8.1 Carrier Setup: Pinion Depth, Bearing Preload, and Backlash Adjustment
Key Takeaways
- Meritor sets pinion depth with the shim pack between the pinion bearing cage and the carrier; new shim pack = old pack − old PC number + new PC number.
- Meritor pinion bearing preload is 5–45 lb-in rotating torque for new bearings and 10–30 lb-in for used bearings in good condition; a thinner spacer increases preload.
- Meritor ring gear backlash is 0.008–0.018 in. (0.012 in. for a new set) for ring gears under 17 in. pitch diameter, and 0.010–0.020 in. (0.015 in. new) at 17 in. and larger.
- Meritor differential bearing preload is 15–35 lb-in, or 0.002–0.009 in. cap spread on RS-140/145/160 carriers and 0.006–0.013 in. on other models.
- Meritor limits ring gear runout to 0.008 in. maximum.
8.1 Carrier Setup: Pinion Depth, Bearing Preload, and Backlash Adjustment
Differential Carrier Bench Setup and Overhaul Overview
Heavy-duty drive axles such as Meritor's RS single-axle and RT tandem series use single-reduction hypoid carriers. Overhauling one is a precision bench job that needs cleanliness, the right fixtures, and a fixed sequence of adjustments. Carriers are heavy, so mount them in a carrier repair stand before teardown or assembly. The specifications below are from Meritor Maintenance Manual 5A (single-reduction carriers); other makes publish their own.
Setting up a commercial drive axle carrier requires five sequential mechanical operations. Because each adjustment mechanically alters subsequent measurements, technicians must strictly adhere to the standardized order of operations:
+-------------------------------------------------------------------------+
| Sequential Carrier Overhaul Order of Operations |
+-------------------------------------------------------------------------+
| Stage 1: Drive Pinion Depth Adjustment |
| (Shim pack under pinion cage, using PC variation numbers) |
| | |
| v |
| Stage 2: Drive Pinion Bearing Preload Adjustment |
| (Meritor: 5-45 lb-in new bearings, set with spacer) |
| | |
| v |
| Stage 3: Differential Case Installation & Initial Backlash |
| (Mounts case in carrier housing legs; sets rough gear mesh) |
| | |
| v |
| Stage 4: Differential Bearing Preload, Backlash & Runout |
| (Meritor: preload by adjusting rings; backlash per Table AA) |
| | |
| v |
| Stage 5: Tooth Contact Pattern Verification |
| (Final validation under load: pattern takes ultimate precedence|
+-------------------------------------------------------------------------+
Skipping or reordering these operations guarantees setup failure. For example, if a technician attempts to adjust ring gear backlash before establishing final pinion bearing preload, the pinion shaft will shift axially when the pinion nut is fully torqued, destroying the backlash setting. Similarly, changing pinion depth after setting backlash forces a complete reset of both backlash and carrier bearing preload.
Drive Pinion Depth: Geometry, Markings, and Shim Calculations
The Geometry of Pinion Depth
The drive pinion and ring gear operate as a matched hypoid gearset where the pinion shaft centerline is offset below the ring gear centerline. Pinion depth defines the exact axial distance from the centerline of the ring gear to the front face (or mounting shoulder) of the drive pinion head.
Proper pinion depth places the pinion gear teeth exactly on the design cone center of the ring gear. If the pinion is set too shallow (too far from the ring gear centerline), tooth contact will ride high along the crown (face) of the teeth. If the pinion is set too deep (too close to the ring gear centerline), tooth contact will jam deep into the root (flank) of the teeth. Either condition causes severe stress concentration, gear tooth spalling, whining noise, and premature tooth fracture.
Hypoid Pinion Depth Geometry
Ring Gear Centerline
|
v
+---------------+---------------+
| [ Ring Gear ] |
+---------------+---------------+
^
| Mounting Distance
| (Cone Center Depth)
v
[Pinion Head]
|
+--[Inner Bearing]--+
| (Depth Shims) |
+-------------------+
|
Pinion Shaft
Pinion Depth Shim Pack Location
Where the pinion depth shims sit depends on the design, and that decides which way a shim change moves the pinion:
- Under the Pinion Bearing Cage (Meritor): The pinion, its bearings and spacer are assembled in a bearing cage that bolts to the carrier, with the depth shim pack between the cage and the carrier. A thicker shim pack moves the pinion away from the ring gear; a thinner pack moves it toward the ring gear.
