Chapter 25

Steering and

Alignment

Objectives (1 of 2)

• Identify the components of the steering system of a heavy-duty truck.

• Describe the procedure for inspecting front axle components for wear.

• Explain how toe, camber, caster, axle inclination, turning radius, and axle alignment affect tire wear, directional stability, and handling.

• Describe the components and operation of a worm and sector shaft and a recirculating ball-type steering gear.

• Explain how to check and adjust a manual steering gear preload and backlash.

Objectives (2 of 2)

• Identify the components of a power steering gear and pump and explain the operation of a power steering system.

• Describe the components and operation of a pneumatic steering system.

• Describe the components and operation of an electronically variable power steering system.

• Describe the components and operation of a load sensing power assist steering system.

• The steering system in a heavy-duty truck is expected to:– Deliver precise directional

control of the chassis at both gross and unloaded vehicle weight

– Be able to minimize driver effort while retaining some road feel

• Truck steering systems can be either manual or power-assisted.

• Some hydraulic power-assisted systems are available with electronic or load sensing controls.

• Coupled to the steering column upper shaft by a pair of yokes and the U-joint assembly is the lower shaft assembly.

• The steering column assembly usually supports some other switch and control components. This helps keep the driver’s hands nearby.

• A multifunction stalk switch, which contains the turn signal switch at a minimum, is usually mounted on the left side of the steering column.

• If the vehicle is equipped with a trailer service brake control valve, it is clamped onto the right side of the steering column.

Pitman Arm• A Pitman arm is a steel lever,

splined to the sector (output) shaft of the steering gear.

• The end of the Pitman arm moves through an arc with the sector shaft center forming its center.

• The Pitman arm functions to change the rotary motion of the steering gear sector shaft into linear motion.

• The length of the Pitman arm affects leverage and therefore steering response. – A longer Pitman arm will

generate more steering motion at the front wheels for a given amount of steering wheel movement.

Drag Link

• A drag link is a forged rod that connects the Pitman arm to the steering control arm.

• The drag link can be a one- or two-piece component.

• The length of two-piece design is adjustable, which makes it easy to center the steering gear with the wheels straight ahead.

• The drag link is connected at each end by ball joints. These ball joints help isolate the steering gear and Pitman arm from axle motion.

Steering Column Arm

• The steering control arm connects the drag link to the steering knuckle on the driver side of the vehicle.

• A steering control arm is also known as a steering arm, a control arm, and a steering lever.

• When the drag link is moved in a linear direction, the steering control arm moves the steering knuckle, which changes the angle of the steering knuckle spindle.

• Steering knuckles mount to the front axle by kingpins or knuckle pins.

• Kingpins provide the ability to steer the vehicle.

• Spindle– The steering knuckle

incorporates the spindle onto which wheel bearings and wheel hubs are mounted, plus a flange to which the brake spider is bolted.

• A steering control arm is attached to the left side steering knuckle.

• Ackerman arms are attached to both steering knuckles.

King Pins• Tapered

– Drawn into the axle center and secured by tightening a nut at the upper pin end

– Usually sealed and may not require periodic lubricating

• Straight– Secured to the axle with

tapered draw keys that bear against flats on the pin

– Have a cap on either end (top and bottom) to retain grease.

– Zerk-type grease fittings are used to lubricate steering knuckles.

• Lubed for life– On newly introduced unitized

steer axles, the steering knuckle is lubed-for-life; no attempt should be made to lubricate these.

Ackerman Arm

• An Ackerman or tie-rod arm is the means used to transfer and synchronize steering action on both steer wheels on a steering axle.

• Ackerman arms are drop-forged, tempered steel levers that are angled to define the steering geometry required on turns.

• One end of an Ackerman arm is keyed and bolted to the lower portion of the steering knuckle.

• The other end is taper bored to allow a tie-rod ball stud to be clamped to it.

Front-end Alignment (1 of 2)

• A front end that is properly aligned will result in:– Easier steering – Longer tire life – Directional stability – Less wear on front-end components – Better fuel economy – Increased safety

Front-end Alignment (2 of 2)

• The primary alignment angles are:– Toe– Caster – Camber – Kingpin inclination– Turning angle

Toe (1 of 2)

• The ideal toe angle when a vehicle is running loaded down a highway is zero.

