3.2 Steering Gear Adjustments: Over-Center Lash, Preload & Poppets

Integral Power Steering Gear Architecture: Mechanics & Valving

Commercial integral power steering gears combine the manual recirculating ball mechanical reduction gear, a rotary hydraulic control valve, and a double-acting hydraulic power cylinder inside a single heavy-duty ductile iron housing. The two most prominent manufacturers of heavy-duty integral steering gears in North America are TRW Commercial Steering Systems (TAS series: TAS40, TAS65, TAS85) and R.H. Sheppard Co. (M-series: M80, M100, M110, and D-series).

                  Integral Steering Gear Major Flow Path
  Input Shaft (Rotary Valve) ──► Worm Shaft ──► Recirculating Balls
                                                      │
  Pitman Arm ◄── Sector Shaft (Tapered Teeth) ◄── Rack Piston
                                                      │
  Pressure Chamber [A] ◄── Internal Poppet Valves ──► Pressure Chamber [B]

Primary Internal Subassemblies

1. Rotary Spool Control Valve & Torsion Bar: The steering gear input shaft connects to a hollow valve spool housed inside a precision valve sleeve. A calibrated spring-steel torsion bar runs through the center, pinned to the input shaft at the top and the worm shaft at the bottom. When the driver turns the steering wheel, mechanical drag twists the torsion bar (typically 2° to 7° of rotational deflection). This slight deflection shifts the valve spool relative to the sleeve, aligning precision metering grooves that port high-pressure fluid to one side of the rack piston while opening the opposing side to the return line. When steering input ceases, the torsion bar springs back to neutral, centering the valve and routing fluid directly to the reservoir at low pressure.
2. Worm Shaft & Recirculating Ball Circuit: The worm shaft features precision-ground helical ball grooves. Chrome-alloy steel ball bearings circulate continuously through internal guide tubes between the worm shaft and the rack piston. This arrangement converts input shaft rotation into low-friction, high-force axial movement of the rack piston.
3. Rack Piston & Sector Shaft: The rack piston acts as both a hydraulic piston and a mechanical gear rack. Precision gear teeth cut into the side of the rack piston mesh with the sector shaft gear teeth. The sector shaft is supported by heavy needle roller bearings or bronze bushings and exits the housing to drive the splined pitman arm.

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Adjustment 1: Input Shaft Worm Bearing Preload

The worm shaft is supported at both ends by tapered roller bearings or heavy-duty angular-contact thrust ball bearings to absorb the massive axial thrust generated as the worm drives the rack piston.

Purpose & Engineering Mechanics

• Eliminating Thrust End Play: Worm bearing preload eliminates all axial movement (end play) of the worm shaft while maintaining a calibrated rolling resistance.
• Consequences of Loose Preload: If bearing preload is loose, the worm shaft moves axially back and forth inside the housing before transferring rotary motion to the rack piston. This creates severe steering wheel free play (deadband) and delays the deflection of the rotary control valve.
• Consequences of Overtight Preload: Overtightening exerts excessive compressive force on the bearing rollers, causing roller brinelling, race spalling, stiff steering, and rapid bearing failure.

Step-by-Step Measurement & Adjustment Procedure

1. Draining & Disengagement: Drain hydraulic fluid from the gear. Loosen the sector shaft adjuster screw lock nut and back out the sector shaft adjuster screw completely (2 to 3 turns) to completely disengage sector-to-rack tooth contact. This ensures gear tooth mesh friction does not corrupt the bearing preload measurement.
2. Tooling: Attach an accurate inch-pound dial-indicating torque wrench (0 to 30 or 0 to 50 in-lb range) to the input shaft spline using an adapter socket.
3. Dynamic Torque Measurement: Rotate the input shaft smoothly through 360 degrees off-center. Record the steady dynamic rolling torque required to keep the shaft rotating (ignore initial breakaway torque).
4. Specification & Adjustment: Target rolling torque is typically 4 to 12 in-lb (0.5 to 1.4 Nm), depending on gear model. Turn the threaded input bearing adjuster plug with a specialized spanner socket to adjust preload. Once in specification, tighten the outer spanner lock nut to OEM torque (typically 150 to 250 ft-lb) while holding the adjuster plug stationary, and re-verify rolling torque.

