Electric Motor Mechanics and Manufacturing Methods--Adhesives

Designers and manufacturers will discover many benefits and opportunities associated with adhesives and related materials in the assembly and fabrication of small electric motors. Adhesives can provide design advantages, improve overall product performance, speed assembly time, and reduce costs of small motor manufacturing as indicated in Ill. 100.

This section addresses a wide range of topics. Discussion includes an introduction to adhesives and sealants and an overview of existing chemistries. It addresses a variety of adhesive applications: threadlocking, gasketing, sealing porosity, retaining cylindrical assemblies, bonding, and specialty electronic and electrical applications. The section concludes with a discussion of processing equipment and safety issues.

Introduction to Adhesives

Use of adhesives and related sealing and coating technologies in small motor design and manufacturing has increased dramatically in recent years, replacing many mechanical techniques such as clipping motor magnets and press-fitting components onto shafts. Motor manufacturers' efforts to reduce costs and improve product performance and warranty life have contributed to the growing number of adhesive applications. Adhesive suppliers have also invested significant research in developing safe and reliable products that allow small motor manufacturers to move away from hostile chemical processes.

Adhesives offer significant benefits over mechanical fastening methods. Rather than concentrating stress at a single point, adhesives distribute stress load over a broader area, resulting in a more even stress distribution. A joint bonded with adhesive resists flex and vibration stresses better than a mechanical joint. Adhesives form a seal as well as a bond, eliminating corrosion. They easily join irregularly shaped surfaces, negligibly increase the weight of an assembly, and create virtually no change in part dimensions or geometry.

ILL. 100 Small motor application guide.

It’s important to note that there is no single adhesive that will perform under all circumstances. Limitations of adhesives include setting and curing time (the amount of time it takes for the adhesive to fixture and strengthen fully), surface preparation requirements, and the potential need for joint disassembly. Proper adhesive selection and component design will often overcome these limitations.

A good adhesive is safe for the manufacturer and the end user and is environ mentally friendly. It will adhere to the substrate, but won’t attack or degrade it. A good adhesive is less costly than any mechanical fastener available. In its liquid state, the adhesive must be capable of flowing into the bond joint, and it must allow easy processing. Solidification or curing of the adhesive should occur in a predictable time frame and minimize work in process. The bonded joint must be strong enough to last the life of the assembly, and every assembly must offer consistent quality and performance.

Joint design is a critical factor in bond strength-a well-designed joint allows for the maximum possible bond area and combines mechanical locking methods with adhesive bonding. The adhesive application and curing processes must remain constant to ensure consistent performance joint after joint, assembly after assembly.

There are four types of failure that can occur in an adhesive joint. Adhesion failure occurs when the adhesive completely separates from the face of one substrate.

When the adhesive splits from itself and remnants can be found on both substrates, the adhesive joint has experienced cohesion failure.

With adhesion failure, the weak point of the bond is the boundary layer between the bonded part and the adhesive. Either the material is not suited for bonding, or there is dirt on the bonding face. In both cases, bond strength will increase with suit able pretreatment of the surface.

ILL. 101 Mechanism of bond failure: (a) adhesive failure, (b) cohesive failure, (c) combination adhesive and cohesive failure, and (d) substrate failure.

In a cohesion failure, the adhesive is overstressed by external factors- For example, temperature, aging, or stress spikes. Either the adhesive is not suitable for the application, or design changes should be made to the bonding geometry.

The third failure mode is a combination of adhesive and cohesive failure, while the fourth mode is failure of the substrate itself.

The best small motor designs that incorporate adhesives and sealants result when designers consider four important questions before specifying the final design and the mass production process.

These questions are also important when troubleshooting an existing application.

1. Does the adhesive bond to the substrate surfaces? Will the material maintain adhesion over time? Ill. 101 illustrates the various types of failure that may occur in an adhesive joint. In evaluating an adhesive design, it’s important to identify the normal failure mode. Periodic monitoring of the nor mal failure mode will help to ensure consistency. For example, if the joint is evaluated and cohesive failure is determined to be the normal failure mode, then a sudden change to adhesive failure might indicate that surface contamination has been introduced to the manufacturing process.

