5.1 Auxiliary Gearing: Range and Splitter Section Design and Power Flow

Auxiliary Gearing: Range and Splitter Section Design and Power Flow

Commercial medium- and heavy-duty vehicles require a broad overall transmission ratio spread to reconcile two conflicting operational demands: delivering maximum tractive effort and gradeability when launching an 80,000-pound Gross Combination Weight Rating (GCWR) rig on an uphill incline, while maintaining optimal engine fuel efficiency and cruising speeds at highway velocity. To achieve between 9 and 18 distinct forward gear ratios without engineering an impossibly long transmission case, commercial truck manufacturers employ a compound transmission architecture consisting of a primary main transmission box coupled to an auxiliary transmission section.

---

1. Purpose and Layout of the Auxiliary Transmission Section

The auxiliary section is an independent cast-iron or aluminum housing that bolts directly to the rear face of the main transmission case. Rather than housing all 10, 13, or 18 gear reductions along a single, continuous mainshaft—which would induce severe shaft deflection, introduce excessive case length, compromise driveline universal joint working angles, and create cumbersome multi-rail shift linkages—the compound design utilizes an auxiliary gearset to multiply the ratios produced in the main box.

+------------------------------------+-------------------------------------+
|        MAIN TRANSMISSION CASE      |      AUXILIARY TRANSMISSION CASE    |
|                                    |                                     |
|  - Input Shaft & Drive Gear        |  - Auxiliary Drive Gear             |
|  - Twin Countershafts              |  - Dual Auxiliary Countershafts     |
|  - Floating Mainshaft              |  - Range Reduction Gearing          |
|  - 4 or 5 Forward Sliding Clutches |  - Pin-Type Range Synchronizer      |
|  - Non-Synchronized Dog Clutches   |  - Splitter Sliding Clutch (13/18)  |
|  - Mechanical Shift Cane & Rails   |  - Output Shaft (Tailshaft)         |
+------------------------------------+-------------------------------------+
                                     ^
                             Bolted Mating Face

In this compound arrangement, the main transmission box functions as a standard 4-speed or 5-speed gearbox with an unsynchronized floating mainshaft and twin countershafts. The rear of the main transmission mainshaft extends directly into the auxiliary housing, where it drives the auxiliary gearing. By routing main box output torque through various combinations of auxiliary reduction and direct drive gear paths, a 5-speed main box produces:

• 9-Speed or 10-Speed Configuration: Utilizes a two-speed Range section (Low Range and High Range) to double the main box ratios (5 ratios × 2 ranges = 10 forward speeds, or 5 ratios in low range plus 4 in high range with one low crawler ratio = 9 speeds).
• 13-Speed Configuration: Combines a two-speed Range section with an auxiliary Splitter section. In High Range, each main box ratio is split into Direct and Overdrive (5 low-range ratios + 4 main box ratios × 2 splitter positions in high range = 13 forward speeds).
• 18-Speed Configuration: Employs Range multiplication and Splitter operation across both Low Range and High Range, splitting every main box ratio and reverse gear (Low gear split + 4 low-range gears split + 4 high-range gears split = 18 forward speeds, plus 4 reverse speeds).

---

2. Range Section Mechanical Architecture and Power Flow

The range section is a two-speed auxiliary gearset that provides either a substantial gear reduction (Low Range) or a straight-through direct coupling (High Range) between the mainshaft and the transmission output shaft.

Mechanical Components of the Range Section

• Auxiliary Drive Gear: Splined directly to the rear of the main transmission mainshaft. It transmits all torque exiting the main box into the auxiliary section.
• Auxiliary Countershafts: Two countershafts mounted 180 degrees apart in the auxiliary case, supported by heavy-duty tapered roller bearing assemblies. Each auxiliary countershaft carries an auxiliary countershaft drive gear and an auxiliary countershaft reduction pinion.
• Range Reduction Gear: A large-diameter helical gear that floats on the transmission output shaft (tailshaft) until engaged by the range sliding clutch.
• Output Shaft: The heavy-duty splined shaft that transfers torque out of the auxiliary case to the driveline companion flange or end yoke.
• Range Sliding Clutch and Synchronizer Assembly: Positioned on splines between the auxiliary drive gear and the range reduction gear, moving axially forward or rearward to select the operating range.

