Ultrasonic Cleaning – More About Baskets and Fixtures

This blog is in response to recently received reader questions regarding the best designs for baskets and fixtures used in ultrasonic cleaning applications.  The blog Reader Questions – Baskets for Ultrasonic Cleaning addressed previous questions.  Today’s question concerns the use of metallic fasteners in the construction of baskets and fixtures to hold parts for ultrasonic cleaning.

The use of fasteners including bolts, nuts, cotter pins, various kinds of clips and other metallic fasteners in basket and rack construction is usually motivated by the desire for either simplicity of fabrication or flexibility in use.  Although valid, these goals should not, if possible, involve metallic fasteners.  There is one pretty obvious reason to avoid metallic fasteners as well as a few that are not so obvious.  First off, fasteners, especially nuts and bolts, have a tendency to loosen over time as a result of ultrasonic exposure.  Although we haven’t really talked about it yet in the blog, ultrasonic vibration is an excellent friction reducer.  The high vibrational energy delivered by ultrasonic sound waves overcomes the static friction between the bolt and a nut or threaded hole that normally holds a bolted connection in place.  Constant and recurrent ultrasonic vibration prevents the hold of static friction required to keep a bolted connection secure.

Quick Physics Brush-Up – You may remember from high school physics when you put a block of wood on a surface and measured the force required to start it moving vs. the force required to keep it moving once in motion.  The force required to start motion is required to overcome static friction.  Once the block is moving, overcoming sliding friction requires much less force than that required to initiate motion in the first place.

Even bolted connections with lock washers can be loosened by ultrasonic vibration!

The not-so-obvious problems with metallic fasteners involve wear.  As two surfaces move relative to one another, the sliding contact between them eventually results in wear at the point(s) of contact.  We have all seen wear on metal and other surfaces – shafts that have worn and become loose, brake pads and rotors and (if you’re old enough) clutches in cars.  Not only does wear affect function, but whatever is worn away becomes particles!  Particles, of course, are something we really want to avoid in an ultrasonic cleaning or rinse tank.  Wear under conditions of ultrasonic vibration is exacerbated by the fact that there is no lubrication possible.  A fastener like a cotter pin or “C” clip may last for a considerable length of time if lubrication is present.  But, lubricants are not normally present in ultrasonic cleaning and rinsing baths.

Most baskets and fixtures used for ultrasonic cleaning are fabricated of welded stainless steel.  Only in extreme cases are welds affected by ultrasonic vibration.  Whenever possible, basket and rack designs also minimize the contact between the parts being cleaned and the fixturing for the same reasons described above.  Proper basket and/or rack design is an integral part of in the development of a successful ultrasonic cleaning process.  Most reliable suppliers of ultrasonic cleaning equipment will include custom basket or rack design (if required) as part of the overall system design process.  It is recommended that users take advantage of the manufacturers’ expertise whenever possible!

–  FJF  –