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The main purpose of a gripper is to securely handle a workpiece, eventually releasing it after a move or process. There are many factors that should be considered when selecting the proper gripper for a given application. We will focus on the grip force aspect.
Most mechanical grippers generate grip force using compressed air. The compressed air drives a piston which transfers the force to the gripper’s base jaws, either directly via the piston rod or through a wedge; the wedge provides a mechanical advantage.
First, calculate the required grip force based on the application parameters. A basic gripping scenario is a friction grip. Figure 1 shows a 2-finger gripper with flat fingers gripping at the part’s center of gravity. The workpiece is shown in a state of equilibrium, so all existing forces are counteracted by equal and opposite forces (Newton’s 3rd law of motion). The vertical forces (Fa, G) are determined by the product of the workpiece mass and the acceleration (Newton’s 2nd law of motion) and summed up. The counteracting vertical friction forces, FR, are generated at the contact surface between the gripper fingers and the workpiece. These friction forces prevent the workpiece from slipping in the direction of gravity. The grip force per finger, F, being generated by the gripper, is the normal force acting perpendicularly to the friction force, FR. Coulomb’s law of friction relates the friction force, grip force and coefficient of friction as FR= µF. Thus, we obtain the following formula for total grip force in Newtons:
Grip Force = m (g + a) S
m = mass (kg).
g = acceleration due to gravity (m/s2).
a = maximum acceleration due to robot (m/s2).
S = factor of safety.
µ = coefficient of friction.
Since the grip force and the coefficient of friction are inversely proportional, increasing the coefficient of friction is usually the most economical way to decrease the required grip force. This can be done by using a rubber gripping pad, a serrated insert with pointed tooth surface or similar solution. Coefficient of friction values can be found in engineering handbooks or determined from actual tests. Reducing the required grip force ultimately reduces the gripper size and cost, and potentially that of all subsequent components to which the gripper is attached.
Another gripping scenario is a partial capture grip from a v-type finger design. In this case, the above mentioned grip force formula is altered to take into account the angular nature of the contact forces. This finger design is common for gripping shafts or parts with cylindrical shaped grip locations.
Depending on the application and the workpiece’s motion path, the worst case required grip force must be determined. Other factors can require more grip force such as gripping at a location other than at the center of gravity or high acceleration causing the finger to back drive the base jaws.
Next, select a gripper with available grip force equal to the required grip force calculated. This is done by referring to gripper catalog data. Grip force values are based on operating pressure and finger length. Typically this relationship is presented in charts for the grip force at various finger lengths and operating pressures. If grip force versus operating pressure data is not available, direct interpolation can be done to estimate the available grip force value.
For more information, see the book Grippers in Motion, ISBN 3-540-25657-1 or visit us at www.us.schunk.com.
Applications Engineer -
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