Gripper Pad Coefficient of Friction
Gripper Pads provide 50% – 400% more grip than metal to metal alone. Frictional effects depend on part material, surface finish, and clamping force. Lighter forces and rougher finish typically achieve the most grip strength, while any ability to conform to a part shape further enhances performance.
The coefficient of friction for an application in which steel fingers grip a steel or aluminum part is estimated to be .28 and .32, respectively. ASTM D 1894 friction test of a PFA 60 durometer NBR knurled pad at light forces (loading a sample at .07 lbs/in2 on a polished surface) demonstrated a coefficient of friction of 1.22.
Historic test data (chart below) of PFA knurled pads (loading full pads at 1.4 lbs/in2 on a ground surface) showed a coefficient of friction of .53 (on steel) and .78 (on aluminum), respectively. Values in the range of .48 to 1.0 provide a good approximation for untested applications and a good starting point for calculations. Gripper pads of different materials should behave somewhat similarly at the same durometer, however, different durometers may show more variation. Material and durometer differences, fluids on parts, and other environmental or unexpected conditions may impact actual performance. Friction coefficient values provide a good approximation of relative benefit when adding gripper pads to an application, however, PFA recommends testing for intended form, fit, and function.
The coefficient of friction is used in conjunction with tooling weight and robot acceleration to calculate the required grip force for a specific application. The following formula can be applied as a good approximation when attempting to determine the minimum grip force. An additional safety factor of 10X may be required depending upon the application.
Grip Force (lbs) = [Tooling Weight (lbs) + Dynamic Force (lbs)] / [Coefficient of Friction]
||Friction Coefficient for Steel Part
||Friction Coefficient for Aluminum Part
||Friction Coefficient ASTM D 1894
The coefficient of friction test for two sample part materials was conducted for the three gripper pad types. The test results were generated under ideal laboratory conditions. Actual performance may differ. In this test situation a metal sheet with a 63 microinch ground finish was placed between two 72 square inch gripper pads and a compressive load of 200 lbs was applied perpendicular to the contact area. All surfaces were clean and dry. In other situations the coefficient may be lower due to lubricants introduced into the system; or much higher for low clamp forces or if the rubber is able to conform to the part.
More on the Challenges of Defining Rubber Friction Coefficients
Typical metal on metal or similar hard contact surface materials, show behaviors that make defining a usable friction coefficient much easier. For more typical materials (Not rubber, elastomers, plastics, or ceramics) the following soft “rules” have been determined for friction that support the use of a Coefficient of Friction (CoF)1:
Friction is proportional to the normal load.
Friction is independent of the area of contact.
Friction is independent of sliding velocity.
Friction is independent of temperature.
Friction is independent of surface roughness.
Due to the unique flex inherent in rubbers and the more dramatic effects of how a softer and more compliant material interacts with a surface, rubber exhibits the the following behaviors relateive to the more typical material aspects mentioned above:
Friction is NOT proportional to the normal load.
Friction is NOT independent of the area of contact.
Friction is NOT independent of sliding velocity.
Friction is NOT independent of temperature.
Friction is NOT independent of surface roughness.
Thus, while an attempt has been made to provide a feel for typical COF values and methods to use those values in approximating application performance, these values and methods should only be used as a first approximation in design. Prior to use, all product selections should be followed up with testing to ascertain a successful specific form, fit, and function, for the application.
1Ref: A. G. Plint, “Notes on Rubber Friction”, 2011. For a quick yet more detailed technical review by Plint, visit the link at Phoenix Tribology: http://www.phoenix-tribology.com/wp-content/uploads/guidance/Guidance-Rubber-Friction.pdf