Power Flow in Low Range

When the driver selects Low Range (range switch down on the shift knob), regulated pneumatic pressure directs the range shift cylinder to move the range sliding clutch rearward into engagement with the clutching teeth of the range reduction gear:

1. Engine torque travels from the main transmission mainshaft into the auxiliary drive gear.
2. The auxiliary drive gear meshes with and drives both auxiliary countershaft drive gears, splitting torque 50/50 across the two auxiliary countershafts.
3. The auxiliary countershaft reduction pinions drive the large range reduction gear.
4. The range reduction gear transfers torque through the range sliding clutch into the transmission output shaft.
5. Mechanical Result: The range reduction gearset provides a deep reduction ratio (typically 3.2:1 to 3.5:1). This deep mechanical reduction multiplies the torque of every main box gear, providing exceptional pulling capability at low vehicle speeds.

Power Flow in High Range

When the driver shifts into High Range (range switch flipped up), pneumatic pressure forces the range shift cylinder forward:

1. The range sliding clutch disengages from the range reduction gear and moves forward across the synchronizer assembly.
2. The sliding clutch engages the clutching teeth on the rear face of the auxiliary drive gear.
3. This locks the auxiliary drive gear directly to the transmission output shaft.
4. Mechanical Result: Power flows straight through at a 1:1 direct drive ratio. The auxiliary countershafts and range reduction gear continue to freewheel with zero torque load, and all main box gear ratios are transmitted directly to the driveline at road speed.

---

3. Range Synchronizer Assembly: Pin-Type Design

A critical design distinction in commercial heavy-duty transmissions is that while the main transmission box is completely non-synchronized (utilizing sliding dog clutches that require driver double-clutching or float-shifting to rev-match), the auxiliary range section relies on a heavy-duty pin-type synchronizer with a LO-range ring and a HI-range ring.

                     PIN-TYPE RANGE SYNCHRONIZER

        Auxiliary Drive Gear             Sliding Clutch, Rings & Blocker Pins
       +--------------------+                   +--------------------+
       |                    |                   |                    |
  ====>| [Clutching Teeth]  |<-- [HI Ring] ---- | [Sliding Clutch]   |===> Output
       |                    |    [LO Ring]      | [Blocker Pins]     |     Shaft
       +--------------------+                   +--------------------+

Why the Range Shift Requires Synchronization

Drivers make range shifts while the vehicle is moving — for example, from 5th to 6th in a 10-speed. In the main box, floating gears can be matched to shaft speed through engine throttle control because the mainshaft can decouple from the driveline. However, the transmission output shaft is permanently coupled to the drive axles and rotating wheels. When transitioning from Low Range (3.5:1 reduction) to High Range (1:1 direct), the relative rotational speed between the auxiliary drive gear and the output shaft changes by a factor of 3.5 instantaneously.

Without a mechanical synchronizer, forcing a sliding clutch into engagement across this extreme speed differential while rolling would cause violent gear clash, shear clutch teeth, and fracture transmission shafts.

Synchronizer Anatomy and Synchronizing Cycle

The Eaton Fuller range synchronizer is a pin-type assembly:

• LO Range Synchronizer Ring: The larger ring, with friction material that contacts a cup on the LO-range gear.
• HI Range Synchronizer Ring: Joined to the LO ring by blocker pins; three springs sit under pressure at the HI-range pin locations (Eaton warns to cover the assembly with a rag during disassembly so they are not lost).
• Sliding Clutch: Rides on the pins between the two rings and carries the clutching teeth.
• Overhaul Inspection (Eaton): Check for burrs, uneven or excessive wear at the contact surfaces, and metal particles. Check blocker pins for excessive wear or looseness, and check the synchronizer cups for wear. Excessively worn friction material causes range grind.

