We are not the definitive source for “everything you ever wanted to know about vacuum but were afraid to ask”! However, we have compiled some basic information about vacuum and how it is utilized for materials handling. There is a variety of other sources on the Internet and in your local library that may be of additional help. If you have information that you feel would benefit others, please don’t hesitate to let us know.
WHAT IS VACUUM?
Strictly defined, a vacuum is a space from which all matter, including air and other gas, has been totally removed. A vacuum exists whenever the pressure within the space is less than the pressure that surrounds it. A vacuum is created when air is removed from within a vacuum chamber. As air is removed, fewer air molecules are present to push on the vacuum chamber walls, so the pressure within the vacuum chamber is reduced. It is impossible to create a perfect vacuum because, even in space, there are small amounts of gas present. Scientists have created vacuums with pressures as low as 10-14 mm of mercury. However, even at this very low pressure, there are still thousands of gas molecules present.
One method of creating a vacuum is to remove air from a vacuum chamber by pumping it out with some type of mechanical pump. Such pumps can operate manually or by AC, DC or compressed air power. Another method used to create a vacuum is to reduce internal pressure by increasing the size of the vacuum chamber. This method is illustrated by the small, clear plastic cups used to hold items on windows, and by cups incorporating a lever, such as those found on the base of some pencil sharpeners.
HOW A VACUUM CUP WORKS
A vacuum cup adheres to a surface due to the weight of the air (atmospheric pressure) pressing against the cup: When air is removed from between the surface and the sealing edge of the cup (the vacuum chamber), the difference in air pressure inside and outside the vacuum chamber causes the cup to adhere tightly to the surface. The weight of air outside the cup is normally 14.7 pounds per square inch (psi), or over 1 ton per square foot, at sea level! If the seal between the cup and the surface is broken, the pressure difference between the two equalizes and the vacuum cup no longer adheres to the surface.
HOW ATMOSPHERIC PRESSURE AND VACUUM ARE MEASURED
Pressure and vacuum (negative pressure) can be measured in several different units. Common values include inches or millimeters of mercury (in. Hg or mm Hg), pounds per square inch (psi), millibars (mb), kilopascals (kPa), percentage of vacuum (%) and torr. Earlier we said that normal atmospheric pressure at sea level is 14.7 psi. Other measurements of normal atmospheric pressure at sea level include: 29.92 inches (760 mm) of mercury, 1013.20 mb, 101.32 kilopascals, 0% and 760 torr. Various methods of measurement are used depending on the application. For example, slight changes in barometric pressure (measured in millibars) help meteorologists to forecast the weather. Firemen use percentage of vacuum when they draft water from reservoirs. For materials handling applications, inches (or millimeters) of mercury are often used. Vacuum is usually measured as gauge pressure, which indicates the difference between the pressure within the vacuum chamber and atmospheric pressure.
HOW ELEVATION AFFECTS VACUUM AND VACUUM HANDLING DEVICES
Just as weather affects barometric pressure, elevation affects atmospheric pressure. Both of these factors (barometric and atmospheric pressure) affect the performance of vacuum handling devices used for materials handling. As mentioned earlier, the weight of the atmosphere at sea level is about 14.7 psi. As elevation increases, the weight of the air above an object decreases. Consequently, the higher the elevation, the less pressure it exerts on the object. Up to about 6500 ft., atmospheric pressure decreases by about 1% for every 330 ft. of elevation. This effectively reduces the weight that a vacuum handling device can manage. In order to calculate the capacity of a vacuum handling device at a particular elevation, a number of factors must be taken into consideration: the degree of vacuum the vacuum pump generates at the given elevation, the size and quantity of vacuum pads, and the required safety factor.
* This formula can be used to provide a rough guide for calculating the theoretical lifting capacity at a given elevation. However, many factors are involved, including contact surface conditions and safety factors. Contact your vacuum handling Technical Sales Representative for consultation when using vacuum handling equipment at elevations above 5000 ft.
