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Space Systems Lab
University of Maryland Three-Dimensional Sprags and Rollers

Sprag and roller clutches are two types of overrunning clutches that automatically engage to transmit torque by relative rotation in one direction and automatically disengage for overrunning by relative motion in the other direction. They are used in rotating mechanical systems when torque must be transmitted in one direction only or where the driven member is to be permitted to "overrun" the driver

Sprag clutches have a cylindrical outer and inner race and a series of specially contoured and processed sprags. The inner and outer races are concentric and the sprags are installed in the annular space between them as shown in Figure 1. Each sprag is designed with a geometry that makes its two diagonals of different length. One direction of relative rotation between the races causes the sprags to rotate so that the longest diagonal is in contact with the two races. The length of this diagonal is greater than the annular distance between the races, so the sprags are wedged between the races, providing a solid connection for transmission of torque. Relative rotation between the races in the opposite direction causes the sprags to rotate so that the shortest diagonal is in contact with the two races. Its effective length is such that the clutch is disengaged and free to overrun.

Figure 1 Traditional "Two-Dimensional (2-D)" Sprag and Races

Roller clutches have one race that provides a cylindrical race surface and a second race that has a series of wedge-shaped friction surfaces spaced about its circumference. The rollers are installed between these concentric races at the wedge-shaped friction surfaces. Engagement and disengagement are determined by the direction of relative rotation. Relative rotation in one direction causes the rollers to roll up the wedge surface, engaging the clutch, wedging the rollers between the races, and providing a solid link for transmission of torque. Rotation in the other direction causes the roller to roll down the wedge surface, disengaging the clutch for overrunning.

NASA/Goddard Space Flight Center (GSFC) has developed and patented an innovative three-dimensional (3-D) sprag that replaces the concentric, cylindrical surfaces of the inner and outer races with grooves into which the tapered periphery of the 3-D sprag fits as shown in Figure 2.

Figure 2 NASA/GSFC 3-D Sprag and Races

The 3-D sprags were originally developed as a robotic joint brake for NASA/GSFC robotic activity and their benefits over the traditional 2-D sprags and rollers are:

  • They require no lubricants so they will not slip in thermal vacuum.
  • They create four point contacts (two between the outer taper of the 3-D sprag and the outer grooved race and two between the inner taper of the 3-D sprag and the inner grooved race) instead of the two line contacts of the 2-D sprag, thus:
    • Allowing loose tolerances
    • Doubling the number of contact points
    • Increasing locking efficiency
    • Allowing larger contact angles, which reduce the level of sprag-to-race contact stress

Additional information on sprags can be found in:

  • NASA/Goddard Space Flight Center's Technology CommercializationOffice Three-Dimensional Roller Locking SpragTechnology Opportunity
  • "Clutch Combined With Screw-Released Roller Brake," NASA Tech Briefs, December 1996.
  • "Roller Unlocking Sprags," NASA Tech Briefs, December 1996.
  • "Three-Dimensional Roller Locking Sprags," NASA Tech Briefs, February 1997.
  • "Sprag DesignAdds New Dimension," Design News, 3 March 1997.
  • Poland, J. and B. Roberts, "Development and Analysis of Three-Dimensional Roller LockingSprags," In NASA/Goddard Space Flight Center's Director's Discretionary Funding AnnualReport, January 1998.

Three-dimensional roller locking sprags have spun off many patents.

Three-Dimensional Roller Locking Sprags

Inventor: John M. Vranish
Case Number: GSC-13,617 Date Patent Filed: 27 July 1994
Date Patent Issued: 9 January 1996
Patent Number: 5,482,144 or 5,482,144

Spin Bearings/Sprag Bearings

Inventor: John M. Vranish
Invention Disclosure Submitted: 25 August 1994
Case Number: GSC 13,679

Roll-Unlocking Sprags

Inventor: John M. Vranish
Invention Disclosure Submitted: 25 November 1994
Case Number: GSC 13,692

Clutch Combined With Screw-Released Roller Brake

Inventor: John M. Vranish
Case Number: GSC-13,674

3-D Sprag Ratcheting Tools

Inventors: Michael O. Wade and James W. Poland, Jr.
Invention Disclosure Submitted: 27 June 1996
Case Number: GSC 13,802

Linear 3-D Roller Locking Sprags

Inventors: John M. Vranish, Michael O. Wade, James W. Poland, Jr.
Invention Disclosure Submitted: 9 September 1996
Case Number: GSC 13,885

2-Way 3-D Roller Locking Sprags

Inventor: John M. Vranish
Invention Disclosure Submitted: 5 December 1996
Case Number: GSC 13,910

1-Way Bearing

Inventor: John M. Vranish
Invention Disclosure Submitted: 5 March 1997
Case Number: 13,905

Screw-Released Roller Brake

Inventor: John M. Vranish
Patent Number: 5,868,226
Date of Patent: February 9, 1999

EVA Wrench Home 3-D Sprags and Rollers Commercial 3-D Roller Wrench EVA 3-D Roller Wrench
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