drive shaft.pdf

8/11/2019 drive shaft.pdf

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DRIVESHAFTS ANDUNIVERSAL JOINTS

The term "driveshaft" and "propeller shaft" aresynonymous, since both describe the basic functionthat they are designed for, namely to transmit drivefrom the output shaft of the transmission to the rearaxle assembly in order to drive, or propel, the vehicle

forward.

The three major functions of a driveshaft are as follows:

1. Since the driveshaft transmits both torque and speedvariations from the output shaft of the vehicle transmission to the axle, the shaft must be structurallystrong enough to handle the maximum low gear torque from the engine on through the transmission.Consider for example that a typical Class 8 truck/tractor, using one of the newer high horsepowerengines, produces a peak torque around an average of 1250 lb. ft. (1694.75 Nm), when rated at the430-450 BHP (320.6-335.5 kW) range. When coupled to a transmission (automatic or standard), lowgear ratio can be as much as 13 or 16:1 reduction. This kind of ratio would therefore produce 16 x

1250 lb. ft. breakaway torque to the driveshaft, which computes out to 20,000 lb. ft. of torque ortwisting force. The initial shock loads imposed on the driveline to move a heavily laden vehicle mustalso be absorbed by the driveshaft. Therefore the driveshaft should be made as strong as possible tobe able to handle these loads and have a significant safety factor built in to handle shock loads. Driveshafts are usually constructed with minimum practical weight, strong but as light as possible.In addition, the shaft must be capable of transmitting the torque at the maximum vehicle speed in allratios of the transmission.Critical SpeedEvery driveshaft has a critical speed. Critical speed is the point at which a rotating driveshaft beginsto bow off its normal rotating centerline.Drive shafts begin to vibrate as they approach critical speed. If they are operated at near critical

speed for an extended period, they often fail. This can damage the vehicle and possibly injurepersons nearby.

A drive shafts safe operating speed can be raised by increasing its tube diameter or by shorteningthe installed center-to-center length of the driveshaft. Changing the installed length of a driveshaft willrequire the use of multiple drive shafts with center bearings.Important: The critical speed of an assembly can be affected by driveshaft imbalance, improperuniversal joint operating angles, or improperly phased drive shafts. (A properly phased driveshaft hasthe in-board yokes of the shaft in line with each other.) Since critical speed can ultimately cause driveshaft failure, it is extremely important to be very precise in all applications.

2. With the driveshaft connected

from the transmission output shaftto the axle, it must operate throughconstantly changing anglesbecause the axles of the vehicle,as they move up and down, mustby necessity create a change inthe angle of drive.


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3. The driveshaft length must be capable of changing as it rotates and transmits torque due to thisnecessary axle movement, torque and brake reaction, road surface changes, etc.

Component IdentificationThe driveshaftconsists of universal

joints, connectingshafts, andattaching flanges oryokes. The term"Cardan joint is thecorrect name forwhat we usually calla "universal joint."The concept wasfirst thought of by an

Italian mathematician named Cardano he never developed the joint anyfurther. A British inventor named Hooke expanded on Cardanos workand actually produced the joint to support a compass, Hooke called his

invention a universal joint. In 1903 Clarence Spicer was the first personto use the U-Joint to invent an automobile drive shaft, automobiles untilthat time were driven by sprockets and chains.

The driveshaft isconstructed of special

high strength tubing with minimum practical weight,which is then welded to its end members. Roller typeneedle bearings are used on the U-Joint journal crosssince they are capable of withstanding high capacityloads in a limited space while oscillating at high drive

shaft speeds.Each bearing assembly is sealed in order to retain itslubricant, as well as to prevent foreign material frompenetrating the bearing area. Any dust, dirt, or watermixed with the bearing lubricant will seriously affect bearing life. Synthetic rubber seals are now usedon most commercial vehicle installations. These seals are designed to allow grease to purge from the

joint without allowing contaminants in.


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Since drive shafts must change operating length because ofsuspension articulation and torque windup, a slip joint mustbe used. This consists of an internal and external splinearrangement that allows the length to change as necessary.