In addition to matching the adhesive to the specific bonding surfaces, it’s important to be aware of factors that influence wetting. Wetting is the degree to which an adhesive penetrates the surface of a substrate, spreading out and filling surface irregularities, as shown in Ill. 102. Good wetting is essential for developing reliable bonds. If wetting does not occur, the adhesive does not spread but forms a round droplet on the surface, the way water beads up on a newly waxed car, as shown in Ill. 103. Factors that affect surface wettability include substrate porosity, roughness, polarity, cleanliness, surface tension, and adhesive viscosity.

ILL. 102 Proper adhesive wetting.

ILL. 103 Improper adhesive wetting.

ILL. 104 Blind hole example of adhesive placement. (a) Wrong-adhesive forced out of bond joint by air during assembly. (b) Right-adhesive forced into bond joint during assembly.

Cleanliness is crucial to wetting. All dirt, residues, and oils should be removed from the substrates using detergents, solvents, chemical cleaning systems, or mechanical abrasion.

2. Can the adhesive be consistently dispensed into the bond joint and remain there until solidification occurs? It’s often difficult to ensure that adhesive is present in the bond joint. In small motor manufacturing, adhesive migration can cause a range of problems, from loss of electrical contact to failure of the painting processes. Ill. 104 illustrates a common problem experienced when bonding cylindrical parts into blind holes; it also illustrates the solution, which takes advantage of the flow of both air and adhesive.

3. Is the adhesive solidifying or curing? Has the adhesive solidified enough to ensure its performance in the application? It’s important that the individuals who control the manufacturing process have a basic understanding of the factors that affect adhesive curing. Frequently, in small motor manufacturing, chemically reacting adhesives are used. These products solidify or cure through a chemical reaction to form highly durable polymeric materials. For example, anaerobic adhesives, used to bond cylindrical commutators to shafts, will solidify faster at elevated temperatures or if trace amounts of copper are present on the shaft surface to catalyze the curing reaction.

4. Will the adhesive perform throughout the assembly's life? Will the adhesive meet the performance required for the manufacturing process and the end-use environment? Small motor assembly utilizes many commercially proven adhesive applications- For example, motor magnet bonding, threadlocking, cylindrical retaining, and laminate sealing and unitizing. Adhesive suppliers have created commercial formulations specifically designed to fulfill the requirements of these applications.

Design engineers who specify proven adhesive formulations in their designs can minimize the risks associated with adhesive specification. However, since all designs possess unique requirements, both short- and long-term performance testing is suggested to ensure a design's success.

To ensure a good, long-lasting bond, the properties of the adhesive must match the requirements of the manufacturing process and those of the end-user application. For example, if the adhesive used is not formulated for solvent resistance, and the manufacturing or end-use environments subject the assembly to solvents, the bond will fail.

The manufacturer should consistently test devices for a series of performance factors once the adhesive is fully cured and the assembled device is ready for end use. A designer concerned about thermal performance, For example, must consider the amount of time that the exposed part will be subjected to high temperatures. Ill. 105 graphically illustrates heat aging and hot strength. Hot strength tests are used to predict bond strength at a selected temperature. Heat aging tests are used to determine long-term durability after exposure to elevated temperatures.

Most reputable adhesive suppliers will assist in developing testing programs, and in many cases, will provide testing services at no charge.

ILL. 105 Hot strength and heat aging are both important measures. (a) Hot strength-bond tested at temperature.

(b) Heat aging-bond tested at room temperature.

Joint Design Guidelines

Understanding the basic types of mechanical stresses will help in joint design. Typical joint stresses are illustrated in Ill. 106.

_ Tensile stress. Occurs when the force on a joint is pulling the substrates away from the bondline. Tensile stress tends to elongate the object.

