What is barometric pressure?The layer of gases comprising our atmosphere blankets the Earth and exerts a pressure on all objects in relation to depth of the layer at the point of observation. The instrument that measures atmospheric pressure is named a barometer. Thus, atmospheric pressure is also known as barometric pressure. A barometer measures pressure via the displaced height of a liquid which is usually mercury (Hg). The common unit of measurement of vacuum is inches of mercury (inHg) or millimeters of mercury (mmHg).
What is vacuum?The term “vacuum” denotes that a pressure is less-than atmospheric. So, it is negative in relation to atmospheric pressure. Vacuum is created in a sealed chamber by evacuating (pumping out) air molecules which depresses the pressure inside the chamber relative to atmospheric pressure outside of the chamber. The terms “shallow vacuum” and “deep vacuum” are descriptive terms for the magnitude of reduction in pressure (vacuum).
What’s the difference between gauge pressure and absolute pressure?Gauge pressure is measured using atmospheric pressure as a reference. Gauges indicate whether pressure is positive (above) or negative (below) atmospheric pressure. Absolute pressure is always a positive value and Torr is the normal unit of measurement and is numerically equivalent to mmHg. For example; if barometric pressure is 760 mmHg (29.9 inHg) and the vacuum chamber measures 381 mmHg (15 inHg), the equivalent absolute pressure is 760 minus 381, which is 379 Torr. The correct way to differentiate between the two scales is to add an “a” for absolute unites (psia) and a g for gauge units (psig). Common practice assumes that units without the “a” are gauge units. Standard atmospheric pressure is 14.7 psia which is the same as 0 psig or psi.
For industrial vacuum systems, we are mainly concerned with the difference in pressure between the atmosphere outside of a vacuum cup and the reduced pressure (vacuum) within the cup. This differential determines the force which can be developed and thus allows work to be done. Vacuum gauges handily read this pressure differential directly, eliminating unnecessary conversion from an absolute measurement.
What’s the difference between SCFM and CFM?In relation to compressed air, CFM (cubic feet per minute) is the flow of air at a specific air pressure. CFM values cannot be directly compared unless they are all for the same pressure. For example: pump air consumption figures at 72 psi, 80 psi, and 87 psi cannot be compared to determine which pump consumes the most or least compressed air.
SCFM (standard cubic feet per minute) is the flow of air converted to standard conditions of 14.7 psia and 60F. Since the volumes are at the same standard atmospheric pressure, they can be directly compared even though the air consumption figures are provided at different air pressures of 72 psi, 80 psi, and 87 psi. Normally, pump air consumption figures are given at the optimal operating pressure for a specific pump. Any vacuum pump manufacturer who publishes a range of operating pressures (45 – 87 psi) with a single air consumption figure (2 scfm) is being deceitful. Air consumption for a specific pump will increase as air pressure increases. For example: if air consumption is 2.0 scfm at 45 psi, it will be about 3.4 scfm at 87 psi.
Pick and Place
Porosity leakage is a specific form of leakage that is caused by the workpiece being made of a material, such as corrugated cardboard, where air flows through pores when subjected to vacuum. Other system leakage sources can be eliminated or minimized but porosity leakage must be addressed during the system design stage as it can consume pump flow capacity to the point where an adequate vacuum level cannot be achieved for a properly functioning system. The equation force equals pressure (vacuum) times area (F = P x A) works well as a starting point for a non-porous application and a good design pressure range is 18 inHg (9 psi). Total area of the vacuum cups lifting the workpiece is the only variable that would increase force. However, porosity flow increases in proportion to the area exposed to vacuum so the vacuum pump must be sized so it can overcome the increased porosity flow to achieve the desired 15 – 18 inHg vacuum level range. This principle is illustrated by the following example where doubling the cup area only yields 32% greater force.
What is leakage or porosity?
On a non-porous part, VG18-10L pump @ 72 psi air supply develops 24.6 inHg.
On a porous, corrugated box:
Notice that vacuum level decreases from 24.6 inHg to 18.3 inHg when changing from a non-porous part and decreases further to 12 inHg when the same pump is used with two vacuum cups on a porous part. The decrease is caused by a demand for increased vacuum flow that the pump cannot supply to overcome the porosity leakage. The solution is to use a larger capacity vacuum pump.
How does vacuum flow affect vacuum level?Vacuum pumps (generators) can produce high vacuum flow or deep vacuum. They cannot generate both at the same time. Deepest vacuum occurs when vacuum flow is zero and the highest vacuum flowrate occurs at atmospheric pressure (zero vacuum). Graphing flow vs vacuum level at a specific air supply pressure produces a pump performance graph that is very useful for vacuum system design. The area under a pump performance graph from atmospheric pressure (zero vacuum) to a specific vacuum level determines the time it will take to evacuate a system volume to that vacuum level. This is called “evacuation time.” Faster system evacuation time requires a more powerful pump which has a greater area under the performance curve and will use additional energy in the form of air consumption.
