The brake system is designed to stop various mechanisms or vehicles. Another purpose is to prevent movement when the instrument or machine is at rest. There are several varieties of these devices, among which the band brake is one of the most successful. Before using a device with such a mechanism, you need to study in detail its device, types and principles of operation.
What is this
During tripping operations performed by a drawworks, a device such as a band brake is used in gas and oil wells. It looks like an elastic steel strip that goes around the brake pulley. The design of the device is very simple and consists of a braking band with friction pads fixed on the frame, a lever on the crankshaft and a pneumatic cylinder. The last element starts working at the time when the maximum effort of the driller is more than 250 N.
The tape communicates with the running edge fixed on the frame.The other end is passed through the rod and goes to the brake lever. When the belt is pulled, it is attracted to the moving pulley and braking occurs. Some designs involve the use of internal tapes. In this case, when braking, the tape, on the contrary, unclenches. When the hoist brake is fully released, the braking process is performed by pressing a special spring that is pulled on the pedal lever.
Band brakes are divided into several subspecies according to the principle of operation. They may have different ways of working. The main varieties are:
Despite the fact that these designs differ from each other, they have the same principle of operation: in order for the mechanism to completely stop, you need to use a tape device that acts on the brake.
In this view, the axis that rotates the lever is taken as the point of highest tension. A simple band brake has an elementary device. It is a device that performs a one-way operation. When the pulley begins to rotate in the other direction, it already has a closing force, which is created by the weight of the load. The highest tension occurs at the edge of the tape attached to the mail. This force is several times less than when the pulley moves in a straight line. This means that the braking torque will also be weaker. For this reason, a simple view is used for climbs, where it is not necessary that the braking torque when moving back and forth is the same. In this deviceit is possible to increase the braking force during the lowering of the load, since less effort is needed when lifting.
This device has a brake lever on which the two ends of the tape are fixed at both ends from the pivot point. The principle of operation of a differential type band brake is not very complicated. Forces in relation to the axis of rotation of the lever on the brake act disproportionately. The braking torque is calculated using a special formula based on the weight of the load.
If you make a small value of the closing force, then this indicator will tend to infinity. This means that the very tension of the brake band arises due to the frictional force between it and the pulley. The advantages of this type of band brake are the low closing force. It is used very rarely due to a large number of shortcomings, which include:
- grabbing the pulley with jerks;
- small percentage of deceleration when pulley direction changes;
- increased wear on parts.
Also, it cannot be used on machine driven winches due to a noticeable change in braking torque and the device's tendency to self-tighten.
The device is represented by two ends of the tape connected to the stopper for braking on the side where the rotating axle is located. The arms or lengths of the levers on which the force acts are commensurate with the axis of motion. They are both different and equal in size. If equal shoulders are made, then suchan indicator, like a braking torque, is completely independent of which direction the pulley rotates.
The summing band brake is most often used in devices where a stable fixing moment is needed during reverse and forward rotation of the shaft. For example, in industrial machines where turning movement occurs. To create a certain braking torque in this type of device, more force is needed than in the simplest band brake.
Band brakes are often used to create various types of hoists and cranes. Despite the simple device, these mechanisms are very reliable. Design engineers cite the following benefits of band brakes:
- small sizes;
- easy maintenance;
- simple construction;
- possibility to achieve large braking torques with increasing wrap angle.
Of all the varieties, the most popular are simple tape mechanisms. They are easier to regulate. In addition, with the help of simple calculations, you can perform the calculation of the band brake. Calculate the weight of the load and the braking force.
The weak points of band brake structures include rapid wear of parts. Because of these problems, it is often necessary to carry out repairs. Other disadvantages include:
- uneven distribution of pressure on the wrapping arc;
- difficulty in calculating the force that bends the brake shaft;
- depending on which directionpulley rotates;
- frequent damage to the steel belt.
The last breakdown can lead to an accident due to a broken tape. The low operational reliability of tape mechanisms leads to the fact that they have recently been trying to replace them with shoe mechanisms. These brakes last longer and wear out less quickly.
Band brakes are installed on all devices where a reinforced fixing moment is needed. They are widely used in many fields due to the fact that the design is small, easy to maintain, and at the same time able to develop sufficient braking force.
Most often they are installed on various types of crane structures, which include tower cranes, winches, drilling rigs. In addition, band brakes are used on automatic transmissions, lathes, motorcycles and small tractors.
If all systems and mechanisms of the device are working, but there is not enough braking, you need to adjust this device. It is performed in the following sequence:
- First, you should check how worn the friction lining is (if this figure is half of the original thickness, then it should be replaced).
- Use the nuts to adjust the spring, setting the pressure to 71-73mm.
- Tighten bolt 10 until the brake band rests against the brake pulley.
- Then loosen one turn and lock.
- Move the circuit breaker withadjusting screw, make the length from the rocker to the bolt head 11-13 mm.
After the adjustment procedure is complete, check the brakes. To do this, the load with the maximum weight is lifted to a height of 10-20 cm and it is checked how well the band brake works after adjustment. In this case, the valve that connects the hydraulic motor lines on the lifting mechanism must be open.
If the lowering and lifting operations were carried out for a long period, then the pads wear out much faster. In this case, care must be taken to ensure that the work of the two belts required for braking is carried out simultaneously. In case of uneven functioning, alignment must be performed. When problems are diagnosed, you can begin to fix them. Refer to the service manual for reasons for the failure of the band brake parts.
To carry out repair work, the device must first be braked so that the tape is released. Slightly loosen the locknuts, and then pull the bands by turning the zip ties. This ensures the same circular gap of 3-5 mm. It should be between the brake pulleys and pads. After that, braking is again done so that the gaps between the cups of the springs and the balancer are the same. If this indicator is not equal, then the brake is again relaxed and the tie is tightened from the side where the gap is smaller. This is easy to do if you lower the opposite screed tothe same distance. When the gaps are the same, you can tighten the locknuts.
Brake bands must be replaced if the wear of the pads is more than 1 cm. With this indicator, you need to remove the casing and remove the release springs coming from above. Now you can remove the tapes from the pulleys, pull them out. After the brake pads have been replaced, the same steps are performed, only in reverse order, followed by adjustment of the system.
The drum shaft must be repaired if the brake pulleys associated with it are badly worn. In order to determine whether it is necessary to change this spare part or not, measurements should be taken. When the wear of the pulleys is more than 1 cm on each side, they are replaced with new ones. For repairs, it will be necessary to dismantle such elements of the band brake as the clutch, hydraulic brake and winch casing. In addition, the brake bands loosen up to gain access to the pulleys.
If the device on which the band brake stands is operated correctly, then a long service life is provided to it. However, in order to avoid an accident, you need to check the mechanisms every week. When the brake pads wear out, the stroke of the pneumatic cylinder rod is significantly relaxed. In this case, you need to tighten the bands and adjust the brake unit. Another device that needs maintenance on a band brake is the drum shaft. As a rule, it works for a very long time, and if the brake pulleys adjacent to it wear out, then this part is replaced.