How to select the right gripper for your application

These are the standard questions that need to be considered when accessing which gripper is needed for every application; in addition there will be more questions and factors that are specific to your own application and that will have an equal effect on gripper selection. The SCHUNK team know which questions to ask, and which factors to consider to help you quickly and efficiently make the right gripper selection for your specific application. We are happy to talk through everything with you, please reach out.

What are the important factors to consider when selecting the right gripper for your application? There are several questions to ask to help narrow down the selection.  

What is the workpiece?

The size and weight of the workpiece are important as there are significant differences between, gripping lipstick cases versus train wheels. The dimensions and shape of the workpiece is an equally valid consideration. Though the workpiece may have a known weight, it could have dimensions of a suitcase, a shopping cart, or a ladder. Establishing these parameters is the first step in determining what series of grippers to consider and how to begin to visualize handling the part.  

How to grip the workpiece?

This is really a two-part question. First, how do the gripper fingers engage the workpiece? Is it with friction or a capture grip?  A friction grip is when the workpiece is gripped and held with friction alone. A coefficient of friction value, represented by the symbol µ, between µ = 0.1 and 0.4 is typical for most automation applications.  A capture grip is when a workpiece has a groove, ridge, or hole that form-fit fingers can positively engage.  An example of this would be an automotive engine block with holes and ribs that form-fit fingers can engage.  With capture grip, the force required to safely hold the workpiece is less than if held by friction alone so a smaller gripper can be selected than would be appropriate for a friction grip.  Imagine two steel cylinders standing on a table, one is dry and one is coated with oil.  More force is required to pick up the oily cylinder than is required to pick up the dry cylinder because the coefficient of friction of the oily cylinder is lower than that for the dry cylinder. Coefficient of friction has the largest impact on the amount of grip force required.  If the grip force required at µ = 0.1 is 2000N, increasing the value to µ = 0.2 will reduce the required grip force to 1000N, or half the previous value.  Therefore, when evaluating friction grip applications, gripper size can be significantly reduced by increasing the coefficient of friction.  This can be done by adding rubber pads, fiberglass inserts, or carbide material between the fingers and the part.

What is the finger length requirement?

Grip force diminishes as finger length increases. Imagine a soda can standing upright on a table and two rulers extended toward the soda can to pick it up.  This can be done easily with 12” rulers but if the rulers are exchanged for yardsticks, picking up the soda can will become more difficult. So first, calculate the grip force required, then evaluate the finger length to make sure the gripper has enough grip force at the required finger length.

What is the jaw stroke?

Now that the search has been narrowed down by answering the previous questions, the jaw stroke must be considered.  There are generally two categories of grippers; long stroke/low force and short stroke/high force. Some applications may require the use of the same gripper on a range of part sizes. In this case, a long stroke gripper may be needed to handle the part range and, if V-jaws are used, then additional stroke is required to open around the part and still have enough travel to close.  Other applications handle only one part and can take advantage of a smaller gripper with higher force. Why not just use long stroke on everything?  Extra stroke requires additional time and energy to move the fingers longer distances, not to mention the additional size, weight, and cost of a longer stroke gripper.

What is the robot acceleration?

The next question that needs an answer is robot acceleration, this is often confused with speed. It is important to clarify that speed is a constant and does not affect the required grip force.  Acceleration is a measure of a change in speed and imparts a force on all masses that are slowing down or speeding up.  For example, holding a soda can and handing it across the table to a friend doesn’t require much force. However, throwing the can to a friend requires more force to hold the can and keep it from slipping before it is released. A part being held stationary has 1G of acceleration acting on it due to gravity and one G is equal to 9.8m/s².  If the robot moves the part upward at 1G, there is now 2G of acceleration acting on the part, 1G for gravity and 1G for the robot. Typically, robot acceleration values tend to be between 0.25G and 3G for automation applications.