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Adjustment 2: Sector Shaft Over-Center Mesh (Lash Adjustment)

The mesh between the sector shaft teeth and the rack piston teeth is the most critical mechanical adjustment on a commercial steering gear.

                  Over-Center Tooth Taper Profiling
               [High-Point Center: Zero Lash / Preload]
                                  ▲
                                ┌─┴─┐
  [Off-Center Left: +0.010" Lash] │ [Off-Center Right: +0.010" Lash]
  ◄───────────────────────────────┴───────────────────────────────►

The Geometry of Over-Center Tooth Profiling

Why commercial steering gears are cut with an over-center high point:

• Crowned Tooth Design: The gear teeth on the sector shaft and rack piston are machined with a progressive taper. The center tooth of the sector shaft is slightly thicker than the outer flank teeth.
• Straight-Ahead Precision: At the exact straight-ahead center position (the "high point"), tooth clearance is engineered to be zero lash (or a slight interference preload of 0.0005" to 0.001"). This zero-clearance zone prevents vehicle wandering, eliminates chucking, and provides firm directional stability during highway cruising.
• Off-Center Clearance: As the gear rotates off-center into a turn, tooth clearance progressively increases by 0.008" to 0.015". This prevents tooth binding across the turning sweep and accommodates normal frame rail deflection and road shocks.
• MANDATORY RULE: Sector shaft lash MUST be adjusted at the EXACT center high-point position! If adjusted off-center, the wider center tooth will violently bind and jam as it crosses through straight ahead.

Step-by-Step Over-Center Adjustment Protocol

1. Mechanical Isolation: Disconnect the drag link from the pitman arm. The steering gear must be completely isolated from kingpin friction, tie rod friction, and tire scrub resistance.
2. Locate Exact Center High Point: Rotate the input shaft smoothly from full lock to full lock, counting total revolutions (e.g., exactly 4.2 turns). Divide by two (2.1 turns) and rotate the input shaft back to exact geometric center. Verify that the alignment timing marks on the sector shaft and housing face are aligned.
3. Back Out Lash Screw: Loosen the sector shaft adjuster screw jam nut. Back out the sector adjuster screw counterclockwise 2 full turns.
4. Initial Contact: Gently turn the adjuster screw clockwise until light tooth contact is felt.
5. Torque-to-Turn Measurement Sweep: Attach an inch-pound dial torque wrench to the input shaft. Sweep the input shaft approximately 90 degrees to each side of the center position.
6. Set Over-Center Drag:
   • Note the off-center rolling torque (e.g., 6 in-lb).
   • Gradually tighten the adjuster screw until the peak rolling torque sweeping across the center high point increases by the manufacturer's specified over-center drag (typically +4 to +8 in-lb above off-center drag, yielding a total high-point rolling torque of 10 to 18 in-lb).
7. Lock Nut Torquing: Hold the adjuster screw stationary with an Allen hex key to prevent it from turning, and torque the jam nut to specification (typically 40 to 50 ft-lb). Re-sweep across center with the torque wrench to ensure tightening the nut did not alter the adjustment.

Diagnostic Consequences of Lash Misadjustment

• Excessive Lash (Adjustment Too Loose): Causes vehicle wandering, road darting, constant steering wheel correction, driver fatigue, and excessive free play at the steering wheel rim. Federal Motor Carrier Safety Regulation 49 CFR § 393.209(b)(1) caps steering wheel free play by wheel diameter and by system type: a 20-inch wheel is limited to 2 1/2 inches (64 mm) on a manual system and 5 1/4 inches (133 mm) on a power steering system, equivalent to roughly 14 and 30 degrees of rotation respectively. See Section 2.1 for the full table.
• Insufficient Lash (Adjustment Overtightened — Center Binding): Causes center-notch binding, a tight/stiff steering feel, accelerated tooth wear, and a critical LOSS OF RETURNABILITY. Positive caster angle normally creates self-aligning torque that pulls the front wheels back to straight ahead after completing a turn. Overtight sector mesh generates frictional resistance exceeding caster self-centering torque, forcing the driver to physically muscle the steering wheel back to center.

