There is a classic “driverless” scene in the film “Crazy Stone”, and everyone just laughed it off after watching it. In fact, this kind of thing happens to trucks in reality. In the hot topic of “car sliding” discussed by card friends in the forum a few days ago, some card friends warned everyone with their own experience (for details, please read the post “Dongfeng Tianlong can be driverless”). To be exact, the “driverless” events frequently occur on semi trailers for more complex reasons, and such situations may occur on any traction model.
[before reading this article, first state that similar events have nothing to do with the Dongfeng Tianlong design of the party concerned. Please sit down according to the number. You should treat them rationally and seek truth from facts.]
Today’s vehicles are equipped with spring parking system, which is commonly known as “air brake”. After the air pressure drops to 4.5-3bar or the parking handle in the cab is operated, the spring chamber of the brake wheel cylinder acts to lock the brake drum to achieve the purpose of locking braking. The essence of the commonly understood “rolling” phenomenon is that the braking force acting on the tire fails or fails. For a semi-trailer, most people think that operating the parking brake handle is a deflated state, and the spring parking system of the trailer and tractor is working, which is wrong and the root cause of sliding.
Compared with the parking control valve of truck and tractor, although the appearance is the same, its internal principle is fundamentally different. Generally, the parking brake valve of truck has only two pipeline interfaces, which only connect and deflate the wheel cylinder (parking brake chamber) and act on the spring brake chamber of the brake wheel cylinder.
The parking control valve of the tractor has three pipeline interfaces. Note that there are two different functions here. When the manual control valve is operated to the parking position, the parking pipeline in the brake wheel cylinder of the main vehicle deflates and the parking spring starts to work; At the same time, the trailer control pipeline is inflated, and the trailer is in full braking state. When the manual control valve is in the parking state, cut off the air brake for the main vehicle and inflate the brake for the trailer. In the parking state at this time, it is the action of the main parking Spring + the normal braking action of the trailer.
So why did the Dongfeng Tianlong semi-trailer “driverless” of netizens in the truck Home Forum slip away? Listen to Xiaobian and analyze it slowly.
BBCA’s “once brilliant” tractor is a 6×2 model, which is supervised by the customs. Usually, the total mass of the vehicle is about 24 tons, and the goods are basically only 4-5 tons, which eliminates the possibility of overload. On the day of the incident, due to the certain slope of the parking position, leave the vehicle after determining that the main parking driver control valve is in the parking state. The next day, I found that the vehicle slipped. Fortunately, it did not cause a malignant accident and was in danger.
Let’s analyze the reasons according to the time point:
When kayou stops, the parking state of the vehicle is: main vehicle driving wheel (2 brake drums) parking spring braking + Trailer (6 brake drums) full braking. At this time, although the vehicle does not slip, it is actually in a “false” parking state.
① In the first case, after 2-6 hours, the trailer control pipeline leaks and loses air pressure, the trailer braking is released, and the parking state of the vehicle is that only the parking spring braking of the main vehicle driving wheel (2 brake drums) works.
② In the second case, after 2-6 hours, the main air circuit system of the trailer leaks, and there is no air pressure in the air reservoir of the trailer. The parking state of the vehicle is that the parking spring braking of the driving wheel (2 brake drums) of the main vehicle + the parking spring of the brake wheel cylinder (4 brake drums) of the trailer works.
It is more complicated to call card friends to understand the actual situation
I talked to kayou over the phone about the “driverless” incident. I learned that the vehicle condition at that time was air leakage, and the brake clearance of the drive axle had not been adjusted for a long time, The braking force is insufficient (affecting the effectiveness of parking braking); only one spring brake chamber of the trailer brake wheel cylinder works. After 2-6 hours, the parking state is that only the driving wheel of the main vehicle (poor braking of 2 brake drums) parking spring braking + the trailer has no braking or only (1 brake drum) parking spring acts. Then the reason for “unmanned driving” is basically clear.