- Behind the Inner Bearing Cup or Cone (other designs): On carriers without a cage, the shims sit behind the inner bearing cup or cone, and adding shims moves the pinion toward the ring gear — the opposite of the cage design. Always confirm the shim location in the OEM manual before applying a formula.
Reading the Pinion Cone (PC) Variation Number and Calculating the Shim Pack
Each Meritor gear set is tested at the factory, and the drive pinion is marked with a pinion cone (PC) variation number, such as PC +3, PC −3, +3, or −3. Meritor defines these as thousandths of an inch (0.003 in.); metric markings such as +0.03 mm are hundredths of a millimeter. The PC number is not used to check that a gear set is matched — it is used only to set pinion depth.
Meritor's shim-pack calculation for a pinion-cage carrier, when replacing the gear set and reusing the carrier:
- Measure the old shim pack with a micrometer.
- Read the old PC number. If it is plus (+), subtract it from the old pack; if minus (−), add it. The result is the standard shim pack thickness.
- Read the new PC number. If it is plus (+), add it to the standard pack; if minus (−), subtract it.
- If the old PC number is missing or unreadable, install a new shim pack the same thickness as the old one.
[!NOTE] All markings must retain their algebraic signs ($+$ or $-$) during calculation. Subtracting a negative number results in addition.
Practical Calculation Examples
Example 1 (Meritor Table J): old shim pack 0.030 in., old PC +2, new PC +5.
- Standard pack = 0.030 − 0.002 = 0.028 in.
- New pack = 0.028 + 0.005 = 0.033 in.
Example 2 (Meritor Table J): old shim pack 0.030 in., old PC −2, new PC +5.
- Standard pack = 0.030 + 0.002 = 0.032 in.
- New pack = 0.032 + 0.005 = 0.037 in.
Example 3 (Meritor Table J): old shim pack 0.030 in., old PC +2, new PC −5.
- Standard pack = 0.030 − 0.002 = 0.028 in.; new pack = 0.028 − 0.005 = 0.023 in.
The logic follows from the cage design: a plus number means that pinion runs best slightly farther from the ring gear, and on a cage carrier a thicker shim pack moves the pinion farther away. The final check is always the tooth contact pattern.
If the carrier housing casting itself is replaced, nominal housing variations prevent using the old shim pack as a reference. The technician must employ a specialized pinion depth gauge set (master arbor bar, micrometer depth gauge, and dial indicator) to measure the housing bore depth directly from the carrier bearing saddle bores to the inner pinion bearing seat.
Pinion Bearing Preload: Solid Spacers and Rolling Torque
Purpose of Pinion Bearing Preload
The drive pinion shaft is supported by two opposing tapered roller bearings: a large inner bearing (adjacent to the gear head) and a smaller outer bearing (adjacent to the companion flange). Under forward vehicle acceleration, the spiral hypoid teeth generate tremendous axial thrust forces that push the pinion gear inward toward the differential. Under deceleration and engine compression braking (coast), thrust reverses, pulling the pinion gear outward.
To prevent axial chucking and radial deflection under these reversing shock loads, the tapered roller bearings must be assembled under a predetermined bearing preload. Preload eliminates all internal bearing clearance and compresses the rollers and raceways into continuous elastohydrodynamic contact. Insufficient preload allows the pinion to walk along the ring gear teeth, destroying tooth contact and causing gear jump or tooth shear. Excessive preload causes overheating, roller spalling, and early bearing failure.
Pinion Shaft Bearing Assembly
Flange Nut Outer Bearing Selective Spacer Inner Bearing Pinion Head
[OEM torque] [Cone/Cup] (sets preload) [Cone/Cup] [Hypoid Teeth]
| | | | |
v v v v v
(==)====[Seal]====>|<<====[Selective Spacer]===>>|<==============[Gear Head]
|| Shaft [Spacer] Shaft |
++--------------------------------------------------------------------+
Selective Spacers vs. Collapsible Crush Sleeves
Light-duty axles often use crush sleeves that yield when the pinion nut is tightened. Meritor heavy-duty carriers instead control pinion bearing preload with a selective bearing spacer between the inner and outer bearing cones:
- To increase preload: install a thinner bearing spacer (the cones are drawn closer together).
