Hoist cable shreaded!

[olddog]

I work in a factory that uses cable hoists to remove (1500 lb +/-) rolls of plastic film from a winder. I can count 21 hoists in my head, two of which are 5 ton hoists that reach the first floor and are located 40 to 50 feet above that. The rest are 1 and 2 ton units that are 20 foot high or less.

I have seen quite a few cables fail over the 41 years I have worked there. It's always scary and never pretty. Every failure, the root cause was the operators failure to properly check the hoist before use, which they are required to do and document every shift, by law. The problem is cables fail slowly over time and checking every shift (law) causes people to assume that if the previous guy did a good job checking, he doesn't have to. Pretty soon everyone is not doing a good inspection.

The simplest check is to let all the weight off the cable so you can flex it into a small loop. Use gloves for this. Then check for whiskers sticking out of the cable. Drag a rag up and down the cable and see if the rag gets caught on whiskers sticking out. The bend will expose the whiskers long before you will ever see them under tension. Always try to check an area on the cable that sees a lot of work bending around pulleys.

We are generally lifting 2 roll per hour 24 x 7 x 365. We size the hoist so the lift is no more than 3/4 the hoist rating, but target 2/3 the hoist rating. We replace the cable every 2 years, and sooner if whiskers show up. We will not operate a hoist with whiskers present. No failures since implementing this policy about 30 years ago. About 5 or 6 failures the 10 year period before that.

One more tid bit. How quickly the motor stops and starts puts a jerk factor onto the cable. The jerk can easily double the tension on the cable. If you are starting the motor with slack in the cable and the motor gets to full speed before the slack is out, the jerk factor could be 3 or 4 or 5 times the weight it is lifting, depending on the speed.