One of the key responsibilities of the competent specialist of the fleet responsible for functioning and / or maintenance of rolling stock, select such friction pairs (brake shoe + brake disc), which most best meet specific objectives. Only in this case it will be possible to ensure maximum savings and unconditional road safety. How to do it?
Expert Articles: Andrey Dolgikh, Representative in the CIS countries for sales and after-sales service of Meritor
Talk about brake discs
Consider several typical questions:
When is it time to change brake discs?
It would seem that the answer is obvious. Each disk shows its minimum allowable thickness. Reached the wear of this critical mark — change the disk. Everything is simple! But it is «simple» — for the ordinary people. And we agreed to approach the question professionally.
Let’s figure out what is actually due to the value of the minimum permissible thickness of the working part of the brake disc.
The first is obvious
Second — professional
The task of the working brake mechanism of the vehicle is reduced to the conversion of kinetic energy (motion energy) into thermal energy with its subsequent scattering into the atmosphere. Will there be warmth? The simplest calculation will show us that when stopping at the traffic light, a total weight of 44 tons from a speed of 90 km / h (and often do we often go without overload or without exceeding speed?) 13.75 million Joule heat is distinguished. Approximately the same can be obtained if the Porsche 911 sports car slows down on the track from a speed of 440 km / h. Such a number of heat leads to a serious heating of the brake mechanism. Even with the «civilian» mode of movement in a smooth highway, it is easily possible to obtain a temperature of 350 degrees Celsius in contact of rubbing surfaces.
In order not to have critical overheating — the braking mechanism must «be able to» somewhere quickly «children» obtained warmth. The man will say — no problems — there are ventilation channels in the disk. Unfortunately, for the brake mechanism, the air is a very bad thermal conductory (remember the windows with double-glazed windows), and even if we ensure a good blowing of the disc (which, by the way, is not always possible. For example, at the end of the braking disk no longer rotates and the counter wind does not match ) This will not allow us to quickly dispel heat. And you need to quickly. Otherwise, the shoe will begin to «burn». What will help us? Metal brake disc. Metal has high-quality brake discs, thermal conductivity is significantly higher than that of the air. But thermal conductivity only allows the heat to quickly «stroke» from the contact zone of the pad and disk. And where to «flock»? Again in the metal, but it should be a lot! It’s like a task about the pool — if you pour water quickly in it, and the pool container is small, then it will soon overflow. It turns out the thickness of the brake disc to the fore. What it is thicker — the greater the amount of heat he will be able to «absorb» in his senses, and only after later, after the driver stops putting pressure on the brake, slowly (and it will not work quickly, because air is a bad thermal conductory) disperse heat into the atmosphere. The thicker disk — the easier it is to work the block. Such is thermodynamics.
So, we understand that the minimum permissible thickness of the brake disc is due to its strength and ability to cool well. And therefore, the greater the residual thickness of the disc — the longer the block will «go» with other things being equal. Saving!
But an example of anti-economic (Fig. 1). Often to the disk in the stage «still like», the pads are bought by the pads «cheaper», believing that the disk, and the pads in this case «end» just at the same time — then change. In most cases, such an idea ends with a typical pattern of destruction.
Do not forget that the pads are «cheaper», as a rule, have a simplified design and smaller, compared with a high-quality block, heat resistance. They do not suffer overheating.
How to measure the residual thickness of the brake disc?
Obvious — the minimum allowable thickness is indicated on the brake disk (Fig. 2).
Do we know how we measure the residual thickness of the brake disc? What could be easier — will tell you the «specialist» — the main thing, do not forget that there are non-worn zones on the external and internal radius on the working surface of the disk. And with a smart species, it will attach to the middle part of the working area of the disk of the caliper, and maybe even a micrometer. And it will be deeply wrong.
Why? This is already professional. Tell me in the next article