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Space Systems Lab
University of Maryland Commercial Three-Dimensional Roller Wrench

Several ratchetless designs have been developed over the years. Some use rollers confined between ramps as a one-way clutch, but all have failed American National Standards Institute cyclic and overload tests. Contact stress has been the main drawback of all lashless wrench designs, therefore high torques could not be transmitted by the wrenches.

NASA/Goddard Space Flight Center (GSFC) has patented the use of three-dimensional (3-D) sprags in all ratcheting devices. In particular, they have developed a commercial wrench that uses 3-D sprags in place of the traditional ratchet mechanism as shown in Figure 1 below.

Figure 1 Commercial 3-D Sprag Wrench

This unique 3-D sprag geometry permits construction of compact locking mechanisms with a simple design that can withstand large loads because the sprags are very small but have large contact radii that reduce sprag-to-race contact stress. The design also allows the locking mechanism to have an infinitely small "ratchet" step. It can lock in the tighten and loosen directions, but unlike the traditional ratchet wrench, the 3-D sprag wrench can also lock in both directions simultaneously. The sprag wrench has a lower backdrive torque than ratchet wrenches which leads to a lower perceived mental workload when operating the wrench compared with ratchet wrenches.

The American National Standards Committee B107, Socket Wrenches and Drives, under the sponsorship of the American Society of Mechanical Engineers (ASME), developed ASME B107.10M-1996, Handles and Attachments for Hand Socket Wrenches - Inch and Metric Series, which governs traditional ratchet wrenches. The standard specifies that the minimum test load for type 1 (handles), class 2 (ratchet, reversible), 3/8" drive hand socket wrenches is 1,800 in-lbf. The commercial version of the sprag wrench will be tested using this standard.

Wrenches are tested by placing the male end into the coupler which allowed the 3/8" male drive of the wrenches to fit into the 3/4" female end of the torque sensor. The base plate was mounted to a tensile test machine and the handle bracket was pulled in the direction shown until the wrench failed. Data from the torque sensor were collected.

Video of the test set-up described above.

Video of setting up the wrench, running the test, and analyzing the results.

Before testing the sprag wrenches, three commercial ratchet wrenches from three different manufacturers were tested to failure. This was done to verify the test set-up and test procedure described above, quantify by how much commercial wrenches exceed the minimum test load, and to determine the failure modes of these brands of commercial wrenches.

Brand A
A close-up of the area in the blue box is shown below. The tooth shown circled on the right was removed from the position shown circled on the left side.

Failure mode: Teeth sheared off and male drive began to fail under torsion
Average maximum torque: 2,600 in-lbf

View torque versus time data
Brand B

A close-up of the area in the blue box is shown below. The tooth shown circled on the right was removed from the position shown circled on the left side.

Failure mode: Teeth sheared off
Average maximum torque: 1,800 in-lbf

View torque versus time data
Brand C

Failure mode: Male drive failed under torsion
Average maximum torque: 2,400 in-lbf

View torque versus time data

Details on the manufacturing and testing requirements for a wrench using 3-D rollers in place of the traditional ratchet mechanism can be found in a NASA Contractor Report that I am currently working on -- NASA/CR-1998-206849, "Manufacturing and Testing Requirements for a Reversible Hand Socket Wrench Using Three-Dimensional Rollers."


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