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  Hydraulic Brake - Overrun Brake



That's a bit difficult now, because actually the overrun brake does not fit into a book on the topic: hydraulic brake. But where it's going else? It certainly does not suit to the compressed-air brake and, after all, it is so characteristic of the brakes of car trailers as the hydraulic brake for cars.

Even in the Netherlands, for example, there are usually four-wheeled light trailers with a large wheelbase, drawbar and hydraulic brake, as the pressure from the towing vehicle's system is transferred mechanically to the trailer's independent system.


But we continue here with overrun brakes, whereby the principle for one and two axles is the same. When the brake of the towing vehicle is actuated, the drawbar is in principle compressed, which accommodates a special mechanism and converts into a pulling force for two or four (picture above) Bowden cables. The drum brakes are thus mechanically operated.


Here again the principle for a single-axle trailer. The pan in the tiller head is slidably mounted relative to the drawbar tube. It is moved to the rear when braking the towing vehicle. Their bearings include a deflection and a locking mechanism, the handbrake (below image).


The deflection makes from the pressure a tension that is passed through a rod with joint, an adjustment mechanism and a rocker to the two Bowden cables to the wheel brakes. The rocker compensates small differences when operating the two Bowden cables. Within the drum brakes, the mechanism acts on an extension of the connecting rod between the two brake shoes.


Now we are in the middle of the braking process. Clearly visible is the principle of the mechanism by which the connecting rods are now longer and press the brake shoes against the drums. If you have difficulties with understanding, the comparison with the previous picture helps. So this is now the position either by a overrunning trailer or a pulled handbrake.


At first glance, nothing spectacular is seen when the brake drum is removed: two brake shoes, each with a pivot point at the bottom and at the top, a spreader lever that presses both. It results in a rotation of the drum in the direction of arrow a primary brakeshoe left and a secondary right.


But if the journey goes backwards, then something strange happens. Clear that a reverse drive with overrun brake is difficult to impossible. Because the brake responds immediately, can not distinguish between overrunning and backward riding, or does it?

Because something is different now. The wheel has changed its direction of rotation while maintaining it during overrunning. As a result, now the right is the primary and the left is the secondary. This affects the lower right pivot point, which shifts by the braking force in the current direction of rotation and inward. As a result, the brake is nearly fully released, although not completely.


This is a picture of the spreading lever with the top of the hook, where the end of the Bowden cable end is hung and the pivot point, which provides for an extension. Below is also the practice taken picture of a trailer shoe brake. In contrast to the schematic diagram here is the fixed stop point above.


In addition to the spreading lever below here is the division of the left brake shoe of importance. Their rear part remains in its place except for minor movements when braking, while the actual part with brake pad above has plenty of freedom of movement to fold while backwards riding. Incidentally, the then existing, low friction is necessary to bring this part when driving forward back to the normal braking position.







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