See also also bike pic for other bike-related technical discussions and more failed parts.
Disclaimer: these notes are under the "fail" section more out of convenience than true failure.
These are a few notes on some crank designs.
Bullseye cranks have the left arm is attached with a spline and a pinch clamp, and the right arm permanently connected (welded) to the spindle. Parts of this design had been used or described before (e.g., Patent 3,906,811 from 1974 describes a pinch spline), but the Bullseye crank was the first such crank to be used widely, and has inspired many later cranks of similar design.
The following photos show a Bullseye crank that is old enough it lacks an axial adjuster bolt, new enough it has a hole for a "key" to hold the arm from walking off. At a guess, it may have beeen made around 1984-6.
The fractional-inch pinch bolt threads have been tapped enough that a standard metric bolt fits. There is "some" traction on the remaining threads, but the pinch bolt load is carried using a separate nut.
This crank predates the Bullseye axial adjuster; instead it uses an IRD "keeper" (more below).
The second picture shows more clearly that the arm section is rectangular.
The left arm is held with a fine spline that goes inside the pinch clamp. The original design had problems with the arm "creeping" or "walking" off the end of the crank, so later versions use a hole in the crank that is engaged by a small "key". That keeps the arm from falling off, but does not allow axial bearing adjustment.
Here is the spline, the hole, and a "key". For the key, the pinch bolt goes through the hole, and the tab on the right goes through the hole in the crank. If the crank tries to fall off the spindle, the tab jams the key in to the pinch bolt and keeps the crank from moving further out.
The key prevents fall-off, but sometimes axial adjustment is needed (a) to keep the chain from rubbing on the derailleur, (b) to keep the bearings in adjustment when using the newer angular-contact Bullseye bottom bracket bearings, or (c) because bottom bracket "slop" annoys the rider.
The IRD "keeper" solves the problem by providing enough axial force to keep the arm from creeping. Note the two ends are different diameters since the left cap overlaps the spindle to press on the left arm.
Note the key and the keeper are workarounds for a design defect: the pinch clamp should be tight enough the arm does not "creep", even without a key or "keeper". Preventing creep would mean using more and/or more-stout pinch bolts.
Even with a creep-free design, it may still be useful to have axial adjustment, to keep the bearings adjusted. For example, the top cap on an Aheadset™ design does not hold the stem in place, but it does allow headset bearing adjustment.
The crank is a "2-piece" design, with the left arm removable from the spindle, but the right arm and spindle welded together. Here is an underside view showing right arm welded to spindle.
Newer Bullseye cranks have a plate welded on the end of the spindle, and the plate has a threaded hole for an axial bolt. That effectively replaces the IRD "keeper" and is more convenient to install and remove, except that the bolt is fractional-inch not metric, whereas most standard bicycle wrenches are metric.
The welded-in plate is at the end of the crank, so it does not allow a recessed "cap" like the IRD keeper. When the axial-bolt model was first sold by Bullseye, it used a hex-head bolt which developed a reputation as an ankle-biter. Later versions use a button-head screw which sticks out as far, but is rounded. This picture shows an aftermarket adaptation using a countersink screw and washer from a hardware store. The countersink screw is slighlty lower profile than the Bullseye button-head screw.
Below is a later Bullseye crank model, the "Safety" crank, which uses an oversize right arm that bolts on to a flanged spindle.
Bullseye cranks inspired many other crank designs — here is a BMX 3-piece crank using two arms that both bolt on with pinch clamps; it also uses an axial bolt. The pinch bolt uses fine threads and a much larger bolt that can be much tighter than the Bullseye pinch bolt.
Shimano 2-piece cranks use a similar design, including a key and an axial bolt. Shimano specifies the end bolt should be tightened between 0.7 N-m and 1.5 N-m. However, the axial bolt takes a Shimano-only tool, and there is no way to connect that tool to a torque wrench to ensure proper torque. Also, it is one more thing to carry around, either for field service or when traveling if the cranks are removed for packing (removing the cranks may be easier than removing the pedals for packing).
The following photo shows an Ultegra SL axial bolt modification. Here, the axial bolt is drilled and tapped, then an ordinary bolt taking a 4mm Allen key is run through with LocTite™ and secured using a nut on the back side. The Shimano tool no longer fits, but this modification allows use of a torque wrench and also means field service no longer requres the special tool. It also means thieves don't need the special tool... Weight before modification was 4.3 grams, after7.5 grams.
There may be lighter ways to do this; some aftermarket bolts have a hex hole built-in. Ultimately, Torx™ is gaining popularity on bicycles and is probably more durable than hex, especially in bolts made of aluminum. A disadvantage of a simple hole is water and dirt can easily get in the hollow center of the spindle. A rubber plug or similar may be useful.
In the photo above, visible on the left side in the "pinch" area is a black piece, which is an edge-on view of the "key". This is not needed in normal use, but should one or both bolts come loose or break, and even if the axial bolt fails, the key may still help keep the arm from falling off. It probably weighs around 1 g so is "cheap insurance".
A few photos of a Mavic 631 "starfish" crank, which is a distictive-looking design, but also flexy. See HERE at MOBI (the bicycle museum of bad ideas) for more.