Progressive Safety Gears and Engineering

I parked my car on a busy street in Ankara. I had two problems at that moment. One of them was to take the elevator construction documents to the building audit company and have them approved by the engineers there, and the other was to get in my car before it is pulled over. I entered the building almost breathlessly, but there was no one there except a secretary for the building audit company. I was going to leave the documents there and then get them back after they were approved by the mechanical and electrical engineers. I was about to hand them to the secretary and leave the office when she asked me to wait and went to the next room. The noise of three stamps broke the short silence. The secretary returned and gave me the documents and told me it was done. I checked the documents; the project was really approved completely. I was happy to finish it off so quickly, but I was also surprised because the signatures were taken, but there were no engineers around!

The lines above describe an incident I experienced 3-4 years ago, when I was working at an elevator installation company. For me, it is a noteworthy incident as it indicates how the engineers are positioned within the industry. Considering all the calculations, designs, projects and so forth, the elevator industry could be a wonderful field; however, in our country, where there are thousands of elevator companies, the situation of the sector and the position of the engineers do not seem good at all!  Although there are good examples where the engineers offer added value and demonstrate all their engineering skills, the general condition at the elevator companies is not much different than the example above. It is tragic that there are some companies who use the freelance consultancy and engineering certificates of the retired engineers, giving them the half of the minimum wage,  and my retired colleagues accepting such a situation. By doing this, they lower the wage band and contribute to the fact that many engineers who are active in the industry but do not get the wages they deserve change sectors at the first opportunity, and the newly graduated engineers are unemployed.

A parachute system, which is vital for the safety of elevators, is a complete engineering subject, as it requires delicate calculations, design and legislation knowledge.  Contributing to the industry with original designs and creating brand-new products is directly related to the employment of engineers. This is why I started this article with such an introduction.

Parachute systems, which are vital in high-speed elevator in terms of passenger comfort and safety, are not appreciated as much as they should be. Finding cheap safety gears in the market is an indicator of this. When the manufacturers and the contractors negotiate, safety gears are generally in the shadow of cabin model choices. However, the less-dangerous parts of the elevators are the parts that are visible to the end users, while the invisible parts — like safety gears — are the parts that come with more risks. In this article, we will talk about the progressive safety gears that are obligatory for elevators with a rated speed exceeds 1 m/s.   

Directive 2014/33/EU defines six products as safety components. The overspeed governor and brake system, which are two of them, work together to maintain safety. The function of the overspeed governor is to keep the safety rope stretched enough and to stop the elevator when the rope loosens. If the suspension ropes carrying the elevator shear off, or the downward speed increases extremely, a regulator rope, which is conjugated with the cabin, one side of which is connected to the cabin, is pulled. On one hand, the electrical safety device on the safety gear cuts of the supply of the traction system and stops the machine room and, on the other hand, the progressive safety gear grips the guide rails, claming the elevator to the rails.

The simplest way to stop a moving object within a certain period of time by absorbing its kinetic energy is to convert its kinetic energy into heat through friction. In general, when a safety gear mechanism is activated, one half of the safety gear that is connected to the cabin is contacted with the rail through springs, and its other half is pressed to the rail through the wheelhouse, which is called “roller.” The rail is clamped between these two parts. The normal force that is generated turns into a friction force and starts using the kinetic energy of the cabin. The friction continues until the cabin stops.

Progressive safety gear is a mechanical system. It can be installed to the lower or upper supporting girders of the cabin suspension. If there is a living space like a residence or office under the travel range of the counterweight, the counterweight should also be equipped with progressive safety gear.

 Acceleration is the amount of speed that changes over time. In order to maintain the safety of life and property, braking acceleration should be between 0.2.gn and 1.gn. Ideally, the brake should engage with an average acceleration 0.6.gn (gn, which is gravitational acceleration, is equal to 9,81 m/s²). High acceleration braking means the cab is stopped in a short distance. For example, if an elevator brakes with a braking acceleration of 1.gn , it will stop sharply as the shifting distance is short. If the acceleration increases more, sudden braking generates is a great risk for children, the elderly, the sick and pregnant women. If the braking acceleration is lower than 0,2 gn, then the shifting distance gets longer. Therefore, the directive mandates a braking acceleration that is 0.2 – 1 times more than the acceleration of free fall.

Safety Gear Tests at Registry Inspections

With the full load braking test, rails, console connections and the conformity of the machine frame installation are tested in addition to the safety gear before the elevator is out into service. During the safety gear tests in registry inspections, the shifting distance has always been the subject of debate.

For the registry inspection of elevators that are installed in accordance with 2014/33/EU Directive, by article 6.3.4 of TS EN 81-20, the safety gear test is made while the cabin is loaded with the 125% of its rated load.

The braking distance based on the activation of the overspeed regulator at a rated speed of 1 m/s is calculated with the above formula.

After the safety gear test, the cabin is positioned in a way that allows for measurement of the marks on the rails. The height and width of the mark on the rails is measured. It is checked that the brake leaves a mark on both sides and grips the rail. It is observed if there is any distortion or breakdown on the cabin. According to the results of the load test, an elevator that brakes with a rated speed of 1 m/s is considered compliant if the safety gear slides 5 cm minimum and 25 cm maximum.

In order to release of a safety gear on the car, a command on the opposite direction should be sufficient. In this way, it should automatically reset its pre-test mode, and the cabin and its frame should be freed from the rails. The elevator should not operate right after the safety brake is released; commissioning should only be possible after the intervention of competent maintenance personnel.

Each safety gear block that forms the brake band is connected to each other with transfer rods that help the engagement of the brake blocks simultaneously. The synchronization of two brake blocks will be better if a solid material is used in these connections.

As an engineer working at manufacturing industry, for me it is necessary that the cabin manufacturers also manufacture the safety gear that will be used in their cabins. In this way, when there is a problem with the safety gear, the customers will be able to find a single responsible party, and the manufacturer will have to find a solution.

I strongly recommend that safety gear manufacturers should accompany the Type Inspection Tests of their safety brakes that will be carried out by a Notified Body. It may sound strange, but the first tests should fizzle out. This means the manufacturer will be able to see that the braking acceleration of the tested safety gear is beyond the ranges allowed by the standard. They should even see that the brakes fail and the elevator crashes. In this way, the manufacturer can understand the importance of their business and will have the opportunity to see the negative scenarios that may be experienced in the future before the product is put on the market. Seeing that a safety gear is tested dozens of times, the manufacturers will learn from their failures and change their approach and design and, as a result, will manufacture a product that is closest to ideal. Just the way Edison invented the lightbulb after thousands of failures! Because, disasters are irreparable after the product is placed on the market.

References

TS EN 81-20:2014
TS EN 81-50:2014
2014/33/EU Lift Directive