ART-13 Line Shaft Maintenance Crank

The AN/ART-13 manuals all speak rather blithely about a special crank for use in aligning the five Autotune units on the front of the transmitter, a tool which is "included in the spare parts", according to the documents. It is inserted into the line shaft from the right side of the transmitter looking from the front, shown below, and fastened with a screw for the alignment procedure.

Line shaft end, showing the keyway that must be driven to coordinate all of the Autotune assemblies.
Because the crank isn't common, this one appears to reflect the use of a flat blade screwdriver to perform the
function - a tiring activity because the line shaft must be turned many revolutions in the process. (Photo by K4CHE)

These cranks must have been few and far between, as many people have never seen one. The purpose of this particular page is to show what it looks like, and what it takes to make one with a small lathe.

Line shaft end of the original crank, showing the key that engages the keyway in the end of the line shaft.

Business end of the crank.

The photos above show an original crank, less the 4-40 screw that holds it onto the end of the line shaft when in use. Interestingly enough, I noticed that an earlier Signal Corps tuning device used the same knob casting and lever arm...the MC-204 shown below. There seems to be a lot of cross pollenation for hardware pieces going on during the war and the runup to it.

MC-204 crank, used in the SCR-263 and SCR-280 navigation receiver sets.

The shaft of the crank is made from steel, Type I (silver) cadmium plated - you could use either 1/2" drill rod or cold rolled and it would work fine. The crank arm and knob pivot shaft are stainless steel. The knob itself appears to be a casting, probably from a tin based pot metal. Since I enjoy working in stainless, I prefer to make the entire crank assembly from that metal. Below is a dimensioned sketch for fabrication.

The OD dimension on the crank lever end of the knob as shown is slightly larger (.020") than that of the cast knob, because a slight taper is present in the cast knob tail end, probably to allow for easy removal from the casting mould. I made one with the taper just for fun, but the 1.2 difference between tapered and straight is so slight as to be almost unnoticeable, so I don't recommend it unless you want to be a purist about it. It is only a tool, after all. You should shoot for an axial clearance of about .005"-.007" after riveting the knob shaft onto the crank arm to avoid binding.

I used a small milling cutter to remove the sides of that little cylindrical boss as it comes off the lathe, but one could easily use a Dremel with a cutoff wheel to do the same job - the dimensions are not very critical. The resultant 1/16"x 3/16" key simply has to fit snugly into the slot on the end of the line shaft.

Below are all the piece parts required, ready to rivet together. A dollop of teflon grease on the spinner knob shaft will finish it off nicely. By the way, there is nothing functionally essential about using the rivet technique to attach all these parts together. I did it that way simply because that's the way the original part is made. You could use a screw to attach the spinner knob shaft, and a nut to hold the keyed drive shaft onto the crank arm. Do whatever you think easiest. I will point out that the crank arm will probably be the touchiest thing to make because of it's small size...drilling the two holes 1.5" apart while you can chuck the rectangular starting strip in the vise is a good idea. Trying to hold a tapered object is always an exercise in patience. The rest you can finish with a good eye and a belt sander. It's also a good idea to put a generous radius on one side of the large hole in crank arm in order to provide a radius for peening over the .275"OD boss that sticks through it. It will reduce or eliminate cracking of the steel rim while you apply a series of increasing angle conical punches to it. Stainless does not like to be formed, so go easy and take your time on this riveting step. If a crack or two does appear, it's not the end of the world - I notice that the original cranks and other similar fabrications I've seen often have a few cracks around the periphery.

And finally, the completed assembly. The thumbscrew is optional, described below.

I don't know how other folks might feel about it, but that pesky little 4-40 retaining screw seems like another one of those ultimate Murphy's Law objects that disappear when you need it most. K7IRK creatively suggested threading an additional 4-40 hole in the crank arm shown in the sketch to provide a handy storage space for the screw. Personally, using a machine screw takes a bit of fiddling to get it started in the end of the line shaft because you can't see the screwdriver socket buried in the crank drive body. As a result, I decided to fabricate a knurled thumbscrew to make it far easier and quicker to attach or remove the crank. Overkill, perhaps, but not a big job. I like to think Art Collins would have approved. The same storage location works fine for the thumbscrew if you decide to make one.

By the way, the #4-40 x 1/2" screw recommended in the manual only engages the end of the line shaft to the bottom of the keyway - a recipe for disaster. Thanks to some careful measurement by WF2U and K4CHE for this project, the typical line shaft is only threaded for a total of about 3/16" beyond the bottom of the keyway, for a total of 1/4", but it's best to use every bit of that if at all possible. That means using a screw with a total length of 11/16" (less perhaps .005" to ensure it doesn't bottom out before cinching the tool to the end of the line shaft.) The knurled thumbscrew below protrudes from the face of the line shaft down into its thread a total of .245", which is about as much as is safe before bottoming.

Sometimes I wonder if all those missing cranks went into the trash bin because attempts to use them caused stripped line shaft threads and fervent cursing...

Crank tool showing the thumbscrew in its stowed position.

Whichever way you choose to fasten the crank to the lineshaft, do it "firmly" so that it does not rock on the end while you are turning it. That can pretty quickly chew up any key and keyway arrangement. There are limits to the torque you can apply to a 4-40 screw, so you have to just make sure that it is not loose. Below is a fabrication sketch for the thumbscrew.

Finally, here's the assembly doing what it was designed to do on the end of an ART-13 line shaft:

Line shaft end with the crank attached. (Photo by K4CHE)

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