Why is a car able to stop properly the moment you step on the brakes?
Part of the answer lies in a precision metal part just a few millimeters in diameter. Seiko Instruments supports the world's automotive braking systems behind the scenes, based on the precision processing technology cultivated in watch manufacturing. With a sense of responsibility toward protecting people’s lives through technology that cannot be seen, we continue to deliver high-quality automotive parts to the world today.
We spoke with two employees from the Precision Devices Division.
The three major elements for a car to run safely are running, turning, and stopping. Among these, stopping is a function that affects the lives of the occupants.
When you brake suddenly, the tires can lock up and cause you to skid. Once a skid begins, not only does the braking distance increase, but you also lose steering control. It is extremely difficult for a driver in a state of panic to respond calmly. The ABS (anti-lock braking system) was created to solve this problem.
Figure: Conceptual diagram of the hydraulic unit in an ABS
ABS is a system that prevents tire lock-up by controlling the hydraulic pressure of the brake fluid at high speed. Multiple valves (also called electromagnetic valves or solenoid valves) are incorporated at the heart of the system, and each valve within the hydraulic unit precisely controls the hydraulic pressure to optimize the braking force for each of the four wheels. Furthermore, modern cars are also equipped with ESC (electronic stability control), which corrects not only for sudden braking but also for skidding during cornering and unstable vehicle behavior. ESC is a system that is a further evolution of ABS, and with numerous sensors such as a yaw-rate sensor (a gyro sensor that detects vehicle skidding), it monitors the vehicle's behavior in real time while performing more precise hydraulic control.
In recent years, the development of autonomous driving technology has been accelerating around the world, and in Japan, from 2025 onwards, field testing and commercial operation of Level 4 autonomous driving have begun. In autonomous vehicles, in order to finely adjust the speed even when driving straight, the frequency of brake operation increases significantly compared to conventional vehicles. Because of this, further durability is required for the parts.
At the core of these systems, the valves that function as on/off switches for hydraulic pressure are precision metal parts manufactured by Seiko Instruments.
Many people, when they hear the word “seal,” may think of rubber O-rings or gaskets, and the word “sealing” may evoke caulking paste. However, the technology used in ABS valves is “metal sealing” technology, which controls the flow of liquid by bringing metal parts into direct contact. The valves have the role of independently controlling the brake pressure for each of the four wheels.
The specific mechanism is simple and rational. A part with a spherical surface (approx. 1.6 mm in diameter, approx. 23 mm in total length) and a part with saucer-like surface (approx. 3.7 mm in diameter, approx. 7 mm in total length) are combined, and the flow of brake fluid is controlled at their point of contact. Since the contact between the surfaces of the sphere and the saucer is “line contact,” it is easier to achieve a high-precision seal than by fitting two flat surfaces together. With contact between two flat surfaces, a minute angular deviation immediately leads to a loss of sealing performance, but a feature of the sphere and saucer combination is that it is less susceptible to this effect.
Also, if a valve becomes stuck in a sealed state, it can no longer be opened or closed. With line contact, precise opening and closing control is possible in a short amount of time, which supports the high-speed responsiveness of the ABS. This combination of sphere and saucer surfaces has become the world standard and is widely used in ABS units from various companies around the world.
The biggest difference from rubber seals is that it does not presuppose deformation. In ABS and ESC, the valve repeatedly opens and closes frequently, so resin materials like rubber would wear out and could not last for the lifespan of the car. Therefore, it is necessary to ensure a seal with metal, which hardly deforms. In other words, the precision of the part itself and the smoothness of its surface are directly linked to the sealing performance. The precision is in the world of microns (1/1,000th of a millimeter) to submicrons (and smaller). If there are even slight irregularities on the surface, oil will leak from them. The applications for such precision parts are not limited to ABS. Similar metal sealing technology is also used in the injector, a device that supplies fuel to the engine, and insufficient precision carries the risk of fire from fuel leaks. Brakes and engines are both critical safety components where malfunction is not permissible.
Although metal sealing has a simple structure, high machining precision that satisfies multiple requirements simultaneously is required to ensure that it functions reliably.
First is the balance between sealing force and opening/closing performance. If the seal is too strong, a large force will be required to open the valve, making the high-speed and precise brake control required for ABS impossible. It is necessary to simultaneously meet the contradictory requirements of creating a secure seal with the minimum necessary pressure while also being able to open it instantly.
Next is the finishing precision. Any microscopic irregularities on the metal surface will become a path for oil leakage, so it is necessary to polish the surface to a near-mirror finish in finishing processes such as grinding and barrel finishing.
To guarantee this precision, it is essential to have high-precision measuring equipment capable of submicron level measurement, and the experience and skills of inspectors who can detect abnormalities in measured values. The prerequisite for quality assurance is not only being able to stably make parts with high precision, but also being able to measure them correctly.
