Steering gearbox (vehicle steering gear)

01

Size the gear by axle load, output torque, ratio and pressure

Bosch's Servocom is a recirculating-ball hydraulic steering gear for heavy commercial vehicles: the steering spindle turns a worm, a recirculating ball thread moves a piston axially, and a gear arrangement turns the sector shaft that carries the drop arm. A rugged cast housing integrates the mechanical gear, the control valve and the power cylinder, and an integrated hydraulic steering limiter protects the linkage, wheel stops and pump. For the main variant 8098 Bosch lists a front axle load above 6.0 t (depending on axle parameters), a variable transmission ratio of 22.2–26.2, a pressure of 185 + 15 bar and a net weight of 43 kg. [1]

Bosch's Servotwin adds an electric motor and worm gear to the Servocom gear. Its listed data are a steering axle load of 5,500–8,500 kg, a hydraulic torque of 7,050–8,294 Nm at an efficiency of 0.9, a maximum oil pressure of 120–185 bar and a net weight of 27.8–55.1 kg; electronic end stops can be programmed to protect the system and the axle at maximum wheel angle. [2]

TRW's TAS integral power steering gears combine a manual steering mechanism, a rotary control valve and a power cylinder. Maximum operating pressure is 2,175 psi (150 bar) for TAS40, 55, 65 and 85 and 2,320 psi (160 bar) for TAS66, and maximum flow is 8 gal/min (30.3 L/min). Front axle ratings run from 9,000 lb (4,100 kg) for TAS40 through 12,000 lb (5,400 kg), 14,300 lb (6,500 kg) for TAS65 and TAS66, to 18,000 lb (8,200 kg) for TAS85; recommended minimum pump flow at 1½ steering wheel turns per second ranges from 2.2 gal/min (8.3 L/min) for TAS40 to 3.6 gal/min (13.6 L/min) for TAS85. [4]

For passenger cars, Bosch's column-mounted electric power steering (EPSc) delivers up to 100 Nm output assist torque through a worm gear on the steering column, feeding a mechanical rack-and-pinion gear via the intermediate steering shaft. [3]

Editorial calculation: TRW limits input during poppet setting to 178 N rim pull on a 508 mm steering wheel, which is 178 N × 0.254 m ≈ 45 N·m at the steering wheel; output torque at the sector shaft comes from the hydraulic piston and is quoted separately, as in Bosch's 7,050–8,294 Nm figure. [4] [2]

Give the supplier the vehicle type and GVWR, front (steer) axle load, required output torque, wheel cut and axle stop angles, target ratio (constant or variable) and steering wheel turns lock to lock, pump pressure and flow or electrical supply and assist torque, dual-circuit need, mounting position and frame bolt pattern, input and output shaft splines, pitman arm or rack interface, operating temperature range and quantity.

02

Check the pitman arm, linkage and steering column

TRW's TAS manual tells installers to bolt the gear to the frame, connect the steering column to the input shaft and install the pitman arm on the output (sector) shaft, each to the vehicle manufacturer's torque. The gear is in the straight-ahead position when the timing marks on the housing trunnion and sector shaft line up, and pitman arm timing marks must be aligned to prevent internal bottoming of the gear. TRW warns against using a torch or a hammer to remove a pitman arm, against welding or straightening any steering component, and asks for the grease pack behind the output shaft dirt and water seal to be maintained at least twice a year with NLGI grade 2 or 3 chassis lube. [4]

Unloading (poppet) valves at each end of the rack piston trip as the steered wheels approach the axle stops, reducing pressure, pump heat and linkage forces; they are set automatically at the first full right and left turn after installation, with no more than 40 lb (178 N) rim pull on a 20 in. (508 mm) wheel. A relief valve, where fitted, limits supply pressure but does not reduce pressure near the axle stops. [4]

FMVSS 204 defines the steering column as the structural housing around the steering shaft, and the steering shaft as the part that transmits steering torque from the steering wheel to the steering gear. For vehicles made on or after 1 September 1991 with a GVWR of 4,536 kg or less and an unloaded weight of 2,495 kg or less, the upper end of the column and shaft must not move more than 127 mm rearward in a 48 km/h perpendicular barrier impact. [6]

FMVSS 203 applies to passenger cars and to multipurpose passenger vehicles, trucks and buses with a GVWR of 4,536 kg or less: when the steering control system is struck by a body block at 24 km/h per SAE J944, the chest force must not exceed 11,120 N except for intervals totalling no more than 3 ms, and no part may catch the driver's clothing or jewelry. [5]

These are vehicle standards certified by the vehicle manufacturer. Ask a column supplier for the test or simulation data it provides in support, the collapse or energy-absorbing design, tilt and telescope range, column tube and shaft dimensions, universal joint angles and spline sizes.

03

Rack-and-pinion and EPS supply, and what to ask any gear maker

Chongqing XEPS Electrical and Mechanical Co., Ltd. lists rack-and-pinion steering racks, EPS systems and electric power steering columns for vehicle OEMs.

When requesting quotations, ask each supplier which gear type it builds (recirculating-ball hydraulic, rack-and-pinion, column, pinion or rack EPS), its axle-load and torque ratings, pressure and flow or assist data, test reports, and drawings of the input shaft, output shaft and mounting.

04

Inside a recirculating-ball gear, from input shaft to pitman arm

05

Servocom, Servotwin and TAS ratings against the gear you need

Torque, ratio, axle load, pressure and weight can be read from the Bosch and TRW data; shaft diameters and operating temperature are not published in these sources and belong on the supplier's drawing.

06

Send the axle, torque and interface data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vehicle: [type; GVWR; front axle load]
Gear: [recirculating-ball; rack-and-pinion; EPS]
Output: [torque N·m; ratio; turns lock to lock]
Hydraulics or EPS: [pressure; flow; assist torque; supply]
Interfaces: [input spline; sector shaft; pitman arm; mounting]
Column: [FMVSS 203/204 data; tilt; telescope]
Quantity: [units]

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