The time frame involved with tractor over-turns
Tractor overturns cause many farm fatalities, because the way that tractors are designed makes them unstable. Tractors can roll over as a result of any one of the following stability factors or as a result of a combination of these factors. The most common ways for tractors to roll over are Sideways Roll Overs and Rearward Roll Overs and there are several types of each of these.
Sideways roll overs
Tipping axis
The tipping axis is the line that a tractor pivots about during tipping. In a crawler tractor this is the outside edges of the track. The tipping axis of a wheeled tractor is more complicated, because the front axle is attached to the tractor chassis by a central pivot point. As the tractor tips it has two tipping axes: the first and most important is in a line from the outside edge of the rear wheel to the central pivot point of the axle and chassis. The second occurs when the axle hits the chassis and then the tipping axis moves to the outer edge of the front and rear wheels. When the tractor reaches this second axis it has usually passed the point of no return.
The effect of implements on centre of gravity
Centre of gravity
The centre of gravity is the point where all parts of the tractor balance. The position of the centre of gravity changes if any implement is added, removed or changes position. The centre of gravity moves towards the increased weight. As the centre of gravity is raised it takes a lesser slope to breach the tipping axis. Once the centre of gravity lies outside the tipping axis a roll over is inevitable.
Centrifugal force
When aroll over occurs during fast cornering, a force called centrifugal force is introduced. This force tries to pivot the tractor on its outside wheels during the cornering. This pushes the tractor towards a roll over. An example of centrifugal force is the force that pushes you to the outside of the car when going around a corner. The faster you go, the greater the force.
Rearward roll overs
Rear-axle torque
The tipping axis of a wheeled tractor.
Rear-axle torque is the transfer of energy between the engine and the rear wheels of the tractor. This occurs when the pinion gear in the differential meshes with the crown wheel of the axle. Rear overturn can happen in as little as 3/4 of a second. At this speed there is little chance that the operator can take evasive action.
Drawbar leverage
Drawbar leverage describes the forces trying to pull the tractor rearwards when it is towing or pulling an object. The effects on tractor stability vary according to the weight, draft, hitching point used, resistance to movement and angle of pull. Tractors are specifically designed to pull objects from the drawbar. When an object is being pulled there is a transfer of weight from the front of the tractor to the rear
Tractor ballasting
Ballast can be added to a tractor as water in the wheels or cast iron weights or both.
Some three-point linkage implements and some heavy trailed ploughs and rippers can impose heavy downward forces on the tractor drawbar or the draft links. This causes additional weight to be added to the rear of the tractor and increases the weight transfer.
Tractor ballasting is designed to counteract this transfer of weight. Cast iron weights or water in the tyres are both used to ballast tractors. Do not exceed the manufacturer’s recommendation. The main purpose of tractor ballasting is to reduce wheel slip, thus aiding increased tyre traction under high drawbar loads. Add weight to the front to increase steering stability when using mounted implements. There are specific recommendations for adding weight to tractors and these guidelines should be carefully followed if the tractor and implement are to operate efficiently.
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