Analysis and Research on the safety performance of

With the development of the construction industry and the continuous increase of high-rise buildings, as a high-rise building exterior wall coating, decoration and property management cleaning, the high-altitude work basket has been adopted by the majority of users. The market of renting hanging baskets for work at heights is gradually taking shape in China's urban and rural construction market. However, due to a variety of reasons, accidents of high-altitude operation baskets falling occur from time to time in construction. According to the statistical data of the safety supervision department of Xuhui District, Shanghai, there were 10 accidents due to high-altitude operation equipment (including tower cranes, elevators and baskets) in 1995, 9 in 1996 and 13 in 1998, respectively, accounting for an increasing percentage of the total number of accidents this year, especially in 1998, which accounted for 16.5% of the total number of accidents this year, Although the basket is rarely used, there are four falling accidents caused by broken ropes. This not only causes personal casualties, economic losses and social impact, but also in the face of the increasing use of hanging baskets for high-altitude operations today, the safety and quality of hanging baskets has become the focus of attention. This requires that the design and manufacturing departments engaged in high-altitude operation baskets should strengthen the development of the safety performance of baskets. At the same time, it is imperative to formulate the safety and quality indicators of our baskets and standardize the domestic market behavior of baskets

this paper focuses on the further analysis and Research on the safety performance of the safety lock used by the basket of the overhead crane with the rapid development of science and technology all over the world

1 braking principle of safety lock

the braking structure of the common safety lock in China is a plane four-bar mechanism composed of a set of plates clamped by ropes, plus a double semicircular wedge-shaped groove mechanism formed at both ends of the connecting rod. The braking principle is that when the basket slips or falls, the trigger mechanism rotates the double crank of the four-bar mechanism clockwise through the elastic force of the torsion spring, forcing the rope clamp to clamp the steel wire rope to form the initial friction force. At the same time, the basket continues to move down, and the double crank continues to rotate clockwise under the action of the double semicircular wedge groove. This kind of braking process time is quite short. It is often called instantaneous braking, and its torque is proportional to the load of the basket, Finally, it meets the requirements of rope clamp a set of plate brake basket falling

according to the kinetic energy theorem, when the falling object wants to stop, that is, vertical braking, the main load is the inertial kinetic energy and gravitational work of the falling object's linear motion. The work done when the hanging basket breaks or slides down in the working process should be balanced by the braking kinetic energy of the safety lock, part of which is consumed to work against gravity. This part of energy is borne by the external clamping force of the automatic drawing mechanism composed of the drawing drum of the rope clamping machine, the magnifying pulley and ejector rod on the side of the jaw under the main machine, and the adjusting rod of the jaw on the main machine; The other part is consumed by the initial speed braking kinetic energy, which is borne by the material impact strength of the rope clamping mechanism components. Now take the 800kg hanging basket as an example. When the braking speed is 12m/min and the braking distance is within 100mm, the rope clamp safety lock is used for braking, and the estimated kinetic energy/potential energy is about 1/4. It can be seen that the design of the safety lock braking mechanism must select the mechanism form and verify the strength of the mechanism according to its different energy distribution characteristics

although the above view is only qualitative analysis, its braking phenomenon has been confirmed by the dynamic performance test of safety lock. In the test, it was found that due to the heat treatment defect of the rope clamp, the rope clamp could not be punched for 3 times, and its lug had been broken, and the semicircle of the sleeve plate was seriously deformed; The clamping energy of the safety lock is enough to make a new steel wire rope break after 7-10 tests, showing great friction effect

2 safety lock test technology

according to the requirements of safety performance assessment of safety locks, we designed and manufactured a set of test devices with novel ideas, simple structure, safety, reliability and high efficiency, and carried out a series of tests on various safety locks, such as free fall, sliding and static force

2.1 free fall test

1) test purpose: simulate the braking performance of the basket under the condition of broken rope

2) test device: mainly composed of test frame, lifting test bench with variable loading, decoupling device, shock absorber, accelerometer, electric hoist and other mechanisms. The height of the test stand is 2500mm, the outline dimension of the test stand is l100mmx600mmx700mm, and the maximum loading weight is 800kg

3) test method: manually install the safety lock, manually control the electric hoist to lift the test bench, manually control the decoupling device, conduct the braking test on the safety lock under the condition of free fall of residual internal stress in the extruded layer tissue under different loads, and record the test curve with an accelerometer

2.2 slip test

1) test purpose: simulate the braking performance when the machine fails and slides during the working process of the hoist