The main considerations for selecting a vacuum pump for a sealed (non-porous) pick and place system are the required vacuum level and time; first to evacuate the system to the design vacuum level, then time to release the workpiece by dissipating system vacuum. If cycle rate is unimportant, a small-capacity pump can suffice. Unassisted release time will take about twice as long as the system evacuation time. A compressed air assisted release (blow-off) will greatly reduce release time.
Vacuum level and time are also important for porous systems but pump size is still the most important factor. If the pump is incapable of producing enough vacuum flow, the design vacuum level target cannot be reached. The effect of porosity leakage is apparent when a system demand graph is superimposed over a pump performance graph.
Why should I design systems for 18 inHg?A vacuum pump (generator) that is 85% efficient can produce a vacuum of 25.4 inHg when tested at an elevation of zero feet (St. Louis, MO) when atmospheric pressure is 29.9 inHg. However, that same pump can only produce 21.1 inHg at an elevation of 5,000 feet (Denver, Co or Mexico City) where normal atmospheric pressure is only 24.9 inHg. If a vacuum system on a machine was designed to require 25 inHg, it would work marginally better in St. Louis but would not work in Denver. If the same system was designed to operate at 18 inHg, it would work at either location and would have a greater margin of safety in St. Louis even if the atmospheric pressure drops due to weather conditions.
It is foolish economy to design a system that requires deep vacuum level that is on the bleeding edge of pump performance rather than design for a vacuum level that can be easily attained which will provide trouble-free operation.
How do pick and place vacuum systems differ from vacuum cleaners?Electric vacuum cleaners employ a motor-driven impeller that reduces internal pressure relative to the atmosphere. This creates a vacuum producing high air flow rates within the pick-up head and the connecting hose. The direction of flow is always from high pressure to low pressure which causes atmospheric air to rush into the pick-up head carrying dust and debris along with it into the filter bag. If the hose end is completely blocked, there is no flow and no dust or debris can be picked up even though vacuum will be at the maximum level attainable for that machine.
A pick and place vacuum system should be controlled so that the vacuum source (pump) is applied after the vacuum cup lips are in sealing contact with the workpiece being picked up. This minimizes the airflow into the system. If the vacuum source is applied as the end of arm tooling (EOAT) approaches the workpiece, the system becomes a vacuum cleaner that sucks in airborne particles, dust, and debris lying on the surface of the workpiece. This “Hoovering” effect will increase the need for maintenance or result in system performance problems due to ingested debris.
What is duty-cycle and how does an energy saver work?Duty-cycle for a pick and place system is the percentage of vacuum-on time vs total cycle time.
If a complete cycle, including dwell time, is 20 seconds and the vacuum on time is 5 seconds, then the duty-cycle is 25%. Catalog air consumption rates for air-powered vacuum pumps are for continuous operation. Average air consumption is calculated by multiplying the catalog value by the system duty-cycle. Continuing the example, if catalog air consumption is 4 scfm and duty-cycle is 25%, the average air consumption is 4 X 0.25 = 1 scfm.
An energy saver control will cycle the vacuum pump on and off to maintain vacuum above a specified level. A vacuum check valve (non-return valve) holds vacuum within the system while the pump is turned off. A vacustat turns the pump back on when the system vacuum decays to a preset level. Since the vacuum check valve holds the vacuum within the system, an air-assisted blow-off circuit must be used to dissipate system vacuum to release a workpiece.
An energy saver control can only affect the vacuum on time portion of the total cycle to further reduce average air consumption and the system must be perfectly sealed. Energy saver controls add cost and complexity to the system and must be frequently maintained because any leakage from any source will defeat the energy saver control. Because the duty-cycle already reduces energy consumption, energy saver controls are not practical for pick and place system but can be beneficial for systems where vacuum must be held for long periods of time.
How do I figure out system evacuation time?To calculate system evacuation time, the void volume of all components (tubing, manifolds, vacuum cups, etc.) must be added together to arrive at the total system volume. Vacuum cup volumes are provided in a table next to the dimensional drawing. Just multiply the number of vacuum cups in the system by the figure given. Vacuum pump tables for small pumps give the time in seconds to evacuate a 100 in3 volume and large pumps figures are for a 1,000 in3 volume. Actual system evacuation time is proportional. If the table shows that it takes 7.5 seconds to evacuate 100 in3 to 18 inHg and your system only contains 12 in3, the system evacuation time is (12/100) x 7.5 = 0.9 seconds. If that time is quick enough, the pump or any similar pump will provide adequate performance. If the workpiece being picked up is porous, don’t forget to verify that the pump produces enough vacuum flow to overcome porosity leakage.
