How did this Tractor Flip?
- Thread starter cartod
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ArtMech
Platinum Member
- Joined
- Oct 5, 2010
- Messages
- 918
- Location
- Lithuania, EU
- Tractor
- Present: 2003 Kubota M9000 DTF; 2001 Kubota B72. Sold: 1985 Kubota L2202
My guess is an operator is pulling incorrectly designed trailer. The trailer axle is too far behind the point calculated according to the load distribution rules. I mean the vertical load of a trailer hitch is higher than the tractor's draw bar or 3ph could be able to carry while towing this trailer safely.
CurlyDave
Elite Member
I think I way overstated that. The center of mass rotates about the contact point. The tractor looks like the whole thing rotates about the axle, but look very closely and the rear axle moves backward a small amount before the overturn.
If the tractor really rotated about the axle, and the axle didn't move, hooking up a chain below the axle would always be safe, and it is not. For a 2WD tractor, the forward torque is the weight of the tractor multiplied by the distance of the center of mass in front of the contact point of the rear wheels. The overturning torque is the friction force at the contact point on the ground multiplied by half the height of the chain, or hitch above the ground. Keeping the chain or hitch low is certainly helpful in keeping the overturning torque small, but if the tractor has good traction and a low gear, a hookup below the rear axle can cause an overturn.
A solid drawbar, mounted to the differential or the frame of a tractor is safe because as the tractor rises, the attachment point lowers. This reduces the overturning torque. Any old piece of steel will not always be safe as a drawbar. An engineer at the factory worked out the geometry, and the factory-supplied drawbar is going to lower the chain or hitch, reducing the overturning torque sufficiently to prevent an overturn.
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Locking the rear tires to the ground as in the freezing example, is a different situation. There it really does rotate about the rear axle.
If the tractor really rotated about the axle, and the axle didn't move, hooking up a chain below the axle would always be safe, and it is not. For a 2WD tractor, the forward torque is the weight of the tractor multiplied by the distance of the center of mass in front of the contact point of the rear wheels. The overturning torque is the friction force at the contact point on the ground multiplied by half the height of the chain, or hitch above the ground. Keeping the chain or hitch low is certainly helpful in keeping the overturning torque small, but if the tractor has good traction and a low gear, a hookup below the rear axle can cause an overturn.
A solid drawbar, mounted to the differential or the frame of a tractor is safe because as the tractor rises, the attachment point lowers. This reduces the overturning torque. Any old piece of steel will not always be safe as a drawbar. An engineer at the factory worked out the geometry, and the factory-supplied drawbar is going to lower the chain or hitch, reducing the overturning torque sufficiently to prevent an overturn.
* * * * *
Locking the rear tires to the ground as in the freezing example, is a different situation. There it really does rotate about the rear axle.
Jim Shooz
Veteran Member
I think that the trailer would have been connected to the tractor by the 3ph. The base of the trailer is a series of rollers. When the trailer gets to the loading area the front of the trailer is raised, the bins roll to the back and the tractor/trailer drive out from underneath. I don't think they would have the time/take the time, to unhitch, unload, reload empty boxes, and hitch up again.
