Instron 2712 Series - Grip Mechanical Function

Function M10-14179-EN
1-6
Function
All Instron pneumatic grips operate on the same mechanical principle. Pneumatic
action grips clamp the specimen through a dual lever arm, actuated by air
cylinders built into the grip body. The gripping force can be increased with air
pressure to accommodate materials that are often difficult to hold. This constant
gripping force is maintained on the specimen, and provides follow-up action to
compensate for any decay in the gripping force.
The interchangeable jaw faces are mounted so that one face is free to swivel on a
horizontal axis and the other on a vertical axis. This action allows for self
alignment to compensate for variations in specimen thickness.
The following subsections are general functional descriptions of the grip
components.
Closing the Grip
Applying air pressure to the grip creates a pressure differential on the piston
causing it to move in toward the specimen. The force of the piston cone against
the roller link actuates the transfer levers inward, pushing the jaw face holders
toward the specimen.
The pressure inside the grip increases as the jaw faces contact the specimen.
Gripping force on the specimen is directly proportional to the air pressure applied
to the grip; if pressure increases, gripping force increases. There must be constant
air pressure to the grip to maintain a gripping force on a specimen.
Coupling pins allow one jaw face to swivel on the horizontal axis and the other on
the vertical axis. This swivel action compensates for small variations in load
string alignment and specimen thickness.
The jaw face contact surface affects the grip's hold on the specimen. Jaw faces
with serrations penetrate the specimen, smooth faces do not penetrate the
specimen and rubber faces have a high coefficient of friction without penetrating
the specimen.
Opening the Grip
When you remove the air supply from the grip, there is no longer a pressure
differential on the piston. With no pressure differential, the return spring retracts
the piston from the roller links. The compression spring in each arm retracts the
transfer lever which pulls the jaw face away from the specimen.