_ Compressive stress. Occurs when the force on the joint pushes in toward the bond line, squeezing the assembly together. A bonded joint under compression is extremely strong as long as the adhesive used is resilient and flexible.

_ Shear stress. Occurs when the two substrate surfaces slide over one another in opposite directions, with the majority of the stress concentrated at the edges of the joint.

_ Cleavage stress. Occurs when the joint is being pried open at one end.

Brittle adhesives have poor resistance to cleavage load.

_ Peel stress. Occurs when a flexible substrate is being lifted or peeled away from the other substrate, with the load concentrated along a thin line at one edge of the bond.

Engineers must have a good under standing of how stress is distributed in order to design the strongest possible joint. Two universal guidelines will help during the design process:

_ Maximize shear/minimize peel and cleavage stress. Shear stresses are distributed evenly across the bond, with the ends resisting more stress than the middle. When a cleavage or peel stress is applied to a joint, most of the stress is concentrated at one end, weakening the bond line and the life of the joint.

_ Maximize compression/minimize tensile stress. When a bond experiences a tensile or compressive stress, the joint stress is evenly distributed across the entire bond. In most adhesive bonds, compressive strength is greater than tensile strength; therefore, an adhesive joint experiencing compressive force is less likely to fail than a joint undergoing tension.

Ill. 107 shows several common bond joints that are successfully used in designing and assembling small electric motors.

Common Adhesive Chemistries

The following adhesive, sealant, and coating chemistries react to form solid bond joints. Chemically reacting adhesives have become extremely popular for small motor assembly because of their long-term durability and versatility.

Anaerobic adhesives are commonly used to bond cylindrical metal components such as fans, laminates, and commutators to armature shafts. They are also used as threadlockers and thread sealants designed to seal and minimize loosening of metal fastener assemblies. They are single-component thermoset materials which cure at room temperature when deprived of oxygen in the presence of metal ions. Anaerobics offer high shear strength, good solvent and temperature resistance, rapid curing at room temperature, and easy dispensing. These products are typically used when gaps don’t exceed 0.020 in and temperatures don’t exceed 450°F (232°C).

Two-part, mix or no-mix acrylics are thermoset adhesives that bond well to lightly contaminated surfaces and are flexible and chemically resistant. They offer fast fix ture, good adhesion to many substrates, high peel and impact strength, and good environmental resistance. The latest acrylic adhesives are rubber toughened for increased pliability and resilience. Acrylic adhesives are the most common motor magnet-bonding formulations.

Epoxies are common thermoset structural adhesives available in single component heat-curing or two-part mix systems. They offer a number of benefits, including high bond strength on a wide variety of substrates, good gap-filling capabilities, excellent electrical properties, and good temperature and solvent resistance.

Proper mixing and/or controlled handling and storage conditions are required to ensure successful use of these adhesives.

Cyanoacrylates, also known as instant adhesives or superglues, are one-part, solvent-free, room-temperature-curing thermoset adhesives. When pressed into a thin film between two surfaces, cyanoacrylates cure rapidly with excellent adhesion to most substrates. These adhesives are available in a wide variety of specialty formulations and viscosities, are easy to dispense via automated systems, and offer excellent bond strength, even after thousands of hours of exposure to temperatures as high as 250°F (121°C).

Extremely compatible with a wide variety of substrates, urethanes are available in one- or two-component systems, and cure to a flexible solid when exposed to ambient moisture. They offer excellent toughness and flexibility, even at low temperatures. Urethanes can cure at room temperature or in an oven, and offer good impact resistance. These products are typically used where gaps don’t exceed 0.100 in and temperatures don’t exceed 250°F (121°C).

ILL. 106 Types of joint stress. (a) Tensile stress tends to pull an object apart and tends to elongate an object. (b) Compressive stress tends to squeeze an object together. (c) Shear stress results in two objects sliding over one another. (d) Cleavage stress occurs when a joint is being opened at one end. (e) Peel stress occurs when a flexible substrate is being lifted or peeled from the other substrate.

