The mechanical steering system takes the driver's physical strength as the steering energy, and all the force transmission parts are mechanical. The mechanical steering system consists of steering control mechanism, steering gear and steering transmission mechanism. Steering gear (also often called steering gear) is a group of gear mechanisms that complete the rotation motion to linear motion (or approximate linear motion), and it is also a deceleration transmission device in steering system. Commonly used are rack and pinion type, circulating ball crank finger pin type, worm crank finger pin type, circulating ball-rack tooth fan type, worm roller type and so on. We mainly introduce the first few.
1) Rack-and-pinion steering gear
Rack-and-pinion steering gear is divided into two types: two-end output type and middle (or single-end) output type.
as shown in fig. 4, the rack-and-pinion steering gear with two output ends, the steering gear shaft 11 as the driving part of the transmission pair is installed in the steering gear housing 5 through bearings 12 and 13, and its upper end is connected with the universal joint fork 1 and the steering shaft through splines. The steering rack 4 meshed with the steering gear is horizontally arranged, and both ends are connected with the tie rod 1 through the ball joint seat 3. The spring 7 presses the rack against the gear through the pressing block 9, so as to ensure backlash-free engagement. The pre-tightening force of the spring can be adjusted by adjusting plug 6. When the steering wheel is rotated, the steering gear 11 rotates, so that the rack 4 meshed with it moves in the axial direction, so that the left and right tie rods drive the steering knuckle to rotate left and right, and the steering wheel is deflected, thereby realizing automobile steering. As shown in fig. 5, the rack-and-pinion steering gear with middle output is basically the same in structure and working principle as the rack-and-pinion steering gear with two ends output, except that it is connected to the left and right steering tie rods 7 with bolts 6 in the middle of the steering rack. On the rack-and-pinion steering gear with single-end output, one end of the rack is connected with the tie rod through the inner and outer brackets.
2) circulating ball steering gear
The circulating ball steering gear is one of the most widely used structural types at home and abroad, and generally has two-stage transmission pairs, the first stage is the screw nut transmission pair, and the second stage is the rack and pinion transmission pair. In order to reduce the friction between the steering nuts of the steering screw, their threads are not in direct contact, and a plurality of steel balls are installed between them to realize rolling friction. Both the steering screw and the nut are machined with semi-circular spiral grooves with cross-sectional profiles consisting of two or three different concentric arcs. The spiral grooves of the two can be matched to form a spiral tubular channel with an approximate circular cross section. There are two pairs of through holes on the side of the nut, from which steel balls can be inserted into the spiral channel. There are two steel ball conduits outside the steering nut, and both ends of each conduit are respectively inserted into a pair of through holes on the side of the nut. The catheter is also filled with steel balls. In this way, the two pipes and the spiral tubular channel in the nut are combined into two independent and closed steel ball flow channels. When the steering screw rotates, the force is transmitted to the steering nut through the steel ball, and the nut moves along the axial direction. At the same time, under the action of the friction couple between the screw and nut and the steel balls, all the steel balls roll in the spiral tubular channel to form a ball flow. When the steering gear works, the two rows of steel balls only circulate in their respective closed flow channels and will not come out.
3) Worm-crank finger-pin steering gear
The transmission pair of the worm-crank finger-pin steering gear (the steering worm is the driving part, and its follower is the finger pin installed at the crank end of the rocker arm shaft. When the steering worm rotates, the finger pin engaged with it moves along an arc around the axis of the rocker arm shaft and drives the rocker arm shaft to rotate. The function of the steering transmission mechanism is to transmit the force and motion output by the steering gear to the steering knuckles on both sides of the steering bridge, so as to deflect the steering wheels on both sides and make the deflection angles of the two steering wheels change according to a certain relationship, so as to ensure that the relative sliding between the wheels and the ground is as small as possible when the automobile turns.
1) Steering transmission mechanism with dependent suspension
The steering transmission mechanism with dependent suspension mainly includes steering rocker arm 2, steering straight tie rod 3, knuckle arm 4 and steering trapezoid. When the front axle is only a steering axle, the steering trapezoid composed of the tie rod 6 and the left and right trapezoidal arms 5 is generally arranged behind the front axle, as shown in Figure 9 a. When the steering wheel is in the neutral position corresponding to the straight running of the automobile, the intersection angle between the trapezoidal arm 5 and the tie rod 6 in the plane (horizontal plane) parallel to the road >: 9。
when the engine is in a low position or the steering axle is also a driving axle, the steering trapezoid is often arranged in front of the front axle to avoid motion interference. At this time, the above intersection angle <: 9, as shown in figure 9 b. If the steering rocker arm does not swing back and forth in the longitudinal plane of the car, but swings to the left and right in the plane parallel to the road, the steering straight tie rod 3 can be placed horizontally, and the steering tie rod 6 can be directly driven by the ball pin, thus pushing the trapezoidal arms on both sides to rotate.
2) Steering transmission mechanism with independent suspension
When the steering wheels are suspended independently, each steering wheel needs to move independently relative to the frame, so the steering axle must be disconnected. Accordingly, the steering trapezoid in the steering transmission mechanism must also be disconnected.
3) Steering straight pull rod
The function of the steering straight pull rod is to transmit the force and motion from the steering rocker arm to the steering trapezoidal arm (or steering knuckle arm). It is subjected to both tension and pressure, so the straight pull rods are made of high-quality special steel to ensure reliable work. The typical structure of straight tie rod is shown in Figure 11. When the steering wheel deflects or jumps relative to the frame due to elastic deformation of the suspension, the relative motion of the steering straight rod, the steering rocker arm and the steering knuckle arm is spatial motion, and in order not to interfere with the motion, the ball pin is used for the connection among the three.
4) Steering shock absorber
With the increase of vehicle speed, the steering wheel of modern automobile sometimes shimmies (the steering wheel swings back and forth around the kingpin axis, even causing the vibration of the whole vehicle body), which not only affects the stability of the automobile, but also affects the comfort of the automobile and aggravates the wear of the front tires. Setting steering damper in steering transmission mechanism is an effective measure to overcome steering wheel shimmy. One end of the steering shock absorber is hinged with the vehicle body (or front axle), and the other end is hinged with the steering straight pull rod (or steering gear).