specification but the grease is readily available at most parts stores just read the back of the tube.
The driveshaft needs to be strong enough to handle not only the massive amounts of torque from your muscle car but also the centrifugal loads from rotation at the vehicle’s top speed. The driveshaft needs proper angular alignment between the transmission output and axle input connections. In order to cancel out any vibrations, the driveshaft needs to be balanced correctly and phased as required. Typically, with minimal maintenance, propshafts can easily last the entire life of the vehicle.
Written by Joe Palazzolo and Posted with Permission of CarTechBooks