When the acetal-based material for the nut lead screw used in its dry water massage therapy became unavailable, Aqua Massage International (AMI) initiated a search for a replacement material. The design criteria required minimum radial play, vibrations and backlash between a screw and nut assembly operating in a saturated environment at an elevated temperature. With a length-to-diameter ratio of thread engagement of about 7:1, twice that of standard nuts, maintaining a constant thread fit under all conditions provided a serious design challenge.
Even though the user stays dry in AMI's Aqua Massage (http://rbi.ims.ca/4922-500), 36 pulsating water jets travel along a seven-foot screw shaft to provide either a full body massage or concentrate on a specific area depending on user's selection. The seven foot long, ¾-inch diameter screw uses a long lead to achieve a 2.4-inch/rev advancement allowing the motor's rpm to be relatively low to avoid screw gripping.
Engineers at Kerk Motion Products performed a thorough application analysis and developed a custom solution that uses a newer and improved nut material, combined with a proprietary non-ball lead screw manufactured from 303 stainless steel.
"The thermal expansion coefficient of the nut material had to be roughly equivalent to that of stainless steel to prevent a binding condition while the machine was in operation," notes Bob Hawkins, an application engineer at Kerk Motion Products. "The material that we selected and are currently using was not available during the time of the original machine design."
In the Aqua Massage application, both the screw and the nut are very long. The nut needs to be long to avoid a rocking motion. "The problem with many plastic materials is the expansion coefficient is much greater than stainless steel," says Hawkins. "You have the radial clearance between the nut and screw but, more importantly, you have an axial clearance that you have to compensate for." With a nut more than five inches long, significant shrinkage or expansion can occur due to different thermal expansion coefficients.
The high stability of the new material eliminates a secondary operation (with a thread chasing tap) frequently required with the old material to fine tune the thread prior to assembly and relieve stress.
AMI's situation is not unlike one several companies encounter during the life of a successful product regarding a critical design element — something new could possibly do an even better job. However, with the loss of supply at AMI, the necessity for change became mandatory. Sometimes, if it ain't broke, it is still worth evaluating newly-available design options.
CONTACT:Bob Hawkins, Kerk Motion Products at RHawkins@KerkMotion.com to discuss custom lead screw applications.
For more information on standard Kerk Motion Products lead screw assemblies, go to http://rbi.ims/ca/4922-501