Drive axle Overhaul

Removal of the drive axle assembly varies slightly depending on the type, integral carrier, removable carrier or integral pinion with removable carrier; however all involve removing the drive shaft and the axles. Inspect carrier prior to disassembly for signs of damage It is essential that if a drive gear set is to be reused you must check backlash and contact pattern before disassembly so as not to change wear pattern on re-assembly

Measuring backlash will also indicate amount of wear on gear set (compare measurement to OEM specifications to determine if gear set can be reused) Performing a pattern check will also indicate whether a gear set can be reused.

Repair of a drive axle is usually necessary because of bearing failure or catastrophic failure of the gear set. It is important to note however that bearing failure can lead to irregular gearset wear necessitating its replacement. Inspect gears closely on disassembly to assess wear. Note that drive axle gears have a normal rough looking surface when new and small lines in the face of the gears may be normal as the gearset wears itself in. Do not overlook the differential gearset, excess wear can occur to the spider and side gears due to driver abuse such as spinout.

Before removing the carrier be sure to mark one side bearing cap and leg with a punch so that they are reassembled on the correct side and the bearing adjuster threads line up.

The differential carrier in most heavy duty systems has two halves the case half or plain half and the flange half that has the ring or crown gear attached to it. Before disassembling the case you must mark the halves so they can be reassembled correctly.

In drive axle overhauls there may be several checks and setting such as differential gearset rotating torque and thrust block setting but all axles require four common adjustments and they are; Pinion bearing preload, to ensure that the pinion stays in its correct position, this adjustment ensures rigidity of the gearset Pinion depth setting, to ensure the pinion is meshed with the crown gear correctly, this adjustment will affect tooth contact pattern. Carrier bearing or side bearing preload, to ensure the carrier cannot move away from the pinion under load, this adjustment ensures rigidity of the gearset. Gearset backlash, to provide the correct running clearance for the gearset, this adjustment will affect tooth contact pattern. 1. Pinion Bearing Pre-load. Bearings are preloaded to provide rigidity to pinion Rigidity of pinion controls its alignment to ring or crown gear. There are several methods of setting pinion pre-load but the most common is a spacer of varying thickness between two opposed tapered roller bearings. The bearings are preloaded to a specified rolling torque dimension. Using the press method allows setting of the pre-load without assembling the pinion in the carrier. The press is used to replicate pressure that would be exerted on the bearings when the yoke nut is tightened. If the outer bearing is a slip fit the check may be done with the pinion in the bearing cage as at right. Refer to manual for specifications and procedure. The OEM manual will specify a certain tonnage of press load to represent the torque on the pinion nut. A spring scale is used to measure pounds pulled. The OEM will recommend either a pounds pulled spec or an inch pound torque spec. When an inch/pound specification is given the pounds pulled must be multiplied times the radius of the pinion bearing cage.

If the outer pinion bearing is an interference fit the same procedure is used but without the pinion installed. In both methods increase the press load as you rotate the bearing cage if the cage stops rotating before the correct tonnage is reached investigate the cause or add spacer thickness. Always read the pounds pulled only after the cage is spinning not the amount it takes to start to rotate it. A thicker spacer will decrease pre-load or pounds pulled and a thinner spacer will increase preload or pounds pulled.

With an integral pinion bearing cage as at left the pinion must be temporarily installed in the carrier. The yoke and nut are installed and an inch/pound torque wrench is used to measure pre-load. Usually the rotating torque setting is between 20 and 40 inch pounds. But always refer to the OEM maintenance manual for procedure and specifications

The procedure for light duty integral drive axles is similar to the above but instead of using a selectable preload spacer a collapsible spacer is used.

2. Pinion Depth. A shim pack placed between the pinion flange and the carrier housing controls pinion depth in most drive axles however there again are different methods of setting pinion depth. Some lighter duty systems may use shims between the inner pinion bearing and the pinion or under the inner pinion bearing race in the housing.

