A good view of the stem, while attached to the busted cam after pull-testing it. The important parts of the stem that you can see here are the cable loop, the plug that attaches the cable loop to the axle housing and the rest of the cam, the plastic tubing and molding that covers the cable and makes it nice, and the sewn loop sling to which you would clip carabiners etc.
The stem is the long flexible bit that comes out of the axle housing and ends with a loop and sling to which you can clip carabiners and other climbing equipment. It is the part that you grab onto as you place the cam, and it feels the entire force that is on the cam. This means that it should be comfortable to use, and it needs to be really strong.
We ended up with the same design as a lot of modern cams: a loop of cable swaged together at the axle housing, with a sewn sling at the end to which you can clip carabiners, as well as plastic tubing and molding to make everything sleek and stiff (so the cam doesn't flop around when you grab it by the stem).
We ended up with the same design as a lot of modern cams: a loop of cable swaged together at the axle housing, with a sewn sling at the end to which you can clip carabiners, as well as plastic tubing and molding to make everything sleek and stiff (so the cam doesn't flop around when you grab it by the stem).
The final design of the swaged stems, after failing from drop-testing, without the molding or any other doodads. Notice that on all the test pieces, the end of the loop broke, not where the cable is swaged into the plug, which speaks to the strength of the swage. In the middle piece, you can just see the severed strand of the cable right next to the plug, where I accidentally nicked it while finishing up the surface on the lathe. Also, the cable ends are much cleaner on the left-most piece because it had plastic tubing around it, while the others didn't.
However, we went through a few different iterations before we got to this design. This is because the stem turned out to be one of the trickier parts of the project, mostly because of the attachment to the axle body. At all the other load bearing joints in the cam (between the axle housing and axles, and between axles and lobes), most of the strength comes from the fact that the two pieces are squarely pushing against each other. Here however, we had to deal with tension as the cable wants to pull out of the axle housing, and this is harder to deal with than the compression forces.
At first we contemplated using the system that climbing nuts use, with a loop of cable threaded around the axle housing. However, there wasn't enough room in the axle housing for the cable to be threaded through twice, so we never did actually pursue this option.
Next, we tried to weld the cable into the axle housing. This was because we were hesitant to try anything with swaging. Real cam manufacturers have really big hydraulic presses and well made dies (basically a mold that you squeeze stuff into) that we didn't think we could replicate.
Eventually, when welding didn't work, we went back to trying to swage, and it actually worked excellently with the equipment we had! As you can read below, the swaged stem was comparably strong to those of commercial cams!
During the design phase, in order to actually make sure they were strong enough, I drop-tested the welded and swaged stems by dropping heavy weights on them. That process is explained at the bottom of this post.
At first we contemplated using the system that climbing nuts use, with a loop of cable threaded around the axle housing. However, there wasn't enough room in the axle housing for the cable to be threaded through twice, so we never did actually pursue this option.
Next, we tried to weld the cable into the axle housing. This was because we were hesitant to try anything with swaging. Real cam manufacturers have really big hydraulic presses and well made dies (basically a mold that you squeeze stuff into) that we didn't think we could replicate.
Eventually, when welding didn't work, we went back to trying to swage, and it actually worked excellently with the equipment we had! As you can read below, the swaged stem was comparably strong to those of commercial cams!
During the design phase, in order to actually make sure they were strong enough, I drop-tested the welded and swaged stems by dropping heavy weights on them. That process is explained at the bottom of this post.
Welded Stems
Using the lathe, I turned down some stainless steel stock into the cylindrical shape you see above, and then drilled out the two holes for the cable to fit through. We then MIG and torch welded the cables into them. We ended up making three, so that we could test them and get some idea of consistency. The cable we used was some 5/32 galvanized steel 7x7 cable, rated up to 2 thousand pounds, which doubled up should get us in the range of 14kN/3150lbs that the real cam is rated to. I had never welded before, so for these very important welds I asked experienced welder Henry Zecca '16 to help me. Thanks Henry!
After drop testing these welded stems (explained below), we found out that this method wouldn't be nearly strong enough. We estimated (extremely roughly) that the welds were breaking at around three to five hundred pounds. Not even in the right order of magnitude!
