Front	Rear

Left	Right	Open for Rigging

Technical Details

I made this Storrick Dumschtik f2022.

This Dumschtik is 305 mm. tall, 70 mm. wide, 50 mm. thick, and weighs 893 g.

The spool is made from 6061-T6 aluminum. It is 50 mm. in diameter and 70 mm. long. A 4.7 mm. deep, 12.5-mm. pitch, right-hand helical groove with 1n 11 mm. diameter profile starts on the front and passes through 450° to end on the bottom of the spool. The spool is tapped and threaded for the frame.

The brake bars are commercial bars. The top bar is solid aluminum, Ed Seaman design bar (see The Nylon Highway #9) from Speleoshoppe. The other two are standard 19 mm. aluminum commercial bars.

The frame is made from twp pieces of 9.5 mm. 1018 steel. The ends are threaded 3/8-16 UNC to fit the spool and nuts.

I stamped my logo on the end of the top bar after taking the photos.

Comments

I was intrigued by the Drumstick combining a rack and spool, but logically, that design concept has its flaws. The spool has superior thermal characteristics to a brake bar (or three), but putting it at the bottom where the heat input is minimal fails to utilize the advantage. This is one off my ideas for placing the spool at the top. I christened it the Dumschtick.

To begin with, the spool had to go at the top. Since the rope had to wrap around the spool, a conventional open-frame rack design was unworkable, so I decided to thread the spool to accept a two-piece frame. The idea is not new, Lowell Burkhead used a similar construction on his production Safety Rack. Rather than threading into a blind hole, I let the ends of the frame pieces extend above the spool, where I added two nuts that double as horns to facilitate tie-offs. This idea is not new either; for example, Kyle Isenhart’s SupeRack had the same feature, as did many later U-frame racks.

I made the spool out of aluminum to get the best compromise between light weight and thermal performance. Titanium would be stronger but its thermal conductivity is horrid. Stainless steel might please the clean rope delusionists, but would weigh far too much. Both are much harder to machine than aluminum. There is no down side to the aluminum choice – the idea that aluminum black marks weaken or harm nylon caving rope is a well-busted myth.

A rope wrapped around a spool will tend to ride over itself unless we do something to prevent this. I chose to cut a helical groove to guide the rope. The old H.K. vertical spool used a similar idea. Filing the helical grooves by hand would be exhausting, so I cut it with a Cincinnati universal milling machine equipped with an overarm spindle and a dividing head with drive mechanism. I cut the helical groove is off-center to roughly center the rope entry point near the bottom of the spool.

One and a quarter wraps around the spool has about the same friction angle as three rack brake bars provide. I placed three more bars below to approximate the behavior of a six-bar rack.

The open-frame design facilitates adding or removing the bottom bar while retaining the full friction angle on the middle bar, something that closed-frame racks would not allow. The resulting descender is shorter than a standard open-frame rack but thicker. Its weight is just slightly more than that of a 6-bar open-frame rack and less than a six-bar SupeRack.

Overall, I am pleased with the design. It shows that it is still possible to have a new idea. It works well, but don't take it too seriously, stick with your standard open-frame rack.

Cutting the rope groove

How it works:

1. First I move the table away from me so that the cutter starts the rope groove. This part of the groove is parallel to the frame. I'm watching a digital readout to know how far to move the table.


2. Second, I cut the helical part of the groove by turning the yellow hand wheel. Gears cause the table to slowly move left as the dividing head turns the spool. This makes a helix. I turn the crank 50 turns which turns the part 1-1/4 turns (40:1 ratio).


3. Third, I move the table away from me again, finishing the groove.