ILL. 107 Common bond joint configurations: (a) cylindrical socket, (b) single lap, and (c) dado angle joint.

Silicones are rubberlike polymers which cure at room temperature. They exhibit excellent resistance to high temperatures and moisture and are well suited for out door weathering applications as sealants and caulking compounds. Silicones have superior chemical and electrical resistance, and good gap-filling capabilities, and they are excellent for bonding glass to most other substrates. However, their low tensile strength limits their usefulness in carrying structural loads. It’s important to maintain controlled curing conditions (e.g., ambient temperature and humidity) to ensure optimal results with silicones.

Many of these technologies can be adapted to employ UV or visible light to initiate curing. UV- and visible-light-curing adhesives contain photo-initiators which absorb light energy to begin the cure. This technology is excellent for manufacturing applications, as it allows the user to take as much time as necessary to position parts.

The capability to cure on demand speeds processing and reduces total production costs. Upon exposure to a light source, the adhesive can be fully cured in less than a minute.

UV- and visible-light-curing formulations are becoming increasingly popular in small motor manufacturing for a number of reasons:

_ They contain no solvents and don’t require heat curing.

_ They eliminate the need for holding racks and ovens.

_ They allow flexible work-cell manufacturing configurations.

_ They cure quickly and with precision control.

Light-curing products are currently replacing heat-curing and two-part epoxies, varnishes, trickle coatings, and many other assembly systems commonly found in small motor manufacture.

ILL. 108 Benefits of adhesive to seal inner spaces in threadlocking. (a) Accelerated corrosion (crevice corrosion) causes nut lock or failure. (b) Uniform corrosion begins at exposed surfaces.

Adhesives in Small Electric Motor Assembly

Threadlocking. Mechanical fasteners and adhesives can work together to form a stronger bond than either method alone can provide. Design engineers who want to improve the safety and quality of an assembly will use a mechanical fastener in tandem with a threadlocking adhesive. The anaerobic threadlocker guarantees that the assembly won’t fail or loosen and that corrosion won’t reduce the life of the fastener.

Since threadlocking agents prevent loosening and movement and act as sealants, they can be used on small motor assemblies for tamper-proofing, mounting bolts, locking assembly screws, bolting flanges, and setting adjustment screws. Threadlockers also offer excellent performance when used on severe-duty fasteners since they seal the bond joint and prevent corrosion. They offer excellent performance in high temperature and high-vibration areas.

Threaded assemblies fail for two reasons. One cause is tension relaxation. Temperature changes, For example, cause bolts and substrate materials to expand and contract, reducing bolt tension and lowering the clamping force. The second cause of threaded assembly failure is self-loosening caused by sliding or vibration between contact surfaces.

Threadlocking adhesives prevent bolts from loosening by completely filling microscopic gaps between interfacing threads. In fact, threadlocking agents maintain clamp load better than lock washers in high-vibration environments. Liquid anaerobic threadlockers cure to a tough solid when they contact metal ions in the absence of air. When selecting a threadlocking adhesive, designers should consider end-use operating temperature, thread size, chemical and environmental factors, bolt reusability, and the need for a primer to speed curing.

When applying threadlockers, the total length of the thread must be wetted.

Proper wetting depends on the size of the thread, the adhesive's viscosity, and the size of the parts. Nothing should be present on the assembly that will restrict curing.

Assemblies must be free of all oils or cleaning systems that might impede curing.

Threadlocking adhesives offer a number of benefits. One distinct advantage is bolt reusability. Each adhesive has break-loose torque values ranging from low, which may be disassembled using normal tools, to high, which is difficult to disassemble. Threadlockers with low to medium strength can easily be loosened without damaging bolts. A bolt treated with threadlocker may be reused by removing old adhesive before applying new.

In addition to preventing movement, threadlocking adhesives can also be used to seal the joint, blocking out moisture, gases, fluids, and corrosives which can reduce the life of the assembly, as shown in Ill. 108.