Pinion depth is the distance between the centerline of the ring gear and the end of the pinion (observed in side view). This dimension determines how deep in mesh with the crown gear the pinion must run to operate properly. When pinion and crown gears are manufactured it is impossible to make any two gear sets exactly the same. Gear sets are

run together in the lapping process so no two are exactly alike. Pinion depth dimensions are given in the service manual for the perfect or optimal pinion position. So the distance from the optimal pinion to the centre line of the crown wheel is referred to as nominal mounting distance or dimension. If the pinion depth setting is not nominal the pinion will have a pinion cone variation number, (a plus (+) or a minus (-) followed by a number indicating the thousandths of an inch of variation. This number is inscribed by the manufacturer to indicate a

change, from (nominal), of the mounting distance or dimension. The new pinion must be installed at the nominal dimension plus or minus its variation number. The size of the shim pack that will constitute a nominal mounting must be determined. Although this number can be measured by using a special micrometer, see diagram at right, not too many shops will have the correct adapters for every drive axle. So to get around this we can check the old gear sets pinion variation number. Measure the shim pack thickness and then factor in the old and new variation numbers to ensure shim pack thickness is correct. (Refer to appropriate shop manual).

Basically you must figure out, (from the original pinions variation number), what thickness shim pack would correspond to a nominal mounting distance. To do this look at the old pinion if it does not have a variation number then the shim pack should already be the nominal dimension shim pack. In this case you must merely either add or subtract the new pinions variation number to find the

correct mounting dimension. A plus variation number means the pinion must be mounted further away from the crown wheel so shims must be added to the pack a minus dimension means shims must be removed so the pinion is mounted closer to the crown wheel. If the pinion depth is not set correctly the contact pattern will be poor and the life expectancy of the gear set will be severely shortened. A minimum of three shims must be used with a thin shim on the outside and inside of the pack to act as gaskets or the cage may leak.

3. Carrier Bearing Pre-load. Pre-loading the carrier or side bearings ensures rigidity of differential case in relation to the carrier or housing. Rigidity ensures backlash remains consistent as the ring or crown gear turns under no-load conditions, normal loading will cause an acceptable amount of deflection of the crown wheel thereby increasing backlash. This is expected and the operating clearances for the crown and pinion are set with this expected deflection in mind. Pre-load makes sure that the crown gear is rigidly supported by the carrier or side bearings. Side bearing pre-load can be controlled by one of two ways threaded bearing adjusters are used in some light duty and all heavy duty applications, other light duty systems may use shim packs either under the side bearings or on the outside of the side bearing cups.

Pre-load actually stretches the carrier housing slightly, maintaining a light pressure on carrier bearings. Some manufacturers suggest or require preload and backlash be adjusted at same time. Combined adjustment ensures preload is not compromised during backlash adjustment. Using a dial indicator is the most accurate method of setting preload. Refer to manual for specifications and procedure for the specific unit being serviced. The following is a general procedure and is only one method of adjusting preload. Mount a dial indicator to the carrier so that its pointer contacts a crown gear tooth at 90 degrees to measure gearset backlash. Adjust the crown wheel tooth side bearing adjuster until one thread is showing from under the bearing cap. Then tighten the other bearing adjuster until there is no backlash at the crown wheel, take care to ensure the crown gear is not jammed into the pinion, you

must stop the adjustment precisely at the point where backlash is eliminated. Use the dial indicator to determine this exact point. Turn the crown wheel to determine that there is no binding because of gear runout and then reset zero backlash at the point of closest mesh. Then tighten the tooth side bearing adjuster until gearset backlash of 4 to 6 thousandths of an inch is achieved this will indicate that the bearing caps have been expanded and that preload is correct. Another method of achieving this preload is to actually measure the bearing cap deflection with a large micrometer, be sure to follow the method recommended by the OEM. The normal backlash setting for new gearing is approximately 6 to 20 thousandths of and inch and if using new gearing no further adjustment may be necessary after either of these methods however backlash may need to be adjusted to achieve the correct tooth contact pattern.

4. Gearset Backlash. Backlash is the running clearance between the drive gears. Backlash for a drive gear set must be adjusted to specification; however because gearset backlash is one of the critical adjustments to control tooth contact pattern all OEM’s allow variation in backlash to modify the contact pattern. Backlash is controlled by positioning of the same bearing adjusters used to set preload. To set backlash after the preload has been set the bearing adjusters must be moved in unison notch for notch, loosening one side and tightening the other by the exact same amount to maintain preload. A dial indicator with its plunger positioned against the drive side of the ring gear is used to measure backlash. The pinion gear must be held steady and the ring gear rocked back and forth from the drive side to the coast side.

