FAQ - Vacuum technology

Vacuum technology

Vacuum technology

Why must vacuum generators always be operated with unlubricated compressed air?

If lubricated compressed air is used, dust and dirt particles from the air which is sucked in can settle in the silencer or even in the vacuum suction nozzle, thus impairing the suction performance.

 

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What effects do changes in air pressure have on the vacuum?

Air pressure drops with increased elevation. This leads to a reduction in the maximum pressure difference, which in turn means that the maximum breakaway force of a vacuum gripper will also be reduced.

 

The table below shows the properties of the same vacuum generator and suction cup at heights with a difference of 2000 m:

Height Air pressure Vacuum Absolute pressure Differential pressure
to environment
Breakaway force – Suction cup diameter 50 mm
0 m 1013 70 % 303.9 mbar 709.1 mbar 105.8 N
2000 m 789 70 % 236.7 mbar 552.3 mbar 82.4 N

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What is the difference between H-type and L-type vacuum generators?

H = High vacuum

L = High suction rate (large flow)

Vacuum suction rate

An H-type is optimised for creating high vacuums > -0.4 bar. They are suitable for use in all standard applications.

An L-type is optimised for generating a high suction rate at a moderate vacuum level up to -0.4 bar. They are can be used to great advantage with porous workpieces. The increased suction rate can deal better with leakages.

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What is an air economy circuit?

When a vacuum generator is being used without other additional functions, it requires energy for as long as "Vacuum on" is active.

If a vacuum generator with vacuum monitoring via a sensor and an integrated non-return valve is being used, the vacuum only needs to be created (and thus energy consumed) if the vacuum falls below a predetermined level.

As long as the vacuum in the system remains within the preset limit values, the vacuum generation is automatically deactivated to save energy.

An energy economy circuit like this can be controlled via a PLC. However, the control system can also be fully integrated into a vacuum generator.

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The vacuum collapses because a suction cup in the system has a leakage. What now?

The ISV vacuum security valve closes a valve seat from a certain switched flow, thus limiting the flow through the leak to a defined amount.

This prevents the vacuum from completely collapsing.

However, only a limited amount of ISV vacuum security valves can be used independently of the suction rate of the vacuum generator.

ISV

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Why should open silencers be used for vacuum generators?

Over time, a closed silencer becomes contaminated from inside with dirt particles which are larger than the porosity of the body of the silencer. As the contamination increases, back pressure gradually develops in the vacuum generator (vacuum performance reduced – maintenance necessary).

With an "open silencer", dirt particles can be sucked in via the laval nozzle, leaving the silencer again with the air flow.

Advantages: Reliable, safe to operate and maintenance-free.

 

offener Schalldaempfer

offener Schalldaempfer 11

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How can the vacuum be monitored?

There are a number of different solutions for monitoring vacuums.

  • Vacuum gauge (analogue display), e.g. VMA
  • Vacuum switch (mechanical / electrical switch), e.g. VPEV
  • Vacuum switch (electrical switch), e.g. SDE
  • Pressure sensors integrated into the vacuum generator, e.g. OVEM

 

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What are the advantages of decentralised vacuum generation?
  • Vacuum generation only when needed and directly on the grip area (economic efficiency)
  • Minimum line / tube length and maximum efficiency
  • Fast evacuation and cycle times
  • Controlled setting down due to reliable ejector pulse
  • Due to the short tube lengths, the evacuation times can often be achieved with smaller vacuum generators (lower air consumption)

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What influence does the vacuum and pressure supply tube with its connectors have on the overall vacuum system?
  • The size of the pressure supply tube must fit the air consumption of the vacuum generator.
  • The size of the vacuum tube must fit the suction cup being used.
  • The distributor must be selected to fit the tube and the number of suction cups which have been chosen.
  • The size of the vacuum tube must fit the vacuum generator being used.
  • Long, slim tubes very often form a bottleneck and reduce the efficiency of the vacuum generator. The result of this is a higher input (air consumption of the vacuum generator), but a lower output (increased evacuation time).

  

 

Venturi nozzle 

 

Formula for nominal diameter (mm)

P1 connection (1) ≥ 2 x ≥ Ø Venturi nozzle

V connection (2)   ≥ 3 x ≥ Ø Venturi nozzle = high vacuum

V connection (2)   ≥ 4 x ≥ Ø Venturi nozzle = High suction capacity

Valid for tube length <= 0.5 m

For tubing lengths of > 0.5 m select a larger diameter.

 

 

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How are the holding force and break-away force calculated?

In order to determine the required holding force, you need the calculated mass, the system acceleration and the friction coefficient.

 

The holding force required depends on the load situation. Below are the three main load situations:

 

  • Situation 1: Suction cup in horizontal position, movement in vertical direction (best case)
  • Situation 2: Suction cup in horizontal position, movement in horizontal direction
  • Situation 3: Suction cup in vertical position, movement in vertical direction (worst case)

 

With most pick and place cycles you will find multiple different movement situations. The worst case with the greatest theoretical holding force must always be used for the following calculations.

 

The workpiece weight and acceleration are required in order to calculate the holding force.

 

Situation 1

Suction cup in horizontal position, movement in vertical direction (most favourable case)

Calculation formula 1

 

Situation 2

Suction cup in horizontal position, movement in horizontal direction

Calculation formula 2

 

Situation 3

Suction cup in vertical position, movement in vertical direction (least favourable case)

Calculation formula 3

 

FH = Theoretical holding force of the suction gripper (N)

m = Mass (kg)

g = Acceleration due to gravity (9.81 m/s²)

a = Acceleration of the system (m/s²)

Warning: Note the emergency off acceleration.