After discussing, we think we figured out why. We just did welds on the very ends of the cable, where they stuck out the top of the holes in the plug. This meant that there isn't very much surface area of weld, and that the weld is under tension, with the cable pulling directly outwards from the weld. Also, we were worried that the zinc coating of the galvanized cable could have interfered with the bond. Another option is that the heat messed up the temper of the cable somehow, judging from how the end of the broken cable is clean of any welding material, which means the actual cable broke, not the weld. One solution we thought of was to use acid to get rid of the zinc coating, and then cut slits down the side of the plug, and weld up the sides of the cable. Then there would be a much larger area of weld, and it would be under shear load, instead of tension. There would still the issue of the heat changing the temper and weakening the cable, and we would still have to clean up this weld so that it would fit inside the axle housing. In light of these difficulties, we went back to try the swaging method that the real cam used.
After drop testing these welded stems (explained below), we found out that this method wouldn't be nearly strong enough. We estimated (extremely roughly) that the welds were breaking at around three to five hundred pounds. Not even in the right order of magnitude!
After discussing, we think we figured out why. We just did welds on the very ends of the cable, where they stuck out the top of the holes in the plug. This meant that there isn't very much surface area of weld, and that the weld is under tension, with the cable pulling directly outwards from the weld. Also, we were worried that the zinc coating of the galvanized cable could have interfered with the bond. Another option is that the heat messed up the temper of the cable somehow, judging from how the end of the broken cable is clean of any welding material, which means the actual cable broke, not the weld. One solution we thought of was to use acid to get rid of the zinc coating, and then cut slits down the side of the plug, and weld up the sides of the cable. Then there would be a much larger area of weld, and it would be under shear load, instead of tension. There would still the issue of the heat changing the temper and weakening the cable, and we would still have to clean up this weld so that it would fit inside the axle housing. In light of these difficulties, we went back to try the swaging method that the real cam used.
Swaged Stems
After the failure of the welded stems, we returned to the swaged connections on the real cam that we were using as a model. On the model cam, there was only one strand of cable, swaged at both the sling and axle housing end. We settled on the double strand though, since it would be stronger, as well as simpler, since it would require only one swage, and there would already be a loop formed for the sling.
This time I made the plugs out of aluminum, not steel, since we wanted the metal to deform around the cable. For swaging, I threaded the two ends of the cable into the plug and then they squeezed the bejeezus (fun fact, autocorrect thinks bejeezus is a real word) out of it using the shop's 5-ton press and some homemade dies, so that the metal of the plug actually flowed around the strands of the cable. These sorts of mechanical attachments are supposed to hold up to 100% of the strength of the cable, so if we did it right this is the best we could ask for.
I made the dies out of 5/8 inch steel bar stock. I cut the stock into short lengths, clamped them together in pairs lengthwise, and drilled holes out of the center. I used three different sizes so that the plug would gradually shrink to the final diameter. I found that if you tried to bring down the diameter too much in a single press, the plug would get squished out between the dies, instead of being forced around the cable like I wanted.
This time I made the plugs out of aluminum, not steel, since we wanted the metal to deform around the cable. For swaging, I threaded the two ends of the cable into the plug and then they squeezed the bejeezus (fun fact, autocorrect thinks bejeezus is a real word) out of it using the shop's 5-ton press and some homemade dies, so that the metal of the plug actually flowed around the strands of the cable. These sorts of mechanical attachments are supposed to hold up to 100% of the strength of the cable, so if we did it right this is the best we could ask for.
I made the dies out of 5/8 inch steel bar stock. I cut the stock into short lengths, clamped them together in pairs lengthwise, and drilled holes out of the center. I used three different sizes so that the plug would gradually shrink to the final diameter. I found that if you tried to bring down the diameter too much in a single press, the plug would get squished out between the dies, instead of being forced around the cable like I wanted.
To squeeze them, I used the shop's 5-ton hydraulic hand press. Even after using some cutting fluid as a lubricant, the plugs melded with the dies, and I had to give them a nice love tap with a hammer to separate them. Through the sequence of dies, I ended up shrinking the diameter of the plug from 540 to 435 thousandths of an inch. It seemed to work well, with the plug visibly deformed around the cables.
The cable, after swaging and then cleaning up on the lathe. Notice how the plug has deformed around the cables, and as well as the ripply surface of the face of the plug, caused by the stress during squeezing. We decided that in such a small piece, which is also supported by the surrounding axle housing, we didn't need to worry about the piece weakening from this stress.
After swaging the cables into the plug, the surface of the plug was rough, and wouldn't fit nicely into the axle body. I made it so the final diameter of the swage was about a hundredth of an inch bigger than the corresponding hole in the axle housing, so there was still some material to work with. I stuck the thick end of the plug in the lathe and worked the diameter down to the final dimension, also leaving a nice smooth surface. The cable, axle housing, sling, and tubing, which had to already be threaded on, spun like some demented whisk in the lathe! This was one of the more funnier and more contrived parts of the entire project as we tried to figure out how to contain the janky looking taped up package.
