One of the key responsibilities of the competent specialist of the fleet responsible for the functioning and / or maintenance of rolling stock — select such friction pairs (brake shoe + brake disc), which in the best way to meet specific operational tasks. Only in this case it will be possible to ensure maximum savings and unconditional road safety. How to do it? The cycle of articles on this topic continue the 3rd article.
In the previous article, we found out that the minimum permissible thickness of the brake disc is due to its strength and ability to cool the brake block well. Let’s now understand how to correctly determine the residual thickness of the brake disc. And also consider some more nuances affecting the wear of the pads.
How to measure the residual thickness of the brake disc?
Obvious — the minimum allowable thickness that is indicated on the brake disc.
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 is not to forget that there are non-worn zones on an 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? It is professional. Let’s figure it out.
From the point of view of structural strength, it is important for us to determine the smallest thickness of the brake disc in the most loaded zones. One of these zones is a, it is the same zone of the smallest radii (Fig. 2). Here the disc is experiencing maximum voltages in the wear zone caused by the maximum moment (remember the Archimedes lever). To measure thickness in this zone, we will need a special tool (Fig. 3). Please note that this is not an ordinary caliper. It has elongated sponges so that we can «reach» to zone A, and pointed probe on sponges so that we can measure the thickness of the hoods that appeared as a result of the grooves on the brake disc, i.e. Really minimal thick. Neither the usual caliper nor the micrometer will help us here, because With their assistance it is possible to measure only the thickness on the vertices of the grooves.
The second most loaded zone in the weathered area of the brake disc — zone B, or the zone of the largest radii (Fig. 2). In this zone, the highest relative speed (the speed of moving the disk relative to the pad) and as a result — elevated wear. Partly, this wear is leveled by a large number of material (larger area) of the disk in this zone, but not always. Therefore, the disk design is not rare when a large thickness of the disc itself is provided in the maximum wear zone (Fig. 4). One way or another, in this zone it is also necessary to find the deepest grooves and to measure the thickness of the disk.
In zone with, or medium radius zone, local grooves caused by foreign abrasives (for example, road dust, sand), or the use of poor-quality friction material in brake pads. So in this zone will have to carry out measurements.
In each zone, it is necessary to make several measurements at different points and choose the minimum result from all obtained. Then choose the smallest result from all zones and compare its value with the standard minimally allowable thickness.
If the brake disc is still «in tolerance,» it is worth thinking about the fact that in any case, being worn, it has a smaller heat consumption compared to the new one. Thus, the heat sink from the contact zone with the pad will be worse, which will lead to an increased wear of the brake pad during intensive operation (whether another thing happens in our realities?).
Another interesting fact is found when installing a new pad to a non-aid disk. We noticed that in some cases the wheel drive is intensely covered with gray raids? It is not always connected with low quality pads. The intensive formation of friction wear products is often found at the «wipes» stage, and it is connected with a special form of brake disk wear. If you look close to the worn brake disc, it can be seen that he is «trough», and in this very «trot» we will have to immerse a new brake block (Fig. 5). This immersion process is called the wipe, although it actually talks here about the catastrophic wear of the edges of the clod, they actually burn, perceiving the whole that load that should have to come to the full surface of the block. Not only the edges themselves are burned, but also adjacent friction zones. As a result — a tangible reduction in the brake pad resource. Partly this problem can be solved by taking the brake disc. But the taching is the removal of metal, which leads to a decrease in heat capacity, and here, as they say, «see the first item.»
Thus, we understand that the decision to replace the brake disc — the question is multifaceted and requiring a professional approach. We hope this article will help you correctly evaluate the state of the brake discs and give customers a reasonable assessment of potential savings when replacing them.