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| Abstract Today, safety brakes are an indispensable safety feature in modern elevator drives. Our power transmission, the global market leader of elevator brakes, is highly concerned about elevator safety and strives to contribute to the safety of the elevator industry. Therefore, Our has always designed safe and reliable elevator brakes. All our electromagnetic safety brakes operate according to the fail- safe principle – they are closed in a de-energized condition. But what makes an elevator brake safe and reliable? The following points should be taken into account. 1. The right static friction coefficient Holding the elevator in position requires a certain static torque. Such torques must be provided under all specified ambient conditions (0° to 40° C) throughout the entire lifetime. 2. Friction work The static torque applies when the elevator car is reliably held in position. In the case of an emergency, however, the brake has to slow down the car to a halt. So dynamic torque is important for a safe and reliable brake as well. 3. Switching times A constant switching time throughout the entire lifetime is essential in order to fulfil the requirements of EN81-20. A reliable brake must have a reproducible switching time in order to keep the specified braking distance. 4. Stroke of the brake The stroke of the brake is important to release the brake and guarantee a reliable signal that must be received from the micro switch. The risk of malfunction at high temperature is immense in case of thermal expansion. 5. Noise testing When a brake is new, switching noise is basically nonexistent and lies below 50 dB (A), a value many brakes in the market achieve. However, after several million switching operations the brake should still be silent – a much bigger challenge. |
| 1. Introduction The world of buildings is defined by superlatives – landmark skyscrapers impress people worldwide and the chase for the highest tower never seems to slow down. Well hidden behind steel and glass, innovation does not stop. Leading elevator companies are continuously setting new standards in their industry and pushing the limits of what is possible. This development brings some challenges, especially in the field of elevator drives. The elevator market demands smaller, faster and cost-optimized drives. Obviously, the safety brake is one of the most important drive components for an elevator. With the ongoing trend towards higher power density, the requirements within the specifications of leading elevator manufacturers increase as well. Some requirements are new or higher than in the past and have to be evaluated first. Another point to be considered: The brake has to fulfil the requirements over the entire lifetime, not only when it is brand new. There is no compromise on safety, especially when health and safety of people is at stake. Therefore, a proper qualification process for brakes that enter the market is mandatory, ideally together with drive manufacturers. All new requirements must be checked in order to avoid any problems in the field. On the other hand, we as manufacturer of safety brakes recognize that the elevator industry has to be increasingly cost efficient in order to survive in the aggressive market. That confronts us with some serious questions: How does cost cutting affect safety? More important: Can we shrink budgets and still keep up safety levels? The difficulty of reflecting the full effects of cost cutting is surely one of the important challenges that the elevator market faces. Thus, a complete understanding of how cost and production pressure affect safety is important to be able to prevent the undesired effects that may arise. Cost cutting is necessary, but it should be done with caution. We, as a leading global brake manufacturer, follow the new standards work hand in hand with our partners to find solutions for the new requirements. To achieve a solution for the cost-safetyproblem, however, we have to answer another question first: What makes an elevator brake safe and reliable? 2. How Safe Elevator Brakes Work Today, safety brakes are an indispensable safety feature in modern elevator drives. To be really safe, these brakes have to comply with basic safety principles that we as brake manufacturer consistently follow during development and manufacturing. First of all, the electromagnetic safety brakes operate according to the so-called fail-safe principle - they are closed in a deenergized condition. The braking torque is generated by the force stored in the thrust springs as shown in picture 1. When the magnetic coil is energized, a magnetic field is built up which attracts the armature disk against the force of the springs, thereby releasing the rotor with the friction linings. The brake is released. This condition can also be achieved using an integrated hand release. The brakes developed for the elevator sector are mainly used as holding brakes, which means that they hold the elevator cabin in its position once it stops regularly. However, in case of EMERGENCY STOP braking actions the brakes also have to take on friction work. This kind of dynamic brake action requires deep know-how regarding the whole friction system. This is a very short description of a secure elevator brake. A deeper look into the respective points makes clear what it needs to make an elevator brake safe and reliable. Five key points have to be considered: 3. The Right Friction Coefficient Elevators are getting faster and faster – a clear trend in the elevator market. To keep up with growing speeds, brakes have to be increasingly powerful. The friction system, consisting of brake lining and friction surface, is the basis for the technical dimensioning of the braking system. The most important tribological system parameter is the friction coefficient μ. The brake torque over lifetime is a result of the following formula: M = Fmin x N x μmin x r m [Nm] The variables are as follows: Fmin Spring force [N] N Amount of friction surfaces [-] μ friction coefficient [-] r m Friction radius [m] One of the main tasks for a brake manufacturer developing a new friction material is to achieve a friction coefficient μ that is stable within the framework of the dimensioning. Therefore, they have to attend to the variables influencing the determination of the friction coefficient. The effects of ambient conditions like temperature on the friction coefficient μ have to be considered as well as a period of use spanning more than 20 years. The friction coefficient of friction linings is in most cases tested on a flywheel test stand like the one on picture 2. An electric motor accelerates a flywheel, the tested braking system brakes it down. As a brake manufacturer we are able to test various ambient conditions. It is important that the friction coefficient keeps constant even at low temperatures, as shown in picture 3. The friction lining has to show a constant coefficient between 0 and 40 degree Celsius. 