01 Nov 2009 21:10
In Part 1 we talked about planning and material selection, in part 2 we're going to talk about the design and construction aspects.
Let's get right to it.
Know Your Building Material
This is a crucial point, well one of them. Every material has a weakness. There is not one super material. Commonly used materials include Steel, Wood, Concrete, and Polymers/Plastics.
Steel is as close as you'll get to a super material but it still has its weaknesses. Welds are often susceptible to stress concentrations and weakness. Solid pieces of steel are usually bullet proof and where things typically fail is at a joint, hole or weld. Steel can still wear and fatigue which makes it susceptible to failure and sometimes the repair or replacement can be tricky depending on how you've designed/manufactured the piece.
The strength of wood depends on the orientation of the grain. When the stresses are distributed parallel to the grain, wood is at it strongest. This is just fancy talk for "go with the grain". If you're going to use a piece of wood for pinching deadlift, or lift exercises you should orient the wood so that the grain direction is vertical, not horizontal. Wood wears very easily and thus is not a good candidate for mechanical equipment. Wood with large dimensions (6x6 and up) can make a decent sled though.
Ceramics, concrete, brick and cementitious materials alike are brittle materials. They are not very robust when it comes to impact resistance, meaning if you drop it or expect to drop it don't be surprised if it chips or fractures. I haven't found any uses for concrete, aside from the floor, in a home gym. The cost, labour intensiveness, sheer mass and lack of impact resistance make it a very niche material. Making a kettlebell or blob (or similar grip training implement) requires the ability to not only make spherical moulds/forms but also to ensure they are free of voids and finished with a surface that looks like glass. I'm also aware you could make rectangular kettlebells but then you have the bulky shape and the worry over chipping corners and edges.
Atlas stones are an exception to this rule because of their dimensions, mass, and spherical shape. Chipping usually occurs at an edge/corner whereas fracturing usually occurs at the smallest cross section. Examining the shape of the Atlas stone, there are no corners or edges and right away we've sidestepped that. The mass of the stone means that it will, 99.9% of the time, damage what its dropped on with only minor superficial damage to the stone. In order to fracture a stone we'd have to split it. The large diameter of the stone gives it a large cross sectional area and therefore a large resistance to a tensile/splitting failure.
Tension is a particularly weak aspect of these materials. I mentioned that an atlas stone had a large resistance to tensile failure. Seems contradictory but the Atlas stone does not gain its strength from the material, it gains its strength from the amount of material. Meaning that a smaller diameter Atlas stone would be more susceptible to splitting than a larger diameter one. Unless there is a lot of material there won't be much tensile resistance. In layman's terms this means you shouldn't be pulling on them; meaning you shouldn't make a concrete loading pin.
Why is a concrete loading pin a bad idea? A Loading Pin, on the right, has a shaft and a base. Plates get loaded onto the shaft and when you lift it the base lifts the plates up. From a technical standpoint this means that there is pure axial tension in the shaft and shear in the base (not the same as axial tension but produces a similar effect). From a practical standpoint the strength of these materials is in compression and using it to resist tension is extremely inefficient. With the limitation of a 2" shaft (plates have to fit over it after all) there isn't a lot of room for massive dimensions.
For any of you who are familiar with concrete, I realize you could make a reinforced concrete loading pin or simply a reinforced base with a large diameter reinforcing bar serving as the shaft. BUT anchorage of the shaft is difficult unless you weld something for mechanical anchorage. That combined with the reinforcing and form work makes it more feasible to just weld some steel together and make it all out of steel. Who knows, you may chip an edge on your first bad attempt. Building a loading pin out of reinforced concrete is not a good idea in terms of durability or strength.
Plastics/Polymers aren't typically used for 'heavy' exercises although they can be used as handles. When using any sort of polymer, bolt holes are weak planes as are bearing areas, particularly edges where chain or rope are in loaded contact.
Regardless of the material you use, endeavour to know when/where it is strong and when/where it is weak. I'm not suggesting you pull out a calculator and crunch the numbers, you can over design just about any piece of DIY equipment without any math.
The Weakest Link
We all know the phrase "only as strong as the weakest link" and this is for good reason…It's absolutely true!
Think of 2 pieces of steel butted together (butt joint) with a couple strips of some good ole packing tape. Now we grab each piece of steel and pull, as if we're trying to create a gap in the butt joint. Obviously the steel is not going to fail! The tape is going to either peel off or break with failure right at that joint and the steel just going along for the ride and .
Failure will ALWAYS occur where the resistance is lower than applied force and this typically occurs within a very localized area.
That being said, joints are typical weak points. I wouldn't recommend using anything with a wood joint, unless it is a very lightly loaded piece of equipment. Steel joints are typically welded.
Let's take 2 pieces of different diameter pipe, slide one pipe a few inches into the other, and weld them together. Let's also assume that the joint is as strong as the larger diameter pipe. Now we try and pull them apart. The larger diameter pipe and the weld would be fine but the smaller diameter pipe would break because it's weaker than the larger diameter pipe and the joint. The failure of the smaller pipe would also most likely occur around the area of the joint. Weaknesses commonly occur at places where dimensions change abruptly or where they are the smallest.
