Why Does a Crane's Load Cell Show Different Weights While Lifting and Lowering?

[Marinel]

As you can see in the attached doc, we have two winches on the same shaft of motor M1, two moving pulleys ( 3 and 4 ) and a fixed pulley (5).
The load is 130 Tons, and we can measure that with a load cell at pulley 5 ( 32,5 Tons) when we stop the lifting of the load.
But when the load is rising we measure only 30 tons! Note that we have a cable of steel with 30mm diam. and the rising speed is about 1m/min.
When we lower the load, the load cell measures 36,2 Tons.
Can someone explain this? This is a real case and my problem is to adjust the safety relay to 110% of nominal load, which is impossible with this beahviour of the system!

 

[phinds]

seems to me you must have the figures reversed for raising and lowering the load since raising it should be more than 32.5 and lowering it should be less than 32.5

 

[Marinel]

I understand what you say, but when is raising shows 30 Ton and when go down shows 36,2 Tons.
I have checked the system and load cell at pulley 5 is OK ( I have already changed it by another one ). The pulley doesn't move at all, only slightly when the movement starts.
Also I have checked the repetibility and all time with the same behaviour.When we put load on the floor, load cell goes to zero tons.