Operating angles.Because any U-Joint operating through an anglehas an inherent resistance to motion the allowabledrive shaft operating angle is limited by the speedthe driveshaft is expected to turn. The faster thespeed the lower the angle must be.

Non-constantvelocity

A shaft driventhrough an angleusing a U-Jointexperiences non-constant velocity,that is that thedriven shaft speeds

up and slows downtwice during arevolution. If a shaftwith one U-Jointworking through anangle were

connected to a drive axle the vehicle would speed up and slow down twice through every revolution.The resulting vibration would be unacceptable. The only way to cancel the non-constant or non

uniform speed is to putanother U-Joint at theend of the shaft

operating at close to thesame angle this willcancel out the speedfluctuations. The anglesmust be equal to withinone degree in mostcases to be effective.


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There are two basic setups to accomplish thiscancellation, the parallel or waterfall jointarrangement or the broken back arrangement. Theparallel type has angles that are equal to within 1degree and are opposite. This is the preferred typebecause angles remain equal throughoutsuspension oscillation. The broken back style hasangles that intersect at the midpoint of the drive

shaft length, in this style the angles will not remainconstant during suspension oscillation because asthe length of the shaft changes the angles no longerintersect at the midpoint of the drive shaft lengththerefore it is usually found only in the rear rear axleof a tandem set where there is very little lengthchange.

Driveshaft phasingHeavy-duty two piece drive shafts mustbe phased on installation in order toensure that each u-joint is in the sameportion of its revolution at the sametime. The in board yoke ears must lineup. If this is not done the driveshaft willcause extreme vibration due to the shaft

winding up and unwinding as it travelsthrough a revolution. The reason for thiswinding and unwinding is that joints arenot cancelling each other out at the rightportion of the shafts revolution.

Constant velocity joints.Constant velocity joints eliminate the need for cancellingangles and drive shaft phasing. There are two common typesin use in automotive applications.

Newly developed double Cardan style CV jointCalled the Yordak 1


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Double Cardan jointThe double Cardan joint has twoCardan joints in the same carrier andnon-uniform velocity is automaticallycancelled as the two joints alwayshave the exact opposite angle. The

joint has a centering socket and ball

which means that ensures the equalangles. This joint may be used on ashaft with no U-Joint on the oppositeend, (Companion flange), or the shaft may have a single U-Joint at theopposite end in which case its operating angle must be 1 degree or less toprevent vibration.

Extreme aftermarketDouble Cardan hasContinuous Operatingangle of 40 degrees

Front drive CVjo intThese CV jointscome in twobasic types the

Tripod plungetype and theRzeppa, (bothplunge andfixed).There are manyvariations but all have one or the other principle of operation.

The secret of these typesof CV joints is that theangle formed between theinput and output side

always bisects thecentreline of the six balls orthe three tripod elementswhich then ensure thatthere is constant velocitythrough the joint. These

joints are very popular asdrive shafts for front wheeldrive vehicles. Tripod joint bearing

Arrangement


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There is another type of constant velocityjoint being developed in Australia at themoment known as the Thompson Coupler.Its inner workings are still proprietaryinformation so how exactly it works is notpublic knowledge as yet but its inherentlack of friction and smooth operation meanthat we will more than likely see more of it

in the future.

Drive shaft vibrationDrive shaft vibration can be caused by any problem that affects drive shaft balance, run out(straightness), or u-joint operating angles. Driveline vibration is usually more rapid than vibrationcaused by wheels and tires.Drive shaft vibration may be similar to the vibration produced by an unbalanced clutch, flywheel, orengine crankshaft.When test-driving, drive in high gear at the engine rpm that causes the most vibration then shift intodifferent transmission gears or neutral while maintaining vehicle speed. If there is NOT a change in

the vibration, the vibration may be in the drive shaft. A vibration change indicates the engine, clutch,torque converter, ortransmission is at fault.When vehicle speedremains the same, driveshaft rpm remains thesame.Drive shaft vibration can becaused by undercoating onthe shaft, a mud build up,missing balance weights,

damaged shaft, (dented),worn U-Joints or sharpdrive line angle. Always check for these and other problems.

