Steering gear

The steering gear determines how precisely steering inputs are transmitted and how much power assistance is available under axle load. Excessive steering play, heavy steering when manoeuvring or uneven assistance call for targeted diagnosis: the cause may lie in the steering mechanism, but also in the hydraulic supply, steering column or kingpin bearings. On MAN and Mercedes trucks, buses and vans, the design and test procedure vary by version.

Heavy vehicles, such as MAN TGS and TGX, Mercedes Actros and Arocs, generally use hydraulically assisted recirculating-ball steering boxes. A steering worm with recirculating balls moves the power piston; its teeth rotate the sector shaft and pitman arm. A torsion-bar-controlled valve regulates the power assistance. Vans such as MAN TGE and Mercedes Sprinter generally have rack-and-pinion steering gears, with hydraulic or electromechanical assistance depending on the generation and version. On buses, including MAN Lion’s City and Mercedes Citaro, the axle configuration and any additional steering circuits are also determining factors.

In a rack-and-pinion steering gear, a pinion on the steering input shaft meshes with a rack. This converts the rotational movement of the steering wheel into linear movement of the rack, which turns the road wheels via the track rods. With hydraulic assistance, a control valve directs oil to the appropriate side of a power piston on the rack. With electromechanical assistance, an electric motor provides additional steering force based on factors including the measured steering torque.

Common steering gear brands include ZF, Bosch, RBL and TRW. The brand alone does not determine interchangeability; the version and specifications must match the vehicle.

The designations 6x4, 6x6, 8x4 and 8x8 describe the wheel positions and drive configuration, but do not fully define the number of steered axles. A 6x4 generally has one steered front axle; a 6x6 also has front-axle drive. On an 8x4 or 8x8, two front axles are often steered, but other axle layouts exist. Two steered axles require matching steering geometry and force distribution, potentially with auxiliary cylinders. The steering gear, steering ratio and hydraulic supply must suit that configuration.

On a 6x6 or 8x8, driven steering axles, different tyres and off-road use can affect steering resistance and kickback loads. The required steering force depends primarily on the load on the steered axles, tyres, ground conditions and axle geometry. More driven axles therefore do not automatically mean a higher operating pressure or a heavier-duty steering gear. Check the specified version, including any auxiliary cylinders and additional steering circuits; steering gears from a 6x4 and 6x6 or an 8x4 and 8x8 are not necessarily interchangeable.

A steered pusher axle ahead of the driven rear axle or a steered tag axle behind it also affects the choice of system. A passive self-steering axle is not operated directly by the steering gear. With positive steering, the additional axle can be controlled mechanically, hydraulically or electrohydraulically, with implications for the steering linkage, hydraulic connections or electronic control. A separately supplied system does not load the main steering gear in the same way as a coupled system. During diagnosis and replacement, also check the relative steering angles, centre positions and, where fitted, sensor calibration, axle locking and control when reversing or with the axle raised. A fault in the additional axle steering can disrupt steering behaviour without the main steering gear being defective.

Always identify parts using the vehicle’s chassis number. Visual comparison or selection by model name alone is never sufficient. Also check the part number, steering gear code, left-hand or right-hand drive version, steering ratio, sector shaft splines, mounting pattern and hydraulic connections. Operating pressure, pump flow rate, maximum steering output force and connections for auxiliary cylinders must match the vehicle configuration. Engine variants mainly affect the steering pump version and hydraulic supply; a MAN D26 or Mercedes OM 471 is not a selection criterion on its own.

Wear on ball tracks, sector shaft teeth and bearing bushes causes free play or knocking. Damaged shaft seals cause external leaks; internal leakage past the power piston seals can lead to reduced assistance, particularly when the hydraulic oil is warm. Heavy steering in one direction may indicate a control valve problem, but can also be caused by mechanical resistance. If foaming or steering pump noise occurs, first check for air entering the suction line.

1. Rule out external causes. Check tyre pressures, axle loads, steering ball joints, universal joints, kingpin bearings and chassis mountings. Support the vehicle in accordance with the workshop instructions before disconnecting components.
2. Locate mechanical play. Have a second technician make small steering movements around the centre position. Observe the input shaft, sector shaft and pitman arm in sequence. Measure the specified radial or axial movements with a dial indicator and compare them with the manufacturer’s limits.
3. Measure hydraulic performance. Use a suitable power steering tester with a pressure gauge, flow meter and temperature measurement. Check pump flow rate, pressure build-up and return pressure at the specified engine speed and fluid temperature. Compare left and right; do not hold the steering against full lock for prolonged periods. There is no universal test pressure for all versions.
4. Check electronic assistance. On electromechanical or electronically controlled systems, read fault codes and live data. Check the supply voltage, steering angle signals and torque sensor signals before condemning the steering gear.

Do not tighten an adjustment screw by feel to mask steering play. Preload, gear backlash and turning torque are version-specific; incorrect adjustment causes binding or insufficient self-centring. Reconditioning requires inspection of hardened raceways, shaft surfaces, bearing fits and seal seats, followed by a functional bench test. Replacing seals alone will not resolve worn shafts or internal wear.

Braem has the necessary test equipment to test steering gears reconditioned in our own workshop. This allows us to sell reconditioned steering gears with a warranty on an exchange basis.

Depressurise the hydraulic circuit in accordance with the instructions before removal. If contamination is present, flush the system, replace the specified filter and use only approved hydraulic fluid. Mechanically centre the steering gear, observe the mounting positions and tightening torques, and adjust any hydraulic travel limits according to the manufacturer’s procedure. Bleed the system, check for leaks and check wheel alignment, and carry out sensor calibrations where required. Finish with a controlled functional test and road test; if binding or unexplained play is present, the vehicle must remain out of service.