Control Engineering Online Update for June 16, 2006


Control Engineering Online Update - Technology

June 16, 2006


Sponsored by
Rockwell Automation

A capsule-like machine delivers 10- to 15-minute, full-body water massages to fully clothed clients. This unusual service, available in malls and salons, presented a unique machine design challenge: the spray water jet assembly, which moves back and forth along a linear shaft to position 36 pulsating water jets, has to quickly adjust from 2 to 10 cycles per second without interruption, and move over a distance of 7 ft.—all while providing a smooth and relaxing experience for the customer.

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Motion challenge met; result is 'dry water massage'

Thirty-six jets spray water against a sealed liner in this “cross between a tanning bed and a car wash.”

Machine builders needing unique motion control solutions might want to lie down, relax and get inspired. Aqua Massage International (AMI), Groton, CT, manufacturers hydrotherapy 'dry water' massage machines. Described by some as a cross between a tanning bed and a car wash, these capsule-like machines deliver 10- to 15-minute, full-body massages to fully clothed clients. Thirty-six water jets travel along a shaft, spraying water against a sealed liner that keeps the client completely dry. An adjustable touch-pad lets the client modify massage strength and location.

One secret to the success of the machine—and a unique design challenge—is the spray water jet assembly, which moves back and forth along a linear shaft. Not only did the linear motion assembly have to be accurate and durable enough to meet demanding speed requirements, it also had to meet performance specifications while operating at an elevated temperature 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.'

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Kerk Motion Products, one of the world's largest exclusive manufacturers of non-ball lead screws, took up the challenge. 'The Aqua Massage application posed many obstacles,' said Bob Hawkins, 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.'

Unique nut material

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,' said Hawkins.

The length-to-diameter ratio of thread engagement required for AMI’s machines is about 7:1—twice that of standard nuts

In addition, the nut material could not be hygroscopic, or tending to absorb water. Kerk 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 exclusive precision rolling process.

Lead screws differ from ball screws through the use of sliding, rather than rolling, friction between the nut and screw. Modern materials allow friction to be kept very low (less than 0.10) without any external lubrication. According to Kerk, some of the key advantages of lead screws are:

  • Zero maintenance

  • No lubrication required

  • Lower particulate generation

  • Longer life with non-catastrophic failure

  • Quieter operation (no re-circulating ball noise)

  • High helix/fast leads (greater than 100mm/rev)

  • Very fine threads (as small as 0.5mm/rev)

  • Non-backdriving (self-locking) leads

  • Multi-function nuts

  • Easily customized nut designs

  • Zero-backlash with very light pre-load/low drag

  • Much lower cost (25% to 90% less).

Aristi said AMI was impressed by the cost effectiveness and the high level of engineering of the solution. 'Our product delivery is now excellent, with quicker than average turnaround on our blanket orders of 200 to 250 products,' he adds.

For more information, download Kerk's whitepaper, 'Lead Screw Technology Rolls Over Ball Screws,' available at

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