Get a handle on grippers

Selecting the best gripper technology for your industrial automation application.

From electronics manufacturing to automotive assembly, grippers have become an important part of material handling processes in many industries. Their recent growth is tied to the rise of robotics, including the need for robots to take on special tasks and handle increasingly complex workpieces. The result is you now have more grippers to choose from than ever.

With all the recent developments in robotics and gripping technology, it can be difficult to know which gripper is best for your application.

Understand the demands of the application

To select the right gripper, it’s important to first consider the nature of the task, operating environment and workpiece — including its size, mass and material. Some key considerations and questions to ask include:

• Environmental considerations: What temperature range will the gripper operate in? Will it be exposed to dirt, dust, oil or moisture?
• Application: Does the application involve food or other hygienic workpieces? Will it be exposed to cleaning processes? Does the application require antistatic materials?
• Design constraints: What direction of motion is needed? What is the maximum operating speed and how large is the work space? Will operators be sharing the space with collaborative robots?
   

Other factors to keep in mind include upfront, operating and maintenance costs, as well as energy consumption. Grippers fall into several categories, and the answers to the above questions will help determine which type is needed to get the job done as safely and efficiently as possible.

Mechanical grippers: proven reliability, flexibility and durability

When it comes to handling applications, pneumatic or electromechanical grippers are the most common. Pneumatic grippers, which make up 90% of the market, tend to be more lightweight and cost-effective than their electric counterparts. They also feature higher grip forces, can handle faster cycle rates and are more suitable for harsh environments. Electric grippers, on the other hand, offer greater precision, providing end users with force and travel control. At the same time, they tend to be heavier due to the presence of a motor and other internal components, which also drive up their upfront costs.

Whether electric or pneumatic, mechanical grippers fall into several design classes. Parallel grippers, for example, incorporate fingers that pull directly apart. Two-finger designs are the most common, making up 85% of the mechanical gripper market, while three-finger designs are suitable for handling round objects or performing centring functions. Other examples include radial and angular grippers, which feature fingers that open at an angle. Radial grippers open to 180°, making them suitable for applications involving varying or inconsistent workpiece sizes. Angular grippers tend to be faster than 180° designs and open to roughly 30°.

Figure 1: Comparing the designs of three gripper fingers. For a larger image click here.

Other important considerations when selecting a mechanical gripper include the gripping force, the guiding strength of the jaws and the design of the gripper itself — all of which depend on the nature of the workpiece. In general, the longer the gripper fingers, the longer the lever arm, which exerts more torque on the jaws. In addition, flat finger designs provide a friction-based grip for bulky or robust parts, while encapsulated designs work best for slippery parts requiring more precision.

Enabling new automation with soft and adaptive grippers

Soft and adaptive grippers can handle workpieces of various shapes, sizes and orientations, enabling automation in areas where it previously didn’t exist. Because they don’t have any sharp edges, these types can handle food, glass and other delicate objects without damaging or marking the surface. They’re also ideal for small work areas. Compared to mechanical variants, however, soft and mechanical grippers are less precise and operate at slower speeds. They are also more susceptible to dirt, oil and other contaminants.

Reap the benefits of compressed air with vacuum grippers

Another gripper technology is the vacuum gripper, which combines suction cups and vacuum generators. Compact and flexible, these grippers are ideal for limited work spaces and can handle a variety of objects at high speeds. At the same time, however, vacuum grippers can increase maintenance and operating costs; suction cups are susceptible to quick wear, while generators consume high rates of compressed air and can easily clog in the presence of dust and other contaminants.

When it comes to suction cups, it’s important to consider the nature of the workpiece when selecting a material. Buna suction cups, for example, are ideal for oily or plain workpieces, while silicone is suitable for food, as well as hot or cold objects. In addition, polyurethane is a good choice for oily, plain and rough workpieces; viton is suitable for oily, plain and hot workpieces; and anti-static buna is ideal for electronics.

Figure 2: Suction cup shapes and surfaces for vacuum grippers. For a larger image click here.

Cup shape is also an important factor, especially when it comes to gripping objects that are flat versus round, slim versus large and sturdy versus delicate:

• Standard cups: for flat or slightly undulating surfaces.
• Bellows type: for pliable workpieces, as well as surfaces that are bevelled, domed or round. This type is also suitable for glass bottles, lightbulbs and other delicate objects.
• Oval type: for slim or oblong workpieces.
• Extra deep: for round and domed workpieces.

Vacuum sources fall into two categories: electromechanical vacuum pumps/blowers, as well as compressed air-driven vacuum generators/ejectors. In general, electromechanical pumps and blowers can achieve high vacuum and suction rates — up to 99.99% and 1200 m³ per hour, respectively. At the same time, however, these machines tend to be heavy and large, requiring a reservoir with a complex piping system. Because they run continuously, they also consume a lot of current, which generates heat.

Compressed-air driven generators, especially single-stage units, overcome many of these challenges. Compared to electromechanical pumps and blowers, they are more compact, lightweight and easier to install. They include simpler piping systems, require lower upfront costs and incorporate no electrical connections, eliminating harmful heat build-up. Although these units can run up air consumption rates, many machines now come with energy-saving functions, minimising these effects.

In general, vacuum grippers are ideal for: material handling applications, such as steel fabricators, conveyors, electronic assembly, industrial robotics; food and packaging tasks, including canning, bottling, capping, tray making, filling, bagging and sealing, conveying, box making and labelling; and printing applications, such as sheet feeding and handling.

Magnetic grippers offer unique benefits

Lastly, magnetic grippers can handle metallic objects like sheets of metal and are ideal for tasks like de-stacking, fixture tooling and bin picking. Although these grippers are limited to applications involving ferrous metals, they require minimal air consumption to actuate, achieving energy savings up to 90% compared to suction cup grippers. Other benefits include:

• Strong gripping: Magnetic grippers can use a shallow magnetic field that enables material de-stacking.
• True ON/OFF: Magnets can be switched completely off and stay clean; any metal fillings left behind from production processes instantly fall away.
• Fast: Some magnetic grippers can actuate in 250 ms, saving valuable cycle time.
• Safe: Magnetic grippers can also provide failsafe performance and will not drop parts in the event of a power or air loss.

Getting started

No matter the application, there’s an ideal gripper. Ultimately, the right choice depends on a number of variables, including workpiece size and shape, operating conditions, industry, energy requirements and cost. In some cases, unique handling applications may even require a custom gripping device.

Top image credit: iStock.com/nay