THE CASE FOR LINE PULL TESTING
By: John W. Davis, PE
Many crane manufacturers, users, operators, and certifiers are less than enthusiastic about proof load testing. It may be troublesome but it has the added advantage of providing a risk management tool to the certifier and others who are doing their jobs according to the standards that are applicable. Perhaps one of the most significant aspects of proof load testing is the verification of line pull capability.
There are several ways to express the line pull. Permissible line pull is the value that is derived from applying the design factor to the breaking strength of the wire rope. For standard running ropes on load hoists, the value of the design factor is 3.5. Available line pull is that value that can be delivered by the available winch torque and it may be more or less than the permissible line pull. Finally, rated line pull is that value specified by the manufacturer upon which the required parts of line is based. It is the rated line pull that should be verified during a proof load test.
Lifting capacity for single line operation and reeving for multi-part operation are affected by line pull. If the line pull is not up to specification, single line operations will not lift the maximum rated single line capacity. When using multipart reeving, more parts will be required than indicated as necessary if the line pull is not up to specification. This may also prevent lifting maximum capacity with maximum reeving or require more line than the hoist drum can carry when more than recommended reeving is required for long boom operations. Similarly, if a crane is set up with 100 feet of boom with a maximum capacity for that boom length of 23,000 pounds and the hoist won’t lift it because the line pull is insufficient for the two-part reeving indicated for that lift, the reeving would have to be changed to three-part. For the recommended length of line on the drum, the increase in parts of line may result in the drum not having two wraps left on the drum for all boom angles and heights of operation.
The lifting capacity of hoists driven by hydraulic motors is affected by system hydraulic pressure. If the hydraulic set pressure is lower than specified, the line pull will not reach specification. Similarly, if the hydraulic motor driving the hoist is worn, even if the pressure is set correctly, the hoist will not receive the required amount of torque to produce the rated line pull. Additionally, the engine rpm required to produce needed line pull will gradually increase as the hydraulic motors wear leaving less and less power margin for emergency action.
A line pull test is needed to adequately determine the braking capacity of the hoist. If the line pull is not tested it can not be determined beforehand if the maximum load can be controlled during lowering. This is particularly true for some friction cranes with a power down function. Even if the brake adjustment is checked by measurement, the brake can be deficient because of drum distortion, glazing of the lining or extreme linkage wear. On some hydraulic cranes, the hoist brake is not readily accessible for inspection, so a line pull test is the only way to determine the effectiveness of the hoist brake.
Testing the line pull should be given very serious attention. First, the load chart or owners manual must be consulted to determine the rated line pull. Then a suitable test weight must be selected. If the hoist is reeved single line, a weight should be chosen which is between 100% and 110% of rated line pull. If the hoist is reeved multipart, a test weight must be chosen which will produce between 100% and 110% of rated line pull considering the number of parts of line. After attachment with proper rigging, the test load should be lifted a few inches off the ground and held to test the brakes in the static hold condition. Then lift the load to an appropriate height and exercise the brake by lifting and lowering the load without touching the ground through several cycles. Since the line pull must lift the load and brake performance controls the descending load, it is unacceptable to test this characteristic with an anchor or dead man, which does not allow movement of the load.
The general function that is influenced by line pull is load control, which involves lifting, holding, stopping, and the lowering speed. Therefore, line pull is the essential element in evaluating conformance to rated capacity, reeving configuration, brake capacity and load lowering control specifications. By eliminating the problems that normally cause line pull deficiency, the test may help uncover a frozen sheave or some other reeving deficiency. Line pull tests provide the best method to verify the rated capability of the hoisting functions and should be the primary analytical test utilized by crane inspectors.