There is a separate parking brake handle on the trailer girder, and the principle is consistent with the “air break brake” of the truck. After the switch is turned off, all brake cylinder spring brake chambers of the trailer work, and the trailer is in the real “parking state”. Friends who often drop and hitch transportation are used to closing this handle first when getting off the bus. Here we remind most novices and card friends who do not often remove the trailer that they need to be extra careful. Remember to check the parking brake system of the trailer and close the hand valve in time when parking.
The vast majority of card users do not know that the tractor control valve has a check position. As shown in the figure above, when the handle is operated and in the parking position, Pull backward (6 °) to the bottom, which is the inspection position of parking brake. At this time, the trailer brake is released, and only the parking brake system of the main vehicle acts. In case of sliding, turn off the trailer parking handle alone or select an appropriate parking position. Card friends who stop temporarily should remember to operate the handle to the inspection position to check whether they slide.
For most domestic 6×2 tractors, the front dual steering axles of the main vehicle do not have parking spring braking. If the trailer control air circuit of this kind of tractor leaks, it is very prone to accidents in the parking state. How can “unmanned driving” be avoided when only the drive axle has “parking spring” braking. At present, manufacturers are also actively responding to this problem, and many later production models have noticed this kind of problem.
The trailer braking system is not as simple as 1 + 1. The components including manual control valve, trailer valve and emergency relay valve interact with each other and have a variety of functions and are very complex. For the parking brake of semi-trailer, the main vehicle and trailer should be distinguished to find out the reasons respectively. Here, the owners of 6×2 model are specially reminded that there are two parking brake handles of semi-trailer. When getting off, the separate parking control valve of trailer must be closed. Be familiar with the basic braking principle of the vehicle, form good habits, pay attention to safety, and start with me.
When the handle is within 0 °, all the parking brakes of the vehicle are released and the vehicle is in driving state; When the handle is in 73 ° locking position, the trailer is in full braking state; When the handle is at the 82 ° inspection position, the parking brake of the tractor is in the spring braking state, but the trailer is in the fully released braking state.
When the handle moves from 73 ° to 0 °, the handle cam pushes down the large piston 1, presses down the balance spring 2, pushes the piston 3 down, closes the exhaust valve 4, fully opens the inlet valve 5, opens the inlet valve 6 of the additional valve, and the compressed air in chamber f enters chamber a, which is then divided into two ways. One way enters the spring brake air chamber through port 21 to release the parking brake of the tractor; Enter the trailer valve (main vehicle) through port 22 to release the trailer brake. When the handle is within 0 °, the vehicle parking brake is fully released,
When the handle moves from 0 ° to 55 ° and 73 °, the large piston 1, balance spring 2 and balance piston 3 move upward, exhaust valve 4 opens and inlet valve 5 closes; Additional valve inlet valve 6 is closed. 22 the interface decreases linearly to zero with the increase of handle angle. When the handle is in the range of 55 ° – 73 °, the whole trailer is in full braking state due to the action of trailer valve (main vehicle). When the handle is at 73 °, the handle is locked.
When the handle reaches the 82 ° check position from 73 ° The air inlet valve 6 of the additional valve is opened. Due to the action of the trailer valve (main vehicle), the braking action of the trailer is released. At this time, check whether it is possible. It has the ability to park on the ramp only under the parking braking action of the tractor. When the handle is released, the handle will automatically return to the parking brake locking position.
When the handle is within 0 °, all the parking brakes of the vehicle are released and in the driving state. If the trailer brake chamber is not released, and the higher the air pressure, the greater the pressure in the brake chamber. Explain that the air pressure in chamber a of the manual control valve flows into chamber B and chamber F, and enters the trailer brake chamber through port 21 and port 22 It belongs to the aging of internal piston 0-ring and foreign matters in the cavity, and the internal crosstalk fault is caused by blocking the closed valve. The damaged parts can be inspected and replaced through the lower body and middle body.
When the manual control valve moves from 0 ° to 55 ° and 73 °, if there are sundries inside the manual control valve, the O-ring is damaged, the piston is worn, and the inner cavity or some part is stuck or stuck, the normal braking effect cannot be produced.