Checking Gear Tooth Contact Pattern. Pinion depth and backlash adjustments are always followed up with a tooth contact pattern check. If a pattern check reveals that pinion depth or back-lash are incorrect regardless of what you have calculated from the variation numbers etc., pinion depth and or backlash should be changed to correct pattern. It is absolutely essential that the contact pattern is within the normal range or the gearset will be noisy and its life will be considerably shortened. Refer to shop manual and adjust as per recommended procedure

Gear tooth nomenclature is listed at left. The toe of the crown gear tooth is always to the inside of the circle it forms.

OEMs refer to the contact pattern in different ways, some talk about face width and some

talk about lengthwise bearing, or they talk about profile bearing and depth of contact no matter which description they use they are all talking about the same thing, the point of contact between the pinion gears teeth and the crown gears teeth. In basic gearing we learn that typical spur tooth gears create noise because as the teeth enter and leave mesh they create a clicking sound that is amplified by the speed of the gears. Spiral or helical gears do not create as much noise because they slide in and out of mesh rather than clicking out all at once. In a drive axle it is no different, the gear teeth must not leave mesh at the end of another tooth or that same clicking sound will occur causing a high pitched whine or howl as speed increases.

At the same time drive axle gear teeth are exposed to extreme loads and we must make use of the entire tooth surface to provide strength or the gears would wear out prematurely. So to accomplish both of these tasks the positioning of the gear teeth in relation to each other is critical. Ideally we want the loaded tooth contact pattern to cover most of the tooth face for strength and not to come off the ends to prevent noise. Unfortunately we can not load the gearing sufficiently when checking the pattern so we can only check what is known as a hand rolled pattern. TOOTH CONTACT PATTERN CHECK: is performed to verify the correct "Pinion Position" and "Backlash" setting. Tooth contact is checked on the drive side of the ring gear teeth. After coating the teeth with a suitable gear marking compound or paint. Generally a dozen or so teeth are coated with marking compound and then the painted teeth of the ring gear are rotated back and forth through mesh with the pinion at least six times to obtain a pattern. Remember to coat the drive side of the teeth, Hypoid gearing is driven on the convex side of the tooth. The tooth contact on the ring gear consists of a "Profile" bearing and a "Lengthwise" bearing on the teeth. Profile contact is up and down on the tooth face and is affected by "Pinion Position". The Lengthwise contact appears along the tooth length and is affected by drive gear "Backlash". The picture at right shows a normal contact pattern for new conventional Hypoid gearing. Note the three attributes of a good pattern; 1. The pattern is close to but clear of the toe. 2. The pattern is evenly centred between the top land and the root of the tooth, this is also known as profile bearing 3. The pattern extends at half way along the tooth face width, also known as lengthwise bearing.

A normal hand rolled pattern like the one at left on conventional Hypoid gearing will mean as the gear teeth are loaded up the contact pattern will spread out towards the heel of the tooth giving full contact as the load increases to maximum as can be seen in the loaded pattern diagram at right.

If the contact pattern is not correct you must fix it or noise and gearset damage will occur! Backlash controls the movement of the pattern along the tooth face width or lengthwise bearing. Pinion depth controls the movement of the pattern up and down the face of the tooth or profile bearing. Note that pinion depth adjustments will change backlash and if you are removing shims from the pinion cage to install the pinion closer to the crown gear centreline you will need to increase backlash appropriately or damage to the gearset may occur. Toe concentration When tooth patterns are concentrated in the "Toe" area of the ring gear teeth, the gear set has insufficient "backlash". The pattern will be corrected by increasing backlash, moving the contact away from the "Toe" and increasing the pattern's length.

Heel concentration Heel concentrated patterns are due to excessive "Backlash" between the drive gears. The pattern will be improved by decreasing Backlash, which causes the ring gear's tooth pattern to move towards the Toe area and decrease its length.

Top concentration When tooth patterns are concentrated in the top area of the ring gear teeth, the "Pinion" is positioned too fat away from the ring gear. The shim pack will have to be decreased, allowing the tooth pattern to shift down on the tooth's profile, centralizing the tooth pattern between top and tooth root.

Root concentration Root concentrated tooth patterns are due to the "Pinion" being positioned too deeply into the ring gear. Shims will have to be added to the pinion to move the pattern up the tooth's profile to sit midway between root and tooth top. NOTE: Whenever the pinion is repositioned "backlash" will have to be readjusted at the ring gear.

