OT cable strength

At work we have a 1958 Unit Crane we still use it has 5/8" steel cable with a 4 line block one of our operators wants to change it out to 1/2" blue steel polly rope with a 3 line block I know it will take more power to lift with the 3 line block but will the 1/2" blue steel poly rope be as strong as the 5/8" cable

Depends on the tensile strength of the cable.
Also going from 4 to 3 block will cut the lifting strength by about 1/4. Not a real good idea to do this.

I have a very good back ground in Cranes having worked on them for many years and performed a 10% overload test on them.
Walt

The poly strength needs to be checked for compliance with the crane spec. As Walter indicates, the lift will be 25% less, and faster, but are the chives designed for smaller cable?? if not they will have a line contact with the new cable stressing the inner strands, and making dangerous lifting. With a 25% reduction in lift there will be a tendency to lift as large a load as before, stressing the cable. I would not go there. Using the correct size cable in a new technology would be much better. If there are wear marks in the chives from cable contact, the twist and lay direction and pitch of twist must be the same. If they are smooth, it would be OK to use the same size as specified. Lives depend on your decision. Maybe yours. Jim

thanks for your replys they are a great help

You would have to change all the sheaves to match the new cable including the crown block and not just the traveling block. But more importantly you should look at the drum and see if it is grooved for 5/8" cable. If so, the 1/2" will never lap right and will cross over till it pinches and ruins the cable and maybe somebody's life.

Not sure I'd mess with it and it won't save any money changing out new sheaves and blocks.

I've run a couple cranes and worked in the oilfieds for years drilling on drilling rigs and even ran a cable tool rig once. I've changed and spliced cable and changed winch cables on trucks.

The other part of the equation for grooved or ungrooved drums is that the diameter and and width of the drum is matched to the size of the cable. Even when respooling a drum with new cable, we spend a lot of time with a hammer pounding each wrap in tight. If you don't get the first and each subsequent row tight and with the cable pulled on in the right direction with the right twist, you will never get it to run right on the drum until you end up with a rat's nest and kinked and ruined cable.

For the strength rating of the cables, check the manufacturer. That info is probably on the web somewhere.

With a crane, you have certain safety rules and insurance issues to deal with. You also have to follow the crane mfr recommendations. And it will change all the lift tables and ratings for the crane at each different angle you have the boom set at (an accident waiting to happen for some new operator who doesn't understand all that stuff)

Stick with the 5/8" cable. Or demote that operator to swamper and let him work under that crane.

My 2 cents. Let us know what you do.