Gripper Systems for Material Handling in Mechanical Engineering

Particularly in large production plants and production chains, it is imperative to deal with handling techniques for materials and workpieces. Due to the weight alone, it is often almost impossible to move materials manually. This is where material handling starts: Gripper systems or conveyor belts, for example, facilitate the movement and control of materials and increase efficiency. In the following, we will take a closer look at various gripper systems and how they are utilized.

What exactly is material handling?

Material handling, or also handling technology, makes manufacturing processes easier, safer and more productive. The primary task is the movement, storage, control and management of materials. Material handling systems can save storage space or serve themselves as storage. Therefore, it is all the more important that material handling systems withstand high demands:

• they must be resilient, mobile and flexible.
• they should not incur high costs.

Types of Material Handling Systems

Material handling systems can be, for example: conveyor belts, forklifts, robot systems, cranes or gripper systems on automated production lines. However, they can also be used in manual work, e.g. to support operators in everyday tasks such as moving drums or boxes. In particular, gripping is one of the main tasks in many production plants. By using material handling systems, manual processes previously used can be accelerated and illness-related work absences can be reduced. Here, gripping systems or gripper systems contribute particularly to working ergonomically.

Gripper Systems

Gripper systems support the precise handling and control of materials and workpieces. Gripper systems are available to support manual gripping work, such as barrel grippers, roller grippers or box grippers. But gripper systems are also used on the production lines in manufacturing. Grippers are often used together with robots: They serve as a connecting piece between the industrial robot and the workpiece.

Vacuum Grippers

In the vacuum gripper, a vacuum is generated with a pump between the gripper and the workpiece. The vacuum gripper is placed on the workpiece and the negative pressure on the vacuum gripper causes the workpiece to be suctioned into the vacuum gripper.

• 1 = workpiece
• 2 = vacuum
• 3 = vacuum gripper

The holding force depends on the pressure difference between the environment and the pressure in the vacuum gripper itself. The greater the difference, the stronger the holding force and the heavier loads can be lifted. The workpieces to be transported can range from hard to easily malleable parts with suction-capable surfaces. The suction force decreases if the surface to be sucked in is porous or air permeable. This requires enlarging the effective cross-sectional area of the line or using several smaller suction cups. Soft workpieces such as paper, thin materials, plastic films can be crumpled by the suction force.

In addition to this type of vacuum gripper, there is also the Bernoulli gripper. It uses a somewhat different principle: the Bernoulli principle. The airflow is passed through a narrow nozzle at high speed. Here, too, a negative pressure is created, which generates a suction effect and attracts the object. In addition, there is a slight buoyancy that lifts the object minimally off the surface. The contact between gripper and workpiece is thus reduced to a minimum.

Vacuum gripper systems can be used in various ways: They are suitable for soft and delicate loads such as bags and glasses as well as for heavy loads. Due to the minimal contact between gripper and workpiece, the Bernoulli gripper is generally suitable for particularly delicate materials, such as paper and plastic films, but less so for heavy loads.

Pneumatic Gripper

Pneumatic grippers are compressed air-controlled gripper systems to hold and move components, workpieces and loads. Depending on the shape, weight and necessary movement of the object to be transported, jaw grippers, rotary grippers or clamp grippers in a wide variety of shapes can be used.

Pneumatic grippers are widely used in many sectors and industries due to their reliability, efficiency, and simple design. As a rule, the grippers are designed as two-finger or three-finger grippers. In addition to single-acting grippers with opener (inner grip) or closer function (outer grip), double-acting grippers are also common.

The following figure shows the schematic structure of an active, double-acting parallel gripper:

• A = pneumatic connection
• B = pneumatic connection
• 1 = slotted holes
• 2 = lateral movement of finger mounting plate (available for both directions)
• 3 = closing and opening the fingers

With the double-acting gripper, there are two pneumatic connections, each of which is responsible for opening and closing the gripper. If the pneumatic connection A is pressurized, the piston is pushed downward and the gripper closes. If the pneumatic connection B is pressurized, the piston is pushed upwards and the gripper opens. The assembly jig for finger assembly can be moved laterally on the gripper body (2) via the slotted holes (1). The bearing connected to the piston runs in the middle, diagonal slotted hole. The movement of the finger mounting plate is initiated by guiding it in this slotted hole. If the piston moves in vertical direction, while the horizontal movement of the finger mounting plate is forced to open or close (3).

While jaw grippers are mostly used for particularly wide or large objects, clamp grippers are usually used for certain shapes (e.g. pipes) and finger grippers are used for smaller objects that need to be gripped. Due to the different types of pneumatic grippers, they are suitable for a variety of applications, e.g. in the electronics industry and in the automation of processes. Pneumatic grippers work highly efficiently and reliably. They are characterized by low operating costs and can be used in tight spaces. They are also easy to set up and to handle.