• We set toe angles statically. – The objective of setting toe at a specified angle when

aligning the front end is to have zero toe at highway speeds.

Toe (2 of 2)

• Incorrect toe angles not only accelerate tire wear but also can have an adverse effect on directional stability of the vehicle.

• Incorrect toe angles have the potential to cause more front tire wear than any other incorrect alignment angle.

• Too much toe-in produces a scuffing, or a featheredge, along the inner edges of the tires.

• Excessive toe-out produces a similar wear pattern along the outer edge of the tires.

• When a fully loaded vehicle is moving at highway speeds, there is a slight tendency of steering tires to toe-out.

• Any looseness in the steering linkage and tie-rod assembly also will contribute to the toe-out tendency.

Camber

• Steering tires also are designed to use a positive camber angle setting.

• We will take a closer look at camber later but, because it influences the toe setting, we will define it here.

• Camber is a measure of the angle a wheel leans away or toward the frame. – Positive camber means that the tires lean away from

the truck frame at the top. – A positive camber setting is used to help compensate

for that slight tendency of steering tires to toe-out when the vehicle is moving.

Measuring Toe (1 of 2)

• First check kingpin inclination, camber, and caster. Correct, if necessary.

• You should not make an adjustment to toe angle until the other factors of front-wheel alignment are known to be within specifications.

• Adjustment of toe angle or dimension requires lengthening or shortening the tie-rod dimension. – This is achieved by

loosening the tie-rod end clamp bolts and then rotating the cross tube.

Measuring Toe (2 of 2)

• Neutralize the suspension first.– When measuring toe angle, the front suspension

should be neutralized: • To neutralize the suspension, roll the vehicle back and

forth about a half vehicle length. This relaxes the front suspension and steering linkages.

• Neutralizing the front suspension is important before making front-end adjustments, especially if the vehicle has been jacked up on either side to scribe the tires.

• This operation causes the front wheels to angle as each is returned to the floor.

• Make sure that the wheels are on the ground and fully supporting the vehicle weight. Measure and record the measurements.

Shop Talk

• When tie-rod ends are replaced, they must be threaded into the tie-rod cross tube sufficiently so that clamp pressure is applied directly over the threads under the clamp. The threaded end should extend past the slot in the cross tube.

• CAUTION: Recheck the toe setting after any change in caster or camber angle.

Deep Drop Tie-Rod Ends

• Toe-in tends to increase slightly on some steer axles when loaded and stationary.

• It is therefore important that vehicles with set-back axles and deep drop tie-rod ends are given some special attention.

• Because of the turning geometry that results from set-back and deep drop tie-rod end axles, the tie-rod end clamps if facing forward can interfere with the axle I-beam on a full lock turn. – This interference will not

necessarily impede steering control, but it can reduce the effective turn angle of the vehicle.

Caster (1 of 2)

Caster (2 of 2)

• Caster is the forward or rearward tilt of the kingpin centerline when viewed from the side of the vehicle.

• Zero caster occurs when the centerline of the kingpin is exactly vertical.

• Positive caster indicates the kingpin is tilted rearward.

• Negative caster indicates that the kingpin is tilted forward.

• Caster is a directional stability angle only. Incorrect caster by itself will not affect tire wear.

• Most heavy-duty trucks are designed with some degree of positive caster.

Positive Caster

• Positive caster creates a force in the front wheels, which tends to keep them tracking straight ahead.

• Positive caster tends to make the steering axle wheels want to return to a straight ahead position.

• Positive caster also means that when the front wheels of the truck are turned, one side of the vehicle raises slightly and the other side is lowered.

• When the steering wheel is released, the weight of the vehicle forces the lifted side downward, resulting in the wheels returning to a straight-ahead position.

• Caster settings generally affect steering performance in the following ways:– Too little caster can cause wheel instability, wandering, and poor wheel

recovery. – Too much caster can result in hard steering, darting, oversteer, and low

speed shimmy.

Caster Recommendations• Examples are measured

weight down on a level floor: – Tandem drive axle: ½–1½

degrees positive– Single drive: 1½–2 ½ degrees

positive– No more than 1/2 degree

difference between the left and right wheels

– Positive caster on the left wheel should not be greater than on the right.

• Caster specifications are based on vehicle design load which will usually result in a level frame.

• If the frame is not level when alignment checks are made, this must be factored in the caster measurement.