The patterns and adjustments just shown are for conventional Hypoid and spiral bevel gearsets. Some manufacturers use gears that are slightly different. Mack uses a form of spiral bevel gearing called Durapoid which has a slightly different tooth design which makes it stronger but the contact pattern is different it is more centred along the tooth face. When this type of gearing is loaded the pattern spreads out along the face in both directions rather than just towards the heel.

Rockwell, now Arvin Meritor uses a similar tooth design change in some of its gearsets called Hypoid Generoid and its pattern is similarly closer to the centre on a hand rolled pattern check. The pattern at left is the newest called for in Generoid gearing it is a little closer to the centre of the tooth than conventional. Older Rockwell service manuals call for a pattern similar to the one shown below right much closer to the centre.

Mack’s Durapoid type gearing must also match the pattern at right. These gears are designed to accomplish the same thing as conventional gearing that is to spread the contact over the entire tooth surface when loaded. Always check the manufacturer’s manual to be certain the pattern is correct. The following diagram compares the contact patterns of conventional Hypoid and Hypoid Generoid gearsets.

Some drive axles have a thrust screw installed behind the crown gear at the point of mesh with the pinion to control crown gear deflection under load. This thrust screw may be just a large screw or it may have a thrust block swaged to the end. After the pattern is correct you will need to install and adjust the thrust screw. Typically this involves tightening the screw until it contacts the back face of the crown gear and then backing it out a specified amount usually half to one turn and then tightening the lock nut. This establishes a running clearance for the thrust screw. In normal operation it will not contact the crown gear but it will stop the deflection of the crown gear during heavily loaded conditions. Be careful not to remove the thrust screw completely in an assembled drive axle as you may cause the thrust block to fall into the unit leading to a costly tear down to remove it.

After the drive axle is reassembled the wheel axles need to be installed. There are two types of axles that are used, semi floating and full floating. Semi-floating axles are found in light duty applications, semi-floating axle carry the entire vehicle weight at the outer end. They may be retained by a bearing retainer at the wheel or by a clip arrangement inside the differential. Bearing retainer types will have ball bearing at the wheel to handle axial thrusts while with the clip type axial thrusts are controlled by the axle ends inside the differential. Roller bearing semi floating style at left Ball bearing semi

floating style at right with bearing retainer. The clip used to retain the roller bearing style semi floating axle is shown at left. The clip slides into a groove cut into the inner end of the axle and then the clip and the axle fit into a recess in the side gear. The spider gears pinion shaft is then installed and the axles are held in this outward position. The clips can not be removed until the pinion shaft is removed.

All larger truck drive axles will have full floating axles, these axles are bolted to the wheel hubs and do not carry any of the vehicle weight. The vehicle weight is supported by two opposed tapered roller bearings in the hub as seen at right.

The full floating axles are typically centred in the hub by a series of tapered wedges under the attaching bolts these tend to bind the axles to the hub and make them difficult to remove. To remove the axles loosen the retaining bolts or nut and leave them attached by on or two threads and then hold a brass hammer against the driving lugs of the axle and strike it with a sledge this will free the wedges and allow the axle to be removed, CAUTION do not remove the bolts or nuts completely until the wedges are free they can fly off at tremendous velocity and cause injury to you or others in the shop! The last task is to fill the drive axle with an approved API GL-5 lubricant this lubricant has extreme pressure additives and is suitable for you with Hypoid gears that

are subject to extreme loading at the tooth contact point. API GL-5 is available in different viscosities for operation in different ambient temperatures and you must select the proper viscosity for the intended use. When filling the axle it is critical that the wheel hubs are also filled. To ensure the hubs have sufficient lubrication follow this procedure.

Fill the axle housing with the correct lube until the level is even with the fill hole then jack up one side of the axle and wait for one minute to allow the lube to flow into the hub. Return the axle to level and wait a minute for the fluid to stabilize and then jack up the other side and again wait one minute. Return the axle to level and after one minute recheck the lube oil level and ensure that it is level with the fill hole. Note! Some drive axles may be filled from different locations depending on mounting angle, and tandem drive power dividers may need to filled at more than one location such as the inter axle differential housing. Always check the service manual to be sure.