All mechanical hammers work on a variation of the same principle. A rotating crank shaft lifts the weighted hammer head that is counter balanced, then forces it down on the next half of the revolution. The attachment on other hammer head has to be a spring construction of some sort so that the impact is absorbed in the spring not the crank shaft. The counter weight relieves some of the strain on the motor.

There have been many different configurations of mechanical hammers over the years. Little Giant comes to mind but this is only one style. Others include Helve Hammers etc.  Mechanical hammers are rated by the hammer head rate. So a 25 lb Little Giant has a 25 lb hammer head weight. The heavier the head weight the larger the steel that you can work under it but the larger the motor that you need to run it.

Something to think about. If your shop is in open air but has no electricity you could run a mechanical hammer off a small gasoline engine. A little expensive but compared to the amount of work you could do this way, it might be worth it.

I have only worked a little with mechanical hammers but a 1 hp motor will run up to about 50 lb Hammer head weight.

The beauty of a mechanical hammer is that it is relative simple to build or repair. The concepts of the movement are very simple and easy to follow in slow motion. Mechanical hammers were relatively common in industrial settings in the late 1800's and early 1900's so you may be able to find one for a good price in your area. The downside is that parts may be impossible to find and you may have to fabricate your own.

You can also build your own mechanical hammer. It will take some tinkering but a good working hammer can be made pretty economically. They don't take up a lot of space. Perhaps 2 feet by 3 feet for a small one. They are a bit noisy to run and have an impact sound to them. They do need a good foundation, although a small one can get by with a small foundation. They are a bit limited by the tasks that you can do with them. If you are creative with your tooling you still can do a lot of work and save your arm.