Spring Recovery VS Counterweight

1. May 27, 2014

Bill K

Hi

I am new to this forum. I am an amateur blacksmith/machinist/all around tinkerer. I took a class with a blacksmith and decided I needed a treadle hammer. Most treadle hammers use springs to lift the treadle and hammer after each blow. I decided to build one using a counterweight instead of a spring. It seems to me that there should be no difference. In the end it is a force that does the lifting, produced by either a spring or a counterweight.

I have attached a photo of the red spring model and my unpainted counterweight model. The way they work is demonstrated in these videos:

Some said the counterbalance hammer would be harder to work because you had to overcome the inertia of the counterweight. My argument is that you have to overcome the resistive force of the spring in that model, and there is no difference.

My question to you all is whether or not there is a difference pushing the treadle down against a spring as opposed to a counterweight? Also is there an advantage to the recovery of a stretched spring as opposed to a counterweight.

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2. May 28, 2014

Staff: Mentor

Yes and yes.

The force from a counterweight is constant - a 100 lb counterweight applies 100 pounds of force no matter how high you lift it.

The force from a spring is not constant. The more you stretch it the more force is required; an ideal spring requires a force that is equal to some constant times the amount that it is already stretched (mathematically $F=k\Delta{x}$ where $k$ is the stiffness of the spring) and real physical springs come pretty close to this ideal as long as they're not compressed until the coils bind or stretched until they break.

The advantage of using a stretched spring over a counterweight in an application like a treadle hammer is that when you start the stroke the spring is not stretched much at all so it doesn't exert much force and you can get the hammer really moving before there's any serious resistance to overcome.

3. May 28, 2014

Bill K

I understand what you posted but since the hammer starts in the up position the spring has to be tensioned with enough force to hold it there, say 100 pounds. With my hammer I would use the same 100 pounds to hold it up. As you mentioned, as you depress the treadle on the spring hammer the resistance would increase. On the counterweight hammer it would be constant. So it seems the counterweight model should be more effective.

I compared my hammer to the spring model at the blacksmith shop. I tool a piece of 1/4 inch aluminum and struck a punch. It penetrated 1.5 X deeper with the spring hammer. The spring hammer was also easier to depress, and had a quicker recovery. If my argument above is correct then I think my hammer and treadle are much heavier than the spring hammer I used so I could not accelerate it as fast as the spring model. I understand the quicker recovery as the spring would be stretched even further than its pretension length and so would pull up with much more force.

4. May 28, 2014

Bill_K

Welsome to PF, Bill K. But I hope no one will confuse the two of us!

5. May 28, 2014

Staff: Mentor

Ahhh - right. There is another difference, which sort of follows from the difference between a constant force dependent on the mass of the counterweight and a variable force that increases with the displacement: No matter how heavy you make the counterweight, it will never accelerate at a rate greater than 9.8 meters/sec^2 (the gravitational force on the counterweight has to move the counterweight as well as the hammer).