Malleolus

CLEAR! *thudthud* From a purely engineering perspective, if we're talking straight energy efficiency, it's very hard to beat mechanical to mechanical component efficiency. This is due to the compressibility of gases or flow inefficiencies in liquids. However, there are drawbacks, and many of them have been aforementioned. Unfortunately I'm working on the very same, building my own hammer. From an engineering perspective, one has to consider the whole system, aside from energy efficiency. The problems with any fully mechanical system are thus: Moving parts, friction, tolerance control, and compliance. The biggest issue that many mechanical hammer designers can attest to is the final, compliance. ANY system that interacts with a solid object must include some degree of flexibility, or compliance, in the system or the shock load can sheer linkages, shafts, et cetera. This problem is multiplied in a mechanical hammer because the system is not only designed to impact a solid (if malleable) object with force over many thousands of cycles, these objects change under impact and increase the travel the dies must take before impact (and rebound) occur. You can look at any mechanical hammer and see some form of dial-able travel length, and some form of spring (compliant) element. In the background, however, there are also a plethora of rotating parts, all of which need a bearing and/or bearing surface with (in)consumable friction reduction (grease, babbitt, thrust bearing, roller bearings, etc.). These all also require tight tolerances, axial alignment, and unless your spring element has near perfect shock absorption, all drive components are subject to cyclical shock loads. Any/all off these components, once catastrophic failure occurs, require full overhaul of the hammer, and if poor safety shielding was in place a very unfortunate trip to the local emergency room. Mechanical hammers also have a tendency to take up significant retail space. This isn't much an issue if you're either a hobbyist and use a small amount of equipment or have a surplus of room. Pneumatic hammers, although pricier and less power efficient, have several benefits that far outweigh mechanical hammers. Gaseous fluids have natural compliance, so no spring elements are required in the design, as long as the design of the hammer accounts for compressibles heat and pressure rises. A poorly designed pneumatic hammer requires cooldown cycles because the heat rise, and air volume increase, causes the dies to travel less or stop travelling all together. Pressure vessel failure can be far more catastrophic than mechanical failure in most cases, but again this is due primarily to poor design. Also in a pneumatic system, force is a product of psi rather than velocity, and reciprocation is not dependent upon the rotation of a mechanical linkage attached to some form of flywheel. A properly designed pneumatic hammer can generate significant force over a very small travel distance, allowing the vast majority of the space between the fully open dies to be used as working volume. This is possible with mechanical hammers, but the travel distance must be periodically tuned to the diminishing height of the work piece (there is a range of travel that must be tuned). In the case of tire hammers, the vast majority have a set range of work based on the size of the hub used and the spring travel distance once maximum angular velocity is reached. All of this to say, if you have the equipment and know-how for it, mechanical hammers are the way to go when on a budget. The catch is that it's cheapest if you have a good sized lathe yourself or access to one for a reasonable price, or know a very generous machinist. Also having decent welding skills is very much a plus, and have the experience to properly align your parts and such. Pneumatic hammers have the edge once you start factoring in having to hire a professional machinist or fabrication shop by the hour for parts. You can go to any equipment store and buy a large enough compressor to run a hobby pneumatic hammer for $500USD. Hit a local scrapyard and buy up some sizable I-beam (I built a 750lb ASO for about $350usd at $.30 a pound plus machinists fees to clean up and square off the stock, I did the welding at the local community college for free) and a basic hammer design for another couple hundred and they'll cut it to size usually. Get a cheap stick welder and sticks, a good drill and decent angle grinder for a couple hundred more, plus a pneumatic cylinder of good quality and you are set. It sounds like a lot, and it is, but a machinist will charge easily $25/hr for machining and will have other fee's for getting the frame laid out properly (and you want to drop money here, a machinist will lay out all your parts per your drawings and use his own experience to help you with the design if you're nice). A welder will charge double that probably. Once you buy the stuff you need to make the frame for the hammer you're going to spend as much on the frame for it (at least) as you would a pneumatic hammer. Then you have to buy pillow blocks or some other form of bearings, many of them, and all the drive components plus those machining costs. You save a vast amount of money building mechanical hammers if either you or someone you know can do a lot of the technical labor for free or very cheap, and the motor (or some form of prime mover) can be had for cheap for the thrifty or well connected. Buying all the equipment you will need to build a mechanical hammer will shoot well into the thousands at minimum (mind you, a good lathe can be used for a plethora of projects. If you have the money drop it). It's possible to pick up all the drive components, shafts, etc. from refurbished equipment and other sources so long as you know the parts are true and use them directly with some ingenuity all on the cheap, but that's as much to do with sheer luck than anything. On the other hand, you don't have to get axial alignment, bearings, spring elements, or even squared off corners on some parts of your framework so long as you can lay surfaces flat on each other and run good weld beads for a good, level, robust I-beam frame. Anchor the cylinder firmly and make sure it's vertically square over whatever you are using for the anvil surface, and have it anchored on a sturdy, heavy base, and your pneumatic hammer requires only a couple control components, tubing, and power to the compressor. Yes, you still have to approach it carefully, making sure your frame is plum straight up and down, good anchor points, et cetera, but the technical labor is nothing compared to machining the drive components for any of the mechanical hammer types. You can incorporate increasingly complex valving over time easily to give your hammer more versatility, whereas a mechanical hammer is pretty well set in stone unless you do a major overhaul. You can go on the extreme other side of the spectrum, where I am. I'm frankensteining together a self contained hydro-pneumatic hammer that uses a 17.5hp lawnmower engine, 22gpm 2 stage pump to drive a short stroke, large bore double-acting hydraulic cylinder tied to a tandem shaft pneumatic linear pump of the same bore size, which in turns drives the hammer head via air-tight steel plumbing. The linear pneumatic pump is sized to completely aspirate 80% of the air in the system each cycle using muffled check valves set to certain psi ratings. I can dial these check valves to modulate the impact strength, or clamp the workpiece. The hydraulic cylinder and rod are going to have to be custom machined, and the actual pneumatic hammer cylinder. The thing is, I'm lucky enough to be in a place that has several scrap yards where the local foundries drop off their cast off stock. I can get plenty of frame components for $.30 a pound, and have free access to a welding shop. I can drop several hundred dollars on custom machining, or hopefully find someone I know that has a lathe and mill the pneumatic portions of the pumps myself, and come out on the cheap. This is very much due to my proximity to various industries and resources. For instance, the anvil shaped object I made I picked up a 6 foot section of 5 inch solid round stock and 32x18 inch plate steel, and they had plenty more I wanted to get but physically couldn't transport at the time. Be creative, do your research, learn your area's resources, and make the decision on which one works best for you.