kevinro
Bronze Member
My $0.02: Take extreme care with your hooks. Your test of lifting the tree doesn't cut it.
There are several things to consider here. There are static and dynamic loads. Just because you can lift the tree doesn't really clear you to say that this hook is strong. Lifting the 500 pounds is only part of the story. As you start to move your 500 pound tree, it gains energy. Depending on how fast you are travelling, it can gain a suprising amount of energy. The basic formula is
F = 1/2 * M * V^2
The important thing to see is the v^2, which means as the speed increases, the energy goes up as the square of the speed. Bottom line is the forces involved get really big in a hurry. When you get that energy created by moving the object, it will eventually need to be disapated.
The true killer is starting and especially stopping. This is where the forces involved are at their maximum. The sum of the forces become the 'weight' of the object (500lbs) PLUS the mass of the object times its acceleration. Just as an example, say you go from a walking speed (4mph) to a stop in 1 second. The chain will see nearly 100 pounds of additional force. (F = m * a, F = 15.6 * 5.8ft/s. The mass of a 500 pound thing is 15.6 slugs. F= 90.625lbs) Thats a 20% increase doing a fairly common and seemly minor change of safely stopping. If you did a sudden stop, you could easily see an additional 500 pounds, which is a 100% increase. If it bounces, the forces can easily double that!
I would like to modify Hydraman's statement: If you are lifting 1000lbs, build it for at least 4000lbs.
There are several things to consider here. There are static and dynamic loads. Just because you can lift the tree doesn't really clear you to say that this hook is strong. Lifting the 500 pounds is only part of the story. As you start to move your 500 pound tree, it gains energy. Depending on how fast you are travelling, it can gain a suprising amount of energy. The basic formula is
F = 1/2 * M * V^2
The important thing to see is the v^2, which means as the speed increases, the energy goes up as the square of the speed. Bottom line is the forces involved get really big in a hurry. When you get that energy created by moving the object, it will eventually need to be disapated.
The true killer is starting and especially stopping. This is where the forces involved are at their maximum. The sum of the forces become the 'weight' of the object (500lbs) PLUS the mass of the object times its acceleration. Just as an example, say you go from a walking speed (4mph) to a stop in 1 second. The chain will see nearly 100 pounds of additional force. (F = m * a, F = 15.6 * 5.8ft/s. The mass of a 500 pound thing is 15.6 slugs. F= 90.625lbs) Thats a 20% increase doing a fairly common and seemly minor change of safely stopping. If you did a sudden stop, you could easily see an additional 500 pounds, which is a 100% increase. If it bounces, the forces can easily double that!
I would like to modify Hydraman's statement: If you are lifting 1000lbs, build it for at least 4000lbs.