CAPSTAN GRIPS

Split capstan grips are typically used to tensile test cordage, rope, flat strapping and similar flexible materials that might be otherwise difficult to grip without creating undesirable stress concentrations and premature failures.  The capstan grips can be sized and configured for the particular application.  A pair of 5000 lb. capacity grips for testing flat strips are shown in the photograph below.

Pair of Split Capstan Grips:  5000 lb. Capacity, 3.2”
Diameter Capstans, for Testing Flat Strapping Material

Capstan grips rely primarily on friction to hold the specimen.  The end of the specimen is clamped lightly between the fixed and movable halves of the split capstan.  This holds the specimen in place while it is being installed.   Ample radii are provided where the specimen then wraps around the capstan half.  For maximum holding power the clamped specimen can be wound counterclockwise around the fixed half and then around the movable half, to achieve an almost full 360 degrees of engagement around the circumference of the grip.  If less holding power is required, or when testing shorter specimens, the clamped specimen can be wrapped counterclockwise around the movable half of the split capstan, providing 180 degrees of contact around the circumference.

Typically the circumference and the clamping region of each capstan grip is thermal-sprayed with tungsten carbide particles, of a coarseness appropriate to the particular application.  This coating greatly increases the effective coefficient of friction of the grips, and thus the holding power.  However, for some applications, smooth surfaces are both sufficient and desired.

As the tensile loading is applied, the movable half of the capstan will slip, providing further clamping force to the specimen in the split, and thus additional holding power.

Shown in the above photograph were split capstan grips for testing specimens of flat cross sections.  For specimens of other cross-sectional shapes, the grip surfaces can be shaped accordingly, e.g., semicircular for testing cord or rope.  Capstan grips of two different load capacities and groove radii, for testing 0.25” diameter and 0.40” diameter cordage materials, are shown in the photograph below.

Split Capstan Grips for Testing Cordage Materials
Top:  15,000 lb. Capacity, 5.2” Diameter
Bottom:  5,000 lb. Capacity, 3.2” Diameter