Full Hydraulic vs Mechanical Hydraulic Steering | Howo Mining Truck System
1. Vehicle Model Classification (Official Configuration of Sinotruk)
70t & 90t Howo mining trucks: Mechanically Hydraulic Power Steering (Semi-hydraulic, ZF8118 steering gear, Bosch supporting parts)
105t, 110t, 120t, 135t large-tonnage Howo mining trucks: Full Hydraulic Steering System
Key distinction: Small-tonnage mining trucks retain mechanical rigid connecting rods; for 105-ton and above large-tonnage mining trucks, there is no mechanical tie-rod rigid connection between the steering wheel and the steering axle, and steering signals are transmitted purely by hydraulic pressure.
2. 70/90 ton: ZF8118 Mechanically Hydraulic Power Steering (Semi-hydraulic)
System Composition
Working Principle
Key Features
In case of hydraulic failure (pump damage, pipe burst): The mechanical connecting rod can still perform steering, though it is very heavy, enabling low-speed pull-over to roadside.
Good road feel and favorable steering return performance.
Mature structure and simple maintenance.
Disadvantage: Heavy steering effort when turning in place under heavy load; frame layout is limited by mechanical tie rods.
3. 105t and above: Full Hydraulic Steering System
System Composition
There is no mechanical rigid tie-rod connection between the steering wheel and front wheels, only hydraulic pipelines.
Working Principle
Neutral position: The steering unit locks the cylinder and keeps wheels at current angle.
Steering priority valve: The steering circuit has higher priority than the hoist hydraulic circuit. When steering and hoisting operate simultaneously, oil supply for steering is guaranteed first for safety.
Key Features
Extremely light steering effort, easy for stationary heavy-load steering; high flexibility of frame layout, eliminating complicated tie rods.
When main pump fails or engine stalls: High-pressure oil stored in accumulator can support about 3～5 complete steering cycles for emergency pull-over. After accumulator pressure drains out, turning the steering wheel can drive the built-in gerotor motor of the steering unit, to manually pump oil for steering (extremely laborious).
Weak road feel and poor steering return performance.
Protection: Bidirectional cushion valve absorbs impact pressure on steering cylinder to protect cylinder and frame, and prevents damage to steering unit from tire impact feedback.
4. Comparison of Two Systems
Item | ZF8118 Mechanically Hydraulic Power Steering (70/90t) | Full Hydraulic Steering (105t+) |
Steering wheel to front wheels | With mechanical connecting rods; hydraulic power assistance only | No rigid mechanical connection; hydraulic pressure transmits all steering power |
Hydraulic failure | Mechanical steering available, heavy operation | Rely on accumulator for emergency steering; manual oil pumping after accumulator pressure depletion |
Steering feel | Good road feel, good return performance | Light steering, nearly no road feel, weak return |
Layout | Restricted by tie rod structure | Flexible layout, suitable for ultra-large tonnage mining trucks |
Core components | ZF8118 recirculating ball steering gear | Gerotor full hydraulic steering unit + priority valve + accumulator |
5. Common Fault Points
Semi-hydraulic (ZF8118) common faults
Heavy steering: Worn steering pump, clogged filter element, contaminated oil, worn oil seal/valve inside steering gear.
Steering pull: Worn tie rod ball joint, incorrect toe-in, poor return of steering gear valve body.
Oil leakage: Oil seal of steering gear, steering pump sealing, pipe joints.
Full hydraulic steering common faults
Slow steering, excessive free play: Worn gerotor motor inside full hydraulic steering unit, air entrapment in system.
Alternating heavy & light steering: Stuck priority valve, insufficient nitrogen pressure of accumulator.
Unilateral weak steering: Internal leakage of steering cylinder, jammed one-way cushion valve.
No emergency steering after engine shutdown: Failed accumulator due to air leakage.