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Nevin Davis made the first MAD, or Motorized Ascending Device. This was the first and perhaps only motorized ascender whose details were widely published. Others made earlier claims but never published many details (e.g., one never even mentions what the power source was) so I tend to discount them. The M.A.D., however, was well-documented, and certainly exists. I bought the original MAD from Nevin at the 1989 Old Timers Reunion. Without a doubt, this is the most complex, and most interesting item in my collection.
Nevin wrote that the MAD was developed for people who "do not view vertical caving as an end in itself but instead want only to visit the cave at the pit bottom and then exit in the most comfortable manner." I've spoken to Nevin and feel that there was a second motive: to show that it could be done, and to illustrate that some of the people who had speculated about motorized ascenders had underestimated some of the problems. When he built the MAD, Nevin proved that the engineering difficulties could be overcome.
I acquired Nevin's original M.A.D. Mark III from Nevin in 1989. The M.A.D. Mark III is 330 mm. tall, 280 mm. wide, 280 mm. thick, and weighs 10400 g.
The MAD is powered by a one horsepower Olsen and Rice gasoline engine. The engine has an integral centrifugal clutch and gear box. Further gear reduction is provided by a set of lathe change gears contained in a fabricated gearbox. The original MAD had a motor-cycle grip throttle, but this was later replaced by a smaller and lighter handgrip. A "kill" switch completes the controls.
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Top View, showing upper rope guide and idler pulley. |
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The is threaded much
like an old-fashioned movie projector - i.e., you have to take
a cover plate off and feed the rope by hand. The rope enters the
top of the MAD through a machined rope guide. From there it passes
a Jumar cam that keeps the MAD
from sliding down the rope when the engine is stopped or idling
with the clutch disengaged. Next it loops around the drive pulley.
Finally it passes around an idler pulley that directs the rope
downward and out of the MAD.
The drive pulley has a V-shaped groove with a 30° included angle. This angle is critical, if it is too large it won't grip the rope, while if it is too small there will be too much friction and the rope will bind in the pulley. Nevin designed the pulley for Goldline and Samson 2-in-1, which were the primary caving ropes when the MAD was built. I think that a smaller angle is needed for the modern, stiffer caving ropes, because I find the MAD slips on them unless one hangs about 15 kg (30 pounds) on the rope. Nevin's articles suggested one third that amount for the older ropes.
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Climbing with the MAD is, well, unique. Fill the gas can first - omitting this step might leave you 800 feet off the pit floor with a 22-pound ascender to haul out. Attach the ascender to the rope, and clip your seat harness to the attachment point. Most people will also want a chest harness to hold them upright. Pull the cord to fire the engine. At idle, the centrifugal clutch will disengage, and you'll be hanging from the Jumar cam. Its best to have a backup (float a Gibbs) in case the cam fails, otherwise terminal velocity is determined by the gearbox friction and lubricant viscosity. Add power, and you're off. Motor to the top, walk the edge, and hit the kill switch. With the MAD, you'll be out of El Sótano de las Golondrinas in 25 minutes, just a couple minutes behind the faster cavers, but hours ahead of the tourists.
| Specifications | |||
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| Weight | 10 kg | 21.5 lbs | |
| Engine | 750 Watt | 1 Hp | |
| Fuel Consumption | 860 meters per liter | 2 miles per gallon | |
| Engine Accessories | 6921:1 Integral Compound Gear Reduction and Steel Centrifugal clutch | ||
| Maximum Engine Output Shaft Speed | 900 rpm counter-clockwise | ||
| Gearbox Reduction | High Gear | 12:1 | |
| Low Gear |
24:1 |
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| Maximum Load | High Gear | 90 kg | 200 lbs |
| Low Gear | 180 kg | 400 lbs | |
| Speed | High Gear | 23 meters per minute | 74 feet per minute |
| Low Gear | 11 meters per minute | 37 feet per minute | |
These construction details are from Nevin Davis' article Motorized Ascending Device MAD in The Nylon Highway #19:
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Believe it or not, the 1971 parts cost for the M.A.D. was only $120 U.S!
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