Braking And Tractor Mishaps

Press, Volume CV, Issue 30977, 5 February 1966, Page 10

Braking And Tractor Mishaps

An examination of the records of fatal tractor accidents for the last three years shows that nearly half of them are caused by inadequate braking effort

The driver may be fatally injured because a tractor overturns and crushes him in some manner. Very often control of the tractor is lost before it overturns. The driver loses control because the tractor Slips or his insufficient braking effort to, cope with a certain situation.' This is a common example. A tractor is driven up a slope ta a gear that is too high. The driver attempts to change to a lower gear but he fails to engage the

lower gear before the tractor starts to move backwards. Gear .engagement is now impossible and if the brakes are faulty or inadequate the tractor continues to travel backwards uncontrolled until it overturns. Therefore the real cause of this type of accident is lack of efficient braking, because the tractor would not have overturned if the braking effort was sufficient to allow the driver to change gear successfully on any slope that the tractor can normally negotiate.

;When describing the braking effort of a tractor, it is necessary to consider all the contributing factors. These include the traction or grip of the wheels on the ground as well as the brake equipment

on the tractor. Therefore, the term “braking effort” refers to the ability to slow down and stop within an acceptable distance on the flat or any slope that may be negotiated. If this meaning of the term “braking effort” is accepted, then it is interesting to examine the effect of different factors under various conditions on the amount of braking effort available. The first is an obvious one; the mechanical condition of the braking equipment. Often

[The accompanying article was written by Mr D L. King, senior instructor in the engineering department at Lincoln College ]

the brakes are ineffective due to worn linings, incorrect adjustment, or oil leaks. Many factors are involved in relation to the design of brakes and there are too many to discuss in this article. The general principle is to choose the mini/num size or friction area to produce the required braking force when a reasonable foot pressure is applied to the pedal. Poorly positioned controls or incorrect seat adjustment may mean that the driver cannot apply sufficient force to the pedal. If the brakes are applied after the temperature of the drum or disc rises, this will cause a reduction in the friction between the sliding sur-

faces. Therefore the brakes "fade” or lose their effectiveness if they are used excessively. The mechanical braking equipment stops, or reduces the movement of the wheels or tracks relative to the body of the tractor. These wheels or tracks must have traction or a grip of the ground to slow down, stop and hold the tractor. The traction or grip obtained is dependent on the weight or pressure between the braked wheel and the ground. The value of the tractive force compared with the ground pressure varies with soil conditions. It may be less than 10 per cent in mud or perhaps 80 to 90 per cent under good conditions. The tractive force is rarely greater than the ground pressure. Most farm tractors are fitted with brakes to the rear or driving wheels only. This means that the braking effort is related to the proportion of the tractors total weight that is supported by the rear wheels, which for the average tractor on flat ground is between two thirds and three quarters of the total. The proportion increases when a tractor is pulling a load or is moving up a slope but decreases when a tractor is moving down a slope. The braking effort required to slow down and hold a tractor is proportional to the total weight of the tractor and any equipment associated with it. While the rear wheel brakes may be sufficient to control a

2j-ton tractor by itself, they could be seriously inadequate if the total weight was doubled when pulling a loaded trailer. Since the additional weight is supported mainly on the trailer wheels, this extra weight does not increase the braking effort of the tractor. Accident records show that this situation is a serious hazard. Since the braking effort is limited by traction, the danger could be eliminated by fitting brakes to more load-carrying wheels, that is, the tractor front and the trailer wheels. Most drivers have discovered that a tractor can be stopped sooner if the brakes are applied so that the wheels still revolve slowly but are not locked stationary. A sliding wheel has less grip or traction than a wheel revolving with a minimum slippage. Several fatal accidents have happened because, when a wheel is not revolving at all but sliding, it has no directional stability. It can slide in any direction, perhaps sideways into a ditch or over the edge of a bank. A revolving wheel still will provide some steerage. / Reliance on engine braking only when the normal brakes are defective can also lead to disaster. If an incorrect gear ratio is chosen when descending a slope the tractor increases speed and as soon as a wheel slips or bounces the' engine braking is lost through the differential. Then all control is lost.