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Gripper Pads

Gripper Pads

Overview

Gripper Pads Overview

Nitrile Rubber (NBR Buna-N) Gripper Pads on steel and aluminum plate from PFA Inc.

Durable, non-slip rubber pads for industrial material handling robots and human collaborative robot (Cobot) applications provide strong gripping surfaces for grasping items using lower clamping forces, while also protecting parts during handling and assembly. The rubber elastomers, available in Buna-N, Neoprene, Silicone, and Viton® operate over a variety of broad temperature ranges and resist a variety of oils and corrosive elements.  Pads with backplates are cure-bonded to aluminum, carbon steel, and stainless steel plate for a molecular level integration of rubber to metal, offering a bond stronger than the base rubber itself.

Metal backing provides stiffness and support to the rubber, allowing easy attachment to gripper jaws or other structures. Typically sections are attached to fingers or fixed structures using high strength adhesive tapes available from PFA or with standard fasteners after modification.  Metal backed pads are easily machined to match custom applications, and holes may be drilled to provide quick attachment and removal.

Historically, Gripper pads have been available in 6” x 12” sheets of three surface styles bonded to Steel or Aluminum backing in only one material and durometer (Buna-N, 60 duro).  Along with these standard pads, sheets are now available in smaller 1.2” x 1.2” sections and some other materials and durometers.  Additional offerings are available by special order and may become available as standard products.  Unique pad configurations and styles are also available as bulk material or as finished parts from customer provided drawings or specifications.

Features & Benefits
  • High Friction Surface
  • Maximum Rubber to Metal Bond
  • Improved reliability for gripping parts
  • Compliant surface for a softer touch
  • Easy installation and replacement
  • Saves tooling and reduces waste
  • High return on investment (ROI)

Elastomer Materials Reference Chart

Elastomer Material Properties Reference Chart
Elastomer Designation1 Common Name(s) Composition Min/Max Temerature5
Operating Range ˚C (˚F)		 General Properties2 General Chemical Resistance3
(Resistant to)		 General Chemical Resistance
(Attached by)
NBR Buna-N Nitrile – Butadiene -34˚C/121˚C
(-30˚F/250˚F)		 Excellent resistance to petroleum based fluids, Good physical properties Most hydrocarbons, fats, oils, greases, hydraulic fluids, chemicals Ozone (except PVC blends), ketones, esters, aldehydes, chlorinated and nitro hydrocarbons
FKM/FPM Flouro-elastomer
Viton®4		 Hexaflouropropylene – vinylidene fluoride -23˚C/204˚C
(-10˚F/400˚F)		 Excellent oil and air resistance both at low and high temperatures. Very good chemical resistance All aliphatic, aromatic and halogenated hydrocarbons, acids, animal and vegetable oils. Ketones, low molecular weight esters and nitro containing compounds.
CR Neoprene Chloroprene -34˚C/100˚C
(-30˚F/212˚F)		 Good Weathering Resistance. Flame retarding. Moderate resistance to petroleum-based fluids. Moderate chemicals and acids, ozone, oils, fats, greases, many oils, and solvents. Strong oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons.
SI/Q Silicone Ploysiloxane -62˚C/215˚C
(-80˚F/420˚F)		 Excellent high and low temperature properties. Fair physical properties. Moderate or oxidizing chemicals, ozone, concentrated sodium hydroxide. Many solvents, oils, concentrated acids, dilute sodium hydroxide.
  1. ASTM D 1418-01a Standard Practice for Rubber and Rubber Latices – Nomenclature, ASTM D 1418-79, ISO 1629:2013
  2. From the “Sheet Rubber Handbook – Gasket and Packing Materials” publication #IP-40 of the Rubber Manufacturers Association (RMA).
  3. 1979 Yearbook of the Los Angeles Rubber Group, Inc.
  4. “Viton” is a registered trademarks of E.I. Dupont, Inc.
  5. The temperature range is determined by the base elastomer used. This chart depicts a general maximum temperature range for each elastomer. The temperature range for a specific compound may not reach these maximum limits. Higher temperatures may be considered if exposure is short or intermittent.
Approximate Weight per square inch

Weight is primarily determined by the type of backplate material and thickness of the elastomer. Weights below are based on the typical values for Knurled and Pebbled surface pads made from Buna-N (NBR material at 60 durometer) . Waffled pad weights are about .04 oz/in<sup>2</sup> less than those. Contact PFA for individual pad weight.

Part Type Weight (oz./in.2)
Steel Backed .53” thick .82
Aluminum Backed .25” thick .22
No backplate, .25” thick .17
No backplate, .13” thick .07

Gripper Pad Coefficient of Friction

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]

Finger Material Friction Coefficient for Steel Part Friction Coefficient for Aluminum Part Friction Coefficient ASTM D 1894
Knurled Pad .53 .78 1.22
Waffled Pad .48 .87 -
Pebbled Pad .52 .76 -

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-Tests.pdf

Gripper Pads

PFA Nitrile Rubber (NBR Buna-N) Gripper Pads Profile of Knurled, Pebbled, Waffled
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Click the part number in the chart below to view that specific pad in our store.

