Structurally speaking,
crane brakes are mainly divided into block type, disc type and belt type, among which block type brakes are the most widely used. It generates braking torque by the frictional force between the brake pads and the brake wheel. The magnitude of this frictional force directly depends on the tension of the brake spring, the friction coefficient of the brake pad lining, and the contact area of the brake wheel. Take the commonly used YWZ series block brake as an example. The friction coefficient of its brake shoe lining is usually between 0.35 and 0.45 (under normal temperature and dry conditions). However, when the temperature exceeds 200℃, the friction coefficient will drop sharply to below 0.2, and at this time, the attenuation of braking torque can reach more than 30%. In actual operation, the wear limit of the brake shoe liner is generally set at one-third of its original thickness. For instance, a 15mm thick liner must be replaced when it wears down to 5mm; otherwise, not only will the braking force be insufficient, but the brake wheel may also be scratched due to the liner cracking.
When selecting a model, many people only look at the rated braking torque, which is far from enough. The working level, load size and running speed of the crane should be comprehensively considered. For instance, for A5-level cranes (with moderate and frequent operation), the braking torque of the brake needs to be more than 1.5 times the rated load torque. For A7-level metallurgical cranes, this coefficient needs to be increased to 2.0 times, as high-temperature environments will accelerate the aging of braking components. Take a 10-ton bridge crane as an example. The rated torque of the hoisting mechanism is approximately 4,900 N · m, and the rated braking torque of the matching brake cannot be less than 7,350 N · m. For trolley mechanisms with a running speed exceeding 120m/min, braking deceleration should also be taken into account. Generally, it is required to be between 0.5 and 1.0m/s². If it is too large, it will cause the lifted object to sway; if it is too small, the braking distance will be too long. Both pose safety hazards.
Maintenance and upkeep are of Paramount importance. Many enterprises, in an effort to meet deadlines, often neglect the daily inspection of brakes and only regret it when problems arise. In fact, there are clear parameter standards for daily inspections: the braking clearance should be controlled between 0.8 and 1.5mm. If the clearance is too large, the braking stroke will exceed 15mm, and the response speed will be significantly slowed down. If it is too small, it may cause continuous friction between the brake shoe lining and the brake wheel, resulting in the brake wheel temperature exceeding the allowable value. Generally speaking, the working temperature of cast iron brake wheels should not exceed 250℃. Exceeding this value will cause the wheel surface to anneal, with the hardness dropping from HB200 to below HB150 and the wear resistance significantly reduced. During inspection, an infrared thermometer can be used for real-time monitoring. If any abnormal temperature is detected, the machine must be stopped immediately to cool down.
Hydraulic push rod brakes are the mainstream nowadays. Special attention should be paid to the hydraulic system of this type of brake. The oil level should be maintained at two-thirds of the oil tank mark. The oil quality should be tested every six months. When the kinematic viscosity exceeds 40cSt (at 40℃), it needs to be replaced; otherwise, it will affect the extension and retraction speed of the push rod. Under normal circumstances, the full stroke action time of the push rod should be no more than 0.6 seconds. If it exceeds 1 second, it indicates that there is a jamming phenomenon. In winter, it is also necessary to pay attention to the changes in the viscosity of the oil. When the ambient temperature drops below -10 ℃, low-temperature hydraulic oil (viscosity index ≥140) must be replaced to prevent the push rod from malfunctioning due to the solidification of the oil.
Let me give you a practical case. Last year, a gantry crane at a port suddenly lost its brakes while lifting heavy loads. Fortunately, the driver was experienced and promptly reversed the operation, thus preventing a major accident. Post-inspection revealed that the friction coefficient of the brake shoe lining dropped to 0.18 after being contaminated by oil stains. Coupled with the braking clearance reaching 3.2mm, the actual braking torque was only 40% of the rated value, far below the safety standard. This is a typical hidden danger caused by inadequate maintenance.
In conclusion, the parameter Settings of
crane brakes all follow their safety logic. For instance, the preload deviation of the brake spring should not exceed ±5%, and the radial runout of the brake wheel should be ≤0.1mm/m. Behind these figures lies the safety bottom line earned from countless accident lessons. After working in this line for a long time, you will understand that true professionalism is not about knowing how much theory, but about implementing every parameter in daily operation to ensure that the crane always remains in a state of "stopping and coming to a stable stop".