Twisted Axles

• In a few cases, steering problems can be traced to a twist in the front axle. A twisted steer axle can be caused by an actual structural twist, or one induced by improper use of caster shims.

• A twisted axle should be suspected whenever: – The difference in caster angle exceeds 1/2 degree

from side to side – The caster shims in place differ by 1 degree or more – A low speed shimmy exists, and there is no evidence

of looseness elsewhere in the steering system – A leading or darting condition persists

Camber

• Excessive positive camber causes the tire to wear on its outside shoulder.

• Excessive negative camber causes the tire to wear on its inside shoulder.

• Unequal camber in the front wheels also can cause the steering to lead to the right or left.

• The truck will lead to the side that has the most positive camber.

Kingpin Inclination (KPI)

• The amount that the top of the kingpin inclines away from vertical as viewed from the front of the truck

• In conjunction with camber angle, places the approximate center of the tire tread footprint in contact with the road

• Reduces steering effort and improves the directional stability

• Cannot be adjusted in trucks– Once set, KPI should not

change unless the front axle has been bent.

• Corrections or changes accomplished by replacement of broken, bent, or worn parts

Turning Angle or Radius

• Turning angle or radius is the degree of movement from straight-ahead to either an extreme right or left position.

• Two factors limit the turning angle.– Tire interference with the

chassis and steering gear travel

• To avoid tire interference or bottoming of the steering gear, there are adjustable stop screws on the steering knuckles

• Turning radius or angle should be checked using the radius gauge.

Shop Talk

• The power steering gear pressure relief valve should open just before the steering stop screw contacts the axle stop.

• You may have to adjust the power steering gear so that power-assist stops approximately 1 degree before the steering stops contact. – Failure to do this will result in slamming of the

steering stops on full lock turns.

Ackerman Geometry (1 of 2)

• Ackerman geometry is the means used to steer a vehicle so that the tires track freely during a turn.

• During a turn, the inboard wheel on a steer axle has to track a tighter circle than the outer wheel.

• Ackerman geometry is also known as toe-out during turns. It allows the inner and outer wheel to turn at different angles so that both wheels can negotiate the turn without scrubbing.

Ackerman Geometry (2 of 2)

• Toe-out on turns is accomplished by having the ends of lower steering arms (those that connect to the tie-rods) closer together than the kingpins.

• Actual toe-out during a turn depends on the length and angle of the steering control arms and the length of the cross tube.

• Even if the toe-in setting with the wheels in a straight-ahead position is correctly adjusted, a bent steering arm can cause the toe-out on a turn to be incorrect, causing tire scuffing.

Axle Alignment (1 of 2)

• All of the axles should be perpendicular to the vehicle’s centerline.

• The thrustline thus created is parallel to the vehicle centerline.

• If they are not positioned perpendicular to the vehicle centerline, the rear wheels will not track directly behind the front wheels, and the thrustline of the rear wheels deviates from the centerline of the vehicle.

• The steering fights the vehicle thrustline, resulting in an un-centered steering wheel and accelerated front tire wear.

Axle Alignment (2 of 2)

• On a single-axle vehicle, the rear-axle thrustline can be off if the entire axle is offset or if only one wheel has an improper toe angle.

• On a tandem axle, there are a number of different combinations that can cause incorrect tracking.

• One method of checking a single axle for misalignment is to clamp a straightedge across the frame so that it is square with the frame rails.

• Measure from the center of the hub to the straightedge.

• The distances on each side should be within 1/8 inch of each other.

Trailer Tracking

• It is also possible for the trailer axles to be out of alignment and cause a tracking problem.

• Depending on the severity of the trailer misalignment, it might be possible to see the effects of the misalignment as the trailer travels down the road.

• Usually, the trailer will travel at an angle to the tractor.

• Misalignment also makes it very hard to back up the trailer.

• This is commonly called dog- tracking.

Axle Offset

• Another problem is an axle that is not centered with the centerline of the vehicle.

• When an axle is offset and the vehicle is driven straight down a highway, the steering wheel should be centered and the vehicle will not dog-track. However, as soon as it is cornered, it will oversteer in one direction and understeer in the other.

Electronic Alignment Equipment

• Wheel/axle alignment measurements can be taken in a number of ways.

• A number of electronic alignment systems exist.

• Electronic alignment systems use computers to analyze and display alignment conditions.

