Where does it stop? - Tractor brakes

By Roger McDermott

Brakes are the single most important safety feature on any tractor. Whatever the job the tractor may be required to carry out, every tractor owner realises that having properly working and safe brakes is vital.

As the scale of agricultural machinery being used (both tractors and machines) continues to grow in size, there has never been greater need for safe brake systems on tractors and machines.

Tractor brake performance is a contentious issue for some tractor users. There are reports of problems arising with brake systems and of brakes on machines wearing out very quickly. Repairs can be costly both in terms of workshop costs and downtime for the machines.

Repairs are a cost to the user and they also are a cost for the manufacturer/dealer in that they often have to carry the cost under warranty.

After discussing the issue with numerous agricultural contractors and independent sources of advice it is clear that problems with tractor brakes are ranked high as an issue affecting tractor use in Ireland at the present time. There is plenty of antidotal evidence of brake wear and unsatisfactory performance after relatively short periods of use.

These alledged problems raise a series of questions to all involved in manufacturing both tractors and trailers and those operating and maintaining them. Brake performance is not a clear cut issue and all groups involved have valid questions regarding the design, operation and maintenance of machine braking systems.

In order to express all views of interested parties on this subject and to give all groups a fair opportunity to answer the questions asked of them and to also ask their own questions, this article will be the first of a five-part series on tractor brakes. A representative of a tractor manufacturer, a trailer manufacturer, and a representative for the agricultural contractors will all be given the opportunity to voice their concerns and opinions on this topic over the four articles.

Farm Tractor and Machinery Trade Association (FTMTA) has carried out independent tractor and trailer brake tests in response to queries from both the agricultural machinery industry in Ireland and concerns voiced by machine users. The results of these tests will be examined in an attempt to deal with the issues over the next four weeks.

Increase in scale and speed

The average size of tractors and machinery being used nowadays has increased considerably over the past 10 to 15 years. It is now common that the average size of tractor being used by an agricultural contractor is in the 150 to 160 hp category, and the average size of a farmer's tractor is in the region of 100 to 110 hp.

And there is a speed factor. Ten years ago, the arrival of the 40km/hr gearbox was a major advance. Now this higher speed is common place. Couple this with a larger load and the demands on the tractors braking system faces new challenges.

Machine size has increased too. Take ploughs for example; 15 years ago the average ploughing set-up would have been a three/four furrow reversible plough. Today five to six furrow ploughs are standard. A five furrow plough weighs in the region of 2 tonne. The plough is just one basic example of how machinery size has increased. The same can be said for the vast majority of machines from fertilizer spreaders to cultivators and harrows and so on.

The increased size and capacity of trailers being used and the increase in haulage work which these trailers are being used for is another issue of major importance.

Standard silage trailers are now 5.5 to 6 metres (18 to 20ft) long and are capable of carrying up to 10 to 12 tonne of grass. That can give a gross train weight (tractor + trailer + load) of more than 20 tonnes depending on the grass or maize silage dry matter percentage.

A large number of tractors are now being used to haul dump trailers and low loader trailers. Site clearance work using dump trailers is now common-place and this presents new issues around the braking capacity.

So we now accept that the scale and speed of machinery being used has increased over time. So too it appears, have the number of problems arising with tractor brakes. So why has tractor braking become a big issue? What are the causes of these problems?

This series of articles is not about a blame game. We need to address the issues as they arise for the three groups involved; the tractor manufacturer, the trailer manufacturer and the tractor and trailer user. All have their own questions and answers!

So what are the issues?

A number of tractor manufacturers now manufacture their tractors with the automatic engagement of the 4WD when the brakes are pressed. This system acts a form of speed retarder with the bulk of the true braking provided by the tractor's rear axle disc brakes.

Some of these machines use single disc brake systems in each of the rear stub axles. In the past, the vast majority of tractors were manufactured with multiple brake discs in the rear of the tractor. So is the single disc brake system adequate?The technology available in modern tractors is at an extremely high level. New developments and new products are continuously being launched.

Would the addition of four wheel braking on all tractors rather than the automatic engagement of the 4WD be of benefit to the tractors braking system. Would it also help to spread the brake load when the tractor is being stopped? Some tractor manufacturers fit a version of four wheel braking with a disc calliper on the front drive shaft.

In order to take the pressure off the braking system of the tractor, it is important that the braking system of the trailer is working correctly. It is noticeable, that when using hydraulic trailer brakes, on the majority of tractors there may be a lag of between one to two seconds before the trailer brakes kick in, due to the brake actuation system.

Therefore, is it not the case that for one to two seconds after pressing the brakes the tractor disc brakes are stopping the entire load until the trailer brakes kick in? This means that for one to two seconds, a 150hp tractor pulling a trailer carrying a load of 10 tonnes has to stop a total weight of 21 tonnes (total weight of tractor, trailer and load on trailer).

Can this lag in time before the trailer brakes start to work not be engineered out of the tractor or is this a problem for the trailer manufacturer to address?

And add in the speed factor. Tractors have moved from 30 to 40km/hr in tractor speed over the past 10 years. Increasing the speed by 30% with the same load on the trailer, adds to the momentum of the moving tractor and trailer combination, putting additional pressure on the braking system.

Trailers – more stopping power?

If the design of the braking system on tractors is not the reason behind the reported cases of brakes wearing out reasonable quickly then it can mean that either of two things. The operators of these machines are either putting too great a demand on the tractor brakes or the trailers and implements the tractors are hauling do not have adequate braking systems for the bigger loads and higher speeds.

Heavy haulage work with tractors has increased. As more tractors are being required to haul large trailers, it is important that the braking systems on the trailers are up to the task.

What are the load/weight limits of a hydraulic braking system on a trailer being pulled by a tractor? On the issue of the lag between the actuation of the tractor brakes and the trailer brakes - could the trailer brake system have anything got to do with this? What are the greatest issues affecting brakes on trailers?

Is it the operators fault?

If it's the case that the tractor manufacturers are making a braking system that's suitable for the tasks, which the tractor is designed to carry out and if it's the case that the trailer manufacturers are making and designing the trailers with suitable braking systems - then is it the case that the operator is expecting too much from the tractor and trailer?

Making sound investment decisions in any piece of machinery is vital for the machinery user. Those decisions are based on performance, value and machine output.

However, does the issue of trailer brake specification stress this point to the extreme? It does illustrate just how important it is that every machine is purchased and spec'ed to the correct standard and that includes safety standard, for the tasks in hand.

The tractor manufacturer will argue that haulage work is hard on tractor brakes. Take for example a 14 tonne dump trailer. Due to the design of such a trailer they appear to be a neat and tidy trailer. In a sense this could lead the user to think they are not carrying a big load when in fact they are.

Is it the case that operators are not putting suitable tractors in front of such trailers? The fact that the tractor can pull the load does not guarantee that it can stop it. Is this mismatch putting too much pressure on not only the brakes of the tractor but in fact the entire machine?

There are guidelines for the operation and maximum loads for all machines. Is it up to the operator to keep within these guidelines and to find out what these guidelines are?

In summary, there are a host of questions and a similar number of answers for each issue. Over the next four weeks these will be addressed. The results of various tractor and trailer brake efficiency tests will be examined to help deal with the questions and provide some answers. We would welcome your comments.