Wednesday, September 5, 2012

Helve Hammer Part 3


Helve Hammer Power Cycle

The mechanical helve hammer uses an eccentric actuated crank to raise and lower the Helve beam through an adjustable connecting linkage.  As with any eccentric system the velocity varies with the rotational position. In a given full stroke the eccentric rotation passes through several important points.
If we divide one full revolution of the eccentric into four equal parts or quadrants we can look at the importance of each position. The first position we care about is what is called “Top dead center”or TDC for combustion engine shorthand. Top dead center is the position in the case of the helve hammer that raises the helve beam to its maximum height over the workpiece as shown in the picture above. The next position is 180 degrees opposite from TDC and is called “Bottom dead center” or BDC This is the lowermost position that the eccentric travels.In this position the hammer head is in contact with the workpiece or your soon to be tenderized fingers if your not paying attention.
These two positions represent the points at which the eccentric reverses its direction, and most importantly the eccentric velocity is zero. Any eccentric hammer system, whether it be Helve, Yoder, or Pullmax can be compared to a sine wave pattern.

Starting at zero we trace along the curve and at the peak of the curve we find  the point  TDC. Following the curve all the way to the bottom we then come to the point BDC. On the eccentric mechanism this represents 180 degrees (270-90) of eccentric rotation. The curve on this graph show one full cycle, or 360 degrees of eccentric rotation. As I stated the eccentric velocity is zero at the extreme ends of travel. If the Helve hammer mechanism were rigidly connected to the eccentric motion it would produce a squeezing action as opposed to a hammering at the ends of the stroke. This is fine for things like press brakes and punch presses where we want to produce a large pressure or force but not for power hammers that operate by impacting the work to move material.
For power hammer design the point we care the most about is the point of maximum velocity. That point is halfway between TDC and BDC. This point is both 90 degrees from TDC and BDC. At this point the eccentric shown in the picture is neither speeding up or slowing down it is actually traveling at the same angular velocity as the motor for just a split second.
Power hammers differ from other eccentric or reciprocating systems in that they have an added spring in which to store some of the inertial energy of the hammer head and impact mass generated during the eccentric cycle. The spring compresses on the upstroke, and extends on the down stroke. For the helve hammer I designed and built the flat spring made from heat treated 17-4 PH stainless steel.

Helve hammers have an swinging arm and a hammer head which have a combined mass that is unique to each hammer. This mass rotated by the eccentric has an inertia. The purpose of the spring is to store some of that inertia on the upstroke and release it on the down stroke. So what is really happening is the mass of the hammerhead and beam is traveling slightly behind the eccentric. As the eccentric is passing through that point at BDC where the velocity is zero the hammer head has been accelerated to the maximum velocity and flung past that point of zero velocity (BDC) thereby delivering a impact blow and releasing the stored energy in the spring instead of giving a harsh squeeze.

The spring that stores the energy needs to have the proper rate for the inertial mass of the hammer. If it is too weak the hammer will flail and miss hit dissipating much of the potential hammering energy in a silly looking dance. If the spring is too stiff it will not release the stored energy at the correct rate, squandering it for some meaningful work. When the spring rate is correct the hammer will hit harder and harder as the speed of the eccentric increases, releasing the maximum stored energy and transmitting it to the work. This tuning is important because this is how the force of the hammer blow is regulated. By varying the speed with the clutch the operator controls the force of the blows. The ability to slip the clutch and control the speed between the motor and the eccentric is essential to a successful power hammer design.

These spring rates can be calculated, (tediously) but I recommend building in some adjustability to the spring mechanism that allows some change of the rate of the spring. Power hammer springs show up in many forms. flat, leaf, coil, and elastomeric just to name a few.
There are many variables that effect how a power hammer hits. System mass, storage spring rate, rebound, and eccentric offset all play a part in the hammer system. It is hard to change any one the variables without effecting the others. My advice to anyone who would like to build a power hammer is to engineer in a little adjustability and fudge factor into all of these areas to allow for some tune-ability and to address tooling mass changes or the different rebound characteristics of various anvils and the different materials being worked. 
I would like to caution builders to use common sense and good design practice when constructing and using a power hammer. These are serious metalworking tools and could really injure the unwary and cause life changing injuries if not treated with respect. Imagine trying to pick your nose with your thumb or use your  smart phone with a pirate hook instead of a meat hand.
These machines smash and mangle metal without a second thought. In all pursuits if you hurt yourself take responsibility for your actions. The facts are this, you will get pinched, cut and crunched doing metalwork. If you put your finger in the mouth of a mean dog and it bites down, can you blame the dog for being a dog? Treat all machinery like a mean dog that thinks your body parts are all made from solid bacon.

Tom Lipton

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