Written By Dave Decker, President, Becket Training & Consulting in CraneWorks Magazine
Sometimes during a load test, a hydraulic mobile crane reeved with less than the maximum number of parts of line will fail to pick up the test load. So why can a crane lift its rated test load with a fully reeved block and yet fail to pick the reduced test load when it is not fully reeved? There may be too much wire rope on the drum. Understanding the how and why of line pull can help us determine the best course of action to take in resolving this problem.
Understanding line pull
For each layer of wire rope on the hoist drum, the effective diameter of the drum increases by approximately twice the wire rope diameter. For example, if the wire rope diameter is 3/4″, the effective drum diameter will increase by 1 1/2″ for each layer on the drum. If the diameter of an empty drum is 24″, the first layer makes the effective diameter 25-1/2″. The eighth layer brings the diameter to 36″.
Torque demand, expressed in foot-pounds, or pound-feet is the result of force (line pull) x radius (distance from the drum axis to the outer layer of wire rope). If the line pull remains the same, and the number of layers on the drum increases, the torque required to lift the load (torque demand) increases. For example, assume the line pull resulting from a load on the hook is 16,000 pounds. The radius to the first layer of wire rope is 25 l/2″ -:- 2, or 12 3/4″. Torque required to rotate the drum, equals 17,000 foot pounds (i.e., 16,000 x 12.75 -:- 12). To rotate the drum with eight layers requires 24,000 foot pounds (i.e., 16,000 x 18 -:- 12) of torque to lift the same load. Hydraulic pressure and hydraulic motor efficiency limit the torque available to lift the load. If a crane is not reeved with the maximum parts of line, the rope may build up to a point where the available torque is not sufficient to rotate the drum. This can also happen on fully reeved cranes if there is too much extra rope,
Line Pull-Dealing with the problem
Remove any excess wire rope if possible. If this is not practical, then how can we test the crane? If the crane has the same capacity at a longer boom length, perform the test with the boom extended. If the crane still cannot lift the test load, check the allowable line pull for the particular hoist and layer of rope. If allowable line-pull (available torque) is the limiting factor, reduce the test load based on the allowable line pull. If the crane cannot lift the reduced test load, verify that the system hydraulic pressure and relief valves are set within OEM specifications. If the settings are within specification, contact the crane manufacturer.
Becket Training & Consulting (The Crane Training Expert) can be reached for consultation right here on our website contact form.