Machinedesign 1609 Pulsator Copy 2 0 0

Lead screw assembly helps people relax

May 6, 2008
Aqua Massage International (AMI), Groton, Conn., manufactures hydrotherapy “dry water” massage machines, called the Aqua Massage Machine.

Aqua Massage International (AMI), Groton, Conn., manufactures hydrotherapy “dry water” massage machines, called the Aqua Massage Machine. Typically found in the center alleys of shopping malls, these capsule-like machines are often situated next to various vendor kiosks. They are similar in appearance to an MRI machine, and have been described by some as a cross between a tanning bed and a car wash. As opposed to traditional massage therapy, which can take up to one hour, aqua massage therapy yields results in only 10 to 15 minutes. Clients are able to stay fully clothed during the massage, no physical contact is necessary to complete the session, and an adjustable touch-pad control provides the client with the ability to adjust massage strength and location.

To optimize the quality of its machine’s design, AMI needed a linear motion solution capable of guiding and translating the machine’s spray water jet assembly, which moves back and forth along a linear shaft. The machine’s 36 water jets travel along the shaft, massaging either the full length of the body or a concentrated area depending upon user preference.

The Aqua Massage machine presented an unusual design challenge. Not only did the linear motion assembly have to be accurate and durable enough to meet demanding speed requirements, it also had to meet the performance specifications while operating at an elevated temperature and over a distance of 7 ft.

“Speed is a crucial factor since the pulsating water jets must be able to quickly adjust from 2 to 10 cycles per second without interruption,” said Gerardo Aristi, vice president of operations at AMI. “We needed a linear solution with minimum radial play, vibration, and backlash between the screw and nut to ensure a tight fit, and thus a smooth experience for the customer,” said Aristi. AMI turned to Kerk Motion Products, a manufacturer of non-ball lead screws and a vendor with whom AMI had a previous relationship. The challenge presented to Kerk was to produce a lead screw and nut assembly that could satisfy all these requirements.

“The Aqua Massage application posed many obstacles,” said Bob Hawkins, an applications engineer at Kerk. “The length-to-diameter ratio of thread engagement required for AMI’s machines is about 7:1, twice that of standard nuts. This presented a challenge in maintaining a constant thread fit. The challenge was amplified by the saturated environment in which the nut would operate.”

After a thorough application analysis, Kerk recommended a unique nut material capable of meeting all of AMI’s requirements. 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. In addition, the nut material could not be hygroscopic, meaning a characteristic whereby something tends to absorb water. Kerk quickly came up with a custom solution that relied on a specialized nut, combined with one of the company’s proprietary non-ball lead screws, which are manufactured from 303 stainless steel and produced with Kerk’s precision rolling process.

“Kerk’s solution is significantly more reliable and cost-effective than competitive offerings,” said Aristi. “Our product delivery is now excellent with quicker than average turnaround on our blanket orders of 200 to 250 products. It was a challenge that many other companies could not have met.” Visit Kerk for more information.

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