Air-powered vacuum pumps are also called vacuum generators or ejectors regardless of the number of stages. A single-stage vacuum pump consists of a first nozzle that is shaped to increase the velocity of compressed air passing through it and a second nozzle with a diffuser section where the airflow can expand before exhausting to atmosphere. Usually, the second nozzle is machined into a body having air supply, vacuum, and exhaust connection ports so the complete pump consists of only two parts. The geometry of the two nozzles determines the air supply pressure where maximum (deepest) vacuum will occur or whether the pump is tuned to achieve maximum vacuum level or for high vacuum flow at a lower vacuum level. Basically, a single-stage vacuum pump can be tuned for deep vacuum or for high vacuum flow. It cannot be tuned for both. The main advantages of single-stage vacuum pumps are lower cost and a high tolerance for ingested particles.
What’s the difference between multi-stage and single-stage vacuum pumps?
Multi-stage vacuum pumps can provide both deep vacuum and high vacuum flow by adding additional nozzles in series with the first stage nozzles to allow the airflow to expand further in each stage extracting more energy from the compressed air. The extra stages produce additional vacuum flow but do not affect maximum vacuum level or air consumption. Maximum vacuum level and air consumption are still determined by the first stage nozzles. Pump construction is more complicated because each stage must be isolated from the others by valves which close off as increasing vacuum level causes the stage to become ineffective. For example: a second stage could increase vacuum flow in the range from 0 inHg to 14 inHg while a third stage would contribute increased vacuum flow from 0 inHg to 4 inHg and the fourth stage would provide increased vacuum flow from 0 inHg to 1.5 inHg. The increased cost of manufacturing pumps with additional stages must be weighed against the diminishing returns of additional stages to overall pump performance.
Multi-stage vacuum pumps reduce system evacuation time because of greater vacuum flow and can overcome greater porosity leakage than single-stage vacuum pumps. Because the compressed air expands within the pump as more energy is extracted, multi-stage pumps are also quieter. If greater pump vacuum flow capacity is necessary, one or more sets of nozzles can be arranged in parallel. This is often done without enlarging the body envelope. On the other hand, single-stage pumps must be made longer and longer as flow capacity is increased and they become extremely loud.
The size and shape of the first nozzle determines air consumption at any given air pressure. This said, air-powered vacuum pumps will consume the same amount of compressed air when they are free-flowing at no vacuum as they do when dead-headed at zero vacuum flow and maximum vacuum.
How do electric motors differ from air-powered vacuum pumps?Electric motors can only be stopped and started a few times per hour or they will overheat and fail. Generally, electric vacuum pumps cannot be deadheaded (no vacuum flow) because the pump mechanism will overheat and fail. For these reasons, electric pumps must run continuously which consumes energy. Electric vacuum pumps are heavy and cannot be mounted on the end of arm tooling (EOAT). Because of this, a large diameter hose is routed along the arm to the tool and vacuum on/off is controlled by a large, vacuum-rated solenoid valve that also unloads the vacuum pump to keep it from overheating. Electric vacuum pumps should be considered for systems where vacuum must be maintained for long periods of time or when a compressed air source is unavailable.
Air-powered vacuum pumps are compact and lightweight allowing them to be mounted directly on the EOAT to improve system response time which eliminates the need for large vacuum hoses. The vacuum pump can be turned on by a small, three-way air valve whenever vacuum is required and system vacuum will automatically vent to atmosphere whenever the pump is turned off which makes the system control very simple. Air consumption is minimized by the duty-cycle to improve energy efficiency. In general, air-powered vacuum pumps should be strongly considered for any pick and place system where compressed air is available.
Pump performance graphs depict how much vacuum flow is available at all vacuum levels from zero vacuum to the deepest vacuum that can be attained at a specific air pressure. Maximum vacuum flow is produced at atmospheric pressure (no vacuum) but is of no practical use in an industrial pick and place vacuum system. It may look like an impressive number, but outside of misleading marketing, it serves no purpose. Vacuum flow decreases as vacuum level deepens until flow becomes zero at the deepest vacuum attainable at that air pressure. Since larger multi-stage pumps are constructed using several set of nozzles in parallel, the performance graph is scalable. For this reason, vacuum flow and air consumption are multiplied by a scale factor equal to the number of nozzle sets in the pump. Evacuation time is inversely scalable and is divided by the scale factor. Single-stage pumps have just one nozzle set so their performance figures are not scalable.
How do pump performance graphs work?
A model number code is molded into the top of every vacuum cup that EDCO USA makes. This code describes the cup style and size but does not contain details about the cup material. For example: XP-B50 indicates a bellows style cup having a nominal diameter of 50 mm. Cup color indicates the rubber material it is made of
How can I tell which vacuum cup I have?