Another benefit of threadlocking adhesives is the reduction of overall assembly costs by using standard bolts in place of special locking bolts. Liquid adhesive is effective regardless of bolt size and diameter, eliminating the need for elaborate inventories of specialized mechanical fasteners.

Anaerobic threadlocking adhesives are now available as coatings pre-applied to threaded fasteners. When the fastener is assembled, microcapsules containing adhesive are crushed, releasing the liquid threadlocker. In addition to cost and logistical benefits, pre-applied coatings improve the quality of the assembly by ensuring that a consistent amount of adhesive is dispensed every time.

When selecting an anaerobic threadlocker, small motor designers must take several factors into consideration. The continuous operating temperature and chemical and environmental conditions of the end-use environment will determine the performance criteria required of the adhesive. Thread size will dictate the adhesive's viscosity. The designer must know whether the part will be disassembled in the future in order to determine the appropriate adhesive strength. And the substrate materials used will determine whether a primer will be required to accelerate curing.

Sealing Pores and Laminations. Advanced product designs and new manufacturing techniques require modern methods for filling and sealing the voids, or micro porosity, in substrate surfaces such as metal castings, powdered-metal parts, electronic components, plastic composites, weldments, and other porous materials.

Pore sealants, also called impregnation resins, are used in a variety of applications, including sealing pores in metal housings and other structural components, unitizing commutators and other porous subcomponent parts, sealing electrical connectors, sealing magnets and other nonstructural components, and unitizing windings and laminate stacks.

Today, designers are creating new lightweight, thin-walled die castings for use in applications containing high-pressure fluids. These castings would not be possible without the sophisticated new vacuum-impregnation systems and sealants. Designers seeking to decrease the weight or cost of assemblies rely upon impregnation, which enables them to design thinner-walled die castings or to switch to lower-cost methods, such as powdered metal. Modern resin impregnation systems are so well accepted, effective, and economical that traditional leak testing of heavily machined castings has been phased out in favor of impregnation of parts.

There are two types of porosity. Macroporosity requires re-melting because the gaps affect structural integrity. Microporosity does not affect structural strength and is the natural result of two physical phenomena which occur as molten metal solidifies-crystal formation and shrinkage, and gas absorption. Visible surface defects are usually not improved by pore sealing as the sealant washes out of the surface defects during processing, leaving them unchanged. Porosity that occurs on a microscopic scale can be readily sealed through impregnation. Even if a pore is large scale within the casting, it can be sealed if the surface opening is not large.

The most common reason for impregnation of castings is to enable parts to retain fluids under pressure. Properly impregnated, components such as hydraulic pump and motor parts are permanently sealed and can hold pressures up to the burst strength of the casting. Castings are also impregnated in preparation for metal finishing operations such as painting or plating. In some instances, it may be desirable to seal pores in a casting so that corrosive fluids cannot enter. This is done to prevent corrosion that may originate within the porosity.

Castings should be fully machined, cleaned, and dried before impregnation, and should be at room temperature. Only after all machining is completed can the impregnation sealant reach into all areas that need to be sealed. The pores of clean, dry parts won’t be blocked with foreign materials that might prevent thorough penetration of the impregnation sealant.

Powdered-metal parts should be impregnated after sintering but before any secondary operations. The pores are normally completely open at that point and can be filled by the impregnation sealant.

Plating, painting, or other finishing operations for either castings or powdered metal parts should be done after the impregnation is complete and the sealant has been allowed to fully cure. The cured sealant in the impregnated parts won’t be affected by the various cleaning and etching steps, even when strongly acidic solutions are used.

The unique self-curing capability of anaerobic sealants, along with the ability to regulate the rate of cure, has made anerobics the most reliable family of materials for sealing the porosity of metallic and nonmetallic parts. The ability to cure without heating the parts, combined with the use of an activator rinse to speed curing, gives the anaerobic sealants much greater capability in sealing a wide range of pore sizes, especially larger pores. Bleedout won’t happen with anaerobic sealants, so part fouling does not occur and sealing performance is consistently high.

