The gears in the steering gear assembly not only steer the front wheels but, at the same time, they act as reduction gears, reducing steering wheel turning effort by increasing the output torque. The reduction ratio is called the steering gear ratio and is normally between 18 and 20 : 1. A larger ratio reduces the steering effort but makes it necessary to turn the steering wheel more when going around a curve. There are two types of steering gear systems.
• Rack-and-pinion type
• Recirculating-ball type Currently, the rack-and-pinion type is used in almost all models.
HINT: Steering gear ratio
• Rack-and-pinion type
Amount of steering wheel rotation (in degrees)
Front wheel steering angle (in degrees)
• Recirculating-ball type
Amount of steering wheel rotation (in degrees)
Pitman arm movement (in degrees)
Rack-and-pinion Type
1. Construction
The steering pinion at the lower end of the steering main shaft meshes with the steering rack. As the steering wheel is turned, the steering pinion rotates to move the steering rack to the right or left. The movement of the steering rack is transmitted to the knuckle arms via the steering rack ends and tie rod ends.
2. Features
• Construction is compact, simple, and lightweight. Since the gear box is small, and the rack itself acts as the steering linkage. • Gear meshing is direct, so steering response is very sharp. • There is little sliding and rotational resistance and torque transmission is better, so steering is very light. • The steering gear assembly is completely sealed so it is maintenance free.
Recirculating-ball Type
1. Construction
Spiral grooves are cut into the worm shaft and ball nut and many steel balls recirculate rolling in the worm shaft and ball nut spiral grooves. The side of the ball nut has rack teeth that mesh with the gears on the sector shaft.
2. Features
• Since the rotation of the steering main shaft is transmitted by the rolling contact surface of the balls, the force of friction for the nut sliding is extremely small.
• This construction can withstand large loads.
• Little sliding resistance because the friction between the worm shaft and the sector shaft is very small due to the balls.
• The operating angle is large.