After swaging, the surface of the plug was really rough, and wouldn't fit into the axle housing, so I had to smooth it back out with the lathe. The axle housing, sling, and tubing had to already be threaded onto the cable, so I taped them together and inserted them inside the big white PVC tube on the right so they wouldn't flop all over the place as the lathe spun.
Then, using a mallet, I pounded the plug snugly into the axle housing. Finally, I used this really cool plastic called InstaMorph to make the molding around the stem. When you put it in boiling water it turns into a clay-like consistency that I could mold with my hands, and as it cools it turns into a semi rigid, super tough plastic that is reminiscent of the plastic in milk jugs. It feels exactly like a real climbing cam stem!
These stems did excellently in the testing. Instead of failing at the plug-cable interface like the welded stems, these all failed by the cable breaking at the end of the loop. This implies that I did a good job doing the machining, and we were simply exceeding the inherent strength of the cable. The cable is rated to 2000 pounds, so we figured that the swaged stems could hold at least 3000 pounds, since the cable was doubled up, but maybe not the full 4000 pounds because of the sharp curve in the loop where we clipped the carabiner, which would magnify any stress. For reference, a similarly sized Black Diamond cam is rated to 14 kilonewtons, which is around 3100 pounds.
When we tested the entire cam using my load sensor, the lobes and axles deformed and broke before the stem did at around 14 kN, so our stem did do really well!
These stems did excellently in the testing. Instead of failing at the plug-cable interface like the welded stems, these all failed by the cable breaking at the end of the loop. This implies that I did a good job doing the machining, and we were simply exceeding the inherent strength of the cable. The cable is rated to 2000 pounds, so we figured that the swaged stems could hold at least 3000 pounds, since the cable was doubled up, but maybe not the full 4000 pounds because of the sharp curve in the loop where we clipped the carabiner, which would magnify any stress. For reference, a similarly sized Black Diamond cam is rated to 14 kilonewtons, which is around 3100 pounds.
When we tested the entire cam using my load sensor, the lobes and axles deformed and broke before the stem did at around 14 kN, so our stem did do really well!
Drop Testing the Stems
For testing the stems, I got in touch with Joe Grosjean, mechanical engineer and owner of CityROCK climbing gym. He has a simple drop test setup, on which he has tested many pieces of climbing equipment, including cams. I took the welded and swaged plugs down to CityROCK one Sunday and we put them through the wringer.
We dropped a 165 pound weight (almost exactly the UIAA standard of 80kg) onto the plugs from some different heights, attached with a chain. Since we used a chain, which doesn't stretch at all, instead of a climbing rope, which is designed to stretch a lot, the deceleration is spread over a much shorter time, resulting in much higher forces for the same amount of drop. Even if the cams wouldn't ever experience these huge shock loads in the real world, we could still get a qualitative sense of how strong the plugs were.
We dropped a 165 pound weight (almost exactly the UIAA standard of 80kg) onto the plugs from some different heights, attached with a chain. Since we used a chain, which doesn't stretch at all, instead of a climbing rope, which is designed to stretch a lot, the deceleration is spread over a much shorter time, resulting in much higher forces for the same amount of drop. Even if the cams wouldn't ever experience these huge shock loads in the real world, we could still get a qualitative sense of how strong the plugs were.
Joe's load cell, which could measure the force on the plugs, was up in Boulder. Despite this, Joe had enough experience with the load cell that he could estimate the force on the plugs just by the sound and way the plugs broke.
Even though we got no real numbers from the tests, we could make some qualitative conclusions: The welds on the welded stems weren't nearly strong enough, and that the swages were working quite well, since the cable was failing before them.
You can see the testing and breaking of the welded stems in the video below. I didn't get any video of the swaged stems, but it looked fairly similar, except we had to drop the weight from higher, and the end of the cable broke instead of the plug-cable interface.
Even though we got no real numbers from the tests, we could make some qualitative conclusions: The welds on the welded stems weren't nearly strong enough, and that the swages were working quite well, since the cable was failing before them.
You can see the testing and breaking of the welded stems in the video below. I didn't get any video of the swaged stems, but it looked fairly similar, except we had to drop the weight from higher, and the end of the cable broke instead of the plug-cable interface.