Helve Hammer Design

Spring Balanced Helve

Please excuse my rough sketches but if we are going to get these posted in time to do some good this is how they are going to be.

Above is the spring balanced helve hammer I spoke of on the guru page. The CAM runs continously (no clutch).

Falling weight hammers are not as efficient as propelled weight hammers therfore a helve needs to be heavier to do the same work. The speed the helve operates at is strictly based on the acceleration of gravity (see exception below). If your cam turns over faster than the hammer falls then the hammer will hit the cam instead of the work, damaging the machine. Its been a long time since my physics classes but acceleration is constant (faster and faster) 32 feet per scond per second. The first second it falls 16 feet I think. . . been too long. I'll have to look it up. However it is all relative to lift and fall distances. . . unless . .

On some early helves to increase the operating speed they used springs (actually spring boards) to help stop the upward motion and accelerate the ram downward faster than gravity alone.

Now the dynamics get a little trickier. I'd try 120 blows per minute with an 18" (450mm) drop.

I'm playing with a design for a spring balanced helve that you might like. The helve is held OFF the cam suspended above the work by a long spring. The 'clutch' peddle is attached to the helve by another heavier spring (maybe three of the balance spring). To engage the hammer you step on the peddle pulling the helve into the cam and work. The harder you lean on the peddle the harder it hits (you are adding some of your weight to the force).

The hammer Danny and I are speaking of is a cam operated hammer. The cam lifts the helve and then drops it by having a drop in the cam. This drop in the cam must be further than the travel of the helve so that it is not hit by the helve. The cam can have one or more lifting lugs depending on the size and RPM. A suficiently slow input drive may want to have four or more 'lobes' but a faster drive may only want one. A single lobe has the advantage of being able to be designed and profiled for a very smooth operation. It can also provide sufficient room to use a ball or roller bearing follower.

My idea for a spring balanced helve was for a small hammer of say 15 to 25 pounds. A heavy hammer may have too heavy a counter balance for the operator to overcome. The whole spring balance idea was to give some control to a plain helve which normaly hits the same (heavy) blow every time.

The chart below shows the fall distances and times for a dead weight hammer. The gravity constant used is 32.16 feet per second per second.

Gravity Graph, click for larger.

A drop of 12" (30cm) takes 1/4 second. With equal lift and drop cycles the cam lifts the weight every 1/2 second. Using the single lobe cam shown the RPM wants to be 120. In this case the drawing above is wrong as the dies will not be closed until the cam has rotated 180°. This is important because you want to be sure to clear the cam when the helve drops.

Animated helve from Martinetrobert
Ancient hammers (XIX Century martinets) French

. . . The intresting thing about this drawing is that the original had 12 cogs or lobes as shown. The typical speed of a water wheel sufficient to run this hammer is 10 RPM.

10 RPM x 12 Lobes = 120 blows per minute.

The animated helve drawing above shows a machine that probably had an 18" (45cm) fall that takes 0.30 seconds. We determined earlier that it probably ran 120 RPM. How can this be? Well, you can lift the helve faster than it falls. Not a whole lot because you have inertia problems at the top if the helve is going too fast but 0.20 seconds is possible. The fact that the original drawing of that machine shows a bottom stop block at the sprocket end indicates the helve traveled further than the cogs would move it at a slower speed.

Adding springs to the hammer makes things more complicated. The counter weight spring "lightens" the upward throw while the treadle springs increase it. Since the intent is to float the helve above the cam when not in use the springs cancel out in the disengage mode. However, when the operator presses all the way down on the treadle the springs are helping pull down on the helve. This is great because it hits a lot harder but our chart using the gravity constant is no longer valid.

A Conclusion:

Now we have built a machine that has a range of drop times. Lets say from 1/8 to 1/4 second. As initialy stated the problems occur when the droping helve hits the cam. Our cam design above could be designed to do all the lifting in 1/5 of a second (120° of rotation at 120 RPM). This would leave almost 1/3 of a second for drop. The faster drop when leaning on the treadle doesn't matter.

There are a varity of options that could be tried with this design that we may play with later.

  • Clutch the cam shaft, extra load spring, no balancing spring.
  • Raise and lower the cam/drive platform to increase and decrease the stroke/power.
  • Raise and lower the helve pivot to do the same.

Copyright © 2000 by Jock Dempsey, DEMPSEY'S FORGE All Rights Reserved