4. Friction Work The static brake torque is sufficient to hold the elevator car in position under normal conditions. But in the case of an emergency, it is important that the brake system provides enough dynamic brake torque as well. In these cases, the brake has to engage quickly in order to reduce the acceleration of the car and help bringing it to a standstill. New drive generations are working at much higher speeds than ever before. That leads to higher friction work if an emergency occurs and a dynamic braking is necessary. The latest specifications show substantially higher friction values than in the past. That puts manufacturers in front of some challenges when using standard friction linings: Their braking torque may fade at high-speed braking. The reason is the high temperature on the friction surface that reduces the friction coefficient, as shown in picture 4. The fading of the friction coefficient during the braking can lead to critical situations. It may even happen that the brake is no longer able to decelerate the car – with unforeseeable consequences for material and passengers. The brake manufacturer therefore has to find a friction material with a stable behavior even during excessive friction work. 5. Stroke of the brake The stroke of a brake, i.e. the distance the armature disk has to move before the brake is closed, is an important factor for its reliability. The bigger the stroke, the more reliable micro switches work. As a manufacturer of reliable safety brakes, we therefore recommend a stroke of more than 0.32 mm. But why is that? High temperatures have an effect on the micro switch function. Increasing temperature will lead to expansion of the material and therefore change the operating points of the switch. The smaller the air gap, the more difficult to find the right switching point – if the brake gets only a little too warm and the material expands too much, the micro switch can be triggered unintentionally. This may result in serious consequences, even to the point of malfunctions of switches in the field with all kinds of unwanted effects. In order to provide reliable braking systems, the brake manufacturer therefore has to find the right balance between minimum stroke for short switching times and a sufficient air gap securing the function of the micro switch. Picture 5 shows the interrelation of temperature and stroke. 6. Noise Testing Now we learned that a minimum stroke of >0.3 mm is necessary for the micro switches to work reliably, so the brake is designed to provide that air gap. A larger stroke, however, comes at a price: There is a substantial disadvantage regarding noise levels. The larger the stroke, the louder the brake. Elevator manufacturers, however, require silent brakes to make their products as comfortable as possible. If a brake with a large stroke has to remain silent, ideally over its entire lifetime, a very stable damping system is required. Such a damping system is characterized by the fact that it is not only quiet when new, but also after several millions of switching actions with a stroke of >0.3 mm. A stable damping system can be achieved by adjusting each damper individually, as can be seen in picture 6. This ensures that each brake can be individually adjusted to a low noise level. To get a production release for the brakes, they have to be tested extensively. Each brake goes through five million switching operations in our test lab, shown in picture 7. In a special noise measurement room the noise level of the brake is tested in order to ensure its noise emission is within the limits. The graph in picture 9 clearly shows the results of these efforts. Even after 3.5 million switchings, the noise level of the brake does not exceed 60 dB and therefore lies under the noise level of other drive elements, such as motor or gearbox. Under normal circumstances this means that the brake is not audible within the elevator, even after years of regular use. 7. Switching times ın order to fulfil the requirements of the elevator codes, the switching behavior of the brake must not vary over the entire lifetime. The switching time of the brake has to remain within the limits of the specification in order to calculate the right stopping distance of the elevator car. And here again the noise prevention comes into play: The damping system has a substantial influence on the switching time. This comes with a degeneration process some dampers undergo over time. Temperature for example has a major influence on the aging of the damping system. There are damping systems on the market where aged or completely destroyed dampers can cause a significantly longer switching time. In a worst-case scenario, the brake switches too slow to guarantee the correct stopping distance and the elevator car shoots over the defined halting point. The reasons that switching times change over time, or in turn keep constant, are complex and the difference lies in the detail. To ensure reliable switching times, all brake components have to be dimensioned safely and have to be made of high-quality, known and proven materials. Furthermore, brakes should be equipped with a stable, reliable damping system. Basically, every brake comes with a short switching time when it is new. However, the same is true here as it is for the noise level – the long-term behavior is important. Long-term tests confirm that operational switching time levels remain low even after 300,000 switching actions. This holds true for an even longer lifespan. The switching time of our brakes is still substantially below the level of other elevator brakes in the market after several million switching actions. The graph in picture 10 shows how the switching time hardly changes anymore once the brake is run in. 8. Summary To sum up the criteria detailed before: A safe and reliable elevator brake has a friction system that is made for the purpose, so it provides a constant and sufficient braking torque over its entire lifetime. It has a sufficient stroke that allows for micro switches to work properly, even when the brake heats up. And the brake keeps a constantly low noise level and a switching time compliant to the specification even after several millions of switching actions. Most problems in the field can be traced back to one of the points described: Increasing noise and switching times due to degrading and eventually failing dampers, lack of braking torque due to wrong dimensioning, insufficient torque calculations and unfit friction systems, failing micro switches because of insufficient stroke of less than 0.2 mm. This all shows that innovative solutions and deep know-how make a high-performance density possible and thereby reduces costs as far as possible while still providing safe and reliable brakes. However, there are certain limits. Especially when people are involved, there must not be any compromise on safety. Our aims for the best compromise between safety, reliability and costs. A technologically leading friction system, together with consistently-observed safety principles make us able to produce safe, reliable brakes with an extremely high performance density. In combination with monitoring modules of the latest generation, which enable permanent brake monitoring, this is what we call brake technology 4.0 – a perfectly coordinated complete system. |
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