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The Trautman Hook

The Trautman Hook is an upper-extremity terminal device that was invented in the 1920’s or 1930’s and produced until the company went out of business a few years ago. Kenneth Heide, CPO brought this device to our attention in the interests of getting it produced again, and generously loaned us two used devices and two unused devices for reverse engineering. The Trautman Hook backlocks when closed, uses fewer rubber bands than other models, and packs a high mechanical advantage into a small package. Its simplicity, at three metal parts and two screws, makes it a promising platform for customization.

We reverse engineered the old hooks and made a CAD model in Alibre Design. Below is the original hook and a picture of our CAD model:

The original Trautman Hook CAD model of the Trautman Hook

We made some small changes to the design based on the areas where the used hooks had been broken and welded back together, and there are probably more opportunities for stengthening and weight reduction. As soon as we had finished the model, we emailed it to Bill Watson at Anvil Prototype & Design, who printed it on his Z Corp rapid prototyping machine and filled it with cyanoacrylate (super glue) for strength. We we able to assemble the parts into a moving model to test the design:

ZCorp prototype of the Trautman Hook ZCorp prototype of the Trautman Hook ZCorp prototype of the Trautman Hook

The next step was to try to get the device made without investing a lot in tooling. The best option seems to be rapid manufacturing, so we got the device quoted for different processes and quantities. Here are the resulting quotes:

Prometal (3D printing of stainless steel powder)

Rapid Tool Inc. (SLS Laserform ST-100)

Quickparts.com (plaster casting process)

American Precision Prototyping (investment casting, not including final machining of holes and threads):

Precise Cast (casting with CNC machining):

Synergeering Group (direct fabrication in Titanium)

Note: The rapid prototyping and manufacturing industry is very dynamic, and these prices may not be accurate within a few months of February 2006.

We’re working on getting the device back into production using one or more of these processes. If you want to be notified when the device is available, please contact us.

You can download the CAD model (4.8M zipped) if you want to experiment with the design or STL files (2.8M zipped) if you want to get your own quotes. Warning: This version is largely untested and may not work as intended.

We sent the part to ProMetal for a first functional prototype. Here are some pictures of the parts when we got them:

Trautman Hook parts made with the ProMetal process Trautman Hook parts made with the ProMetal process Closeup of a Trautman Hook part made with the ProMetal process

As you can see in the closeup, the printed surface is a bit grainy, but not unattractive, and vibratory tumbling would help it look smoother. After a little drilling and threading, it went together without a hitch. Here’s the assembled device:

A Trautman Hook made with the ProMetal process A Trautman Hook made with the ProMetal process A Trautman Hook made with the ProMetal process

The action is smooth and the backlock is tight, the only issue is that the grippers don’t line up perfectly, probably due to the fingers warping a little during heat treatment. The total error is about 0.050 inches.

The next step is to test it with a body-powered harness, then we’ll try to break it to see where the it needs to be stronger. Also, we’re already planning on making a few revisions that would make the device more durable. The cost of this prototype was $500 (ProMetal’s minimum order) plus about $50 for finishing tools. Thanks to Kenneth Heide, CPO for funding it.

All content and designs on this site are in the public domain, and we place no restrictions on their use. We encourage any derivative works, but all designs are registered periodically so that our work cannot be kept from the public by patents.