This paper presents a new design concept of buffer for elevators, which is based on dissipating impact energy by means of non-recoverable plastic deformation, but maintaining similar performance requirements of typical hydraulic buffers on a wide range of impacting masses and velocities. Firstly, it is presented an aluminium tube with specific geometry which provides folding behaviour under impact conditions. The design process and geometry optimisation are performed by means of explicit finite element (F.E.) crush simulations with the adequate material model for the aluminium, including strain rate stiffening of the material. The tube geometry is optimised for fulfilling a maximum average acceleration requirement within the specified ranges of cabin mass and impacting velocity, being finally validated with experimental results on full size prototypes. An alternative concept is shown as well, as combination of plastic deformation buffer with a simplified oil-based damper to satisfy critical vertical strokes requirements without modifying general geometry of buffer.
- INTRODUCTION AND OBJECTIVES
Elevators are always equipped with emergency braking systems which ensure that cabin is stopped when an overspeed situation occurs. These systems are calibrated to be activated for stopping the cabin when a speed over a 115% of nominal velocity occurs. However, in other particular situations, the cabin could fall into the pit from the bottom floor position and reach its stroke end before reaching this overspeed limit. In these situations, overspeed brakes would be therefore not actuating, causing a safety risk for elevator users.
To cover these emergency situations, buffers of different types can be disposed in the pit to stop the cabin under safe deceleration conditions. Nowadays, the best technology to achieve this function is based basically on hydraulic buffers, which are usually expensive devices compared to the general cost of elevators.
The present paper proposes a new design concept of buffer for elevators, which is based on dissipating impact energy by means of non-recoverable plastic deformation (progressive folding behaviour), but maintaining similar performance requirements of typical hydraulic buffers on a wide range of impacting masses and velocities, with simple geometries and easy manufacturing processes and, therefore, of reduced cost.
The work is presented in three parts according to their specific objectives: In the first part, the new buffer concept is designed for fulfilling the deceleration specifications (see Figure 1) on a particular case of cabin mass and velocity. The geometry of the buffer is fitted by means of virtual prototyping techniques, in this case, explicit finite element (F.E.) simulations which are later validated by experimental cabin free fall tests on full size samples. The use of F.E. simulation includes to develop specific material models and to adjust the boundary conditions which reproduce adequately the real conditions of the buffer in the test. In the second part, buffer design is optimised for fulfilling same deceleration specifications under defined ranges for cabin mass and velocities using same F.E. techniques. Finally, it is conceptually adapted for specific design conditions which include elevator installations with limited pit depth.
- GENERAL DESIGN SPECIFICATIONS
The general specifications for the new design of plastic deformation buffer concept are detailed below, according to the previous objectives:
- First concept design for fixed cabin mass and velocity:
- Cabin mass: 1000 Kg
- Speed: 115% of 90m/min (103.5 m/min)
- Average sustained deceleration: 1G
- Deceleration peaks under 2.5G or shorter than 0.04 seconds.
- Second optimised concept design for ranges of cabin mass and velocity:
- Cabin mass (m): from 700 to 1300 Kg.
- Speed (v): from 22% to 115% of 90m/min (20 to 103.5 m/min).
- Final concept solution for limited pit depths, adding following specifications:
- Stroke: Maximum, 180 mm.
- Device length: Maximum, 600 mm
- DESIGN OF PROGRESSIVE CRUSH BUFFER BASED ON FE-METHOD FOR FIXED CABIN MASS AND VELOCITY
3.1. Conceptual design: shapes and materials
In the previous section, the general specifications that must fulfil the elevator buffer in this preliminary design phase have been presented. The concept design is based on absorbing the impact energy of the elevator cabin in free fall conditions by means of non-recoverable plastic deformation of the buffer, as it is done in vehicles design, where energy absorption is solved by means of engineered structures with tailored crashworthiness characteristics (Xiang et al. 2006). For this, a novel concept design, newly applied in elevator industry, which is based on an aluminium tube with progressive folding behaviour is presented. The folding behaviour is ensured by means of an specific geometry of the tube. Figure 1 shows the ideal deceleration curve defined as requirement for the component.