What Is a Band Brake?

A band brake is a type of brake that uses the contraction of the brake band around the brake drum to produce a braking effect. Can be used in automotive automatic transmissions, ships, marine anchor winches, winches and mine winches, construction winches and other equipment.

Chinese name: Band brake
Category: Automatic Transmission

Application: car
Internal composition: Brake band with internal friction material

Brief introduction of belt brake concept

A band brake is a type of brake that uses the contraction of the brake band around the drum to produce a braking effect. Can be used in automotive automatic transmissions, ships, marine anchor winches, winches and mine winches, construction winches and other equipment.

In automobiles, the band brake is mostly used in automatic transmissions. It is not used to prevent power output, but according to the characteristics of the planetary gear mechanism, one of the three basic elements is restricted from turning and cooperates with the clutch to achieve different effects. Gear ratio.

The advantages of belt brakes are: good clinging performance; occupying a small space in the transmission; when the brake belt is tightly rotated, a so-called self-energizing wedge effect that stops the brake drum from rotating is generated.

Classification and principle of band brake

The ordinary belt brake can be divided into three types of structure according to the connection form of the brake belt and the lever, namely simple type, differential type and comprehensive type. The principles are briefly described below.

Simple brake

The following figure is a simplified structure of a simple belt brake. One end of the brake belt is fixed on the lever fulcrum A, and the other end is connected to point B on the lever. The brake band will shrink radially under the gravity of the hammer, so it will hoop on the brake drum and brake

The belt will rub against the surface of the brake drum. Because the brake belt cannot rotate, the brake drum will be slowed down or even fixed due to the effect of the friction torque, and it will be in a tight brake state. When the current is turned on, the magnetic force of the electromagnet lifts the lever, and the brake band and the brake drum are separated from each other, that is, the brake is released. This type of brake drum generates a large braking torque when turning in the direction of rotation as shown in the figure, and a reverse rotating braking torque is small, which is used for one-way braking.

Figure 1 Simple structure of simple belt brake

Note that in practice, it is not necessary to use a hammer and an electromagnet as a brake actuating device, and a device such as a hydraulic cylinder may also be used.

Belt brake differential belt brake

The following figure is a schematic diagram of the structure of a differential belt brake. The two ends of the brake belt are respectively connected to points B and C of the lever. Under the action of the braking force P, the lever rotates around point A, point B is tightened and point C Relax. Because AB is greater than AC, that is, the amount of tension is greater than the amount of relaxation, so the entire brake belt is still tightened, the brake belt will shrink radially, hoop on the brake drum, and brake the brake drum. effect. On the contrary, it will be in a released state. It is the same as the simple belt type, which is suitable for one-way braking, but the external braking force required is smaller than that of the simple belt type and the braking stroke is large. Therefore, it is often used for one-way braking controlled by hands or feet.

Figure 2 Schematic diagram of the structure of a differential belt brake

Belt brake integrated belt brake

The figure below is a simplified diagram of the structure of the integrated belt brake. Under the action of the braking force P, points B and C are tightened at the same time, and AB is equal to AC. Therefore, the brake band is tightened and it will shrink radially. Tightly on the brake drum, it acts as a brake on the brake drum. When the brake drum is rotating forward or reverse, the braking torque generated by this brake is the same. It can be used in both forward and reverse rotation and where the same braking torque is required.

Figure 3 Schematic diagram of integrated belt brake

Structure composition of belt brake

Basic composition of belt brake

The belt brake is mainly composed of brake drum, brake belt, hydraulic cylinder and piston, etc. The following figure is a hydraulically actuated belt brake.

Figure 4 The actual type of band brake

The structure diagram of the belt brake is as follows. The inner surface of the brake band is coated with friction material. It is wrapped around the outer surface of the drum. One end of the brake band is fixed to the transmission case, and the other end is connected to the brake cylinder. The pistons are connected. When the brake oil enters the brake cylinder, the compression piston return spring pushes the piston, thereby moving the movable end of the brake band and tightening the brake drum. Since the brake drum is integrated with a part of the planetary gear mechanism, tightening the brake drum means clamping and fixing the part so that it cannot rotate. After the brake oil pressure is released, the return spring resets the piston in the brake cylinder and pulls back the active end of the brake band, thereby releasing the brake drum and releasing the brake.