Certain formulations of cured anaerobic sealants are suitable for continuous ser vice up to 392°F (200°C). Brief exposure to higher temperatures, as might occur in paint ovens, won’t normally cause problems. These sealants can be used success fully in such high-temperature environments as automotive engine blocks, cylinder heads, and coolant pumps. The cured sealant is a thermoset plastic which won’t melt, liquefy, or run out of parts at elevated temperatures. If operated beyond acceptable temperatures, the cured sealant will slowly lose weight and turn to ash.

Impregnation sealants are tested by the manufacturer for chemical resistance to determine their suitability to environments exposed to fuels, lubricants, coolants, cleaners, and other chemicals that may be encountered in automotive, aerospace, and general industrial environments.

Gasketing (Flange Sealing). Gaskets (flange sealants) prevent leakage of liquids or gases by forming impervious barriers between mating flanges. In small motor applications, flange sealants are used for gasketing fluid- and air-tight components, sealing crimped components, sealing flange couplings for improved power transmission, and ensuring the performance of hermetically sealed components.

There are three types of flange gaskets: conventional compression gaskets of cork, paper, rubber, metal, and other asbestos-free materials; cured-in-place liquid compression gaskets cured in seconds with UV light prior to assembly; and formed in-place liquid gaskets cured after the parts are assembled. All three types of gaskets must create and maintain seals, remain impervious to fluid flow, and be compatible with the machinery.

Liquid formed-in-place (FIP) gasketing materials are used to dress conventional rubber, paper, or cork gasketing materials, and can often completely replace cut gaskets. These liquid anaerobic gasket dressings fill surface imperfections in the mating flanges and extend the life of the gasket. When parts are assembled, the flange sealant flows into voids, gaps, and scratch marks, forming a durable seal after curing.

Formed-in-place gaskets offer a number of advantages over precut compression gaskets, including improved reliability, reduced costs, and easier application and service.

Two common types of FIP materials are room-temperature-vulcanizing (RTV) silicones and anaerobic compounds.

Anaerobic FIP materials are generally used on rigid joints made of cast aluminum or iron, such as pumps, engines, and transmissions. Anaerobic gaskets offer several benefits over traditional sealing systems. They won’t relax, so retorquing is never necessary. These materials allow metal-to-metal contact, and therefore don’t require shimming. Anaerobics won’t begin to cure before flanges are mated, and any excess material which squeezes out of the flange faces won’t cure and can be easily wiped away. These high-shear-strength materials demonstrate excellent sol vent resistance and chemical compatibility, and eliminate the need for large inventories of precut gaskets.

RTV silicones cure by reacting with atmospheric moisture and are typically used on high-movement joints made of stamped steel or molded plastic. RTV silicones are applied as beads to the flange area of a component, and flow over the flange to form a gasket that completely fills voids, scratches, and other surface imperfections. They pro vide a small amount of squeeze-out on both the inside and outside of the joint, forming a fillet around the flange which acts as a secondary seal. FIP silicones are capable of filling large gaps, flex easily in response to the movement of the flange, adhere well to many surfaces, and withstand a wide range of temperatures. As with anaerobic materials, silicones eliminate the need for large and costly gasket inventories.

Silicone cured-in-place (CIP) gasketing involves positioning a compression gasket as a permanent part of one flange surface. The gasket is created by a tracing machine which applies precise beads of silicone to the flange surfaces. The beads are cured and bonded to the component flange in 30 s with UV light, or in 7 to 14 days with slow moisture curing. Sealing is achieved through compression of the cured gasket during assembly of the flange joint. CIP compression gaskets offer many advantages over die-cut rubber, die-cut foam rubber, or molded gaskets. Equipment manufactured with CIP-sealed access ports is easy to produce and service. CIP gaskets reduce labor costs, improve product quality, reduce gasket inventories, and allow flexibility in the manufacturing process.

Retaining Cylindrical Assemblies. The term retaining compounds describes adhesives used in cylindrical assemblies joined by inserting one part into another. A typical example is a bearing mounted in an electric motor housing. Retaining compounds enable buyers of new bearings to salvage worn housings.

