A Drop-Test Facility at Marlboro College

• Jim Mahoney (mahoney@marlboro.edu), Physics & Astronomy, Marlboro College
• Dave Custer (custer@mit.edu), MIT's Experimental Study Group
• Randy Knaggs (rlnkaggs@marlboro.edu), Outdoor Program, Marlboro College

We were granted initial funds of $6,000 from the Vermont EPSCoR (Experimental Program to Stimulate Competitive Research) program and $2,000 from Marlboro College.

Here's our proposal.

We are now (still) in the process of acquiring eqiupment and building the facility.

Parts should be up by the end of this term, with most of the rest going up during the winter recess.

A list of what's we have, what's been ordered, budgets, etc. should be posted here eventually.

The distances required for ASTM rope certification and testing are shown here, essentially the test mass needs to be dropped from 2.3 meters above the carabiner anchor point (which is where all the stress from the drop will be concentrated), and fall to a point 2.5 meters (before the rope stretches) below it. Dick Newell (BlueWater Ropes) informs us that the rope will stretch up to 60% over the course of repeated drops, which will require an additional 1.5 m. In addition to these distances, we need room for a) a mechanism to hoist the weight up and release it, b) the load cell (with attached stess sensors) and weight, and c) a "catcher" at the bottom so that we don't destroy the floor.

The available distances in the stairwell of the science building look like this. It will be very tight to fit all this in, but if we can keep the weight's height small, it should fit.

Nov 1, 1996:

We now have beam, winch, and pulley installed on the roof above the stairwell. Here it is while it both during at after construction.

Some notes:

• The entire available stairwell height is 762 cm. (If we push the path of the drop closer to the bottom of the stair, we may get a little more, at the cost of edging closer to the 2nd floor landing.

• The combined 2.3 m above and 2.5 m + 60% below the carabiner is 630 cm, which leaves only 142 cm for (a), (b), and (c) above.

• (a)Rather than put a hoist up at the ceiling, we have decided to put a pulley up high and position a winch elsewhere. This will take up much less space, and have less of an impact on the building itself.

  The release mechanism will attach directly to the winch cable, and includes a lever which (with a second rope attached to it which can be pulled from the ground) will release the weight. Here it is at 36dpi: .

  The pulley will attach on the side of a ceiling beam, and will therefore be right next to the ceiling. Since the release is under 11 cm, the entire release/hoist assembly should be under 30 cm.

• The stress sensor (from NavTech) is on its way; I don't know how big it is yet.

  We still need to determine the weight's size and shape. It will need rings on the outside for the guide-wires which will keep it from swinging sideways. Steel's density is around 8 gm/cm^3, so if it was one solid piece, an 80 kg mass would have a volume of 10 liters, or 10^4 cm^3, or a cube that's 22 cm on each side.

  For now, let's say we can keep this entire assembly (load cell and weight) at under 60 cm, or 2 feet.

• The "catcher" is very open ended; I'm still not sure what we'll manufacture this out of. If we keep to the distances given above, then the total we have left is 142 cm - 30 cm - 60 cm = 52 cm = 1.7 feet. Some of this needs to be a margin (a small one!) for error; the rest will be carpet or some such to absorb the impact.

  Since we will have guide wires running from the ceiling to some kind of wooden floor plate, at least we will be able to choose exactly where it will impact.

  Some of the numbers work out this way: a weight that falls 5 meters will have a velocity of around v = sqrt(2 g 5m) = 10 m/sec = 22 mph. If we want to stop it in a 1 ft distance with an ideal spring ( F = -k x ) then, using (1/2 m v^2) = (1/2 k x^2), k = 8.4 Newton/m = 480 lb/in, and the maximum force is 5600 lb.

  This doesn't sound too bad; its only a static load a few tons.

  And given that, perhaps some big springs or shocks from a car or truck, mounted in parallel on several boards could serve as the catcher.