Elevator Buffers

Elevator buffers are designed to stop descending cars and counterweights from free falling beyond the lowest stopping point to the base of the elevator shaft. They have to meet a number of specifications in different regions worldwide.

One key performance criterion is that the average deceleration must not exceed 1g when stopped in either load condition. This is achieved by precise control of hydraulic oil flow across an orifice throughout the buffer stroke.

Type Tests

Elevator buffers are an essential safety device, which prevent the elevator from falling or crashing in a number of ways. They are also used to absorb energy when an elevator carriage is overrun in the shaft or exceeds its travel limit. These are the main reasons why elevator buffers need to meet certain specifications and be type tested.

There are different technical specifications for elevator buffers in different parts of the world but all employ the same basic performance criteria. They must absorb the kinetic energy of an elevator car, bring it to rest and then retract to its minimum length in order to protect the passengers inside it.

The performance of an elevator buffer is achieved through the careful control of hydraulic oil flow across an orifice during the buffer stroke. This can be done by fine tuning the size of the metering orifices in the plunger, which is controlled by a solenoid.

As with all hydraulic components, oxygen in the atmosphere is a key accelerant in the deterioration of the oil and can accelerate the degeneration process. Therefore, a number of measures are taken to delay the degradation of the buffer oil, which includes the use of high-quality hydraulic oils with low oxidation stability.

These oils are available in various viscosities and have been tested to be compatible with a wide variety of elevator engines. They can also be formulated to resist heat and provide long-term protection against corrosion and wear.

In addition, these high-quality oils have been carefully tested to ensure that they offer outstanding noise and vibration NVH isolation, which can help reduce the cost and weight of elevator buffers by minimizing the impact of engine noise on the passenger cabin.

A simplified FE model for the collision between an elevator and a hydraulic buffer has been established by imposing a loading input in the form of acceleration and velocity curves. This model has been validated through theoretical calculations, FE simulations and experimental tests, which have matched well.

Stroke Requirements

An elevator buffer is a safety device that decelerates an elevator car or counterweight to bring it to a stop. Buffers are usually used on traction elevators that have a rated speed of at least 200 feet per minute and preferably more than that. These are typically mounted beneath the elevator car or counterweight in the pit and require regular maintenance and inspection to ensure they meet their performance specifications.

Ideally, an elevator buffer should decelerate the descending mass with a constant retardation force over its entire stroke. This is achieved by precise control of the hydraulic oil flow across an orifice in the buffer. However, this can be difficult to achieve in practice since the lift car mass is highly variable and the braking force on the cab or counterweight is not uniform over the entire buffer stroke.

As a result, many elevator manufacturers use an oil buffer that uses a combination of springs and oil to cushion the descending mass. This type of buffer is prone to being exposed to water and flooding, so routine cleaning and painting are required to maintain its performance specifications.

Another common type of elevator buffer is a plunger-type oil buffer. These are also more commonly found on traction elevators and are typically mounted beneath the elevator car or counterweight. These types of buffers are also subject to the hazards associated with a descending car or counterweight and must be designed with special care.

The length of an oil buffer must be such elevator buffer that its slenderness ratio (piston length over radius of gyration of piston cross section) is not more than 80. This slenderness ratio is important to ensure that the descending car or counterweight is not contacted by the overhead structure during its braking action, as well as to keep the piston diameters at a reasonable level.

Reduced stroke oil buffers are allowed for new installations where the maximum load on the car or counterweight is rated at less than 4 m/s (800 fpm) and where an emergency terminal speed-limiting device is installed that conforms to 2.25.4.1. A reduced stroke buffer may be one-third the stroke required by 2.22.4.1.1 for rated speeds under 4 m/s (800 fpm), or, for rated speeds exceeding 4 m/s, three-fourths the stroke required by 2.22.4.1.1, whichever is greater.

Height Requirements

The height of a pit buffer arrangement is critical to provide adequate cushioning or energy-absorbing capacity at the bottom of hoistways. Having sufficient buffer size typically requires deepening the pit, which can be expensive and essentially slow to carry out.

The idealised performance of an elevator buffer is to limit peak deceleration force to average 1g over an extended period. This is achieved by precise control of hydraulic oil flow across an orifice throughout the buffer stroke. Unfortunately, this can only be achieved for one specific impact mass that is not realistic in a real world where the elevator car mass varies with passenger load.

Therefore, a compromise must be found. An alternative method is to design a pit buffer arrangement with a number of buffers spaced apart sufficiently so that the vertically moving mass (i.e., a counterweight) can be received between the buffers.

An example solution is illustrated by the example arrangement shown in FIG. 1. This arrangement includes a counterweight 24 positioned on a guide rail 66 that guides its vertical movement in a known manner. The counterweight is also spaced between a plurality of buffers, each having an adequate stroke to provide the required energy-absorbing characteristics within the elevator system.

A stem part is fixed to the base of the buffer and a stop element with an adjustable height position fitted in connection with it. The stop element includes a stopping surface for the buffer of the elevator and an extension part in connection with it for increasing the height position of the stopping surface.

In the example, the stop element is disposed to rest in the normal drive mode of the elevator if the elevator car for some reason drives so far downwards that it collides with the elastomer buffer 3a. In this case the stop flange 9 of the stop element 7 is fitted to receive a collision of the elastomer buffer 3a.

In the test procedure, the speed of the car when it hits the buffer is determined based on its mass and the travel height. The resulting impact speed is then used to calculate the elevator buffer guided travel and determine the maximum buffer clearance between the buffer and the car or counterweight. The results are then compared to those obtained using the corresponding theoretical model. Moreover, relevant test verifications are conducted.

Design

An elevator buffer is a safety device that is mounted at the base of an elevator shaft. These devices must meet certain design requirements and perform as designed. These requirements vary from one region to the next but all employ a basic performance criteria that ensures that elevator cars are brought to rest in a safe manner.

The most common type of elevator buffer is the hydraulic buffer, which is used to cushion the impact of the elevator car against the floor when descending to a lower floor. Hydraulic buffers have a throttle shaft and an orifice that controls the flow of hydraulic oil throughout the buffer stroke, which allows for the smooth and controlled deceleration of the elevator car during the buffering process.

However, hydraulic buffers are expensive to manufacture and difficult to test due to their size and complexity. They are also susceptible to damage and disconnection.

Therefore, a more compact and cost effective solution to the problem of designing an elevator buffer for high speed elevators is required. This can be achieved by implementing an alternative design technique to the traditional hydraulic buffer.

According to this alternative approach, a cab pit buffer assembly is formed by mounting a beam on the guide rails of both the cab and the counterweight. A number of conventional safeties are set on each beam to prevent it from moving downwardly.

The safety and retarding characteristics of the beam are then determined by a combination of its response acceleration, deceleration, velocity and displacement obtained from time response analysis. The results of the analysis are then compared with the desired performance specifications.

This is done by applying a simplified impact model to the 3D FE model of the elevator carriage. This provides a relationship between the elevator carriage’s acceleration and its time during the buffering process, and enables accurate evaluation of the desired performance for different types of input parameters.

This type of design approach is able to match the idealised performance, which is typically achieved by controlling hydraulic oil flow across an orifice for a specific impact mass. Nevertheless, it is not able to meet the demands of various passenger load conditions.