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The walker's leg joints look like they're just bolted together, which of course would make it pretty hard for them to swivel. However, they swivel with virtually no resistance and they're more than strong enough for a servo-based walker.
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The linkage is an important design element; its purpose is to increase the servo motor's torque at the joint. Note that the distance from the servo motor shaft to the motor end of the linkage is one half the distance from the joint's swivel point to the linkage attachment point on the leg. (see photo of leg undergoing testing). This results in 1:2 "gearing", increasing the motor's leverage at the cost of reducing the angle of the joint's travel.
A more subtle design element is that the knee and thigh motors work in tandem to support the bot's weight when the thigh is horizontal, which also increases the lifting power of each leg. The thigh motor pulls up while the knee motor pulls down. At 40 oz-in per motor, this gives us a starting torque of 80 oz-in. The 1:2 linkages double this to give us 160 oz-in. The thigh length of two inches works as a 2:1 "gear" to drop it back down to 80 oz-in per leg. With three legs on the ground, this gives us a theoretical maximum total of 240 oz-in of lift.
The key to making the joints work is that the 1/4" long spacer
is just a shade longer than the sandwiched center acrylic piece is thick.
The result is that when the joint is assembled and the bolt tightened,
the spacer holds the two outer pieces of acrylic apart just far enough
for the center piece to swivel freely.
The center piece has a slightly oversized 1/4" hole drilled in it, through which the spacer passes.
In addition to what you see on the left, you'll also want a washer and
lock washer on the nut end to keep the nut from working loose after
a few dozen hours of walking around.
The space remaining between the center and outer leg pieces, perhaps on the order of 1/64" or so, is still enough to allow for a noticable amount of slop, which is undesirable. To reduce it, what I did was provide for at least 1/2" of material all around the spacer, or to put it another way, I used at least a 1" circular mating surface for a joint, as you can see in the "knee joint front view" above. This is also better for wear, since the more surface area is sliding against each other, the less wear there is at any one point. Another thing that I could have done to reduce slop further would have been to install some paper-thin 1" mylar washers with 1/4" holes in the joint.
All of the parts that I used for the robot body and legs, with the exception of the servo linkages, I found at a local Eagle Hardware. This includes the bolts, nylon 1.4" spacers, acrylic sheet, and the 1" mylar washers which I did not wind up getting. The servo linkages I found at a local hobby shop that sells R/C gear. I have also found the linkage parts online at http://www.towerhobbies.com although it may have been a difficult part to order and get right. I almost fabricated the linkages out of tin and kotter pins, which probably would have worked just fine.