• The electronic methods include: – Light beam alignment – Laser alignment

Beam Alignment Systems

• Laser beam equipment projects light beams onto charts, scales, and sensors to measure toe, caster, and camber.

• The wheel-mounted light projectors can be focused to any length wheelbase and to check alignment angles.

• Two- and four-wheel models are available.

• Mirrors redirect the light beams from the projector back onto scales mounted in the rear toe boxes of the projectors.

Sensor/Computer Alignment (1 of 4)

• This shows an initial analysis made by Hunter WinAlign software as displayed by a Windows driven PC.

• WinAlign automatically calculates the correction for the technician.

• As the adjustment is made, the arrow moves across the bar graph target guiding the technician.

Sensor/Computer Alignment (2 of 4)

• Sensors are mounted at each wheel for fast, precise alignment.

• Alignment readings, specifications, and step-by-step instructions are displayed on a display monitor.

• Keyboard-entered specifications are automatically compared against the actual angles of the vehicle, with the results displayed on the display screen.

• Specifications can be retained in computer memory for future use or on CD.

• As adjustments are made on the truck, these are automatically displayed on the monitor, enabling a high degree of precision.

Sensor/Computer Alignment (3 of 4)

• When the adjustment is within spec, the bar graph changes from red to green.

• A typical system provides for:– Four-wheel alignment with

four sensors– Two-wheel alignment with two

sensors• All wheels are aligned to a

common centerline for precise alignment.

• By moving instruments, the system can also check both rear axles of a tandem drive axle, as well as the front steering axle.

Sensor/Computer Alignment (4 of 4)

• Like the light beam system, computer-controlled systems can be adapted to align trailer axles by using a gauge bar attached to the trailer kingpin, offering a much greater degree of precision.

• Computerized alignment systems make truck alignments an exact science.

• They will also measure and display frame offset angles.

• This allows technicians to true truck and trailer chassis and suspensions.

Warning

• All steering mechanisms are critical safety items.

• A vehicle should be deadlined (OOS report) when a defect is reported.

• It is essential that instructions in the service literature are adhered to. – Failure to observe these procedures may

result in loss of steering with life-threatening results.

Warning

• When a vehicle is operated at temperatures below 30°F with SAE 90 weight oil in a manual steering gear, it can turn to a grease-like consistency, resulting in stiff, sluggish steering.

• This can compromise accident avoidance maneuvers because of slow steering response.

• When operating in temperatures continuously below 30°F, install a lighter oil in manual steering gear, such as SAE 75 weight oil.

Shop Talk

• Before performing the remaining checks, apply the parking brakes and chock the rear tires.

• Raise the vehicle until the front tires leave the pavement and then place safety stands under the frame rails.

• Be sure that the stands will support the weight of the vehicle.

Caution

• Before performing any servicing procedure on the front suspension, set the parking brake and block the drive wheels to prevent the vehicle from moving.

• After jacking the truck up until the steering axle wheels are raised off the ground, support the chassis with safety stands.

• Never work under a vehicle supported only by a jack.

Warning

• Do not drive the vehicle with too much lash in the steering gear.

• Excessive lash is a sign of an improperly adjusted steering gear or worn or otherwise damaged steering gear components.

• Driving the vehicle in this condition could result in a loss of steering control.

Manual Steering Gears

• A steering gear is often referred to as a steering box. – It is an assembly of gears contained within a

housing. – Manual steering gears have lubricant within

the housing. • Two basic types of manual steering gears are

used in heavy-duty trucks.– Worm and sector shaft type – Recirculating ball and worm type

Recirculating Ball Gears

• Mounted on the worm gear is a ball nut with internal spiral grooves containing recirculating ball bearings.

• The ball bearings transmit the turning force from the worm gear to the ball nut, causing the ball nut to move up and down the worm gear.

• Ball return guides are connected to each end of the ball nut grooves. These allow the ball bearings to circulate in a continuous loop.

• The ball nut also has exterior gear teeth that mesh with the sector gear.• As the ball nut moves up or down on the worm gear, it causes the sector gear

to rotate, which, in turn, causes the Pitman arm to swivel back and forth.

Power-assist Steering Systems

• A power steering system must default to manual operation in the event of power-assist circuit failure.

