What is the accuracy of an overload protection system for EOT crane using load pins.
The aim of this document is to analyze where the different measurement errors come from and to define their importance.
The measurement accuracy of an overload protection system is dependent of several factors:
- Accuracy of the load-pin
A load pin has essentially mechanical errors (non-linearity, hysteresis and non-repeatability). They depend on the dimensions of the axis (length / diameter ratio which cannot be too small) as well as the quality of the supports.For standard axes (5300 series), this error represents ± 0.5% of their full scale.If necessary, this error can be reduced to ± 0.2% by profiling the axis (feasibility to check).
On the other hand, if the ratio length / diameter is too small this error can go up to 5%.
As the thermal drifts of zero and sensitivity are less than 0.2% of full scale / 10° C, they are often negligible compared to mechanical errors.
- Accuracy of the load limitation electronics
The accuracy of a load limitation electronics like Crane-Boy is of the order of ± 0.1%.
The error induced by these electronics is therefore generally negligible compared to other errors
- Weight of the hoisting cable
Higher the load smaller is the part of cable supported by the load pin.
The importance of this error depends on the ratio between the cable weight and the nominal load hoisted.
The only way to cancel this error is to always do the measurements at the same height.
To eliminate this error, it will therefore be necessary to measure with the load at the same height or to provide a correction system taking into account the height of this height.
- Errors due to frictions in the pulley blocks
The consequence of frictions in the various pulleys is that the apparent load on the load pin seems lighter when the load is being hoisted and heavier when the load is being lowered.
It is possible to lessen the influence of these frictions by always taking the measurement after the same hoisting or lowering action.
- Errors due to dynamic phenomenon’s
The hoisted loads rarely stay still, and the resulting acceleration is added to that of gravity.
This influence can be lessened by treating the signal of the load pin with an adequate filtering of the electronics and by waiting for the load to be still.
Considering a load pin with favorable dimensions and making calibration and load measurement always at the same height, after a rise in low speed, the accuracy of the measurement will be between ± 1 and 2% of the crane’s capacity.