- To decrease preload: install a thicker bearing spacer.
Because the spacer does not deform, the preload set on the bench stays stable in service.
Preload Measurement: Dynamic Rolling Torque
Pinion bearing preload cannot be measured with a dial indicator or a standard foot-pound clicker torque wrench. It must be measured as dynamic rolling torque using an inch-pound (in-lb) dial-type or beam-type torque wrench:
Meritor allows a press method (pinion and cage in a press with a gauge, applying the specified load) or a yoke/flange method (nut tightened to specification):
- Assemble the pinion, bearings, and selected spacer in the cage. Do not install the pinion seal yet — Meritor's note is to leave it out while adjusting preload, because seal drag would add to the reading.
- For the yoke/flange method, install the yoke or flange and tighten the pinion nut to the specified torque while holding the yoke with a bar. Pinion nut torques are very high (hundreds to over a thousand lb-ft depending on thread size), so use Meritor's torque table.
- Rotate the pinion several turns to seat the rollers.
- Attach an inch-pound torque wrench and read the torque needed to keep the pinion (cage) rotating.
| Bearing Condition | Specified Dynamic Rolling Torque | Measurement Technique |
|---|---|---|
| New Bearings | 5 to 45 lb-in (0.56 to 5.08 N·m) | Meritor; read while turning with an inch-pound wrench |
| Used Bearings in Good Condition | 10 to 30 lb-in (1.13 to 3.39 N·m) | Meritor; read while turning with an inch-pound wrench |
[!WARNING] Never record breakaway torque (the initial spike required to start the shaft rotating from a standstill). Breakaway torque reflects static friction and will cause a technician to under-shim the carrier, resulting in loose bearings and rapid gear failure. Always record the steady rolling torque during active rotation.
Pinion Nut Sealing and Final Installation
Once rolling torque is verified, remove the pinion nut and flange to install the pinion oil seal. Coat the outer diameter of the seal with sealant and use a dedicated driver tool to seat the seal squarely. Apply high-temperature grease to the seal lip.
Meritor's newer carriers use unitized pinion seals installed with the specified seal driver. Follow the OEM instructions for the seal type, yoke installation, and nut torque exactly.
Ring Gear Backlash and Carrier Bearing Preload
With drive pinion depth and preload finalized, the differential case assembly (differential nest, ring gear, and carrier bearings) is installed into the carrier housing bearing saddles.
Ring Gear Backlash Fundamentals
Backlash is the small mechanical clearance between the meshing teeth of the ring gear and drive pinion. Backlash provides clearance for thermal expansion as the gear set heats up and allows lubricant between the tooth flanks.
- Meritor Backlash Specification (Table AA): ring gears under 17 in. pitch diameter: 0.008–0.018 in. (0.20–0.46 mm), set to 0.012 in. for a new gear set. Ring gears 17 in. or larger: 0.010–0.020 in. (0.25–0.51 mm), set to 0.015 in. for a new gear set. An old gear set is reset to the backlash measured before disassembly.
- Too Little Backlash: The mesh closes up as it heats, wiping the oil film and causing scoring, galling, and whine.
- Too Much Backlash: Causes clunk on drive-to-coast transitions and moves contact toward the heel.
- Adjust Within Range for the Pattern: Meritor says to change backlash within the specification to get a good tooth contact pattern.
Ring Gear Backlash Measurement
Dial Indicator Plunger
(Perpendicular to Tooth Face)
|
v
[DI]
||
+---------------------||---------------------+
| \/ |
| [Tooth Flank] |
| / \ |
| [Ring Gear] [Pinion Tooth] |
| \ / |
| \<-Lash-> / |
+--------------------------------------------+
Meritor: 0.008"-0.018" (<17" ring gear)
Measuring Backlash with a Dial Indicator
- Mount a precision dial indicator equipped with a rigid magnetic base firmly to the carrier housing casting.
- Position the indicator plunger stylus strictly perpendicular ($90^\circ$) to the drive side face of a ring gear tooth at the outer pitch diameter.