• Truck hydraulic power-assisted steering systems contain a dedicated pump that delivers hydraulic fluid under pressure to a power-assist steering gear.

• In a gear such as this, the hydraulic control circuit is located within the steering gear so it is known as an integral system.

Power Steering Pump

• The power steering pump is used to produce the hydraulic circuit flow required for the power-assist to the steering gear.

• They can be either gear- or belt-driven, depending on the engine application, but the majority are gear-driven by an engine accessory drive.

• Power steering pumps are usually mounted near the front of the engine.

• Most power steering pumps are similar in construction.

Pump Operation

• Centrifugal force and hydraulic pressure loads the vanes outward against an oval cam ring.

• As the rotor turns, the volume between vanes changes.

• When they move past the inlet port, the volume increases; oil is drawn in.

• As they approach the discharge port, volume is reduced, creating pressure rise and oil is discharged.

Flow Control and Pressure Relief Valves

• The flow control valve regulates pressure and flow output from the pump to provide for consistent power-assist during variations in engine rpm.

• When the engine speed increases, the pump can deliver more flow than is required to meet system requirements.

• When outlet pressure reaches a preset level, the pressure relief ball is forced off its seat, creating a greater pressure differential at the two ends of the flow control valve.

• This allows the flow control valve to open wider, permitting more pump pressure to flow back to the pump inlet, and pressure is held at a safe level.

Power Steering Gears

• Power steering gears are similar to a manual recirculating ball steering gear with the addition of a hydraulic assist control mechanism.

• The power steering gearbox is charged with hydraulic fluid under pressure and uses a rotary control valve to control the flow of fluid.

• The movement of the ball nut is assisted by hydraulic pressure.

• The integral-type power steering has the rotary control valve and a power piston integrated with the gearbox.

• The rotary valve directs the oil pressure to the left or right chamber to steer the vehicle.

• The spool valve is actuated by a lever or a small torsion bar located in the worm gear.

Hydraulic Operation

• The rotary control valve is an open-center type that allows a continuous flow of oil when held in the neutral position by the torsion bar.

• When steering effort is applied, the spring action of the torsion bar results in the input shaft rotating slightly in advance of the ball screw.

• The six pairs of grooves that form the rotary control valve are displaced from their neutral flow position.

• As steering effort increases, so does the amount of displacement. • Depending on the direction steered, the groove displacement of the input shaft

directs hydraulic oil through the appropriate drilled passages in the ball screw to one side or the other of the piston.

Troubleshooting Safety

• Always block vehicle wheels. Stop the engine when working under a vehicle. Keep hands away from pinch points.

• Never connect or disconnect a hose or line containing pressure. Never remove a component or pipe plug unless all system pressure has been depleted. The pressures used in truck hydraulic power-assist steering systems are massive and can peak at 1,500 psi.

• Never exceed recommended pressure and always wear safety glasses. Never attempt to disassemble a component until after reading and understanding recommended procedures.

• Use only OEM-suggested replacement components. Only components, devices, and mounting and attaching hardware specifically designed for use in hydraulic systems should be used. Replacement hardware, such as hoses and fittings, should be of equivalent size, type, and strength as the original equipment.

• Devices with stripped threads or obvious physical damage should be replaced. Repairs requiring machining should not be attempted.

Caution

• As with manual steering gear, the Pitman arm should NEVER be removed from the sector shaft to center the steering. – In some instances, this could require

disassembly of the steering gear.

Shop Talk

• A power steering analyzer is the preferred method of assessing the performance of a hydraulic power-assist steering circuit.

• The power steering analyzer consists of hoses, quick release couplers, a flowmeter, pressure gauge, and flow control valve.

Electronically Variable Steering

• Optimized power steering control has the following features: – Improved steering wheel

road feel

– Directional stability enhanced by a steering effort proportional to driving speed

– Reduced dry park effort at the steering wheel

– Reduced system power consumption

EVS Pump Design

• The EVS steering system uses a solenoid-controlled variable-flow control concept.

• The pump assembly includes a high-efficiency vane pump, flow control, and proportional solenoid-type throttle valve.

• The system is designed with fail-safe features. – When the solenoid is not energized, flow is low

(at around 60 percent of its normal capacity), similar to conventional power steering systems.

EVS Electronic Control Unit (ECU)

• The purpose of the electronic controller is to change the PWM actuation signal to the pump solenoid with respect to the road-speed sensor signal.

