The steering gear is a device for converting the rotary motion of the steering wheel into straight-line motion (of linkage). There are a couple different types of steering gears. The most common are rack-and-pinion and recirculating ball.
Rack-and –pinion S teering
Rack-and –pinion steering is quickly becoming the most common type of steering on cars, small trucks and SUVs. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.
The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle.
The rack-and-pinion gear set does two things:
l It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.
l It provides a gear reduction,making it easier to turn the wheels.
On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far righ).
The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher radio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio.
Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. 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-which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive.
Some cars have variable-ratio steering, which uses a rack-and-pinion gearset that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheel’s turnin limits.
Recirculating-Ball Steering
Recirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system.
Essentially, the steering gear consists os two parts, a worm on the end of the steering shaft and a pitman arm shaft on which there is a gear sector.A nut rides on the worm gear and a sector, is engaged with this nut.
As the steering wheel is rotated, the nut is moved up or down on the worm. Because teeth on the nut are meshed with the teeth on the sector, which causes the sector to rotate. This movement causes the pitman arm, attached to the sector, to swing to the right or left. This action, in turn, pushes or pulls on the tie rods attached to the pitman arm. The steering-knuckle arm, attached to the front wheels, are therefore forced to swing the wheels to the right or left on their pivots.
Instead of the worm directly engaging, the recirculating balls are filled between the nut and the worm. The balls actually serve two purposes: first, they reduce friction and wear in the gear; second, they reduce slop in the gear.
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转向系的作用是把方向盘的旋转运动变为连杆机构的直线运动。转向器有很多类型。最常见的有齿轮齿条式和循环球式。
齿轮齿条式转向器
齿轮齿条式转向器正迅速发展，广泛用于轿车、微型货车和SUV。事实上，它结构简单，操作方便。整个齿轮齿条式转向器装在金属壳内，其齿条从转向器伸出来与转向摇臂连接。拉杆，或称为转向拉杆，与摇臂的另一端连接。
该齿轮连接转向轴。当转动转向盘时，齿轮即转动，而齿条就移动。齿条一端连着拉杆，拉杆与车轮轴上的转向摇臂连接。
齿轮齿条式转向器有两个作用：
把转向盘的旋转运动转变为车轮转向所需要的直线运动
减速增力，使转向更方便容易
在大多数情况下，转向盘转三到四周才能使车轮从一个极限位置转动到另一个极限位置。
转向盘的转角与车轮偏转角的比值，称为转向系的角传动比。例如，方向盘转动一周，使车轮转动了20°，那么其角传动比就是360除以20，为18：1.大的角传动比意味着，对于给定的车轮转角，其转向盘转过的角度更大。可是，高的角传动比可以使转向更轻便。
一般的说，轻型车、赛车的传动系传动比比较低，而重型车和卡车的角传动比较高。传动系角传动比低，则转向灵敏——当你想将车轮转过一定角度时，不必把转向盘转很大的角度——这正是赛车所需要的。而且，这些轻便型和小型车不必过多地考虑其转向轻便问题。
有些车的转向系角传动比是可变的，这种转向器采用齿轮齿条式，它在中间而不是外部，有不同的齿密度。这使得车轮开始转时，转向灵敏，而在车轮接近最大偏转角时减少其灵敏度。
循环球式转向器
循环球式转向器被广泛用于卡车和旅行车。其转动车轮的结构与齿轮齿条式稍有不同。
转向器基本上由两部分组成：装在转向轴末端的蜗杆和转向摇臂，转向摇臂上装有扇形齿轮。螺母在蜗杆上移动，同时，螺母还与扇形齿轮啮合。
当转动轴转动时，带动螺母在蜗杆上轴向移动。螺母上的齿同时与扇形齿轮啮合，带动了扇齿轮转动，这使得与扇齿轮连接的转向摇臂左右摆动。这一运动又依次推动或拉动与转向摇臂相连的转向直拉杆。于是，与前轮相连的转向节臂使车轮在其轴线上左右转动。
转向螺杆不是直接啮合的，在螺母和蜗杆之间装有很多循环滚动的钢球。这些钢球实际上有两个目的，减少转动时的摩擦和磨损，另一个是减少了转向盘的自由行程。