Drive shaft inspectionTo inspect the drive shaft for wear or damage,raise the car on a hoist. Look for undercoating ormud on the drive shaft. Check for missingbalance weights, cracked welds, and other driveshaft problems. To check for worn U-joints, wiggleand rotate each U-joint back and forth. Watch the

universal joint carefully. Try to detect any playbetween the cross and yoke. If the cross movesinside the yoke, the U-joint is worn and should bereplaced.


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Next check for endplay in the U-Joint if there isexcessive endplay, (more than .006), the joint isworn and should be replaced. Always check U-Jointbefore lubricating the shaft as grease can mask aworn joint.

Also, wiggle the slip yoke up and down. If itmoves in the transmission bushing excessivelyin a light duty application, either the yoke itself

or the bushing is worn. In heavy dutyapplication check the slip yoke for looseness ormovement on the splines the max allowable istypically .012. Also inspect all the attaching bolts for tightness, (note that Spicer recommends notreusing any of the attaching hardware when installing U-Joints). Make sure the motor mounts are notbroken. Look for any condition that could upset the operation of the drive shaft.If you fail to find a problem, you may need to measure drive shaft run out (wobble) and checkbalance.

Measuring drive shaft run outDrive shaft run out is caused by a bent drive shaft, damaged yokes, or worn U-joints. A dial indicator

is normally used to measure drive shaft run out.First, sand and clean around the front, center, and rear of the drive shaft. This will give the dialindicator a smooth surface for accurate measurements. Mount the dial indicator perpendicular to theshaft. The indicator base must be placed on a rigid surface floor pan, frame, or special poststand).


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The drive shaft must NOT be in a sharp angle during run out measurement, make sure that thevehicle axles are in their normally weighted positions.With the transmission in neutral, turn the drive shaft. Measure run out at the front, center, and rear ofthe shaft. Compare your measurements to specs. Generally, drive shaft run out should not exceed.010 to .030 in. (0.25 to 0.76 mm).If drive shaft run out is beyond specs, try removing and rotating the shaft 180 degrees in the rearyoke. Make sure the universal joints are in good condition and that the yokes are not damaged.If after eliminating all other causes of vibration and if run out is okay, try rotating the shaft 180

degrees in the rear yoke to possibly lessen vibration, if there is still a vibration present the drive shaftshould be sent out for balancing.

Driveline AngularityDriveline angularity is usually the last thing to look for when checking for vibration unless obviouschanges have been made to the vehicle running height or length etc. but after eliminating otherpossible causes you may need to check the driveline angles.To check the angles you will need a good protractor, most manufacturers print angularity worksheetsthat you can use to record the angles and almost all have computerized angularity programs that willdo the calculating for you.The following are Spicers recommendations.


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When a drive shaft has more than two angles to consider such as with a two piece shaft then all threeangles must be considered when eliminating vibrations. Again most Manufacturers produceworksheets or computer programs to solve these situations. The following is a three angle worksheetfrom Mack.


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When using these sheets it i s important to remember that shaft speed is Max engine RPM fornon overdrive vehicles and engine RPM times the overdrive ratio in overdrive vehicles


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The computer programs that are put out by the manufacturers have greatly reduced the time andeffort required to do these calculations.

LubricationSome U-Joints are sealed and have no grease zerks all others must be lubricated regularly following

the OEM manual recommendations,usually every 10,000 miles or 16,000Kilometres. It is crucial that grease purges

from all 4 bearing cups and it is a goodidea to purge the cups until all of theoriginal lube is gone to avoid incompatiblegreases from being mixed. If grease doesnut purge you must remove the joint andlocate the problem. If you notice moistureor rust purging with the grease you mustreplace the U-Joint.

Most slip yokes must also be lubricated on a regularbasis again usually every 10,000 miles or so but checkthe OEM manual to be sure. When lubricating yokes thegrease should purge from the grease relief hole in theWelch plug on the yoke and at the seal on the other endof the yoke. In cold weather be sure to road test a

vehicle after lubricating the slip yoke to displace anyextra grease, if the grease is allowed to solidify slip yokeaction can pop out the Welch plug leading to loss oflubricant and eventual failure of the slip joint.

Documents

drive shaft.pdf