Hydraulic Gripper

Hydraulic fluid, usually a hydraulic oil, is used for hydraulic grippers. The hydraulic fluid is pressurized via a hydraulic pump and thus transported to the working cylinder where it transfers the force. They function according to a system similar to pneumatic grippers.

Hydraulic grippers allow for very high gripping forces, which is why they can be used for heavy loads. They are also very resistant and also suitable for harsh ambient conditions. However, due to the risk of leakage, they cannot be used to transport materials that must not be contaminated. Their high gripping strength also excludes their use for delicate materials. The construction of hydraulic grippers is generally more complex (e.g., due to oil pumps, etc.) and thus somewhat more maintenance-intensive and expensive than other grippers.

Electrical Gripper

Electrical grippers have a control system with microprocessors, e.g. a PLC control system. This allows selecting the closing speed and gripping force precisely. It is often also indicated whether a workpiece was picked up successfully or not. A rotational movement is generated via an electric motor or electric actuators. This rotary movement of the motor is then transferred to a gear box where it moves the gripping jaws as linear motion. The gripping jaws can, for example, be constructed parallel or at an angle. The electronic control system monitors and controls the processes. Additional sensors are often installed, which measure force, distance and other parameters. Electrical grippers are well suited for high speeds and multi-step actions, as they can be precisely controlled. Unlike, e.g. vacuum or hydraulic grippers, no additional lines for fluid or air are necessary. However, since their gripping surface is often quite hard. Therefore, they are not suitable for delicate objects. In addition, the gripping force is rather low, so that heavy objects can usually only be lifted using the sub-gripper variant.

Gripping Inserts

Gripping inserts can be used on the gripper jaws to optimize the contact surface between gripper and workpiece. Gripping inserts thus improve the grip or adapt to special shapes. A chuck is a special form of gripping insert, which usually consists of steel and thus provides high precision and strength, but is also quite inflexible. Other materials for gripper inserts consist of:

• Rubber/elastomer such as: silicone, rubber, polyurethane. These adhere well, are flexible and protect the surface, but are only partially durable under harsh conditions
• Plastic e.g. polyamide, polyethylene. They are light-weight, affordable and gentle on the surface, but can fail under high loads and temperatures.
• Special materials such as wood, ceramic are for special applications. Ceramic is, for example, highly heat-resistant, while wood is abrasion-resistant. However, due to their specialization, they are usually more expensive than other gripper inserts.

Euro Gripper Tooling System

The Euro Gripper Tooling System was developed by the Automotive Working Group. It is a standardized system for fastening and replacing grippers. It is widely used in Europe and is used not only in the automotive industry but also in other industries and manufacturing. The EGT system is based on an octagonal aluminum section with grid hole drilling for quick component replacement.

Standardized interfaces, quick-change mechanisms and modularity, allow for swift replacement of the broken components, which significantly reduces downtime. The EGT system is compatible to many brands and models. MISUMI has a comprehensive range of EGT system-based components.

Exemplary Design of a Gripper System incl. Components

Depending on the type of gripper, gripper systems consist of the following components:

• Drive unit: It controls the movement of gripper jaws or gripper surfaces. It can be an electric motor (for electric grippers) or a cylinder (hydraulic grippers, pneumatic grippers)
• Control system: In electric grippers, the control system takes over the movement of the gripper and other tasks.
• Power source: Depending on the gripper, this may be: the source of electricity, compressed air or hydraulic pump.
• Sensors: are installed in electric grippers. They monitor pressure, force among other relevant parameters.
• Mounting adapter: these are used to fasten grippers to the robot arm or similar They can be standardized according to the EGT system.

One of the most important components is the robot gripper itself. It has direct contact to the gripping object. Here too, there are different types that are utilized depending on the application. In parallel grippers, for example, the gripping jaws are attached parallel so that the force is evenly distributed on the gripping jaws. This makes it particularly easy to picked up standard shapes. In angle grippers, the grippers are similar to pliers. This is the way they adapt to different shapes. In addition, more than one gripping mechanism can be installed, e.g. in a multiple gripper. It can grip multiple objects at the same time. In addition, there are multifunctional grippers that combine several gripping methods and functions.

MISUMI offers some components that support the construction of a gripper system: e.g. mounting plates or locating pins

Calculation of the gripping force and weight

The gripping force can be used to determine the theoretically possible weight that can be carried. The following applies to parallel grippers, for example: the gripping force is the sum of the individual forces acting on each finger. The following formula is used to calculate the maximum weight of the object to be gripped:

• m = weight of the workpiece in kg
• F_G = arithmetic sum of the gripping forces
• μ = friction coefficient, material-dependent
• g = gravity (9.81 m/s²)
• a = acceleration (m/s²)
• S = safety factor*

* A higher safety factor should be considered in the event of high acceleration, deceleration or shock loads.