Bunting Magnetics

How to Select the Holding Assemblies You Need

Begin by asking yourself the following questions. Your answers will point you in the right direction to find the assemblies that meet your specifications. If you want further advice after reading the following prompts, we’ll be glad to help you. Just call us and describe your application.

What are your shape and size requirements and mounting restrictions?

The Magnetic Holding Assemblies in this catalog are arranged by shape and size. Once you’ve determined what limitations your design imposes and answer some other questions, you can start your search by going to the Table of Contents and looking up the shape(s) that fit. Turn to the appropriate section, and you’ll find the standard stock sizes listed for each shape.

Is the mounting surface magnetic or nonmagnetic?

Certain holding assemblies need to be insulated for maximum holding effectiveness when mounted in ferrous materials. Call for more information.

How will your assemblies be mounted?

Press-fit mountings may require special insulation in ferrous materials. Place the uninsulated holding assembly inside a tightfitting stainless steel, aluminum, or other nonmagnetic sleeve and install. For bolted mountings, you’ll be looking for tapped or thru-hole assembly designs. Nonferrous fasteners are recommended for bolt-on applications.

What is the estimated holding force required for your application? What forces, in addition to gravity, will be acting on the parts while they are being held?

If you know how much your parts weigh and what other factors are likely to be involved, you can begin to make a rough estimate of how much holding power each individual magnet will need to have. In the parts listings, you’ll find magnetic holding values for every standard assembly. These holding values have been established in controlled laboratory tests and indicate vertical pull from flat ground steel.

For example, an assembly with a holding value of 5 pounds can hold a 5-pound steel part with a flat ground surface on a vertical pull under laboratory conditions. But outside the laboratory, you need to consider more than the weight of the parts you will be handling.

The size and shape of parts, as well as their surface condition, motion, vibration, friction, holding angle, and machining forces need to be factored in. Anything that gets in the way of direct magnet-to-metal contact or creates an air gap will also create the need for more magnetic holding force.

What are the size, shape, and weight of the parts you’re handling?

Use the size and shape of the parts to estimate the number of magnetic holding assemblies you’ll need to hold each part in your application. From the number of magnets, weight of the parts, and factors such as the ones noted in this section, you can figure approximately how much each magnet will need to hold.

What is the condition of the part surfaces? Are they flat or curved, smooth or rough cast? Are they clean, oily, rusted, or painted?

Tabulated holding values are based on direct contact with flat metal. Curved surfaces or anything that reduces surface contact or creates an air gap will result in the need for more powerful holding assemblies. As a general rule of thumb, factor in 25% more holding force for parts with slightly curved, rough, or painted surfaces, as well as heavily oiled parts. Parts with sharply curved surfaces and significantly reduced magnet-tometal contact may require 100% or more additional holding power. We recommend experimenting to determine the holding values you require.

Will the parts be above or below the holding assemblies? Will the parts be held horizontally or will you need to factor in shear forces for vertical or inclined applications?

Holding values are based on vertical “pulling power.” To allow for shear forces, additional holding force may be required. Call Bunting® Magnetics for specific information.

What will the temperature of the operating environment be?

For temperatures above 180°F, select assemblies that use Alnico magnets. See the following sections on magnet materials for more temperature information.

Select the Right Magnet Material

Alnico Holding Assemblies are well-suited for high-heat applications. Alnico magnets have the widest range of temperature stability of any standard magnetic material. At temperatures as high as 840°F, they maintain about 78% of their room-temperature magnetic properties. Alnico assemblies are strongly recommended for use in temperature ranges from 180-800°F. We recommend that these magnets be mounted with nonferrous fasteners and insulated when installed in ferrous materials to avoid holding value drops of 20% or more. Graph is representative of typical Alnico Holding Assemblies Specific holding values may vary.

Ceramic Holding Assemblies provide a strong, stable magnetic field. The ceramic magnet material possesses high coercive force and resists demagnetization from heat, vibration, and electrical fields. Ceramic Assemblies are permanently embedded in epoxy plastic. They are typically more economical than Alnico assemblies and can be used in temperatures up to 180°F. Temperatures higher than 180°F can cause the standard epoxy that holds the magnet material to fail. Higher temperature epoxy is available on special order. Because Ceramic assemblies are virtually unaffected by demagnetizing fields, they are ideal for work holders on welding machines and other equipment with strong electric currents. Graph is representative of typical Ceramic Holding Assemblies Specific holding values may vary.

Rare Earth Holding Assemblies offer the ultimate in holding strength. They deliver more magnetic power per ounce than any other assembly. They are well-suited to applications where size and weight must be kept to a minimum. Rare Earth Holding Assemblies are not intended for high-temperature applications and should not be used in environments above 180°F. Temperatures above this level can cause the epoxy plastic that surrounds the magnet material to fail. Graph is representative of typical Rare Earth Holding Assemblies Specific holding values may vary.

Pole Plates Concentrate Magnetic Energy

The pull of conventionally magnetized strips can be increased and directed with a variety of iron or steel pole pieces. The metal forms paths of high magnetic conductivity, concentrating the total magnetic force where it is most useful. Four of the most common types of pole plates are shown, along with the direction of maximum magnetic force each provides.

Order by part number after selecting the type, size, and quantity you need.

Orders for custom sizes and small quantities are welcome.

Channel Conventional Magnetization	Sandwich Conventional Magnetization	Back Plate Conventional or Multipole-Both Sides Magnetization	Disk Conventional Magnetization

Holding Assemblies for Spot Welding Applications Ceramic Holding Assemblies resist demagnetization by vibration and electrical fields. They are ideal for use as work holders on welding machines and other equipment with strong demagnetizing fields. Cylindrical Ceramic Holding Assemblies can be installed by a simple press fit with only a slight reduction in holding values.

1. Install assemblies so that the demagnetizing field is parallel to the pole pieces, as illustrated.
2. Make sure that at least 50% of the holding assembly’s length is outside the maximum demagnetizing field.

Mounting Magnets for Hold or Shear When you need to hold parts securely in place and minimize accidental slippage, be sure to mount holding assemblies so that their pole pieces are at right angles to the strongest force acting on the workpiece. In most cases, the main force will probably be gravity. So the pole pieces should be mounted parallel to the ground, as in Illustration A. The arrow indicates the pull of gravity. To make part removal easier, mount the assemblies with their pole pieces parallel to the direction of release motion. This orientation facilitates part movement and removal.