Grippers grasp and manipulate objects during the manufacturing operation work cycle. The objects grasped are usually workpieces that need to be loaded or unloaded. The task may involve loading or unloading one station or moving the object from one station to another. Typical robot loading or unloading operations include machine tool tending, stamping press loading and unloading, pick and place operations, palletizing activities and assembly operations. Grippers may be standardized or custom-designed to suit the physical specifications of the workpieces they have to grasp.

Basic gripper classifications include:

Type	Description
Mechanical Grippers	Two or more fingers typically actuated by a robot controller to open and close on a workpiece
Vacuum Grippers	Suction cups are used to hold relatively flat objects
Magnetic Grippers	Making use of the principles of magnetism, these devices are used for holding ferrous workpieces
Adhesive Grippers	Deploying adhesive substances, these hold flexible materials, such as fabric
Dual Grippers	Mechanical grippers with two gripping devices in one end effector for machine loading and unloading. Reduce cycle time per part by gripping two workpieces at the same time
Interchangeable Finger Grippers	Mechanical gripper used to accommodate different workpiece sizes by attaching different fingers
Sensory Feedback Finger Grippers	Mechanical grippers with sensory feedback capabilities in the fingers to aid in locating the workpiece and determine the correct grip force to apply (for fragile workpieces)
Multiple Fingered Grippers	Mechanical gripper with the general anatomy of the human hand.
Standard Grippers	Mechanical grippers that are commercially available, thus reducing the need to custom-design a gripper for each robot application

Mechanical grippers are grippers that use mechanical fingers to manipulate objects. They have a distinctive design similar to a crab’s pincers. Mechanical grippers usually come with adjustable force and stroke features, enabling them to perform tasks with human-like precision and dexterity. The number of robot fingers varies depending on the model, although most mechanical grippers have two to four fingers. The fingers are usually replaceable, allowing manufacturing operations to maximize their investment.

Choosing the right end-effector depends upon the needed functionality. For loading-unloading operations grasping the object is required. Other considerations are the object’s weight, material, surface quality conditions and other part-specific issues.

For sheet metal products, vacuum grippers are widely used. They provide gripping using suction cups and are mostly used for handling workpieces with uneven surfaces or irregular shapes. Traditional vacuum grippers utilize external air supply systems which require high maintenance costs. Newer models run on electricity, eliminating the heavy costs in addition to improving the work environment due to reduced noise and dust.

Vacuum grippers are the standard EOAT in manufacturing because of their high level of flexibility. This type of robot gripper uses a rubber or polyurethane suction cup to pick up items. Some vacuum grippers use a closed-cell foam rubber layer, rather than suction cups, to complete the application.

Pneumatic grippers are popular due to their compact size and lightweight. They can be incorporated easily into tight spaces, which is helpful in the manufacturing industry. Pneumatic robot grippers can either be opened or closed, earning them the nickname “bang-bang” actuators, because of the noise created when the metal-on-metal gripper operates.

Hydraulic grippers provide the highest strength and are often used for applications that require significant amounts of force. These robotic grippers generate their strength from pumps that can provide up to 2000 psi. Although they are strong, hydraulic grippers are messier than other grippers due to the hydraulic oil used in the pumps. They may also need more maintenance due to the gripper being damaged because of the force used during the application.

Servo-electric grippers are appearing more and more in industrial settings because they are easy to control. Electric motors control the movement of the gripper jaws. These grippers are highly flexible and allow for different material tolerances when handling parts. Servo-electric grippers are also cost-effective because they are clean and have no air lines.

The most common purpose of a gripper (also referred to as an end effector) is to grasp or enclose parts for transfer, insertion, or assembly in automated manufacturing and systems. Additionally, grippers are used in environments that are hazardous for human presence. Several factors are required for selection and design to ensure proper gripping. The following considerations will help when choosing and sizing the right gripper for your application:

> Part Shape – If the product or part has two opposing flats, a 2-jaw gripper is normally used. If the part is cylindrical, a 3-jaw gripper is typically used. Tooling can be designed to accommodate cylindrical parts with a 2-jaw gripper.

> Accessibility & Part Consistency – Angular grippers are usually low in cost, but the arcing motion of the jaws may require additional tooling clearance and will grip at varying points as part width varies. A parallel gripper is easier to tool to compensate for part size.

> Part Weight – Grip force must be adequate to safely transport the part.

> Orientation & Dimensions – Part orientation and distance from the gripper face affect the gripper selection.

> Size – Nominal gripping dimension indicates approximate gripper size.

> Variation – Variation in gripping location or encapsulation determines minimum gripper jaw travel.

> Air Pressure – The air pressure at the gripper affects gripper sizing and must be taken into account.

> Grip On Open or Close – Grip force varies in each direction due to the effective area of the piston rod on some gripper types. Verify that the gripping direction is taken into account when sizing.

> Velocity – Higher speeds and acceleration/deceleration affect gripper selection.

> Tooling Length – Longer tooling introduces bending moments into the gripper and affects sizing.

> Tooling Configuration – If the part is encapsulated, the required gripping force can be lower than if it is grasped on flats only.

> Product Retention – If part retention upon air loss is desired, springs or locking cams can be specified for the gripper. 

> Environment – For harsh environments, special plating or materials should be specified.

> Synchronous Operation – Most grippers provide synchronized jaw movement. In special circumstances, independent jaw travel is desired and can be supplied.

> Switching Options – Most grippers offer several switching options.

In addition to robot end-of-arm-tooling requirements, automated manufacturing fixturing must now provide enough clearance to allow robot arms to manipulate the parts into the work nest and provide better chip evacuation (in machining operations). Fixtures must also provide the ability to retain the parts in a free state before clamping and unclamping. Lastly, fixtures must have the capability of knowing when a part is not seated properly in the work nest.

To begin the process of the robot with the workholding fixture, the machine loading surface must be clear of any foreign material that would not allow the part to seat properly in the work nest. This may require an air or coolant source to remove chips or debris in the work area. In most cases, the area is flooded with a coolant flush from the machine and then an air blast from the robot arm.

Next, the robot must have the ability to load the part in the nest in such a way that the part does not fall out before it is clamped. This requires a provision for some type of preload mechanism to hold the part in position for the clamping. This may be simple spring clips or a spring-loaded low-pressure clamp mechanism. Once the part is in the preliminary load position on the work nest, the final clamping of the part takes place with the robot retracted.