Adhesives that cure on demand help stepper-motor builders customize designs and speed production.
Santa Clara, Calif.
Senior Application Engineer
James A. Serenson Jr.
Loctite Industrial Adhesives,
Rocky Hill, Conn.
Stepper motors find use in a wide variety of high-precision applications including medical equipment, industrial robots, and video cameras. Many of these uses need motors with custom winding patterns, shaft configurations, housings, and bearings.
Light-curing acrylic adhesives help motor makers balance the demands of small, customized batches and short lead times. Such adhesives fully cure in seconds so work-in-process approaches zero and quality inspections can take place immediately. Because the adhesives apply as liquid, one formula can work on many different motor designs and sizes. This speeds line changeovers and cuts inventories compared with mechanical alternatives such as gaskets that need sizing for each motor design.
In ambient light, all but the fastest light-cure adhesives stay liquid long enough to do multiple part adjustments. Exposing the adhesive to light of a proper intensity and wavelength triggers a photoinitiator that starts the curing process.
Though cure times depend on many factors, 10 to 30 sec of light exposure cures adhesives to a depth of about 0.5 in. (13 mm) or more. But for that to happen, light must reach the full bond line. This may limit their use in motors when bonding metal to metal or two opaque substrates. However, curing excess adhesive around the joint (fillet) rapidly fixtures the components so they can be handled right away. Heat or another secondary curing mechanism can then cure the remaining adhesive.
Stepper motors have a multitude of mounting configurations, making standardization tough or impossible. For example, some motors use a bayonettype mounting bracket that permits manual assembly in a blind location. In this and other custom applications, a light-cure acrylic adhesive bonds the molded stator into a housing. The housings are typically machined from aluminum to customer specs. Adhesives also structurally reinforce the motor body, reducing body thickness.
Other jobs for light-cure adhesives include leadwire tacking and strain relief. Lead wires typically pass through an opening in a motor housing. Improperly secured wires exposed to sharp pull forces or vibration may degrade solder connections. And irregularities in solder joints, such as solder balls or sharp peaks, can abrade wire insulation and cause shorts should wires move freely about inside the motor.
Grommets and clamps are two ways to secure wires. But the devices take up space and may necessitate a larger motor-frame housing, which is highly undesirable. Further, assembling grommets and clamps can be a tedious process that slows production and adds cost.
A better alternative tacks the lead wires with a light-cure acrylic adhesive. A handheld syringe manually dispenses a bead of adhesive over the wire connections. The coated motor assembly then goes on a conveyor where a 30-sec light exposure fully cures the adhesive.
Leaks are another design challenge. Wires passing from outside through a motor casing create a potential leak path into the motor interior. Potting the connection with adhesives or heat-shrink seals are common ways to seal the opening. But heat-shrink seals don't effectively seal wire bundles of four to eight wires, which are typical wire counts for stepper motors. Such seals fit snugly around the outside of the wires, but don't adequately seal between them. In contrast, a light-cure acrylic adhesive of 20,000 cP viscosity seals the opening without flowing