Power Assisted Steering

Power Assisted Steering (PAS) - Overview

Jump to:
1. Overview
2. How It Works – Step by Step
3. Key Components Involved
4. Common Misconceptions
5. Why This Matters

Anchor 1

Plain-English summary: what the system does

Power assisted steering reduces the physical effort required to turn the road wheels, particularly at low vehicle speeds. It does this by using hydraulic pressure to assist the driver’s input while maintaining a direct mechanical connection between the steering wheel and the wheels. If assistance is lost, the vehicle remains steerable, but with increased effort.

How it Works - Step by Step

Anchor 2

Mechanical steering connection
The steering wheel is mechanically connected to the steering gear (rack and pinion or steering box). This connection always exists, regardless of whether hydraulic assistance is available.
Hydraulic pump operation
An engine-driven hydraulic pump circulates power steering fluid continuously while the engine is running. Fluid flows from the pump, through the steering gear, and back to the reservoir.
Neutral (straight-ahead) position
When the steering wheel is not being turned, hydraulic fluid is directed equally to both sides of the power piston within the steering gear. Pressures balance, so no assistance force is applied.
Steering input applied
When the driver turns the steering wheel, resistance at the road wheels causes a small torsional deflection in the steering input shaft.
Rotary control valve operation
This torsional deflection causes a slight relative rotation within a rotary control valve. The valve redirects hydraulic fluid to one side of the power piston.
Assistance force generated
Hydraulic pressure acts on the piston, producing a force that assists the driver in moving the steering gear in the intended direction.
Progressive assistance
The greater the steering effort required, the more the torsion bar deflects, increasing valve opening and hydraulic assistance proportionally.
Pressure and flow regulation
The pump incorporates flow control and pressure relief mechanisms to ensure assistance is appropriate across engine speeds and to prevent excessive system pressure, particularly at full steering lock.
Return to neutral
As steering effort reduces, the torsion bar recentres, the control valve balances pressure again, and assistance reduces smoothly.

Key Components Involved

Anchor 3

Steering wheel and input shaft
Transfers the driver’s steering input into the steering gear. The input shaft provides the mechanical link that ensures the vehicle remains steerable even if hydraulic assistance is lost.
Torsion bar
A slender steel bar integrated into the steering input shaft. It twists slightly under steering load, allowing the system to sense how much assistance is required. The amount of twist directly controls hydraulic assistance.
Rotary control valve
A precision valve mounted around the input shaft. It uses the relative movement created by torsion bar twist to direct hydraulic fluid to the appropriate side of the power piston. In the straight-ahead position, it balances pressure so no assistance is applied.
Hydraulic pump
An engine-driven pump, typically vane-type, that circulates power steering fluid through the system. It provides the energy required for hydraulic assistance but does not determine steering direction.
Fluid reservoir
Stores hydraulic fluid and allows for thermal expansion, deaeration, and fluid return from the steering gear. It ensures a continuous supply of fluid to the pump.
Hydraulic hoses and pipes
Carry pressurised fluid between the pump, steering gear, and reservoir. These lines allow fluid to be directed to either side of the power piston depending on steering input.
Power piston
A hydraulic piston integrated into the steering gear. When pressurised fluid acts on one side of the piston, it generates a force that assists movement of the rack or steering linkage.
Flow control valve
Regulates the volume of fluid delivered by the pump to maintain consistent steering assistance across a range of engine speeds.
Pressure relief valve
Limits maximum system pressure to prevent damage when the steering reaches full lock or hydraulic flow is restricted.

Common Misconceptions

Anchor 4

“Power steering turns the wheels for you”
The system assists the driver’s input; it does not steer independently.
“If power steering fails, steering is lost”
Hydraulic assistance can fail, but mechanical steering remains intact.
“More engine speed always means more assistance”
Flow control valves regulate assistance so it remains appropriate at higher speeds.
“All power steering systems are the same”
Hydraulic, electro-hydraulic, and fully electric systems operate on different principles.

Why This Matters

Anchor 5

Understanding power assisted steering explains:

Why steering effort is light at parking speeds
Why assistance reduces naturally at higher speeds
Why steering becomes heavier but still functional if assistance is lost
How steering feel is deliberately engineered, not accidental

This knowledge underpins wider topics such as steering geometry, vehicle stability, and driver feedback.

Quick Reference

Primary purpose: Reduce steering effort
Energy source: Engine-driven hydraulic pump
Control method: Rotary valve with torsion bar
Fail-safe: Mechanical steering retained
Greatest benefit: Low-speed manoeuvring