2) test device: mainly composed of test frame, lifting basket with variable loading, hoist, transmitter, working pulley, electric control box and other mechanisms. The height of the test frame is 2500m, the outline size of the basket is 700mmg600mmx450mm, and the maximum loading weight is 800kg

3) test method: install the safety lock manually, lift the basket with the electric control hoist, change the braking sliding speed, and trigger the safety lock braking under the monitoring of the speed transmitter including the centrifugal throw block or the electric control

4) test indicators: braking distance and braking speed

2.3 static test

1) test purpose: simulate the braking performance of the safety lock statically locking the basket

2) test device: the same as 2.2

3) test method: manually lock the basket on the safety rope and hang it for 48 hours under the loading condition of the basket

4) test indicators: slip amount, clamping spring stiffness

3 factors affecting the safety performance of safety locks

at present, there are two types of safety locks commonly used in China: centrifugal block clamping rope type and swing arm clamping rope type. Their rope clamping mechanisms are basically the same, which are different from the trigger mechanism. Therefore, the analysis of the factors affecting their safety performance can be summarized as follows:

1) the impact strength of rope clamping components is very important to ensure the safety of safety locks, Therefore, the material and heat treatment process must be strictly controlled

2) the sensitivity of the triggering mechanism (including speed or angle triggering) will affect the braking distance of the safety lock. The shorter the technical distance between the lead wire and lead frame of the brake IC, the greater the impact force, and even cause the damage of the mechanism; If the braking distance is too long, it does not meet the requirements of safety standards. Therefore, it is suggested that the braking distance value corresponding to the best reaction time of the safety lock locking mechanism should be used as the index to evaluate the allowable safety sliding of the safety lock

3) because the clamping spring is in the energy storage elastic force when loosening the clamp, the spring is in the compression or torsional deformation state most of the time in the working process. If its stiffness decreases, that is, the spring force value decreases after long-term compression, which will lead to the failure of the clamping force

4) as the safety lock is used in the open-air construction site, its friction coefficient in the braking force is unstable, resulting in the change of the braking force of the safety lock. Therefore, the safety lock must be regularly calibrated

5) this kind of safety lock is not convenient to operate, which will indirectly lead to human unsafe factors. If it is difficult to open after locking or the safety valve is too sensitive to affect the construction operation, the operators give up using the safety lock

in short, in order to ensure the safety performance of the safety lock, in addition to optimizing the mechanism design and improving the mechanism stiffness, due to the randomness of many influencing factors, great attention should be paid to the dynamic test of the safety lock when assessing the performance of the safety lock

4 ways to improve the safety performance of safety lock

1) select a reasonable rope clamping mechanism to meet the impact strength requirements. According to the comparative drop test of two kinds of rope clamping mechanisms, it is found that the modular rope clamping mechanism is superior to a set of plate structure. After the same five drop tests, the wedge rope clamping mechanism was disassembled and inspected, and there was no obvious deformation and wear, but the rope clamp lug was broken in a set of plate mechanism of the rope clamp. The first reason is that if the manufacturing accuracy and heat treatment requirements of this structure are not strictly controlled, stress concentration is easy to occur at the lug of the rope clamp; Secondly, the wedge rope clamping mechanism changes the stress bearing state, changes the shear stress on the rope clamp lug into the lock cylinder extrusion stress, and expands the bearing area of the lock cylinder, which is equivalent to increasing the impact strength of the lock cylinder rope clamping mechanism

2) to control the stiffness of the rope clamping spring, it is required that the spring force value in the clamping position is in a certain proportion to the self weight of the lock cylinder, so that the rope clamping mechanism can meet the purpose of not automatically loosening the clamp after the safety lock is unloaded; On the other hand, a small amount of impact force can loosen the lock cylinder. The stiffness of the rope clamping spring can be obtained through the static test of the safety lock

3) reasonable selection of trigger mechanism. Although different physical quantity triggers will have different trigger mechanisms, the general requirement for the trigger mechanism should be to meet the sensitivity limit required for accident triggering (making the response of the trigger produce a minimum excitation change value that can be detected). Therefore, evaluating the sensitivity limit of the trigger mechanism has become one of the safety quality indicators of the safety lock

4) in addition to the simple structure, the overall design of the safety lock also requires that the rope clamping mechanism is reliable and the reset mechanism is flexible in place under the environmental conditions such as open-air construction site and adverse weather, so as to achieve the purpose of safe use of the safety lock