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Braking systems: work safely

The requirement for mobility in modern agriculture is more and more pressing with the involvement of loads and speed steadily increasing. What is important in these conditions is maintaining the best possible level of safety

by Stefano Albanesi
March - April 2015 | Back

All farmers, and especially contractors, have hopes of being able to travel more quickly with greater loads because these transfers are notoriously seen as time lost and thus not dedicated to simple work in the field. It is therefore fundamental that this trend must evolve while maintaining – or, better yet, improving – the level of safety in travel, especially the more sensitive maneuvers such as braking.

 

Mechanical, hydraulic or pneumatic?

Brakes for trailers can be mechanical, hydraulic or pneumatic. For their efficiency, hydraulic or pneumatic systems are almost obligatory for models with high load capacities. In any case, what is needed is the best coordination between the braking of the driving vehicle and the towed machine also, and especially, consideration of the transfer of weight caused by the change in the alignment of the two components.

 

Mechanical brakes

Mechanical brakes are typically used to equip farm trailers with low load capacities. In practice, the braking components are the well-known shoes pressing against the drum activated manually with a dedicated lever located to the side of the drivers seat.

The principle crucial feature of this system is the way the control lever, which must be activated by hand, is used in combination of the pedals on the floor of the tractor. This can cause problems for the stability of the tractor and operator working together due to the imperfect synchronization of the two braking procedures.

An interesting development for mechanical solutions for stopping or slowing a trailed implement is the so-called inertial brake. This is a type of brake which is widespread, especially in Germany, for trailers which do not have to be connected directly to the brake system of the vehicle as, for example, a combine harvester which is towing its own cutting head on a bar trolley. The new European Union regulation for the circulation of agricultural machinery, in fact, opens new horizons for the inertial, or overrun brake system which has been around for a long time but has not been seriously considered because the present norm does not include it. ADR in Uboldo, in the Province of Varese, a manufacturers of axles, suspensions, wheels and accessories in the industrial and agricultural fields, has developed a basic version with a DIN towing eye, which meets the German standard, and an Italian version with a CUNA towing eye to be ready for the update of the EU Directive on trailer hitches. The key function of the system is based on mechanical control and a telescopic drawbar. The drawbar moves back and forth relative to the trailer chassis. When the towing vehicle brakes, the inertia of the trailer slides the mechanism, this in turn uses the travel – or inertia, thus the name – of this mechanism to pull on the brake rod which apples the trailer brakes. The efficiency of this system is closely linked to the ratio between the towing vehicle and trailer which must be decisively greater than the vehicle. In the case of carriages carrying harvester cutter heads the value is always 2 to ensure greater stability for road travel.

For tractors, the situation is reversed, sometimes drastically. For these conditions active braking, provided by hydraulic or pneumatic braking, is required. For their inertial brakes, ADR has developed various drawbar shapes, sizes and regulations for fixed trailers and fifth wheel trailers.

 

The Bosch

Rexroth ABS hydrostatic brake system

As tractor bra­kes are of the hydraulic type, the German ma­nufacturer Bosch Rexroth has developed an ABS (Anti-Lock Braking) device which works proportionally through the pressure relief valves to set braking oil pressure for each single wheel. The main problem for the function of the entire system involves the need to ensure the speed necessary for the hydraulics to intervene to manage critical conditions. Provided for this case is the application of suitable pumps and the development of specific software which correctly controls the various tractor components, through the frame itself or mounted implements, and the tractor-trailer combination, a traditional trailer or a 2 or 3 axle dumper trailer with part of the load on the tow hitch.

 

The efficiency of compressed air systems

Pneumatic trailer brakes are very widespread in Italy over hydraulic brakes whereas the opposite is true for other countries, in Germany for example. Among other considerations, new higher power tractors are increasingly built with new generation air brakes as standard equipment to facilitate coupling with trailers with the same type of brake system.

In practice, air is compressed by the tractor engine in a dedicated tank. With a regulator controlled by the tractor floor pedals air is driven through the trailer half joint where a servo distributor actuates the brake system. For greater safety, also the trailer is fitted with a compressed air tank to deal with the prospect of braking in an emergency.

WABCO in the United States manufacturers a range of mixed automatic air brake types with single as well as double lines complete with pressure regulators and load-sensing valves for a more economic use of the systems.

 

Zaccaria’s new solution

Various manufacturers of big trailers are already marketing models with air and hydraulic brake systems fitted with ABS. Zaccaria in Montese, near Modena, has come up with an interesting solution for 2 or 3 close axle dumper trailers which are equipped for towing by vehicles which operate with mixed automatic hydraulic as well as air brake systems. Moreover, the parking brake is automatically engaged without need to activate mechanical couplings as would be needed for air brake systems.