 

S = Safety factor

= at least 1.5 for linear movement

= at least 2 for rotary movement

µ = Friction value

 

Empirical friction value (surface)

Oily µ = 0.1

Wet µ = 0.2 to 0.3

Rough µ = 0.6

Wood µ = 0.5

Metal µ = 0.5

Glass µ = 0.5

Stone µ = 0.5

Warning: These values are average values and should be verified for the workpiece in question.

 

Empirical acceleration values

Electric spindle 6 m/s²

Electric toothed belt 20 m/s²

Servo-pneumatic 25 m/s²

Pneumatic 30 m/s²

Pneumatic rotary drive 40 m/s²

 

 

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Which workpiece properties is it important to observe when planning a vacuum application?
  • Weight
  • Porosity (porous or hermetically sealed)
  • Surface (smooth/rough)

 

The weight and surface play an important role when calculating the holding force and the break-away force (force, friction coefficient).

The porosity of the workpiece must be considered when selecting the required performance (leakage air and consequent vacuum collapse).

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Does Festo also do filters for vacuums?

Yes, our VAF-DB vacuum filter is available in the following sizes: ¼", 3/8", and ½"

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The definition of vacuum

Vacuum is the state of a gas when its particle density is lower than that of the earth’s atmosphere at sea level. In general, pressure in pneumatics is specified as positive or gauge pressure (relative to the ambient pressure). This in turn means that vacuum is always quoted as a negative value (measured down from the ambient pressure level). The units generally used are bars or millibars (mbar) (1 bar = 1000 mbar). This unit is derived from the SI unit for pressure, pascal (Pa). Previously common units of pressure such as Torr, Kp/cm2, at, atm, mWS and mmHg should no longer be used.

 

 

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How does a vacuum generator operate?

 

Vacuum generator

 

The Festo vacuum generator operates on the venturi principle. The compressed air flows from the pressure supply port into the ejector. The constriction in the venturi nozzle increases the flow velocity of the air to a supersonic speed. After exiting the venturi nozzle, the air expands and flows through the receiver nozzle into the outlet port (silencer). In the process, a vacuum is created in the chamber between the venturi and receiver nozzles, which causes air to be drawn in from the vacuum port. The vacuumed air and exhaust air both leave through the outlet port (silencer).

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How large does the hose diameter have to be when VN-... vacuum generators are used?
Vacuum generator Supply port:
tubing outside diameter
Vacuum connection,
high flow rate:
tubing outside diameter
Vacuum connection,
high vacuum:
tubing outside diameter
VN-05 4 4 4
VN-07 4 6 4
VN-10 4 6 6
VN-14 6 8 6
VN-20 6 12 8
VN-30 10 16 12

 

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At what speed does the air flow through a vacuum generator?

The air velocity in a vacuum generator reaches a value greater than Mach 3.

  • Mach 1 = speed of sound
  • Mach 2 = twice the speed of sound
  • Mach 3 = three times the speed of sound etc.

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From which type of material are Festo suction cups made and what are they used for?

 

Suction cup material Colour Temperature range [°C] Resistance to wear Workpiece
Nitrile rubber (N) Black -10 ... +70 ++ Oily and smooth
Polyurethane (U) Blue -20 ... +60 +++ Oily, smooth and rough
Silicone (S) White, transparent -30 ... +180 + Food, hot and cold
Fluorine rubber (F) Grey -10 ... +200 + Oily, smooth and hot
Nitrile rubber, antistatic (NA) Black with white dot -10 ... +70 ++ Electronics, oily
Polyurethane, heat-resistant (T) Brown transparent -20 ... +60 +++ Oily and rough

 

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Which suction cup fits which workpiece?

 

Standard suction cup

For flat, lightly corrugated and curved surfaces

 

 

 

Extra deep suction cup

For round and curved workpieces

 

 

 

Oval

For narrow, oblong workpieces such as profiles and pipes

 

 

 

Bellows

  • For inclined surfaces

  • For curved, round surfaces, flexible workpieces with large surface areas
  • For sensitive workpieces such as glass bottles or bulbs
  • Cost-effective height compensator

 

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What does the air supply time specify?
The air supply time is the time required to reduce a vacuum of 6 bar to a residual vacuum of -0.05 bar for a volume of 1000 cm³. (Air is supplied backwards through the silencer via the laval nozzle.)

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What does the evacuation time specify?
The evacuation time is the time required to evacuate a volume of 1000 cm³ to a particular vacuum.

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What lift do the vacuum generators cover?

Read the further information about the various suction types:

Vacuum suction gripper ESS, oval

Vacuum suction gripper VAS/VASB

Vacuum suction gripper, standard

 

 

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Can the filter cartridge of a VAF-... vacuum filter be replaced?
No, it is not possible to replace the filter cartridge of this component.

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Does the VADMI-...LS-... vacuum generator also work with cables from other valves?

No, the vacuum generator with air saving function requires the special cable set supplied.

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Does the VADMI-... vacuum generator also work with the cables for the VADMI-...-LS-... vacuum generator?

No, the VADMI-... vacuum generator does not work with the cables for the VADMI-...-LS-... vacuum generator with air saving function.

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Are air reservoirs approved for vacuum?
All air reservoirs from the stainless steel series (CRVZS-...) can also be used for vacuums down to -0.95 bar.

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