Which cup material should I use?It’s a good idea to start out with Nitrile or Ameriflex cups unless there is a reason not to. They are the lowest-cost options and work well for packaging and other room-temperature applications. For extremely high or low temperatures, Silicone is a good choice as it provides greater flexibility than Nitrile for thin workpieces such as foil bags. Duramax is our softest cup material and provides similar flexibility to Silicon but should only be used for room-temperature applications.
Can I mix vacuum cup styles or sizes in my system?Stroke is the distance a vacuum cup collapses under vacuum and differs for each size and style of vacuum cup. When mixing cups, it is necessary to install the cups so that all are fully collapsed against the workpiece when vacuum is applied. If there are slight height differences, the larger vacuum cups will smash the small vacuum cups into a workpiece which could cause damage to the workpiece and cause premature wear to the vacuum cup. Also, if some portion of the total vacuum cup force is being used to smash other vacuum cups, that force is not available for picking up a workpiece. Overall, the best practice is to use only one cup style and size per end of arm tool (EOAT).
Are EDCO vacuum cups non-marking?The term “non-marking” can have different meanings. When talking about pick and place systems, it generally means that a vacuum cup will not leave a residue from oils or solvents migrating out of the rubber compound of the cup. For example: PVC vacuum cups are notorious for leaving an oily mark on smooth surfaces when left in contact for a few hours.
Marks can still be left on a workpiece even though the cup itself is non-marking. Ambient aerosols and dust particles tend to collect on cup lips and then are transferred to the workpiece. If cups are cleaned with a product like Armor All, the residue of the cleaner will then transfer to the workpiece. Glass workpieces will also attract and become coated with a layer of dust and aerosols that will become smeared when a vacuum cup contacts the surface. None of these “marks” are caused by the vacuum cup material. Anything that collects on the vacuum cup will transfer to the workpiece being picked up.
If there is any odor or scent in the area, this means aerosols (microscopic solid or liquid particles) are present in the ambient air and can cause marking problems. Machine lubricants, cutting fluids, grease, water, coolants, tobacco smoke, after-shave, perfume, food preparation, cleaning supplies, toilets, out-gassing from stored plastic material, and various other items can be a source of aerosol in your ambient air.
What is a shear load?Vacuum cups are rated for lifting capacity when the vacuum cup is oriented vertically and the workpiece surface is oriented horizontally. Whenever the cup and workpiece are rotated 90 degrees, the workpiece weight must be carried in shear across the face of the cup. Because of this, the coefficient of friction must be considered and the cups must collapse to an essentially rigid position. That said, multi-bellows vacuum cups should not be used for shear loads. Whenever possible, it is good practice to include mechanical edge stops to prevent movement due to shear and inertial forces.
What is the coefficient of friction?The coefficient of friction is a dimensionless ratio of horizontal frictional force (resistance to movement) between two bodies and the perpendicular force (weight) pressing them together. This differs for each pair of materials and can be determined using simple equipment.
Cataloged vacuum cup load ratings are straight, vertical, clean lifts on clean, dry surfaces so the coefficient of friction is an extremely important consideration whenever the workpiece or cup surface is dusty or wet. Reduced frictional force between cups and the workpiece they are lifting can dramatically increase the size or number of vacuum cups required for an end of arm tool (EOAT) to operate safely. Whenever possible, it is good practice to include mechanical edge stops to prevent workpiece movement due to shear and inertial forces.
- Cylindrical Metric Thread – Designated with the letter M (example: M5X0.8)
- Cylindrical Inch Thread (Unified) – Designated with the letters UN (example: 10-32 UNC)
Dry Seal Thread (American System Pipe Thread):
- Conical Thread – Designated with NPT or NPTF (example: 1/4-18 NPTF)
- Cylindrical Thread – Designated with NPSF (example: 1/2-14 NPSF)
G Thread (Whitworth Pipe Thread):
- Cylindrical thread designated with the letter G (example: G 1/4-19)
Which threads are compatible with each other?Some combinations of G (BSPP) threads and NPT threads will mate if the engagement length is short. EDCO uses an odd thread description such as G 1/8” NPSF for a female thread to indicate that either 1/8” NPTF or G 1/8” male threads will mate with it. By using straight threads, the fitting shoulder will bottom out against the mating surface so that all cups are at the same installed height. If tapered threads were used, the cup installed height would vary depending on the length of thread engagement after tightening. Pipe dope sealant is usually unnecessary but will positively eliminate even small leaks. Tape sealant can shred slivers that tend to migrate and cause problem so it’s best to avoid using it.
Please note: Some thread sizes in different systems do not always fit.