By Richard T. Schneider
Atmospheric pressure acting on the exterior of evacuated cup generates lifting force.
Locating cups uniformly around load's center of gravity balances lifting force.
When an application calls for lifting or handling with vacuum-as the interface between the load and the actuator, a suction cup typically is involved. There is a wide variety of cup types and designs available — each with an advantage in a specific area. But before discussing the use of cups in design, it is important to understand how a cup works. A suction cup adheres to the surface of an object to be handled because the pressure under the cup has been lowered. The higher atmospheric pressure outside the cup pushes it down, holding it to the surface to be handled. The lower internal pressure is achieved by connecting the cup to a vacuum system, which evacuates most of the air from under the cup.
When designing a vacuum system that will lift or hold with suction cups, begin the project at the initial point of contact, and work back to the source of the vacuum flow, which determines the force developed. By properly sizing all the components, the best mix of capacity and efficiency can be attained. Before choosing a cup, consider the surface, its texture and temperature, any porosity, load weight and size, and lifting direction.
Surface and texture
What kind of surface does the application present? Is it flat or curved, and what are the overall dimensions? Suction cups are designed to be more effective on one surface than another. Answer this question correctly, or you could be fighting against the wrong cup throughout this design project.
Surface texture is the next consideration. The surface may appear to be perfectly flat and thus, present an easy lift. But a closer examination could reveal a grainy surface texture that allows leakage, or show some other variation that limits the type of cup that can be used. Some applications might require a soft cup material because of the texture of the surface.
Soft or low-durometer cups usually will seal more effectively against grainy or rough-textured surfaces. But a softer durometer will increase the flexibility of the whole cup body. Such increased flexibility can decrease stability during actual handling. A dualdurometer cup — usually with a 30-durometer lip and 50-durometer body — can improve compliance while retaining stability.
The presence of lubricants is another consideration. The cup material must be resistant to the lubricant. Contact with incompatible material can cause the elastomer to become gummy or sticky — or in the worst case, to become brittle and crack. Applications with lubricants call for a cup design with an edge that will act as a squeegee. Such cups may have a slight radius at the point where the lip touches the surface; other designs have the shape of a shallow bowl. (These cups are not suitable for flat, dry surfaces because they would increase drag, possibly curling under.) On flat, lubricated surfaces, the edge has a wiping effect as it slides outward when atmospheric pressure pushes down on the cup. These cups are fitted with cleats that act as treads to reduce the risk of slippage.
The potential for surface marking is another factor in cup selection. Products, such as glass or styrene plastic, with highly polished surfaces can retain a cup imprint — just as a handprint might appear on the same type of surface. A mark-free cup, molded without a release agent, avoids marking. Other cup compounds can leach plastisols that may mark some surfaces. Products that will later be coated or painted may need special cups. Silicone from release agents and silicone rubber itself can leave marks that only become visible when the coating or paint fail to adhere to that location.
Is the material solid and nonporous like glass, or porous like paper? The answer to this question will impact the design from the choice of cups to the decision of what size pump to use. Porosity is defined as the amount of outside atmosphere that passes through an object under vacuum. A solid material with no pores or openings will not allow outside atmosphere in. Paper, for example, is full of tiny pores and leaks a considerable amount of air into any application. Temperature extremes at the contact surface also can effect cup performance. Might the workpiece be coming out of an oven — or a refrigerator? Its best to know this ahead of time to learn how a specific cup material will respond.
Weight and size
It is only logical that the weight and dimensions of the object to be lifted are known. These two indicators determine the number and size of the suction cups needed. Any design should incorporate a safety factor greater than two.
Workpiece size also determines the placement of the cups. Suction cups should always be positioned relative to the objects center of gravity. This allows for proper balance on the lifting actuator.
When the objects weight and dimensions are known, the next logical decision is the type of cup and its size. Remember, there probably is a specific cup design that is most suitable for the surface and texture of the item to be lifted. It is advisable to use as large a cup as possible. The system then can work at a lower level of vacuum force. There are two benefits to this:
- faster evacuation time,
- longer cup life.
Once the components that precede the vacuum pump are selected, it is time to choose and size the vacuum pump. Several factors are involved in this decision:
- Is the system centralized or decentralized?
- What is the speed of the application?
- How much volume must be evacuated?
- How porous is the product being handled?
- What vacuum level is required?
The issue of centralized versus decentralized systems sometimes comes down to the designers personal preference. Some prefer a single large pump, others prefer smaller pumps dedicated to specific applications. Each arrangement has its pros and cons.
Will the centralized pump serve the entire facility, or just one machine? If a single centralized pump serving the entire facility fails, the whole operation will be down while repairs are made. On the other hand, maintenance is easier if there is only a one pump to service. In centralized systems, situations can occur on one machine that negatively impact the entire operation.
For example, suction cups left open to atmosphere on an idle machine can affect the performance of another machine across the plant.
A decentralized system with a number of pumps requires more coordination to synchronize their activities, and multiple pumps need more maintenance. A decentralized system, however, operates independently. One pump does not impact another, nor is one machine affected by the performance of another piece of equipment.
Application speed is a function of flow. By definition, vacuum flow is the amount of outside atmosphere that passes through a vacuum pump per unit of time. To evacuate a volume or suction cup, 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 improved by eliminating leaks in a system.)
Correct line sizing is important to optimize the the vacuum flow that the pump actually generates. Undersized vacuum lines will reduce vacuum flow. A quick way to check for line restrictions: does the vacuum gauge read a level greater than zero with the vacuum cups o[en to atmosphere? If yes, the lines are too small.
Volume is the sum of all space to be evacuated in a system. This includes the suction cups, the vacuum line, and any ancillary volume between the area of application and the vacuum pump (a filter, for example). The greater the volume, the more time required to evacuate to the appropriate vacuum level.
Both porosity and leakage have a major impact on the success of a vacuum application. Products such as corrugated boxes are not solid in construction; they have pores which allow ambient atmosphere to leak into the system. The greater the leakage, the more flow required to achieve a specific level of vacuum — and to maintain that level. A larger pump with more flow is necessary to compensate for porosity. Other leaks should be dealt with by eliminating the source.
Remember, it is best to design for the lowest level of vacuum that will do the job. Energy consumption increases exponentially as vacuum level goes up. To increase the level of vacuum from 18- to 27-in. Hg requires ten times as much energy. And the higher the level of vacuum at which the system operates, the more susceptible it becomes to operating problems due to leakage and porosity.
Ancillary components also are factors in in the vacuum-system equation. Filtration is desirable in almost any vacuum application — particularly if there is the potential for contamination entering the pump. In most vacuum systems, contamination in the pump is the main cause of failure. Adequate filtration can prevent major repairs and lessen downtime.
Some simple controls can enhance many vacuum systems. A vacuum switch that signals a valve once the pre-determined level of vacuum is achieved is one of these. A mechanical pump might require a vacuum valve between the pump and working system that closes once a certain level of vacuum is reached. The pump continues to operate, but the valve shifts, opening to atmosphere. This prevents the pump from overheating by pumping against a closed valve. The valve for an air-driven system is placed on the air supply so that once the predetermined vacuum level is achieved, the pump is turned off. In this case, a check valve between the pump and tank is used to prevent leakage.