Vacuum cups often are the best solution for workpiece handling in robotic or automation applications. Typical applications include removing injection-molded plastics from molds, sheet metal transfer, palletizing of workpieces, nesting fixtures and assembly fixtures.
Typical materials that are handled include plastic, sheet metal, glass, wood, cardboard, paper and electronic components. Choosing the right vacuum cups for the application depends on several characteristics of the workpiece and environment. Vacuum cups come in different forms, sizes and materials to accommodate working conditions.
Some of the forms of vacuum cups can take include, but are not limited to, oval, flat, L5 bellows and 2.5 bellows. Oval cups are useful when the workpiece is long and narrow or if the workpiece has raised ribs or edges. Flat vacuum cups are used for flat workpieces with smooth to slightly textured surfaces. In general, they have a wide range of diameters for different-sized workpieces. They also hold up best to shear when a horizontal load is applied and have a faster response time than the bellows-style cups. The L5 bellows vacuum cups provide a flexible sealing lip for workpieces with irregular, smooth or contoured surfaces. They also work well with slightly flexible surfaces. The bellows provide dampening and help protect sensitive workpieces.
UPRIGHT VACUUM REVIEWS
Vacuum cups are made of different materials, including nitrile, silicone, polyurethane and viton, and the environmental and workpiece characteristics will determine what material is best suited for your application. Polyurethane, for example, is the choice for high abrasion and wear resistance, silicone or viton for higher heat applications and nitrile for good overall performance in a wide variety of conditions.
Once you have chosen the style and materials, you can size your vacuum cups and determine the lift capacity using the following basic formulas. If the workpiece is irregularly shaped or is subject to high accelerations, the lift capacity can be adversely affected.
Lift Capacity of Vacuum Cups: Theoretical lift per pad (N) = [(PxA)/l01] xSx 10.13, where P=vacuum pressure (kPa);A = area of vacuum cup (cm2); and S = safety factory (use 1 for theoretical, 8 for horizontal lift and 4 for vertical lift)
Once the vacuum cups are chosen for a specific application, the task of mounting the cups can be made easier with an adjustable, modular system of clamps, connectors and gripper arms. You can use extruded aluminum framework for flatplate design. Just mount the clamps or connectors to the framework, then mount the vacuum cups directly into the connectors or mount the vacuum cup into a gripper arm, which in turn is mounted into a damp. The gripper arms can be spring-loaded for part compliance or to be used as spring buggers. The gripper arms also have an option to adjust the angle of the vacuum cup to meet workpiece contours.
VACUUM GENERATORS AND HOSES
By Partha Badve and Tom Herndon
Applications Engineers Schmalz Inc.
SELECTION OF VACUM GENERATORS
Different types of vacuum generators are available. They are broadly classified as electrically operated (vacuum pumps and blowers) or pneumatically operated (ejectors). Pumps offer a high vacuum but a low suction capacity. Low suction is defined as the amount of air pumped out in a specified amount of time. Blowers, on the other hand, have a high suction capacity but reach lower vacuum levels. So if, for example, you have to lift a workpiece that is porous, it may make sense to use a blower instead of a pump, because with the blower you are pumping out roughly twice the amount of air that is leaking out of your porous workpiece, thus maintaining a stable vacuum in the system. Ejectors would be used where space and weight are considerations, such as in robot-mounted vacuum systems. Ejectors are small and light and reduce the cycle times of an operation considerably. Compressed air is more expensive than electricity, so running costs of electrically operated vacuum generators are less than the pneumatically o perated generators, though the initial investment for the capital cost may be higher.
SELECTION OF HOSES
To reduce the flow resistance of the air, the internal diameters of vacuum hoses must match the sizes of the suction pads they are to evacuate. If the diameter of the hose is smaller, then the resistance to the flow is higher, which leads to energy losses. Hoses are available in different materials, including polyurethane and PVC. When selecting hoses, it is important to allow for shortening by 5 percent to 10 percent caused by vacuum. Also, flexible hoses should be used where the bending radiuses are smaller to reduce flow resistance. Some hoses are reinforced with spiral wires, which make them stronger for applications requiring high strength.
DESIGNING A VACUUM SYSTEM
By Mike Tuohey
Marketing Communications Manager
PIAB Vacuum Products
Porous applications pass atmospheric air through an object at the vacuum point. Examples include paper handling and lifting corrugated boxes, which can be done with robotic arms. The important factor in these applications is the amount of vacuum flow a pump generates. In porous applications, the pump has to compensate for leaking air to sustain a given vacuum level.
Other considerations are:
- Surface and Texture: These will determine the size and type of cup you choose.
- Porosity: Is the material solid and nonporous, like glass, or porous, like cardboard? This will affect the choice of cups and the size of the pump.
- Material: This determines the type of material the cup is made of.
- Weight and Size: These two indicators determine the size and number of suction cups. Any design should incorporate a safety factor greater than 2.
Application speed is a function of flow. To evacuate a volume or suction cup quickly, flow is needed. For faster evacuation or greater cycle rates, more flow is required. For greater flow, a larger vacuum pump is normally used. Flow can also be increased by eliminating leaks in a system.
Volume is the sum of all space to be evacuated in a system. This includes the vessel or suction cups, vacuum line and all ancillary volume between the area of application and the vacuum pump (e.g., the filter). The greater the volume, the more time required to evacuate to the appropriate level.
Porosity, or leakage, is critical to the success of a vacuum application. Products such as corrugated boxes have pores that allow outside atmosphere to leak into the system. The greater the leakage, the more flow required to achieve and maintain a desired level of vacuum. Porosity can be compensated by a larger pump with more flow, or by eliminating leaks as much as possible.
It is best to work at as low a level of mercury as possible. Energy consumption increases asymptotically. To increase the level of vacuum from 18 inches to 27 inches, Hg requires 10 times as much energy. The higher the level of mercury designed into the system, the more it becomes susceptible to leakage and porosity.
- Use the right cup for the application
- Pay attention to the type of material and the surface texture
- Determine which cup material is best for the application. Always design with a greater than 2 safety factor
- Be aware of additional dynamic forces that could affect the lifting ability of the cup
- Cup distribution should be uniform in relation to the center of gravity
- Different styles of cups demand different placement
- Use the proper accessories for the best performance
- Consider the type of surface finish
CENTRALIZED VS. DECENTRALIZED
A centralized vacuum system uses one large pump that serves a whole machine or an entire facility. One disadvantage of centralized systems is if the pump fails, the entire operation is down until repairs are made. Maintenance is easier, however, because there is only one pump.
A decentralized system has several pumps dedicated to specific applications or machines. These systems often require synchronized operations and, therefore, have the potential for higher maintenance. Because components of decentralized systems operate independently, however, when faulty pumps need service, all other pumps can keep running.
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