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Adjustment 3: Internal Poppet Valves (Hydraulic Unloader Valves)

Integral steering gears incorporate internal hydraulic poppet valves (also known as unloader valves or stroke-limiting valves) positioned within the rack piston.

                     Poppet Unloader Valve Operation
[Rack Piston Travel ──►] ───► [Poppet Plunger Contacts Housing Stop]
                                        │
                                        ▼ (Lifts Off Seat)
[High-Pressure Chamber] ──► Internal Bypass ──► [Low-Pressure Return Line]
   (Vents 2,400 PSI down to 300 PSI ~1/4" before axle stop contact)

Purpose & Engineering Mechanics

• Pressure Unloading Before Mechanical Stop: Poppet valves are designed to trip and vent high hydraulic pressure approximately 1/4 to 1/2 inch (or 1/8 to 1/4 inch before the axle stop bolt contacts the axle pad) before the steering linkage reaches full mechanical cramp.
• Component Protection: Relieves hydraulic circuit pressure from 2,200+ PSI down to approximately 200 to 400 PSI before mechanical stop contact.
  • Prevents the pump from deadheading against its relief valve at full lock, stopping fluid boiling and cavitation.
  • Prevents the gear from generating tens of thousands of pounds of mechanical force against axle stops, pitman arms, drag links, and steering knuckles, avoiding bent linkages, sheared stop bolts, and cracked gear housings.

Setting Procedures: Automatic vs. Manual Systems

1. Automatic Poppets (TRW TAS Series)

• Factory / Reman State: Replacement gears ship with internal poppet plungers fully extended (unset).
• Setting Protocol:
  1. Install the gear and connect all steering linkage. Ensure front axle stop bolts are set to the correct wheel-cut angle.
  2. Raise the front axle off the floor so steer tires clear the ground.
  3. Start the engine and run at idle.
  4. Slowly turn the steering wheel firmly to full lock in one direction until the axle stop bolt contacts the axle stop pad. Apply approximately 30 to 40 lbs of rim force against the steering wheel to firmly seat the poppet.
  5. Steer firmly to full lock in the opposite direction, repeating the procedure.
  6. Self-Setting Action: As the rack piston travels to full stroke, the extended poppet plungers strike the housing end covers and are pushed inward into their friction sleeves to the exact travel limit of that chassis.
• Poppet Reset Requirement: Automatic poppet plungers only push inward; they cannot self-extend. If axle stop bolts are backed out to increase wheel cut, or if the gear is moved to another chassis with greater wheel travel, the poppets must be manually pulled outward using a poppet reset tool (or serviced internally). Failure to reset poppets after adjusting axle stops results in a permanent loss of turning radius.

2. Manual Screw-Adjustable Poppets (Sheppard M-Series)

• Architecture: Sheppard gears utilize external slotted or hex poppet adjusting screws located on the front and rear covers.
• Adjustment Protocol: Turning the adjusting screws clockwise (inward) causes the poppet to contact the stop earlier, unloading hydraulic pressure earlier in the stroke. Backing the screw counterclockwise (outward) unloads pressure later in the stroke. Adjusted with a pressure gauge installed to confirm pressure drops right before the axle stop bolt touches the axle beam pad.

Diagnostic Symptoms of Poppet Misadjustment

• Poppets Tripped Too Early (Premature Unload): Hydraulic assist cuts out before full wheel cramp is achieved. The driver complains of sudden, heavy manual effort near the end of steering travel and a severely reduced turning radius (truck cannot make sharp docking turns or negotiate tight intersections).
• Poppets Tripped Too Late or Inoperative: Hydraulic pressure does not unload before the axle stop contacts the axle pad. At full wheel lock, the pump squeals loudly, engine idle drops, fluid overheats rapidly, and immense hydraulic pressure spikes bend drag links, flex frame rails, or snap knuckle steering arms.

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Steering Gear Mechanical & Hydraulic Adjustments Summary