Though they work independently, the hydraulic and air cylinders both work through the same pedal that moves the brake components. On the other hand, when the pressure in the system is down to zero a solid spring on the rear axis of the cylinders activates the parking brake. As opposed to what one might think, the ABS greatly increases braking safety without a full load, when the trailer is empty. If weight is reduced, in fact, the trailer wheels are immediately blocked with braking causing dangerous skidding, especially when navigating a curve. This situation is emphasized due precisely to the typical regulation of the force of braking on the trailer which is obviously certified as equipment for a full load. Exactly when driving on critical road conditions, on wet, muddy or icy surfaces, there are definite advantages with a trailer equipped with ABS to ensure driving in combination to avoid disastrous slipping off the road, as can happen even with braking which is not especially strong. In this connection, and looking especially to the future when maximum speed allowed on roads will be greater than the 40 km/h at present, Zaccaria has carried out various tests in collaboration with New Holland with the company’s trailers hitched to NH model tractors equipped with ABS.

 

The ForCon 201 by Fliegl

Though this trailer model built by the German manufacturer Fliegl Agrartechnik GmbH, the ForCon (Force Control) 201, does not introduce a specific agricultural trailer braking system, electronically controlled forced steering provides much better handling, including braking. The innovation, which won a silver medal at Agritechnica 2007, ensures much improved maneuverability of tandem and tri-axle trailers on the road, on difficult terrains and in restricted spaces. In practice, the new forced steering makes it round the curves without mechanical linkage. Instead of this a gyroscopic instrument determines the curve radius and consequently the steering angle. This steering angle is passed on to the rotary encoder on the axle, where a hydraulic cylinder controls the steering axle bringing it to the optimal steering angle. The gyroscope can be mounted anywhere on the tractor, for example on the mud guard or wing of the rear axle. The only point of contact between the towing vehicle and the trailer is the coupling device, which can be a towing hook, ball-type, piton fix or similar device. The system provides many advantages thanks to the optimal steering angle of the wheels on all axles maintained by electronic vertical and horizontal control which guarantees smooth forward and reverse driving on every terrain, even over bumps or gullies; greater stability in braking; reduced tire wear even in the tightest curves and security when driving on slopes; better distribution of weight through the tires. All this is monitored and displayed to indicate the direction of movement and steering and show any possible malfunction.

ABS on tractors

 

The combination of ABS on the tractor as well as on the trailer is the best solution for safety when breaking with heavy loads in precarious conditions. The leading manufacturers in the sector have been equipped for this for some time.

The ABS system is standard equipment on the Fendt 900 Vario tractors: 4 sensors on each wheel constantly control braking and the speed of rolling to prevent blocking. Moreover, at speeds over 20 km/h the FSC (Fendt Stability Control) blocks compensation of loads from right to left to ensure precision steering on curves and braking as well. Compensation is returned at speeds of less than 15 km/h for better contact with the ground. And finally, there is self-levelling front axle suspension. The hydropneumatic individual wheel suspension is fitted with a double wishbone axle, which optimizes the steering angle. The fully automatic levelling control ensures that both sides have the same suspension travel to enhance precision and driveability on roads. 

The new SuperSteer by New Holland has been added to their tractors with ABS for the individual management of each wheel in braking. The conventional doubled two pedal system has been replaced by a single combined pedal with a tradition black central section and two lengthened lateral sections in orange for easily identifying them. This arrangement means the tractor can be steered by using only one side of the brake. To prevent accidental braking, this is avoided by automatically disactivating the system over a certain speed. 

Moreover, control of tires slipping is provided by automatically coupling them according to the steering angle to reduce the turning radius without applying the brakes. With this coordination, the tractor hinges around one of the two rear wheels with a braking action which can be set to avoid damage to the ground surface.

And finally, the brakes are automatically engaged on steep slopes so that the tractor will not slip back when forward traction is applied. This means the use of the clutch is greatly reduced, especially when frequently coping with heavy loads towed.  

Knorr-Bremse brakes for farm trailers

 

With the incessant increase in weight and speed of travel, the requirements of agricultural trailers for road travel are becoming more and more similar to those for big commercial truck trailer rigs as regards safety, also braking safety.

Knorr-Bremse, a German multi-national, developed the Trailer-EBS TEBS G2.1 in 2011 for Jumbo Pöttinger trailers. This electronic braking system is made up of a set of sensors, pneumatic braking components and the required CPU. At the heart of the system is the ABS for avoiding locking with overbraking or travel on a road surface with scarce traction to prevent skidding or jackknifing. Moreover, because the high center of gravity of the trailer involves the risk of tipping over in critical driving conditions, the Trailer-EBS TEBS G2.1 is also equipped with RSP (Roll Stability Program) for automatically correcting the direction of travel and speed of the complete combination in dangerous situations to reduce risky lateral acceleration, and the ALB, that is, braking with efficiency automatically calculated to the load.

The system also includes the recording of operational data, ODR (Operational Data Recorder) for evaluating data on the vehicle and braking.

Trailers equipped with the TEBS G2.1 can be towed by conventional tractors with ABS and EBS (Electronic Braking Systems). In case of a lack of current, the trailer can be stopped by using the pneumatic brake alone, even without the ABS and ALB functions.

 

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