Figure 5 Structure of the belt brake

When braking, allow a slight friction between the brake belt and the brake drum, so that the planetary gear mechanism components that are being braked will not stop suddenly, because a very sudden stop will have an impact and may affect Transmission damage. On the other hand, too much slip between the brake belt and the brake drum, that is, the brake belt slips, will also cause the brake belt to wear or ablate. The degree of slippage of the brake belt generally increases with the wear of the friction material lined on its inner surface and the increase of the gap between the brake belt and the brake drum, which means that the brake belt needs to be adjusted from time to time. In fact, most of the early automotive automatic transmissions had to make this adjustment on a regular basis, but with the improvement of the design of the brake band, most automatic transmissions produced in the 1990s no longer need to periodically adjust the brakes Driven.

In the new type of automotive automatic transmission, the release of the braking effect is usually completed by a return spring and oil pressure, that is, the brake oil pressure on one side of the piston is cut off and released, and an additional brake release is provided on the other side. Oil pressure to assist the return spring to release the brake as soon as possible. When the piston is fully reset, the brake release oil pressure will continue to work to ensure that the brake band is fully relaxed.

Belt brake

Brake bands are the key components of belt brakes. According to their deformation ability, brake bands can be divided into rigid brake bands and flexible brake bands. The rigid brake belt is thicker than the flexible brake belt, has greater strength and heat capacity, and has the disadvantage that it cannot generate deformation that is compatible with the brake hub. The flexible brake belt can fully fit the brake hub during operation, and the price is low.

According to the structure, there are two types of brakes: unilateral and bilateral. The double-sided brake band can better fit the outer surface of the rotating drum, so it can provide greater braking friction torque under the condition of a certain force at the movable end; at the same time, the engagement of the double-sided brake band and the rotating drum It is also more stable than the unilateral braking band, which makes the shifting action softer. Mostly used for low gear and reverse brake with large torque. However, the unilateral brake belt in the automatic transmission is lower than the bilateral brake belt in terms of manufacturing cost, and its performance is quite satisfactory in many applications. Therefore, most new automotive automatic transmissions use Unilateral brake belt with good flexibility, light weight, low cost and simple manufacture.

The brake belt is made of a friction material bonded to the coiled steel belt base plate, as shown in the figure below. The thickness of the steel belt is about 0.76 to 2.64mm. The thick steel belt can generate a large clamping force and is used in automatic transmissions with high engine power. The thin steel belt can exert a small clamping force, but because of its good flexibility and strong self-increase effect, it can generate a large braking force.

Figure 6 Brake band

The friction properties of the friction material bonded to the inner surface of the steel belt are very important to the performance of the automatic transmission. There are many types of friction materials used in automatic transmissions. In commercial vehicles, copper-based powder metallurgical materials and semi-metal friction materials with higher hardness are generally used, and paper-based friction materials are used in passenger cars. Paper-based friction materials are composed of cellulose fibers, phenolic resins and fillers. Phenolic resin is used as a binder to connect cellulose fibers into a continuous matrix. Fillers are used to increase the strength of the material, improve friction and wear resistance. Fillers for friction materials in automatic transmissions are powders of graphite, metal, and ceramic materials.

Modern paper-based friction materials can already be used as friction elements working under heavy loads. The friction performance is stable, and the cellulose fiber is rich in resources and low in cost. The process of manufacturing friction materials is also simple, which can reduce the cost of automatic transmissions. Wide application.

Belt brake wear and adjustment

For most straight-rod or lever-type links that need to be adjusted after the brake band is worn, the gap between the brake band and the drum is determined by the adjustment bolts that are the fixed ends of the brake band. This adjusting bolt is screwed into the threaded hole penetrating the casing of the automatic transmission, so the gap between the brake band and the drum can be adjusted outside the casing, and after adjusting, it is locked with a lock nut.

However, for caliper lever transmission, the brake band adjustment screw and lock nut are located at one end of the rocker arm. Therefore, the clearance between the brake band and the drum must be adjusted after removing the oil pan of the automatic transmission. ^[1]