• The controller incorporates switches that allow for three potential operating modes with an LED display to assist in troubleshooting. – ECONOMY MODE: In economy mode, flow control is at maximum

below 15 mph (24 km/h); at its minimum, above 50 mph (80 km/h); and it decreases proportionally from 15 to 50 mph.

– NORMAL MODE: The normal mode has high flow at low speeds and decreases as speed increases (similar to economy mode), except the minimum flow is set at 13.6 L/min and is reached at about 40 mph (64 km/h). In this mode, the steering system acts much like a conventional power steering system with 13.6 L/min flow control.

– STANDBY MODE: If the vehicle is stationary for longer than 10 minutes, the system switches into a standby mode. This mode reduces flow to minimum power-assist as long as the speedometer indicates zero. When road speed is detected, the system automatically switches to its original mode.

PWM Actuation

• The PWM actuation circuit operates the solenoid valve by a fixed-frequency, pulse-width-modulated signal with closed-loop duty cycle control that uses the solenoid current as feedback.

• This ensures that the solenoid positioning is relatively constant with changes in vehicle battery voltage.

• It also allows the controller to provide current to the solenoid directly from the battery without need for high-current voltage regulators.

Load-sensing Power Steering• HYDRAULICALLY

CONTROLLED: This non-electronic hydraulically controlled load-sensing system has a demand-type flow control that uses steering load pressure to control its variable-flow power-assist.

• SPEED PROPORTIONING: This system provides for speed proportioning of flow to the steering gear from a flow-controlled pump.

• Instead, the system relies on a control strategy in which the pressure requirement for power steering is a function of vehicle speed.

• The load-sensing system is similar to a conventional hydraulic power-assist circuit, except that a modification has been made to the flow-control section of the pump.

Combination Flow/Relief/Load-sensing Valve

Summary (1 of 6)• Steering systems used in trucks must deliver precise

directional control of the vehicle and its load, in both loaded and unloaded conditions, and at highway and park/stall speeds.

• Truck steering systems are either manual or power-assisted. – Power-assist systems are required to default to

manual operation in the event of a loss in the power-assist circuit.

– Power-assist systems can use either hydraulic or air-assist circuits.

• Improper steering adjustments and front-end alignment can lead to suspension and tire wear problems.

Summary (2 of 6)

• A properly aligned front-end results in:– Easier steering

– Increased tire life

– Directional stability

– Less wear and maintenance on front-end components

– Better fuel economy

– Increased safety

• Ackerman geometry provides toe-out on turns, permitting tires to roll freely during turns when each travels through a different arc.

Summary (3 of 6)

• Axle alignment measurements can be taken in a number of ways. – Tram gauges and measuring tapes may be used as

can light or laser beam alignment systems with computerized sensors and analysis.

• The most accurate and easiest to use alignment systems in use today are computer-controlled and feature in-memory specifications, step-by-step instructions, and user-friendly displays. Keyboard-entered specifications are automatically correlated to the actual angles measured on a vehicle, with the results displayed on the monitor screen.

Summary (4 of 6)

• Steering axle components should be inspected and lubricated routinely on a preventive maintenance schedule.

• Two general types of manual steering gear are used in heavy-duty trucks. – They are the worm and sector shaft type and the

recirculating ball and worm type. • Some worm shaft preload is necessary to prevent

unwanted random movement within a steering system. • Two types of steering gear preloads have to be

checked.– Worm bearing preload and total mesh preload

Summary (5 of 6)

• Insufficient preload allows some lost motion in the gearset, requiring the driver to turn the steering wheel farther to get a steering response from the front wheels. – Excessive preloads result in hard steering, darting,

and oversteer. • An integral power-assist steering system contains a

pump that delivers hydraulic fluid under pressure to a power steering gear. – The steering gear has an integral hydraulic control

valve that measures steering effort and responds to provide the appropriate amount of assist in turning the wheels.

Summary (6 of 6)

• A power-assist steering gear is fundamentally similar to a manual recirculating ball steering gearbox with the addition of a hydraulic assist: It must default to manual operation in the event of a hydraulic circuit failure.

• Air-assisted steering systems use the vehicle compressed air supply as a means of supplementing manual steering operation. This is an external system consisting of an air torque valve contained in a dedicated drag link assembly and a double-acting power cylinder.