For another example we can simply take a piece of chain. One of the links in the chain is substantially smaller than the others. If we pull on this chain hard enough the smallest link will eventually break because it isn't as big as the other links.
Bolts & Holes
Holes or places where material has been removed are weak areas. This subtly ties in with the abrupt dimension change that I mentioned but also opens the door for bolts. Bolts can also be weak areas. Its a bit of a catch 22. If you drill a small hole you retain the strength of what you drilled in to but you have a very small bolt. If you drill a large hole you will have a very strong bolt but you remove a lot of material from the thing you drilled. The trick is to get a happy medium between bolt size and the amount of material you remove.
A good starting point for DIY equipment is to pick up a catalog, do a google search or visit a manufacturer's website. I'm not saying that you blatantly copy someone else's design, but that is usually the easiest way. Manufacturer's websites can be the most helpful. Just take a look at the item and brainstorm from there.
When looking at any design or the manufacturer's design always think of where you can add strength. Generally you cannot take away strength but more often than not you can add strength. Even if you cannot 'out-do' the manufacturer's claims, you can figure out where your construction may be weakest or where you may want to have some extra peace of mind.
I have a Rolling Thunder deadlift handle and a carabiner from IronMind, however I made my own loading pin. The base of my loading pin is about 2-3" of solid steel. I can drop that until I'm blue in the face and it will keep smiling at me. With the added thickness I don't have to worry about it dropping funny either. It always lands on the base and very quickly finds vertical. I don't have to worry about dropping it cockeyed on a plate (and possibly damaging my pin or plate) or having it roll onto my foot.
The loop at the top of my loading pin, where I attach the handle, is made from a small diameter piece of rebar and has some ribs on it. The ribs on the bar would undoubtedly cause excessive wear and premature life span of my carabiner. So to prevent this I bought a steel shackle and now that takes the wear from the rebar and allows me to hook my carabiner onto something. You may be wondering why I didn't just attach the handle right to the shackle and the answer to that is because the shackle is a hard steel whereas the deadlift handle is either at par or a slight bit harder. If I attached the shackle right to the handle it could wear out my handle! The carabiner seems to me made from a soft metal and so far there are a few wear marks on it and none on my handle.
Simple Training Sled
Examining a sled, the rails and the point where you attach your harness are going to be taking a lot of abuse. The loading pin can also take some abuse depending on how much weight is on it. The rails obviously take a beating because they're being dragged along the ground and the attachment point will wear because of movement or excessive stress. A lot of people use sleds outdoors and on grassy surfaces, parking lots, some even use sand (think of sand paper on the rails of your sled). Uneven surfaces or jerky movements can wear out that attachment point because of the continual shifting, can damage your loading pin because of all the plates shifting around, and can wear down or damage the rails. Those 3 points are important places to reinforce.
As you can see, durability (and the ability to take a bit of abuse) is a pretty big one for me. I don't want to spend a lot of time and effort in something only to have it break because I dropped it, was too aggressive with it or used it too much! For that reason I typically make things heavier and bigger (thicknesses, welds, etc) than they need to be.
When adding strength or reinforcing something the answer is usually to add more material. A bigger piece of steel is stronger than a smaller one. When it comes to durability (reinforcing wear points) there are a couple options. We can add material so that there is more to wear, you can control what wears and make sure that whatever wears out is easily repaired/replaced and economical. In the deadlift handle example I used the latter of the two, a shackle costs about 50 cents. I can afford to wear out at least 6 of them.
For impact resistance for a log, you can simply wrap some rope around the ends. Because of the way the weight is distributed the log will either fall straight down or on one of the ends. Wrapping the ends with rope safeguards against, approximately, 99.9% of the ways it can land, unless you caber toss it..To protect your equipment from damage due to missed attempts or dropping, etc. reinforce/cushion the points which will bear the impact.
Existing Equipment Modifications
Take a sled for example. You don't like the way the rails are performing, maybe the sled is upsetting if it goes over a bump or uneven ground. If the rails were bolted rather than welded it would make it a lot easier to put a new set of rails on. You should try and make your equipment in such a way that you can change it or modify it once it's been in use. You may find one area that could be improved and its always nice to be able to just improve it rather than start from scratch.
Hopefully you find that your training involves heavier and heavier weights or more and more demand out of your equipment. Today you may design a log capable of going up to 260lbs but one day you might be able to do 280lbs. While that would be a great day, it would suck to have to re-make or buy a new log. By the same token you may find that you made something too heavy. Being able to shave a couple of pounds of it means the difference between starting over and starting to use it.
Now that I've finished boring you about general material properties and design I'm going to be concluding this series with some DIY examples. It will be some of my stuff plus some of the stuff I've seen on the web. If you have any suggestion or question….my comments section gets lonely sometimes, feel free to keep it company.
This page created 01 Nov 2009 21:10
Last updated 24 Jul 2016 18:13