by Gary Bond
A lot of outrageous claims are made when it comes to vacuum lifting devices: If the pads are bigger, it's obviously better. And if the frame is heavier, it will obviously lift more.
Various countries and regions throughout the world have different standards and regulations regarding the design, construction and use of vacuum lifters. Some of these are legal requirements and others are developed by groups to provide guidelines and recommendations. Vacuum lifters sold in the European Union, for instance, are required to comply with EN 13155:2003, Cranes / Safety / Non-fixed load lifting attachments.
In the United States, the predominant standard is ASME B30.20-2006, Below-the-hook Lifting Devices. This is written as a consensus standard and adopted by ANSI as reasonable guidelines. Compliance to ASME B30.20 is not required by law. However, Wood's Powr-Grip uses these guidelines to evaluate products, in order to minimize risks associated with overhead lifting.
One subject commonly addressed by standards and regulations is the rated capacity. There are two primary areas related to capacity of vacuum lifters, the vacuum capacity and the structural capacity. Various design factors are applied in order to determine the appropriate capacity rating.
There are a number of considerations that influence the vacuum capacity of a lifting device. These factors include:
- pad size and design,
- the friction coefficient between the pad and the load as it pertains to vertical lifts and tilting,
- the vacuum generated by the pump,
- the thickness of load material,
- the type and condition of the load surface (smooth/rough, porous/nonporous, flat/curved, dry/wet),
- and the number of vacuum pads in use.
Wood's Powr-Grip has chosen to rate standard lifters at a vacuum level of 16 inches of mercury [-54 kPa]. This level of vacuum can be achieved with reliable, cost-effective vacuum pumps at elevations up to 6,000 feet [1828 meters]. This allows the same lifter to be used in many different locations. Some manufacturers rate their vacuum capacity at a higher vacuum, but their lifters may not be usable at the needed elevation.
When the vacuum pad is vertical, the coefficient of friction is a critical component to the vacuum capacity. Different rubber compounds and different load surfaces combine for a variety of coefficients of friction. The ASME B30.20 design factor of 4:1 for vertical vacuum ratings provides for more variation and unknowns than the 2:1 design factor required in EN13155. The design factor also allows for variations such as aging rubber, manufacturing differences, contamination on the load, and deflection of the pad under load. While some manufacturers may boast a higher load capacity, that may not necessarily be the case, depending on the design factor they use.
Although vacuum capacity is important, just adding vacuum pads doesn't always increase lifter capacity. The frame-work that holds the vacuum pads together must be adequate to handle the capacity as well. Some of the items that must be considered when determining the structural capacity are:
- Strength of materials,
- bending potential,
- and weld sizes.
The truth of the matter is that nothing is more important to the performance of a vacuum lifter than design and manufacturing done by qualified engineers with a true understanding of vacuum and structural capacity. With experience dating back to 1964, Wood's Powr-Grip strives to provide its customers reliable products manufactured with a higher design factor for increased product safety and dependability.
Gary Bond was Chief Engineer, managing the Wood's Powr-Grip Product Engineering Department. He served on the ASME B 30.20 and the BTH standards committees. He was an engineer at Wood's Powr-Grip for over 30 years.