- Lock the drive pinion stationary using a holding tool or flange bar.
- Rock the ring gear gently back and forth by hand and record the total needle deflection on the dial indicator.
- Rotate the ring gear and repeat the measurement at four equidistant locations ($90^\circ$ apart) around the ring gear circumference.
- Backlash must be within specification at every check point. Significant variation around the gear points to ring gear runout, so check it directly.
Ring Gear Runout
Mount a dial indicator against the back face of the ring gear and rotate the differential. Meritor's maximum ring gear runout is 0.008 in. (0.20 mm). Excess runout usually comes from dirt or burrs between the ring gear and case flange, a damaged case flange, or bad differential bearings. Correct the cause before setting backlash.
Threaded Adjuster Rings and Lateral Differential Movement
Heavy-duty commercial carriers utilize threaded adjuster rings (adjuster nuts / spanner rings) threaded into the carrier bearing cap pedestals on both sides of the differential case. Turning these adjuster rings moves the differential case laterally across the carrier housing:
Differential Lateral Adjustment via Threaded Rings
[Left Adjuster Ring] [Differential Case] [Right Adjuster Ring]
| | |
v v v
( @ )===> [Bearing]====[Ring Gear]====[Bearing] <===( @ )
Loosen Left Moves Right Tighten Right
--> Backlash Decreases <--
- To Decrease Backlash (move ring gear closer to pinion): Loosen the adjuster ring on the back side of the ring gear, and tighten the adjuster ring on the tooth side of the ring gear by an identical amount.
- To Increase Backlash (move ring gear farther from pinion): Loosen the adjuster ring on the tooth side of the ring gear, and tighten the adjuster ring on the back side by an identical amount.
Setting Differential Carrier Bearing Preload (Housing Spread)
Differential carrier bearings are large tapered roller bearings that support the heavy differential case. Like the pinion bearings, they must operate under controlled preload to maintain rigid ring gear alignment under peak hypoid thrust.
Meritor states that differential bearing preload is controlled by tightening both adjusting rings after zero end play is reached:
- Establish Zero End Play: Tighten the adjusting rings until all side play of the differential case is just removed, keeping some backlash between the ring gear and pinion.
- Preload the Bearings: Tighten both rings further to preload the bearings.
- Verify Preload (Meritor Table Y): either 15–35 lb-in (1.7–3.9 N·m) of rotating torque, or expansion between the bearing caps measured with a dial indicator: 0.002–0.009 in. on RS-140, RS-145 and RS-160 carriers, and 0.006–0.013 in. on all other carrier models.
- Recheck Backlash: If backlash must change, loosen one ring and tighten the other by the same amount so the preload is not changed.
Carrier Bearing Preload (Housing Spread)
[Bearing Cap] [Bearing Cap]
| | | |
|<--Pad | | Pad-->|
| | | |
+-----------+ +-----------+
^ ^
| |
+------- [ Dial Indicator ] --------+
Meritor: 0.002"-0.009" (RS-140/145/160)
Securing Bearing Caps and Adjusters
Once backlash and housing spread are verified:
- Torque the bearing cap capscrews to the OEM specification in the specified sequence.
- Install the stamped steel adjuster lock brackets or cotter pins into the matching castellated slots of the adjuster rings.
- Torque the lock bracket retaining bolts to specification and bend locking tabs over bolt heads. Never leave an adjuster ring unpinned, as normal operational vibration will rotate the ring, destroying backlash and housing preload.
A technician replaces the ring and pinion in a Meritor single-reduction carrier that uses a shim pack under the pinion bearing cage. The old shim pack measures 0.038 in., the old pinion is marked PC +2, and the new pinion is marked PC −1. Using Meritor's procedure, what shim pack should be installed?
Which of the following procedures correctly describes measuring drive pinion bearing preload during the bench overhaul of a heavy-duty commercial axle carrier?
Technician A says that to increase ring gear backlash, the technician should loosen the adjusting ring on the tooth side of the ring gear and tighten the ring on the back side by the same amount. Technician B says Meritor establishes differential bearing preload by tightening both adjusting rings after zero end play is reached, and verifies it by rotating torque or by bearing cap spread measured with a dial indicator. Who is right?