Incorporating retaining compounds into the assembly of small electric motors pro vides a number of performance benefits. Retaining compounds can augment, improve, or replace normal press fits and can increase the strength of heavy press fits. They also eliminate distortion when installing drill bushings. Retaining creates an essential seal which eliminates fretting corrosion and seizure. Finally, retaining compounds help to reduce stress in parts and enhance the combined performance of substrate materials.

Retaining compounds are used in a variety of small motor applications: mounting bearings in housings or on shafts; attaching pulleys, cams, fans, and gears; replacing or augmenting interference fits, keyways, and splines; retaining commutators and mounting flanges; and assembling armatures. Retaining compounds work extremely well on cylindrical components made of dissimilar materials, as well as on parts that must be protected from corrosion and backlash.

Retaining compounds are primarily anaerobic materials, used to bond metal to metal or metal to a nonmetallic substrate. However, for nonmetallic substrates such as plastics, cyanoacrylate adhesives can be used.

Two key formulation variables exist for anaerobic retaining compounds. The viscosity of the adhesive determines its ability to fill gaps in the assembly-the higher the viscosity of the material, the greater its ability to fill gaps. Like threadlockers, anaerobic retaining compounds are formulated in different strengths-from low, which may be easily disassembled, to high, which is difficult to disassemble. An assembly treated with the appropriate-strength anaerobic retaining compound may be disassembled and reused.

As with all adhesives, it’s extremely important that substrates be thoroughly cleaned before retaining compounds are applied. Contamination will greatly inhibit curing, especially on metallic substrates.

Retaining compounds are crucial in two types of cylindrical assemblies. In bonded slip-fit assemblies, the shaft is inserted into the hub with no force required.

Cured adhesive located between the shaft and the hub transmits the load or torque.

In bonded interference-fit assemblies, friction and the adhesive combine to transmit the load or torque. There are two types of interference fit assemblies-bonded shrink fits, where the hub is heated to allow insertion of the shaft, and bonded press fits, where axial force allows insertion of the shaft In selecting an appropriate retaining compound, designers should consider the type of assembly being bonded, the thermal expansion of the individual substrates, the radial clearance of the bonded slip-fit assemblies, the required shear strength and ease of disassembly, and the continuous operating temperature of the end-use environment.

Bonding. A number of adhesive technologies are commonly used for bonding a variety of substrate surfaces to one another. Common surface-bonding opportunities for small motor assembly include flat-face bonding, plastic and sheet-metal bonding, motor magnet bonding, affixing nameplates or identification tags, attaching the housing to the base, bonding armature segments, and bonding the cooling fins to the heat sink.

When bonding assemblies with large gaps, high-viscosity adhesives can act as fillers, occupying space between substrate surfaces. However, there is an inverse relationship between the size of the gap and the overall performance of the adhesive. Large gaps reduce the adhesive's bond strength and durability and increase the fixture time required, diminishing the adhesive's effectiveness.

Magnet Bonding. Acrylic adhesives are now used extensively to bond magnet segments in motor manufacturing. Millions of motors are operating successfully in demanding environments using only adhesive systems to reliably secure their magnets.

Magnet bonding has evolved significantly over the past decade. Ten to fifteen years ago, it was common to use spring steel clips to fix magnet segments into position. In addition to the relatively high cost of materials, magnet clipping was difficult to automate and required relatively complex parts-handling systems and insertion.

In addition, clips could loosen or shift, allowed corrosion to occur between the mag net and the motor assembly, and did nothing to prevent noise and vibration.

As cost and performance became increasingly important, many motor manufacturers who were skeptical of adhesives' bonding performance began to use epoxy adhesives to prevent corrosion and reduce noise, but still augmented the bond using clips. Adhesive suppliers began to develop formulations specifically for magnet bonding applications. Specialty acrylic adhesives that were tough and environmentally resistant were introduced to bond magnets without clips.