During a range shift, the pneumatic shift cylinder applies axial thrust to the synchronizer sleeve:

1. Frictional Engagement: The blocker ring friction cone makes initial contact with the mating gear taper. The friction between the cone and cup immediately begins matching the rotational speed of the auxiliary drive gear to the output shaft.
2. Blocking Action: The torque created by the speed differential clocks the blocker pins within their chamfered holes, mechanically blocking the sliding clutch sleeve from advancing toward the clutching teeth.
3. Index and Lockup: The instant the friction surfaces equalize the speeds (synchronous speed achieved), frictional torque drops to zero. The chamfered edges on the sleeve push the blocker pins into alignment, allowing the sliding clutch collar to glide smoothly over the clutching teeth without a sound.

---

4. Splitter Section Operation: Direct vs. Overdrive Ratios

While a range section doubles the main-section ratios with one large high/low step, a splitter divides individual gear steps into smaller intermediate steps.

             TYPICAL 18-SPEED GEAR STEPPING (SPLITTER + RANGE)

  Ratio:  1st-Dir   1st-OD    2nd-Dir   2nd-OD    3rd-Dir   3rd-OD ...
  RPM:    [======]  [======]  [======]  [======]  [======]  [======]
  Step:     half     half      half      half      half      half
  (Each split is roughly half a full gear step, keeping the engine in its torque band)

The Operational Purpose of the Splitter

Heavy-duty diesels are built to pull hard in a fairly narrow rpm band. On steep grades or with heavy loads, a full gear step can drop engine speed below that band, costing boost and road speed. The splitter lets the driver shift in smaller steps — roughly half a full gear step — so the engine stays in its best torque range.

Splitter Mechanical Configuration and Power Flow

The splitter gearing is integrated into the front of the auxiliary section:

• Splitter Gear: Mounted on the mainshaft behind the main box rear wall.
• Splitter Countershaft Gears: Meshed constantly with the splitter gear on the auxiliary countershafts.
• Splitter Sliding Clutch: An unsynchronized double-sided sliding dog clutch driven by a pneumatic splitter shift cylinder. Unlike the range section, the splitter does not use a brass-cone synchronizer; it relies on floating dog-tooth engagement executed when driveline torque is momentarily relieved by throttle dip or clutch depression.

In an overdrive-split design, the rearward (direct) splitter position lets torque pass at the direct split, and the forward position routes torque through the splitter gear set to produce the overdrive split. The exact gear paths and ratios depend on the model, so check the power flow diagram in the OEM service manual.

Comparing 13-Speed and 18-Speed Transmission Architectures

---

5. Auxiliary Countershaft Timing and Torque Equalization

Like the main section, the auxiliary section uses twin countershafts 180 degrees apart to share torque between two gear meshes, so its gear sets must also be timed at assembly.

                   AUXILIARY COUNTERSHAFT TIMING

                 Aux Countershaft 1 Drive Gear
                     [Marked Tooth: 1]
                            |
                            v
  Aux Drive Gear -> [ : Marked Teeth : ] <- Tooth Meshes Must Align Exactly
                            ^
                            |
                     [Marked Tooth: 2]
                 Aux Countershaft 2 Drive Gear

Timing Mark Verification During Assembly

Eaton requires both countershafts of the auxiliary section to be timed, just like the front section, so the load is shared evenly. Which gear set is timed depends on the model: the LO-range, deep-reduction, or splitter gear set. The method is the same as the front section:

1. Marked Tooth on Each Countershaft Gear: A single marked tooth on each auxiliary countershaft gear of the timed set.
2. Two Marked Pairs on the Mating Gear: Two adjacent teeth are marked, then the two adjacent teeth directly opposite, with an equal number of unmarked teeth between the pairs.
3. Meshing: Each countershaft's marked tooth meshes between one marked pair when the auxiliary section is assembled.

Consequences of Incorrect Auxiliary Timing

If a technician installs the auxiliary section without verifying timing marks or misaligns the countershaft gears by even a single tooth:

• One countershaft will absorb 100% of the drive torque while the opposite countershaft freewheels with tooth clearance.
• The overloaded countershaft gears will experience immediate tooth pitting, gear face spalling, and catastrophic tooth fracture under heavy engine torque.
• Severe radial side-thrust will be introduced to the output shaft, causing rapid bearing failure, excessive operating temperature, and audible gear howl during acceleration.