Hair entanglement is a major concern in the pool industry.

Anti-Hair Entanglement

Cantilevered Grating Elements

by Peter J. Poczynok,* Adam K. Dybek,** and Ralph L. Barnett

I. INTRODUCTION

Hair entanglement /entrapment is a failure mode shared by swimming pools, wading pools, spas, hot tubs, and whirlpool bathtub appliances. When hair is entrained in the discharge flow through a suction fitting or drain, its withdrawal will generally be resisted by gravity, drag, friction, buoyancy and interference. Large withdrawal forces obviously lead to a horrifying safety problem. In an attempt to manage the magnitude of these forces, the pool industry introduced their first standard relative to hair entrapment on November 3,1987; ASME/ANSI A112.19.8M -1987, Suction Fittings for Use in Swimming Pools, Wading Pools, Spas, Hot Tubs, and Whirlpool Bathtub Appliances. 

In section 5.2 of this standard, a test protocol is outlined for measuring the withdrawal force required to extricate a 16’ long strand of hair that has previously been fed into a test suction fitting. Figure 1 depicts the test status at the last stage where the pull force is measured. Quoting paragraph 5.2~) from the standard:

  "With the pump still operating, test for the amount of pull necessary to free the hair from the fitting. Measure the force of entanglement by pulling the scale and dowel vertically. Repeat the test ten times." 

The performance requirement for an acceptable fitting is given in paragraph 5.3:"  A pull of 5 lb. or greater on any one of the ten tests, including the weight of the saturated test apparatus, shall be deemed a failure..." 

There are several implications of the ANSI standard that are worth noting:

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 The standard assumes that a force level exists that will free hair from a fitting. It is called the "force of entanglement." Unfortunately, the ANSI standard is incorrect. Two modes of entanglement are shown in Fig. 2. If the knot and the wrap are self-locking (the more you pull the tighter they become) one cannot escape the entanglement without fracturing the hair strands.

 

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 The likelihood of achieving a self-locking type of entanglement cannot be explored with a sample size often. Furthermore, the test protocol specifies hair samples of medium to fine straight hair, which is kept tangle-free by periodic brushing.

 

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 When self-locking modes of entrapment are not present, the force of entanglement is installation specific. It will depend on the precise piping geometry, the orientation of the suction fitting, the exact materials of construction, etc. The latest standard for Whirlpool Bathtub Appliances, ASME Al 12.1 9.7M - 1995, solves this simulation dilemma by specifying that the tests be conducted using an exemplar whirlpool bathtub appliance installed per the manufacturer’s instructions.

 

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 The 1987 ANSI standard does not recognize self-locking types of entanglement. Nevertheless, 30 cases were reported to the Consumer Products Safety Commission from January 1990 through May 1996 where hair entanglement occurred because of tangling rather than strong suction forces [Ref. 1]. An additional four cases of entanglement are reported by the CPSC from April, 1981 through February, 1985.

Senior Mechanical Engineer, Triodyne Inc., Niles, IL. Master’s Candidate, Illinois Institute of Technology, Chicago. Professor, Mechanical and Aerospace Engineering, Illinois Institute of Technology, Chicago, and Chairman, Triodyne Inc., Niles, IL.

This paper was published in the Proceedings of the American Society of Mechanical Engineers’ International Mechanical Engineering Congress and Exposition in November 1999. 

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