Rack & Pinion Power Steering
Rack & Pinion Steering
Rack & Pinion steering is steadily becoming the typical kind of steering on cars, small trucks, and SUVs. It is actually a pretty simple mechanism. A rack and pinion gearset is encircled in a metal tube, with each end of the rack poking out from the tube. A rod, called a tie rod, hooks up to each end of the rack.
The pinion gear is connected to the steering shaft. When you twist the steering wheel, the gear spins and moves the rack. The tie rod at each end of the rack hooks up to the steering arm on the spindle (see diagram above).
The rack and pinion gearset performs two jobs:
- It changes the rotating motion of the steering wheel into the back and forth motion needed to turn the wheels while turning
- It makes it easier to turn the wheels
This is why, on most vehicles, it requires three to four complete spins of the steering wheel to make the wheels turn from far left to far right.
The steering ratio is the ratio between how far you can spin the steering wheel to how far the wheels actually turn. A higher ratio means that the steering wheel has to do more revolutions to get the vehicle wheels to turn a certain distance. The higher ratio also means less effort is needed on the steering wheel because of the higher gear ratio.
Unsurprisingly, the lower ratio gives the steering a quicker response — you don’t have to turn the steering wheel as much to get the wheels to turn a given distance. Generally, sportier cars lower steering ratios than larger cars and trucks. This is because smaller cars are light-weight by comparison so that, even with the lower ratio, it doesn’t take much to turn the steering wheel.
Some vehicles have variable-ratio steering; this uses a rack and pinion gearset with a different tooth pitch (number of teeth per inch) in the center than it does on the exterior. This way, the vehicle responds very quickly when starting a turn.
Power Rack & Pinion
The rack has somewhat different design when it is in a power-steering system.In the rack, there is a cylinder with a piston in its middle. The piston connects to the rack. There are two ports for fluids, one to either side of the piston. Higher-pressure fluid to one side of the piston forces the other piston to move, which in turn, makes the rack move, and that provides the power assist.
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