Air brakes versus hydraulics set-ups

ALL tractors built to travel at speed in excess of 40kph (25mph) should be fitted with air brakes – along with the trailers or implements they are towing, insists air brake specialist, Michael Erentraut.

He says air brakes provide more efficient braking and are more controllable than hydraulic systems.

Based at Waddington, Lincolnshire, Mr Erentraut’s company, Erentek, specialises in providing kits to provide agricultural trailers with air braking – either as a retrofit to existing units or for fitting to new trailers by their manufacturers. He also provides kits for tractors to enable them to operate trailers fitted with air brakes.

“There is now increasing awareness of the importance of having good, efficient trailer brakes,” he says.

“The industry appears to be finally waking up to the fact that it needs to be seen to be responsible when taking tractors and trailed equipment on the road, and this means having trailer brakes which work.”

But why air brakes and not hydraulic? According to Mr Erentraut, when a vehicle or trailer is heavier than about seven tonnes, the braking power required is such that, for hydraulic systems, brakes tend to be applied abruptly with little – if any – room for progressive application.

“When the brakes are applied it is like opening a tap,” he explains. “It is either open or shut with nothing in between and when it is open, oil just flows to the rams and activates the brakes. Tyre wear is high as is the stress on the trailer generally. The only solution is to fit an automatic hydraulic load sensing valve which will reduce the oil pressure and gradually increase it in the line when trailer weight increases.”

He says one of the great advantages of air brakes is that if there is a leak, the compressor continues to top up the air supply and the brakes continue to work. A leaking hydraulic system eventually empties itself with the result that there will be no brakes.

So, with air brakes the assumed choice, the first point most will want to know is how much will it cost?

Depending on the type of trailer, the parts will cost between £600 and £800 – fitting them takes about a day and can be performed on farm by a competent person.

If ABS is required – and this is not usually considered to be really necessary unless the trailer is spending an inordinate amount of time on the road – the cost will rise by about £1,000, mainly due to another axle requiring pole rings and extra sensors.

Should the tractor need to be equipped with a compressor, air tank and operating system, the kit will cost in the region of £1,500. It is perhaps worth commenting at this stage that this air supply can also be used for tyre inflation and operating power tools.

Before the air brake kit is fitted to the trailer, it makes sense to ensure the drum brakes are in good order, tyres are not excessively worn, and general condition of the trailer is acceptable. There is little point planning to spend over £2,000 on new brakes if the rest of the trailer is dropping to bits – air brakes can not make bad brakes good.

About air systems

Time then for an overview then of how trailer air braking works with the kit supplied by Erentek.

Two pipes connect the system to the tractor’s air supply - one red (usually described as the ‘emergency’ line) and the other yellow (the ‘service’ control line).

The red line has two functions: it feeds air to the trailer’s air reservoir tank via the emergency relay valve (ERV) and ensures that, under normal operating conditions, the reservoir is always under pressure. Its second role is to apply the brakes should the tractor and trailer become parted – the ERV detects the pressure drop in the line and uses the air in the reservoir to apply the brakes – effectively ‘a dead man’s handle’.

The yellow service line has no active flow of air through it, but is used as a pressure control for the ERV – the air pressure is produced by the tractor’s air reservoir and released proportionally by a valve opening in respect of the pressure exerted on the tractor’s brake pedal. This valve is fitted in the tractor’s hydraulic brake line.

A pressure increase causes the ERV to release air from the trailer reservoir to activate the brakes – the more pressure it is provided with from the service line, the greater the pressure released to the brakes – as the ERV functions as a relay.

Before moving on, it is important to fix in the mind the sequence of attachment and detachment of the two pipes from the tractor. To attach, it is yellow first and then red; to detach, it is red first and then the yellow.

Beware of air discharge due to the couplings being under pressure and always apply the handbrake on the tractor and trailer before attaching or detaching.

Removal of the red pipe from the tractor applies the brakes just as if the tractor had broken free from the trailer – but they should never be relied on for parking brakes – the mechanical handbrake must always be applied.

So that the disconnected trailer can be moved around the yard – perhaps by a tractor not fitted with air brakes – there is a valve (Shunt Valve) which can be operated which directs air from the trailer reservoir into the emergency port of the ERV. This ‘tricks’ it into believing a tractor has been connected and releases the emergency brake.

The braking requirement of a trailer changes with the amount of load it carries – a light load with heavy braking can cause the wheels to lock up, and so on. To compensate for such load changes, a combined load sensing valve (LSV) is fitted – a linkage between the chassis and the suspension is connected to the LSV. Linkage position changes with the amount of load and activates the LSV to progressively change the pressure in the brake cylinders depending on weight imposed on the trailer.

The initial setting of this feature is clearly important and is made using a series of calculations which take into account matters such as weight, wheel diameters, brake drum size and so on.

Fitting an air brake system to a trailer does not restrict its use behind tractors fitted with air braking. Some brake actuators have a dual action which allows them to be also operated by a hydraulic system.

Air or hydraulic

A clever system, the main push rod has a hydraulic ram incorporated in to it. When air is being used, the push rod moves as a solid rod, the force provided by the air pushing on a diaphragm.

If oil is used, it is connected to the ram section and it is this which extends the length of the pushrod to apply the drum brakes. It is imperative air and hydraulic brakes are not operated at the same time.

Incidentally, it is worth noting that Mr Erentraut does not consider disc brakes for trailers or trailed implements have a place in agriculture.

“The conditions are too dirty for them,” he says, “they turn too slowly to operate effectively and, while drum brakes might suffer from a little rust when they have not been used for a while, it is nothing when compared with the rust on discs, disc brakes have a habit of seizing up when not used regularly”.

The kit for tractors comprises a compressor which, in the Erentek version, is belt driven, but there are electric versions on the market. Air is stored at a pressure of about 120psi – a safety valve ensures this pressure is not exceeded and there is a gauge and audible warning system for the operator to warn of any malfunction.

The heart of the system – as far as the tractor is concerned, is the Fluid Reaction Valve (FRV) which is tee’d off the tractor’s hydraulic braking circuit. Supplied with air directly from the reservoir, it provides an output control to the trailer via the yellow service line and then, as we have already noted, to provide proportional braking using the trailer’s air reservoir and the ERV relay.

Meanwhile, the trailer’s reservoir is constantly topped up with air from the tractor’s reservoir via the red emergency line.

Where the trailer is fitted with ABS, it is usually powered from a dedicated supply, and as a backup can be powered via the brake lights – a convenient source that, by definition, is only available when the brakes are applied. A green light on the trailer headboard indicates correct functioning.

Air braking clearly has its advantages but it also has its costs and, on past history at least, it would seem purchasers of trailers have been reluctant to pay for any ‘extras’ in the braking department.

The converse, of course, is that trailer manufacturers will cater for these demands and continue to build their wares economically so that they only just meet legal requirements in terms of braking.

To be fair though, several manufacturers are now offering trailers equipped with industrial axles and braking to suit.

Change could be induced by tractor manufacturers who are now questioning the validity of warranty claims for brake problems on their machines when they are deemed to be caused by operating towed implements with poor brakes.

There is also now increased awareness by the industry for the need to be operating safe, road-going tractors and trailers and there is pending European legislation which could impose tighter controls and greater demands on braking standards.

Whether these factors will persuade authorities to sanction an increase in the Gross Train Weight above the current 24,390kg remains to be seen but, one suspects, it is almost certain there will be no increase without a significant improvement in trailer braking systems, in conjunction with maintenance schedules and regular inspections.

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