What is in this article?:
- Handling vacuum design
- Flexibility and energy efficiency
Material handling applications present interesting challenges for vacuum systems.
Designing or implementing a vacuum system for material handling applications is an important and often costly expenditure. System designer may receive mixed messages, making the right choice unclear. Engineers should consider the following design parameters: reliability, product safety, efficiency, response time, flexibility, and maintenance issues.
A centralized system consists of one vacuum source, usually mounted remotely, that provides vacuum to multiple use points. This occurs in varying degrees, such as a plantwide supply of vacuum provided by a single central system, individual machines having their own dedicated vacuum pump, or a system where a machine may have pockets or cells of multiple cups operating from a single vacuum pump. Centralized systems are a common design and have some advantages: ease of design and installation, ability to use a vacuum sensor for part presence, no extra hose/line needed for blow-off, and little weight added to an end-of-arm tool setup.
A decentralized system, Figure 1, locates the vacuum pumps closer to the points of use, and can range from a zoned system, where groups of vacuum cups that work together are isolated, to a system where each cup is completely independent of the others. Newer technology and advances in pumps (such as more compact designs) allow for a vacuum generator to be installed at each vacuum cup.
The value of vacuum flow
Vacuum flow is important to understand when choosing a vacuum system, yet it is commonly neglected. As seen in Figure 2, a vacuum pump has its highest flow as it operates at or near atmospheric pressure. Vacuum flow then decreases as the vacuum level increases, while the air within the system is evacuated.
When a vacuum cup first makes contact with a part, the flow creates the initial grip that securely grabs the part. Leaks in the system, such as those produced when handling porous parts or materials with a textured surface, rely on flow to maintain vacuum levels. Some designers in the packaging industry recognize the importance of vacuum flow and view it with greater importance than vacuum level. The flow of a system also determines response — and therefore cycle — time.
Although it is easy to understand the downside of restricting vacuum flow, many vacuum systems do restrict flow, particularly in material handling applications. For example, most companies that use centralized systems channel the vacuum flow through tubing and manifolds. These restrictions are probably the greatest factors in reducing system performance and reliability.
Conductor size: a Catch-22
Flow restriction also creates the need to oversize vacuum pumps in order to compensate for line losses. Of course, installing larger pumps increases the system's energy consumption with no additional benefit at the cups. Tubing and manifolds in a centralized system also create additional volume that needs to be evacuated, and then returned to atmospheric pressure during each cycle. Evacuation and release time combined determine the cycle time for a handling application.
Thus, it may seem logical to simply use smaller tubing to decrease system volume and create faster cycle times. However, this only impairs system performance by creating pressure drops and reducing available flow. The resulting flow loss can mean increased response times. Larger size vacuum tubing actually increases system performance by allowing the maximum amount of flow to pass.
With pressure drop through the system caused by restrictions, the vacuum level seen at the pump may not be the same as that seen at the cup, especially with porous objects. It can be misleading to assume that the vacuum level present at the pump is the same as the vacuum level present through the entire system. This causes erratic performance and troubleshooting problems. In fact, vacuum flows and levels at each cup will likely be different if tubing lengths are different, based on their proximity to the pump.
A decentralized approach, with little or no vacuum tubing, minimizes or eliminates the effects of line losses and pressure drops. Higher flows are realized at the cups, and cycle times are decreased due to increased flow and reduced volume. The increased flow adds to the reliability of the system and gives a higher safety margin. Very often, the pumps can be downsized to provide similar performance, and less operating energy will be expended.
Reliability, safety, and maintenance issues
The reliability and safety of a vacuum system is critical — problems may includedropping parts or missing picks. With a centralized design, all of the cups are tied together into one volume, and therefore, when a low flow or low vacuum situation occurs, it affects all cups in the system. For example, if one cup is damaged, or leakage is otherwise present, the resulting vacuum loss will be seen at every cup in the system. Because symptoms occur throughout the entire system, troubleshooting such an event is more difficult and identifying the cause requires time and effort.
Conversely, in a decentralized system, each pump-and-cup combination works independently of — and has no influence on — the others. A leak at one cup can be readily identified and fixed, and will have no bearing on the capacity of the rest of the system. Safe product handling is not as great of an issue with leaks, and downtime due to troubleshooting and maintenance is lower.