I have a customer that has purchased and installed a 20-ton hydraulic boom crane at the side of a wharf. It was a good deal as it came from an equipment auction.

The wharf is on wooden pilings and it is only rated for 10 tons.

To top it off, the hydraulic requirements put us into a 25-30hp power pack. Hmmm... that's expensive eh? So is the 300' of cable to get to get to the nearest 3 phase supply.

What do I tell these people? Is there some magical way to load-limit the crane?

Because you touched it last, you'll have some liability if anything goes wrong and that effects the decisions you make.

But if this was my crane and I was hooking it up myself...

Just because the crane's structure can handle 20 tons doesn't mean it has to lift 20 tons. I would say the lifting capacity of the crane is limited by the hp at the hydraulic pump. If your motor can only generate enough hydraulic pressure to lift 10 tons, it simply becomes a 10 ton crane.

Higher lifting force means higher hydraulic pressure. There are lots of hydraulic pressure switches available that could be used as limit switches to shut down the crane if the weight limit was exceeded. Calculating how much pressure produces the lift you want could be tricky though.

I wouldn't even go the pressure switch route because of the liability issues. An adjustable pressure relief or bypass valve would arguably be more reliable.

Do both

I thought about undersizing the powerpack, but that's already 90% built. I wouldn't be comfortable with making the judgment over "x horsepower = x ton capacity".

I did talk to a crane company and they make load sensing devices that attach to the winch rope. That made my day... and the project much more viable IMO.