The chart below illustrates the effects of increasing elevation on an eight-inch diameter vacuum pad that normally produces a vacuum of 25″ hg. at sea level. The chart includes the Rated Capacity based on a 4:1 safety factor. ANSI Standards require the use of safety factors for many vacuum handling applications.Contact your vacuum handling Technical Sales Representative for consultation when calculating capacities for any vacuum handling application.
HOW VACUUM LIFTER CAPACITY IS DETERMINED
There are many factors that must be taken into account to determine the capacity rating for any vacuum lifting device.
A formula is used for calculating a “THEORETICAL” capacity of a vacuum pad. The result is theoretical because there are a number of factors that may influence the ability of the vacuum pad to actually lift the weight (pad size and design, load surface porosity, contaminates on the load surface, the degree of vacuum generated by the pump, etc.).
Although the formula is used as a basis in calculating capacities, other factors such as ANSI Standards (which apply differently to below-the-hook lifters handling materials in flat or upright positions, and hand-held cups and mounts), safety factors, the style of vacuum pump, the thickness and type of load surface (smooth/rough, porous/nonporous, flat/curved), etc. must be considered as well. On vacuum cups used for equipment mounting applications, the capacity is reduced when the attaching point is away from the center of the pad.
This formula and chart provide theoretical lifting capacities only. Contact your vacuum handling Technical Sales Representative for consultation when calculating capacities for any vacuum handling application.
A BRIEF HISTORY OF VACUUM
Aristotle (384-322 BC), the Greek philosopher and scientist, proposed that air had no weight or pressure, and until the 16th Century that was the scientific belief. The invention of the vacuum pump was connected to the need for a device used in chemistry, physics and physiology to remove all gas from an enclosed space. To this end, the “Father of Modern Science”, Galileo Galilei (1564-1642), was the first to design a simple vacuum pump. His first design was never tested and a second design, tested just prior to his death, apparently did not work very well.
Following a suggestion of Galileo, Evangelista Torricelli (1608-1647), a student, secretary and assistant of Galileo, in 1643 filled a 4-foot-long glass tube with mercury and inverted the tube into a dish. He noted that some of the mercury flowed out of the tube, but that about 30 inches remained, supported by nothing. This “nothing” was the first sustained vacuum and proof that air does, in fact, exert pressure. In the course of his experiments, he observed that the height of the mercury varied from day to day and deduced that these changes were a direct result of changes in the barometric pressure. Originally known as Torricelli’s Tube, his invention, measuring the variation in barometric pressure, is now known as the barometer.
After hearing of Torricelli’s tube, Blaise Pascal (1623-1662) theorized that, if mercury was forced up in the tube by the pressure of the air, the column of mercury should be shorter at higher elevations, where there is less air. In 1648 Pascal’s brother-in-law climbed the mountain of Puy-de-Dôme and reported that the mercury had dropped three inches.
Around 1650, Otto von Guericke (1602-1687) invented the first successful air pump and used it to study vacuum. In 1654, the “Magdeburg Sphere” experiment demonstrated the actual force of atmospheric pressure (14.7 psia or 1 ton/sq. ft.). Two hemispheres with roughly a 20″ diameter were joined together and then exhausted to a high vacuum. Although eight horses were hitched to each half of the sphere, they were unable to pull the sphere apart.
Chemist Robert Boyle, who was trying to prove that air is used up by both breathing and combustion, heard of von Guericke’s air pump. In need of this type of device to continue his studies, he employed Robert Hooke in 1655 to construct his air pump. Although it was based on von Guericke’s original pump, the new pump was a great improvement over the previous design.
Boyle continued his improvements of the pump with Christiaan Huygens in the 1660’s. Their improved version, which incorporated a container into which items could be placed and studied under vacuum, produced the bell jar. In the 1670’s, Huygens, along with French inventor Denis Papin, worked together on a two-way tap and valve system which allowed air to be removed continuously. These improvements resulted in the vacuum pump that was used until the late 19th century.
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