Starter Clutch
As mentioned in an earlier post above, I wanted to come up with a suitable means of starting the engine whilst it is mounted in the test stand. I had already rejected the hex-socket-on-the-flywheel-bolt method as being a bit too risky, and the pony motor idea as too elaborate and space-consuming. What I finally arrived at is a simple type of self-ejecting clutch. It consists of two parts: 1) a male
driver, which is basically a 1/2" dia. shank, cross-drilled for a 3/16 dowel pin, and 2) a female-headed
driven bolt with two helical cutouts, which replaces the standard 3/8-24 LH flywheel bolt.
Here's a CAD view showing the clutch at full engagement:
When the engine starts up, exceeding the speed of the drill, the driver half is automatically disengaged:
Due to the amount of torque being transmitted when the clutch is in operation, I wanted to use a high-strength steel to make the parts. I ended up using a special grade of 4140 steel sometimes referred to as "ETD-150". The 'ETD' stands for Elevated Temperature Drawn, and I believe that the '150' refers to (nominally) 150,000 psi tensile strength. It's listed as having a yield strength of 130,000 psi, which is 2-3 times that of regular 1018 cold rolled steel. I have used this steel before on other projects, and it machines like a dream. Very nice stuff to work with.
Here's a quick rundown of the machining process:
After cutting the stock to length, one end is turned down to .500" dia. This will end up being the 3/8-24 LH threaded shank, but for now the .500" simplifies the head machining operations:
The part is then flipped around for the turning operations on the O.D. of the head:
A 33/64" hole is drilled in the head. This provides a nice working clearance for the 1/2" dia. driver shank:
At this point the part is removed from the lathe, and mounted in a 6-sided 5C collet fixture. This type of fixture is useful for machining hex flats on a round part. Once the mill is setup to cut one flat, the fixture is simply repositioned in the vise to cut the next flat:
Next, the part is transferred to a 4-sided 5C collet fixture for machining the two helical cutouts. First step here is to drill and ream a 13/64" cross hole, providing an easy way to align the mill's spindle to the part for milling the helical cutouts:
The vise is then rotated to an angle of 48.5 degrees. The spindle is centered over the previously reamed hole, and then a 33/64 end mill is used to finish the first cutout. The 5C fixture is then flipped over for the second cutout:
The final step for this part is to chuck the head in the lathe and cut the 3/8-24 left-hand thread:
The driver (male portion of the clutch) is a simple part to make, so I didn't take pictures of the operations. It's just a 1/2" dia. piece of ETD-150, cross-drilled on one end for a 3/16" dowel pin, LocTited in place. I cut three flats on the opposite end to keep it from slipping in the drill chuck.
Here's a look at the finished starter clutch parts on the engine:
Almost ready to try starting 'er up!