8 March 1996

Air brake specialists will

pull you up sharp!

Stopping on a sixpence may be the aim but many tractor/trailer combinations do well to stop in the length of the Royal Mint. Is air braking the answer?

TRACTORS get bigger and heavier; trailers get larger and able to carry ever heavier loads. It is one thing to pull a heavily loaded trailer but quite another to stop it.

Most would agree that relying on the tractors braking ability alone to stop a loaded trailer is folly indeed, particularly if it is of the four-wheel variety.

The majority of modern agricultural trailers are now designed to be braked using hydraulic power sourced from the tractor but there is a growing trend to fit trailers with air braking equipment – a system used widely by heavy goods vehicles.

A company which now specialises in retro-fitting agricultural tractors with air braking equipment is Hull-based Tractair.

Fronted by Martin Constantine, the company reckons to either fit or supply conversion kits for about 300 tractors a year – 60% of them to the home market and the remainder to countries throughout Europe and the Middle East.

"There is no doubt there is a greater awareness in the industry for properly braked trailers," says Mr Constantine. "It is a situation helped, to some degree, by the introduction of the new generation of fast tractors – although it should be said these tractors are usually equipped with air braking as part of their specification."

Kits are now available for most makes of tractors including Massey Ferguson, New Holland, Case, John Deere, Valmet, Same and Zetor. If specified, Tractair can fit the kits at its own site or, if a manufacturer has say, five or six tractors ordered with air braking, the company takes its expertise to the factory.

So what does an air braking conversion entail? The basic components comprise a compressor, air reservoir, fluid reaction valve, unloader valve, trailer control valve, hose couplings and dashboard mounted air pressure gauge.

"Fitting is not that easy on some tractors," says Mr Constantine. "Tractor manufacturers are not renowned for leaving volumes of space beneath the bonnet and, nice as it would be, you cant just stick components anywhere. There are things like tractor loaders and the free movement of rear linkages to consider."

Re-siting components

For a Case 4230 – the latest tractor to go through the workshop – the attachment of the compressor and its Vee-belt drive taken from the engine tailshaft required the re-siting of the alternator to the top of the engine and, as a result of this relocation, moving the air-cleaner to the side of the engine.

And that was just for starters. The air reservoir tank finds a place below the cab steps and, ignoring for the moment the plumbing routes for air pipes, the rear air-line connections for the trailer have to find room between the usual hydraulic spool valve outlets.

"Despite these problems, we feel we do a pretty tidy job," insists Mr Constantine. And on that score it would be hard to disagree having seen the finished article.

The operation of an air braking system is relatively simple. Air from the compressor – the system works at about 100psi – passes first to an unloader valve which incorporates a non return valve and a take-off point for supplying auxiliary air for tyre inflation etc. The unloader valve is also responsible for monitoring and controlling the output of air from the compressor to maintain a full supply of pressurised air in the reservoir tank.

The fluid reaction valve: This is basically the interface between the tractors existing hydraulic braking and the air braking system. It senses the hydraulic pressure applied by the brake pedal and apportions the correct pressure signal for the new air braking system. There is also the ability for this valve to receive a mechanical link from the tractors hand brake to provide a secondary brake for the trailer.

Signals from the fluid reaction valve also pass to the trailer control valve – sequencing ensures the trailer brakes are applied before those of the tractors.

Operator information regarding the pressure and volume of air stored is displayed on a dashboard mounted dial.

When it comes to trailer hose connection there are three options, depending on the type of braking system involved. Most common is the EC two-line, one for the trailers reservoir and the other a service line activated from the brake pedal. Earlier versions may have a three-line system – one for the reservoir, one for the service and a third for the handbrake. Some eastern bloc sourced trailers are fitted with a single line to operate what is known as an inverse system – the air pressure is used to hold the brakes off; when the air pressure is released heavy springs pull the brakes on.

Mr Constantine recommends customers have what he terms an "EC two plus one" system fitted which allows a variety of trailer types to be used – two work on the EC two-line system and the third is able to operate an inverse system.

Price of fitting an air braking system to agricultural tractor ranges from £1400 to £1800 depending on tractor model and takes between one and two days to complete. &#42

It takes about two days to fit an air braking system to an agricultural tractor. Apparatus seen behind the tractor is used to test the system.

Tractairs Martin Constantine: "We supply and fit air braking kits to about 300 tractors a year."