14 April 2000



As the silage season draws

ever closer, it is essential

to make sure the trailers

brakes are going to

perform when the

tractors brake pedal is

depressed. Geoff Ashcroft

takes some advice from

Stuart Minter of John Deere

IMPROVEMENTS to tractor brakes in recent years have perhaps, done much to reassure drivers that whatever the load being handled, they will feel sharp enough to bring the whole outfit to a safe stop. But are they?

When pulling a fully loaded trailer having badly adjusted or even disconnected brakes, even the most efficient of tractor brakes will be tested to the limit when the driver jumps on the pedal in search of some stopping power.

"Although there is a degree of spare capacity in tractor brakes, it is asking too much to stop a tractor and trailer combination with a 24-tonne gross vehicle weight from 40kph using oil-immersed disc brakes in the back axle," explains Stuart Minter, John Deeres division service manager.

"At the very least, the service life of the brakes will be shortened dramatically – at the worst, the outfit just wont stop when you really need it to."

Rear end

But its what goes on inside the tractors rear end that is cause for most concern. All the energy from travelling at speed needs to be absorbed by the tractors brakes and while energy can only be changed from one form to another, the friction created from applying the brakes turns all this energy into heat.

"Bringing a fully loaded, unbraked trailer to a halt can cause a tractors brakes to generate as much heat as 220 single bar electric fires," says Mr Minter. "Its enough to fry brake components and destroy the additives in the tractors oil."

And it doesnt stop there. That same oil – complete with metal filings and fragments of brake lining – often circulates through elaborate and finely engineered powershift transmissions, steering systems and hydraulic control valves, with costly effect.

"Where tractor brakes feel man enough to handle the weight of the tractor and its load, most drivers would assume all is well," warns Mr Minter. "Then comes the day when you really need to make that emergency stop."

Simple prevention

Preventing the massive and sudden heat build-up with the tractors brakes is simple according to Mr Minter – just get the trailer to share the burden. UK law requires hydraulically operated trailer braking systems to provide a minimum of 25% braking efficiency with 150 bar line pressure – that is, for example, the brakes of a trailer with a 12t gross weight must develop 3t of stopping force.

In the commercial haulage industry, trucks and trailers are tested independently on an annual basis to ensure they meet a minimum braking efficiency standard. For agricultural vehicles, no such annual test requirement yet exists, so the onus is on drivers and owners to check and adjust trailer brakes to make sure they work with the tractor, when theyre meant to.

"Just watching the brake actuators move isnt enough," says Mr Minter. "If theres any doubt as to how trailer brakes are working, take the outfit to a commercial vehicle test centre and get a brake test carried out. At least youll know which wheels, if any, are doing the work." &#42

Fail-safe braking

Peterborough firm Euro Safe has developed a fail-safe braking system for agricultural applications, which automatically applies a trailers service brakes if it becomes detached from the towing tractor.

The system uses an hydraulic accumulator which is fitted to the trailer and an electrical control box in the tractors cab. The accumulator stores energy every time the brake pedal is depressed and stores enough hydraulic pressure to apply the brakes should the tractors engine fail.

Disconnecting the fail-safe systems electrical and hydraulic connections when unhitching the trailer also causes the accumulator to apply the brakes, preventing trailers from running away.

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