Adhesives proved successful in motor magnet bonding because they were designed to process quickly and bond reliably in less-than-ideal industrial environments. In addition, adhesive bonds last through the severest environments that a motor may encounter.

Factors relating to the magnet and the housing play a critical role in successful magnet bonding. Magnets must be dust-free and be formed or machined to ensure that gaps between the magnet and the housing remain small. Most magnet-bonding adhesives solidify faster and are stronger when gaps are less than .010 in. For curved magnet segments, some motor manufacturers have moved from single-radius segments to the tri-arc configuration seen in Ill. 109. This design helps to reduce movement of the magnet in the fixture, and can help reduce the gap due to tolerance differences between the housing and the magnet. Examination of the tri-arc bond joints shows that the adhesive fixtures or solidifies quickly at the two points of con tact on the magnet surface.

The surface and shape of the housing can be crucial to the success of the bonding design. The surfaces of housings are often fabricated so that they resist corrosion and can be painted. Conversion coating processes such as chromating, phosphating, galvanizing, and anodizing are used to fabricate housing surfaces. In most cases, pre paint coatings such as zinc phosphate or chromic acid anodizing are the best for adhesive bonds. Coating processes that leave weak surface layers can be problematic. Such processes include galvanizing or yellow zinc dichromating. Some adhesive manufacturers have developed primers that repair weak coating surfaces.

The method used to fabricate the housing will often affect its ultimate dimensional tolerance. Tighter control of housing dimensions can be used to ensure small bondline gaps. Drawn or extruded housings are typically the most stable from a dimensional standpoint. Magnet bonded housings are commonly roll-formed housings, which are more difficult to control dimensionally. When rolled housings are used, it’s important that the seam be smooth and properly fitted. Misalignment or tabs that cause large gaps are unacceptable. The surface finish range that optimizes adhesive strength is a 62- to 125-µin RA finish.

ILL. 109 Triarc magnet assembly.

Other Applications

Adhesive chemistry and technology have been used in a variety of less traditional applications. Specialty adhesives have been formulated to provide a number of benefits in electrical and electronic device assembly, including thermal conductivity for heat sinks, coil termination and strain relief, potting and encapsulation, conformal coatings, wire tacking, and armature balancing.

Processing Equipment

The capital expenditure for setting up production-line bonding systems will pay for itself in materials and labor cost savings, particularly on high-volume production lines. A broad range of dispensing equipment is available, from manual systems for small-batch production or repairs to fully automated systems. The adhesive used determines the type of dispensing equipment required.

Viscosity is one decisive factor in choosing a dispensing system. The adhesive's flow properties determine the control technique and the valve choice for adhesive application. In order to properly wet the bond faces, the properties of both the adhesive and the surfaces must be controlled. With sufficient experience, an adhesive manufacturer can integrate individual components of each dispensing unit into a complete system that can cover most dispensing tasks.

Increasingly, automatic dispensing systems are being used to ensure a well bonded joint by consistently applying the correct quantity of adhesive in the proper place. Automatic systems can easily be monitored and checked for correct positioning and adhesive quantity.

Safety

Employee safety and environmental friendliness are two issues of concern through out the industrialized world. Manufacturers are striving to develop production processes which ensure compliance with the stricter safety and environmental standards in place today. The trend toward safety has created both challenges and opportunities for the adhesives, sealants, and coatings industry.

Although most adhesives have evolved to meet stringent safety requirements, improper handling and process design will yield unnecessary problems and expenses.

It’s possible to achieve a safe and trouble-free adhesive process if safe handling and use issues are attended to during the process design stage. Implementing an appropriate safety and ventilation strategy can make the difference between success and failure.

Extensive information concerning the safe handling of chemicals in the workplace exists, but it’s scattered and difficult to gather, filter, and apply to specific operations.

Your adhesive supplier should be able to provide guidance on material safety; safe shipping and storage of unopened containers; minimizing and monitoring exposure to potentially dangerous materials; and waste collection, storage, and disposal.