What is in this article?:
- Getting a grip on vacuum-handling systems
- Mounting elements
Here’s a step-by-step process for designing a cost-effective vacuum system.
Normally, the customer specifies how the suction pads are mounted. However, there may be special reasons which make a specific mounting element mandatory in certain cases, namely:
• Uneven or sloping surfaces. The suction pad must be able to adapt itself to the slope with a flexible mounting.
• Differing heights or thicknesses. The suction pads must be spring-mounted to compensate for workpieces of varying heights.
In the example, steel sheets are stacked on a pallet. If the sheets are larger than the pallet, they may droop at the ends. This means the suction pads must compensate for height differences and slope angles.
Two mounting elements solve the problem. A 0.25-in. Flexolink flexible mounting element lets the suction pad adjust for sloping workpiece surfaces. And 0.25-in. spring plungers with a 75-mm stroke cope with the hanging ends of the steel sheets. Always ensure that mounting elements can be screwed onto the suction pads — that is, the threads match and are the same size. Also note the load-carrying capacities of the mounting elements, found in the manufacturer’s technical literature.
Hose and distribution
The size of the vacuum hose must match the suction pads being used. As with other components, recommendations for the cross-sectional size of the vacuum hose can be found in manufacturers’ technical data. For our example, a vacuum hose with a 6-mm ID provides sufficient flow.
Likewise, we must select a distributor to match the hose and number of suction pads. Here, we’ll use a ¼-in. hose with an ID of 6 mm and a ¼-in. thread; a VTR 9-station distributor with a ¼-in. thread, nine inputs, and one output; and three ¼-in. sealing screws to seal the open ports.
Choosing between an ejector, pump, or blower to produce the vacuum depends on various factors, as shown in the Generator selection table. These include:
• material is porous or air-tight.
• power supply is electricity or compressed-air.
• restrictions on size and weight.
• required cycle times. Use ejectors for short cycle times; pumps or blowers for long distances between the vacuum generator and the suction pads.
Our example requires short gripping and release times, so vacuum should be generated with an ejector, particularly because the workpiece material is air-tight. First, the vacuum generator size is calculated. The values in the Suction Capacity Table give general ranges, but a more-precise number for 95-mm suction pads is 15 lpm. The recommended suction capacity is for a single suction pad and valid only for smooth, air-tight surfaces. For porous surfaces, experts recommend conducting a suitable test before selecting the vacuum generator.
Calculate required suction capacity V (in m3/hr or lpm) based on:
V = n × VS
where n = number of suction pads and
VS = required suction capacity for a single suction pad. Thus,
V = 6 × 15 = 90 lpm.
In this case, a good choice is a SCP compact ejector with a suction capacity of 116 lpm.
Solenoid valves are only needed if a compact ejector does not have integrated valves. In a complete vacuum circuit, solenoid valves control the “grip” (vacuum on) and “release” (vacuum off) functions. They are normally used in vacuum circuits where a pump or blower generates the vacuum. Selection criteria for solenoid valves include:
• suction capacity of the vacuum generator.
• available control voltage.
• operating mode of the solenoid valve (NO or NC).
Nominal flow rate of the solenoid valve must always equal or exceed the vacuum generator’s suction capacity. In this case, the solenoid valve should have a nominal flow rate ≥ 116 lpm. However, because the SCP 20 compact ejector (selected in the previous step) has integrated valves), no separate valves are needed in this example.
Switches and manometers
Vacuum switches and manometers are normally selected based on application requirements and switching frequency. Available functions include:
• adjustable switching point.
• fixed or adjustable hysteresis.
• digital or analog output.
• status LED.
• display with input keypad.
• threaded, flange, or plug-in tube vacuum connection.
• supply-voltage and signal connections via cable or screw-in connectors.
Finally, we need to ensure that the system, as designed, meets the required pick-up time of < 1 sec. First, determine the total volume to be evacuated VG, which is the sum of the volumes of all the components. This includes:
Suction pads: 6 x 32 = 192 cm3.
Mounting elements: 6 × 9.5 = 57 cm3.
Vacuum hoses: 6 × 43 = 258 cm3.
Distributor volume: 1 × 38.5 = 38.5 cm3.
Total volume, VG = 545.5 cm3, or 0.000546 m3.
Also, don’t forget volumes of any filters or solenoid valves that may be used in a system.
Knowing the total volume, we can calculate the evacuation time, t, from:
t = [VG × ln (Pa/Pe ) × 1.3]/V
where ln = natural logarithm;
Pa = initial absolute pressure (1013 mbar);
Pe = final absolute pressure, (mbar); and
V = suction capacity of the vacuum generator (m3/hr).
For a vacuum generator producing a 60% vacuum, Pe is about 400 mbar absolute. Therefore:
t = [0.000546 × ln(1013/400] × 1.3)/6.95, or
t = 0.0000949 hr = 0.34 sec.
Results show the system as constructed falls well within the application specifications.
The vacuum components selected should make for a well-designed system that operates reliably. However, even if you are confident that the results of the system-design are correct, experts still recommend carrying out tests with original workpieces, to be on the safe side. Nonetheless, the theoretical system design gives a good idea of the general parameters for the intended application.
Markus Schmider is Product Manager and Applications Engineer at Schmalz Inc., Raleigh, N. C. For more information, call (919) 713-0880, or visit www.schmalz.com.