Hydraulic Power Steering (HPS) is a type of power steering system that uses hydraulic pressure to assist the driver in turning the steering wheel. It is one of the oldest and most common forms of power steering used in vehicles before the advent of electric power steering (EPS). Hydraulic power steering systems are especially popular in trucks, larger vehicles, and older passenger cars, providing the necessary assistance to make steering easier and more responsive.
Steering Wheel:
The driver’s input device, where the steering wheel is turned to change the direction of the vehicle.
Steering Shaft:
A shaft that transmits the rotational movement from the steering wheel to the steering gearbox.
Steering Gearbox (or Rack and Pinion with Power Assist):
The steering gearbox or rack and pinion assembly (depending on the vehicle) directs the movement of the wheels. The hydraulic pressure is used to assist the driver in turning the steering wheel with minimal effort.
Hydraulic Pump:
The pump generates the hydraulic pressure required for the system. It is typically driven by the engine via a belt and is responsible for circulating hydraulic fluid through the system.
Hydraulic Fluid:
The hydraulic fluid (often a special power steering fluid) is used to transmit pressure throughout the system. It also lubricates the components and helps keep the system cool.
Control Valve:
The control valve regulates the flow of hydraulic fluid to different parts of the system based on the driver’s input. It adjusts the fluid pressure, directing it to assist in turning the steering wheel.
Ram or Cylinder:
In some systems (typically in recirculating ball steering), a hydraulic ram or cylinder is used to apply pressure to the steering mechanism, helping to steer the wheels.
Reservoir:
The reservoir holds the hydraulic fluid when the system is not in use. It also helps maintain the proper fluid levels for the system to function.
Return Line:
After the fluid has been used to assist in steering, it returns to the reservoir through the return line.
High-Pressure Line:
This line carries the pressurized hydraulic fluid from the pump to the steering gearbox or rack.
Driver Input:
When the driver turns the steering wheel, the steering shaft rotates. This rotation is transferred to the steering gear (either the rack and pinion or steering gearbox).
Hydraulic Pump Activation:
As the steering wheel is turned, the hydraulic pump (which is belt-driven from the engine) circulates hydraulic fluid under pressure throughout the system. The pump creates pressure that flows into the steering gear.
Fluid Pressure Assistance:
The control valve inside the steering gear directs the hydraulic fluid to assist with turning the steering. Depending on the direction of the turn, the valve adjusts the fluid flow to either side of the steering mechanism.
This hydraulic pressure helps to move the rack or pitman arm (depending on the type of steering system) with less effort required from the driver.
Steering Assistance:
When the system is engaged, the ram or hydraulic cylinder (if used) pushes against the steering components to provide additional force, making it easier to turn the wheels. This assistance is particularly useful at low speeds or when parking.
Return Fluid:
Once the fluid has assisted in the turn, it returns to the reservoir via the return line. The system is pressurized again by the pump for the next turn.
Easier Steering:
The primary benefit of hydraulic power steering is that it significantly reduces the effort required to turn the steering wheel, especially at low speeds, such as when parking. It makes steering much easier, particularly in large vehicles or when navigating tight spaces.
Better Control:
The system provides consistent steering effort and better control, offering the driver more precise and responsive handling, particularly when turning at low speeds or in heavy traffic.
No Need for Electric Motors:
Unlike electric power steering systems, hydraulic systems don't rely on electric motors, making them suitable for heavy-duty applications and vehicles that require a lot of steering power.
Durability:
Hydraulic power steering systems are generally durable and well-suited for long-term use in a variety of vehicle types, especially trucks, SUVs, and older cars.
Feedback from the Road:
Hydraulic systems provide more "road feel" and feedback to the driver than electric systems. This is because the hydraulic pressure is connected to the force being applied, giving the driver a more tactile sense of the road surface.
Increased Fuel Consumption:
Since the hydraulic pump is driven by the engine via a belt, it consumes engine power, which can lead to increased fuel consumption. The system is always operating, even when the steering effort is minimal.
Complexity and Maintenance:
Hydraulic systems are more complex than electric systems, with more components such as the pump, hoses, reservoir, and control valve that may require maintenance or repairs over time.
Leaks and Wear:
Hydraulic power steering systems can develop fluid leaks over time, particularly at the hoses or pump seals. If hydraulic fluid levels become too low, the system can fail to provide adequate assistance.
Heavy Components:
Hydraulic power steering systems are generally heavier and bulkier than electric power steering systems, adding more weight to the vehicle, which can affect fuel efficiency.
More Maintenance:
While hydraulic power steering systems are durable, they require more maintenance compared to electric power steering systems. The hydraulic fluid needs to be replaced periodically, and any leaks or pressure issues must be fixed to keep the system functioning properly.
Trucks and SUVs: Hydraulic power steering is commonly used in larger vehicles, especially trucks and off-road SUVs, due to the higher force required for steering.
Older Passenger Cars: Many older vehicles, particularly those built before the rise of electric power steering (EPS), still use hydraulic power steering due to its reliability and effectiveness.
Heavy-Duty Vehicles: Commercial trucks, buses, and construction vehicles often use hydraulic power steering systems to manage the increased steering loads and provide ease of operation under heavy conditions.