Braked trailer Couplings: Operation and Maintenance

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On large trailers, brakes and couplings are the most important to the running of a trailer; we aim to help you keep your trailer in good running order with the help of this guide.


The Law

By law a trailer with a gross weight of over 750kg and under 3500kg must have brakes. Normally these are the overrun type. There must also be a breakaway cable that operates the brake in the event of the trailer becoming uncoupled whilst towing.


Braked couplings and the principles of how they work

The operation of an overrun braked coupling is fairly simple and easy to maintain.

On the front of a coupling there is a coupling head. This can either be a cast steel 50mm ball, pressed steel 50mm ball or a cast eye (ring) of varying sizes. These essentially need to be kept clean and lubricated (for more information please see our guide on unbraked trailer couplings, as the same principles apply).

The coupling head is connected to a draw tube which is a round tube that allows the coupling head to move in and out of the main body and swivel. Inside the draw tube there is a hydraulic damper (this is not fitted on all couplings, but is fitted to most) that slows the movement in and out of the coupling and holds the head as far out as it will go.

Behind the draw tube there normally is a linkage. This is a point that you fix your brake rod to. The linkage is pushed by the back of the draw tube and it has a pivot below. The lowest point of the linkage then pulls the brake rob rather than pushing.

Handbrakes vary on overrun couplings. Most of the modern larger trailers have an over-centre handbrake and some smaller couplings tend to have a ratchet type. On most of the over-centre type handbrakes there is a bar fastened to the handbrake that moves the linkage and then pulls the brake on. Ratchet type handbrakes have an 'L' shaped bracket that threads over the brake rod and a damper that sits within the 'L' shaped bracket. A nut is needed on the brake rod approximately 2mm in front of the damper when the handbrake is fully down. This means that when the handbrake is applied it can pull the brake rod independently.



There are a few different types of compensator from a single axle compensator that is essentially a piece of steel with three holes to the more complicated twin axle compensators.

Single axle compensators can be used on single, twin or tri axle braked trailers. A single axle trailer is simple. The threaded bar (brake rod) from the coupling passes through the centre hole of the compensator. The cable ends from the brakes are then secured through the other two holes, either side of the centre hole. A nut is tightened onto the brake rod behind the compensator so that the rod can pull the compensator and thus, the brakes. Another nut is also tightened against the initial nut to lock them into position.

A twin axle compensator




To be completed soon...

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