Dimensions mm (inches)
Part No.SurfaceLength
A**
mm (inch)		Width
B**
mm (inch)		Overall Thickness
C
mm (inch)		Pad Thickness
D
mm (inch)		Plate Thickness
E
mm (inch)		Durometer***
±5 Shore A		Elastomer***Plate Material***
GP-702-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-702-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-704-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-704-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-706-1A-6.0-12.0*Knurled304.8
(12.0)		152.4
(6.0)		–6.4
(0.25)		–60NBR-BunaN
(Black)		Elastomer Only**
GP-706-2A-6.0-12.0*Knurled304.8
(12.0)		152.4
(6.0)		–3.3
(0.13)		–60NBR-BunaN
(Black)		Elastomer Only**
GP-154-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		50SI-Silicone
(Blue)		Aluminum
GP-154-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		50SI-Silicone
(Blue)		Aluminum
GP-334-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		30CR-Neoprene
(Brown)		Aluminum
GP-334-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		30CR-Neoprene
(Brown)		Aluminum
GP-574-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		70FKM-Viton®
(Orange)		Aluminum
GP-574-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		70FKM-Viton®
(Orange)		Aluminum
GP-578-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		70FKM-Viton®
(Orange)		316 Stainless
GP-578-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		70FKM-Viton®
(Orange)		316 Stainless
GP-742-1A-1.2-1.2Knurled304.8
(12.0)		152.4
(6.0)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		45NBR-BunaN
(Black)		Carbon Steel
GP-742-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		45NBR-BunaN
(Black)		Carbon Steel
GP-744-1A-1.2-1.2Knurled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		45NBR-BunaN
(Black)		Aluminum
GP-744-1A-6.0-12.0Knurled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		45NBR-BunaN
(Black)		Aluminum
GP-701-1-1.2-1.2Waffled30.48
(1.2)		30.48
(1.2)		13.5
(0.50)		10.2
(0.37)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-701-1-6.0-12.0Waffled304.8
(12.0)		152.4
(6.0)		12.7
(0.50)		9.4
(0.37)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-703-1-1.2-1.2Waffled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-703-1-6.0-12.0Waffled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-705-1-6.0-12.0*Waffled304.8
(12.0)		152.4
(6.0)		–6.4
(0.25)		–60NBR-BunaN
(Black)		Elastomer Only**
GP-733-1-1.2-1.2Waffled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		35NBR-BunaN
(Black)		Aluminum
GP-733-1-6.0-12.0Waffled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		35NBR-BunaN
(Black)		Aluminum
GP-801-1-1.2-1.2Pebbled30.48
(1.2)		30.48
(1.2)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-801-1-6.0-12.0Pebbled304.8
(12.0)		152.4
(6.0)		13.5
(0.53)		10.2
(0.40)		3.3
(0.13)		60NBR-BunaN
(Black)		Carbon Steel
GP-803-1-1.2-1.2Pebbled30.48
(1.2)		30.48
(1.2)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-803-1-6.0-12.0Pebbled304.8
(12.0)		152.4
(6.0)		6.4
(0.25)		4.8
(0.19)		1.5
(0.06)		60NBR-BunaN
(Black)		Aluminum
GP-805-1-6.0-12.0*Pebbled304.8
(12.0)		152.4
(6.0)		–6.4
(0.25)		–60NBR-BunaN
(Black)		Elastomer Only**
GP-805-2-6.0-12.0*Pebbled304.8
(12.0)		152.4
(6.0)		–3.3
(0.13)		–60NBR-BunaN
(Black)		Elastomer Only**
*No metal plate

**Elastomer only pads may show some dimensional contraction. Use A=11-5/8” and B=5-3/4” actual for planning on 6” x 12” pads and assume nominal A & B dimensions for all metal backed pads (+.050/- .090”), nominal thickness (+/-.050”), and nominal squareness for smaller sizes as these are cut from standard nominal 6” x 12” pads. For custom machined sizes and make to print items, contact PFA.

***Durometer specifications are nominal +/- 5 to +/- 10 Shore A as shown. Durometers are rounded from actual on various materials – consult PFA for more specific durometer information. Polymer Type Abbreviations: CR = Neoprene (Chloroprene), NBR = Nitrile Butadiene Rubber (Buna-N), FKM = (Also FPM) High Temperature Fluoropolymer (Viton®), SI = Silicone (Polysiloxane), backing plate material is Aluminum (6061-T6), low carbon steel (10 gauge), or 316 Stainless Steel (or similar).
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