“Making one perfect part is relatively easy. The difficult part is continuing to make tens of millions, or hundreds of millions of them per year with exactly the same quality and within the same tolerances,” says Mr. Sekiguchi of the PMD Sales Section.
One automobile uses 8 to 12 ABS valves. Since they are installed in several million cars worldwide per year, a simple calculation shows that tens of millions of the same parts are needed annually. Moreover, each part must exhibit the same performance without any variation.
The strength of Seiko Instruments lies in its ability to perform integrated in-house production, from cutting and cleaning to heat treatment, barrel processing, grinding, plating, and automated inspection. This is a rare system even for the industry, and in the unlikely event that a problem occurs, we can quickly investigate the cause and respond. It is also a major advantage in terms of cost and lead time.
One of the biggest challenges in mass production is the heat treatment process. Metal parts are heat-treated after cutting to increase their hardness, but during this process, the metal structure changes, causing slight dimensional changes and distortion. In the case of complex shapes like ABS valves, this deformation directly affects sealing performance.
The countermeasures are multifaceted. First, the precision settings in the previous process are determined by calculating backwards from the required values of the final product. Next, to minimize deformation during heat treatment, we select the processing conditions and equipment that are suited to the part's shape and material, and then perform the heat treatment. After that, the shape after heat treatment is adjusted by grinding or barrel finishing (a process of putting the parts in a container with abrasives and rotating it to polish the surface). Furthermore, we inspect all parts by conducting an air leak test, in which we press a reference sphere against the saucer-shaped part and apply air pressure to measure the amount of leakage.
“There are aspects of distortion from heat treatment that are difficult to capture numerically. Experience and intuition are also important factors, and if the final sealing performance cannot be guaranteed, it is extremely important to accurately identify the cause,” explains Mr. Sato of the PMD Engineering Section 2.
There are also challenges specific to mass production in the cutting process. As cutting tools (blades) are used continuously, they gradually wear down and their shape changes. Even a slight change in shape will affect metal sealing parts that require submicron precision. A management system for replacing tools at the right time and maintaining quality consistency before and after replacement is the foundation that supports mass production quality.
Contamination of the braking system with foreign matter is also a major risk. If fine dust or metal fragments enter the valve, the sealing performance will be compromised. At our factories, we implement multiple countermeasures, including wearing work clothes that minimize contamination by foreign matter, thorough management of gloves, precision cleaning of parts using dedicated cleaning equipment, and even contaminant inspections before shipment.
Another particularly noteworthy point is the establishment of self-management points that exceed the customer's drawing requirements. For example, there are cases where we manage the customer's specified tolerance at half the value, or manage surface roughness that is not specified in the drawings according to our own standards. While management strictly according to the drawings might be acceptable for a single prototype, in mass production of tens of millions of units, the accumulation of variations within the tolerance will affect quality. To catch early signs of quality abnormalities, we have also established a system within our factories to monitor production data online in real time, enabling the immediate detection of anomalies.
As the practical application of autonomous driving technology advances, the requirements for braking systems are also changing. In self-driving cars, the automatic brakes are activated frequently, so improved durability of parts is required. There is already starting to be demand for the support for non-conventional surface treatment technologies such as diamond coating.
In addition, the increase in vehicle weight due to the development of electric vehicles (EVs) and hybrid vehicles (HVs) is also creating new demand. There is a growing demand for more precise control of suspensions that support heavier vehicle bodies, and the application of Seiko Instruments' precision parts to hydraulic and pneumatic suspension control systems is expected.
Furthermore, our sights are also turning to the space industry. As the Japanese government promotes the expansion of the space economy, we are exploring expansion into the space field as a strategy to diversify our business and break away from dependence on the automotive industry's business cycle. The fields where precision machining and metal sealing technology can be utilized, such as medical equipment, aerospace, and robots, are still vast.
Seiko's precision cutting parts sector has production bases in China (Dalian) and Thailand (near Bangkok). The entire mass production process is completed at our overseas factories, and we have a system in place to ship directly to automobile manufacturers around the world. Our customers include major global automotive parts manufacturers.
“Our parts support the safety of cars all over the world. “It may seem dull, but I feel that the thorough pursuit of quality will lead to initiatives that lay the foundation for people around the world to live with peace of mind,” Mr. Sekiguchi says emphatically.
It may not be flashy, but there is a definite sense of pride and purpose. Precision machining technology that supports global safety behind the scenes. Its stage continues to expand globally.
Interview support
Left to right: Mr. Sekiguchi of the PMD Sales Section, Precision Devices Division; Mr. Sato of the PMD Engineering Section 2.