September 2015 Suction cups are by no means new. However, until recently they were all based on the same design principle. This changed when a new, multi piece suction concept was brought to the market. In a special study, vacuum technology specialist Schmalz, a leader in its field, compared the performance of single piece suction cups against multi part suction cups. The result: Single piece suction cups perform significantly better in many important respects.
Single piece suction cups consist of a single elastomer part. Every suction cup has a specific geometry, which is adapted during the development stage to meet the process requirements. Multi-piece suction cups, on the other hand, are made up of different individual components such as a sealing lip and a base body. A large number of variants can thus be created using a small number of single components. Both concepts are available on the market, but which to choose?
Multi-piece suction cups can frequently be found in packaging processes. During testing, Schmalz compared these with its own 1.5 bellows suction cups from the SPB1 series which are geared toward such applications in the industry. In a bid to determine suitability for use in the best possible way, the company examined performance features that are especially important in packaging processes, including dynamics, service life and ease of product selection. “Extensive experiments enabled us to draw clear conclusions for and against the different suction concepts,” explains Dennis Kilb, industry manager for packaging at Schmalz, who coordinated the series of tests. When compared in its entirety, single piece suction cups perform better with regard to all features – despite the wide choice of variants offered by the multi-piece concept. (Picture 1)
Efficient and dynamic
To evaluate the dynamics of the suction cups, test engineers concentrated on the holding power, shear force and relationship between suction cup stroke and height. Holding power is a pivotal factor that determines how many suction cups are needed for a handling task. This force is calculated from the vacuum level and the area that has been effectively evacuated. Essentially, the higher the vacuum level and the larger the effective vacuum area are, the stronger the holding power will be. For applications in the packaging industry, the default vacuum level lies between 400 and -600 millibar. In the case of bellows suction cups, the internal diameter between the folds when suction is applied determines the holding force area. “Because of the required plug connections, the internal diameter of multi-piece suction cups was lower by an average of 40 percent in our tests,” Kilb explains. At a given vacuum level, the single piece suction cups achieved greater holding power by a factor of 272 percent. (Picture 2)
High accelerations are needed in order to facilitate fast cycle times. In this respect, suction cups must be able to withstand the lateral forces that occur. On an airtight surface, the single piece suction cups that were tested achieved lateral forces up to 175 percent higher than comparable multi-piece suction cups. For example, the 1.5 bellows Schmalz suction cup SPB1-25 in the test series showed a maximum lateral force of 23.4 newtons (N). The comparable multi-piece suction cup achieved a maximum lateral force of 9 N. “When subjected to higher lateral forces, in some cases the plug connections of the multi-piece bellows suction cups actually came apart,” comments Kilb, the chief test engineer.
This result can be attributed to the physical properties of single piece suction cups optimized for packaging applications. A thin sealing lip and a reinforced bellows guarantee maximum stability when suction is applied. During pick-up, a special suction cup geometry causes the bellows and sealing lip to interlock. Lastly, the suction cup material Elastodur ED-65 shows high friction values despite its special elasticity. What this means in practice is that single piece suction cups allow significantly higher speeds without causing the gripped piece to slip.
The lower a bellows suction cup operates, the greater its stability is. Therefore, when suction is applied, the fewest possible layers of elastomer should lie on each other, and these layers should be as thin as possible. As a rule of thumb, the thicker the walls are and the greater the height is, the more unstable the suction cup will be when operated. If, due to the nature of the work piece and its surface, a bellows suction cup with a large stroke is needed, this must be attached so that the folds completely cushion one another to ensure product stability. “In the size range that was examined between 25 and 50 millimeter diameters, the single piece suction cups developed by Schmalz boast a relationship between suction cup stroke and size that is greater than multi piece suction cups by a factor of over 200% on average,” notes Kilb. The lesser ratio achieved by multi piece suction cups is due to their mechanical interfaces, which lead to additional height but do so without improving the stroke properties. (Picture 3)
Long service life equals high process reliability
Abrasion and adhesion tendency are two factors that have a significant impact on the service life of suction cups. In a test involving 1,000,000 pick-up cycles, Schmalz determined the service life of suction cups with similar shore hardness by applying them to a suction tight surface and pulling them off under vacuum to simulate the maximum load imaginable in practice. The single piece Schmalz suction cup SPB1 showed slight abrasion at the end of the test series but remained fully operational. “Our endurance tests established a maximum service life of only 800,000 suction cycles for the multi piece suction cups,” says Kilb. After this, as a result of abrasion, increased adhesion tendency and the rapid disappearance of the multi piece suction cup sealing lip, a reliable suction process is no longer possible. In addition, clear signs of abrasion were visible on the mechanical plug connection between the bellows and the sealing lip. (Picture 4)
How easy is it to select the right product?
Single piece suction cups are developed for industry-specific handling tasks. This defines the number of variants required. Combining every series, the Schmalz product range includes 24 types of packaging suction cups for cardboard handling and 12 types for bag and pouch handling applications. Multi piece suction cups, in contrast, allow a considerably higher number of variants. “The series examined allows several thousand suction cup combinations for designers and buyers to choose from,” explains Kilb. These also cover all common packaging processes. Since the single piece suction cups that were examined manage with far fewer variants, the process of product selection is faster and less prone to error, saving both time and money.
Dynamics, service life, and ease of product selection – when these core requirements in the packaging industry are taken into consideration, the following is certain: Multiple pieces allow for more variants, but to achieve maximum performance a suction cup must be matched perfectly to a specific application. Single piece suction cups are clearly the winners in this regard.
Picture 1: Test objects: Single piece suction cups and multi piece suction cups
Picture 2: Internal diameter compared which determines holding force
Picture 3: Relationship between suction cup stroke and height
Picture 4: Service life compared
Picture 5: Multi-piece cup with tied on flexible sealing lip to prevent separation
Picture 6: Schmalz SPB4f with flexible sealing lip in a single elastomer part design
Picture 7: Justin Katz, Applications Engineer, Schmalz Inc.
About SchmalzSchmalz is a global leader in the field of industrial automation as it pertains to vacuum automation, robotic palletizing vacuum grippers, material handling vacuum lift assist devices and CNC machine vacuum clamping. The company is known for its innovative quality products, which give its customers decisive productivity improvements in their manufacturing processes, as well as its commitment to the environment and its employees. The forward looking 105 year young privately held manufacturer has continually reinvented itself with each generation and has to its credit over 450 patents to date.
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