Trying to find an estimated weight for a mobiles crane with a 5 tonne capacity ?
If the crane HAS a 5-ton capacity, then to be able to estimate the weight you would need to know the length of its lifting arm at maximum extension w.r.t the C.G. of the body of the crane. So suppose we have an arm length of (say) 6 mtrs, then the max turning moment at the fulcrum (from front wheels of crane ? ) due to the Load alone = 5 x 6 = 30 You need to add the moment for the arm itself. Lets say the arm is around 1/2 ton. And its c.g. is approx 1/3 the distance from the fulcrum. Then this adds another = 1/2 * 1/3 *6 = 1 So total moment is 30 + 1 = 31 The body of the crane must be MORE than this. Suppose the C.G. of the crane is 2 meters behind the same fulcrum. Then weight of the body must be 31/ 2 = 15.5 tons. Lets say the weight is 16 tons. Then the TOTAL mobile cranes weight would be 16 (body) + 1/2 (arm) ~ 16.5 tons It all depends on the dimensions of the crane, and where the c.g. is located. If the crane has ballast added right at the back, then it's overall weight can be made less since it's c.g. would get shifted further away from the fulcrum
It is not the weight of the crane alone that matters. But to some extend if the crane is light, this could create a torque when carrying a heavy load and the crane could collapse. Therefore I am speculating that inorder to lift 5Tonnes, the crane must be more than 5 tonnes to balance out the moments resulting in a torque The internal structure of bolts and trusses must be able to handle heavy loads and their are several calculations done to find each forces acting on the members of a truss. For eg: By using Pin Joint Analysis
