Abstract:
A device for the determination of the frictional characteristics of large surfaces comprising: a frame; a drive motor mounted atop the frame; a drive train; a horizontal measurement arm attached to the drive train and capable of circular rotation; a spherical frictional slider attached to the distal end of the measurement arm that contacts and slides along a surface under evaluation; the frictional slider comprising a spherical member that directly engages the surface under evaluation; and a tangential force detector on the measurement arm to measure the resistance encountered by the spherical member as it slides along the surface under evaluation.

Description:
FIELD OF THE INVENTION 
   The present invention relates to devices for determining the friction characteristics of surfaces, and more particularly to such a device for the determination of the frictional characteristics of large surfaces such as floor, landing areas and the like. 
   BACKGROUND OF THE INVENTION 
   The determination of the frictional characteristics of surfaces has long been and remains critical in many areas of industry, the military etc. A particularly important application of such devices is in the determination of the frictional characteristics of surfaces aboard ships where the movement of personnel over surfaces that may be wet under less than ideal conditions is extremely important and may be the determinant factor in the safety of military personnel. 
   As will be apparent to those skilled in the arts related to the determination of the frictional characteristics of surfaces, a large number of devices and a great deal of effort has been devoted to the development of improved such devices and no extended discussion of such efforts and the resultant devices is required here. Suffice it to say that numerous such devices have been designed, manufactured and applied to obtain accurate measurements of the frictional characteristics of large surfaces. 
   In spite of these efforts, there remains a need for further improved such devices, especially as they apply to the aforementioned shipboard surfaces under less than ideal conditions as are routinely encountered by military personnel aboard ships, especially on the decks of, for example, aircraft carriers. 
   OBJECT OF THE INVENTION 
   It is therefore an object of the present invention to provide an improved device for the determination of the frictional characteristics of large surfaces. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided a device for the determination of the frictional characteristics of large surfaces comprising: a frame; a drive train; a horizontal measurement arm attached to the rotary motion transfer mechanism and capable of circular rotation; a spherical frictional slider attached to the distal end of the measurement arm that contacts and slides along a surface under evaluation; the spherical frictional slider comprising a spherical member that directly engages the surface under evaluation; and a tangential force detector on the measurement arm to measure the resistance encountered by the spherical frictional slider as it slides along the surface under evaluation. According to various alternative preferred embodiments, the system further includes an angular motion detector to determine the relative angular position of the measurement arm, a vertical deviation detector to measure changes in the topography of the surface under evaluation and a lift mechanism to raise and lower the frictional slider into and out of contact with the surface under evaluation. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of the friction testing device of the present invention. 
       FIG. 2  is a top plan view of the friction testing device of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1  that depicts a side view of the friction testing device of the present invention, the friction testing device  10  of the present invention comprises a frame  12 , a drive motor  14  mounted in said frame, a mechanism  16  for transferring motion produced by motor  14  to a measurement arm  22 , mechanism  16  is identified as the drive train hereinafter and comprises a mechanism  24  for transferring motion produced by motor  14  to a drive shaft  18 , and a mechanism  20  for transferring the motion from drive shaft  18  to measurement arm  22 , at the distal end of measurement arm  22 , a spherical frictional slider  24  that contacts a surface  26  that is under frictional evaluation; and a tangential force detector  28  mounted on said measurement arm. 
   Frame  12  is, of course provides support for motor  14  and may, according to various alternative preferred embodiments described below, incorporate additional handling enhancement and protective members. Motor  14  may be of any design or derive power from any suitable source such as an AC or DC electrical power source  49  or even a gasoline engine capable of delivering a consistent and reliable source of rotary motion. Drive train  16  may also be of any of a variety of designs and configurations so long as it is capable of delivering reliable and consistent rotary motion to measurement arm  22 . In the embodiment depicted in  FIGS. 1 and 2 , drive train  16  comprises a gear assembly  34  that transfers rotary motion from motor  14  to drive shaft  18  and a yoke and bearing assembly  20  that transfers rotary motion from drive shaft  18  to measurement arm  22 . The bearing assembly allows for appropriate rotary engagement of drive shaft  18  with measurement arm  22  while yoke  30  allows measurement arm  22  to float over or be pressed against surface  26 . At the distal end of measurement arm  22  is spherical frictional slider  24 . In the embodiment depicted in the accompanying Figures, spherical frictional slider  24  is ball shaped and frictionally or otherwise, i.e. with screws, bolts or other securing devices, secured in a holder  32  that limits the ability of spherical frictional slider  24  to rotate as it slides across surface  26 . The use of holder  32  allows spherical frictional slider  24  to be adjusted or replaced easily in the field without the use of tools so that, for example, a non-used/non-worn area on spherical frictional slider  24  can be brought into contact with surface  26  for multiple or repetitive testing. 
   Measurement arm  22  is capable of nearly 360° travel in a circular path along surface  26 . According to the embodiment depicted in the Figures, after travel in a first direction, because of the design of gear assembly  34 , measurement arm  22  reverses direction and returns to its starting position. Using such a circular path minimizes dependence of the coefficient of friction on any directional or anisotropic properties of surface  26 . 
   Measurement arm  22  may be fabricated from any suitable material such as a thin metallic strip or polymeric strip that can undergo limited bending under the load conditions anticipated in the course of any frictional measurement. Thus, tangential force detector  28  is incorporated into measurement arm  22  to measure the frictional force caused by spherical friction slider  24  traversing surface  26  as manifested by limited bending of measurement arm  22 . One such suitable tangential force detector  28 , and that depicted in the Figures, comprises one or an array of strain gauges mounted on the surface of measurement arm  22  parallel to the normal plane of surface  26  to form a Wheatstone bridge. 
   As is well known to the practioners of the tribological arts, the coefficient of friction or COF is not an absolute value, but rather a relative measure of the force of resistance encountered when two surfaces of the same or different materials slide over one another. Thus, for example, the COF of a leather shoe on a freshly waxed floor can be expressed as a value, but will have no relationship to the value determined when the leather shoe is slid over a steel stair step. Thus, spherical slider  24  may be fabricated from an almost infinite number of materials depending upon the particular surface under evaluation  26  and the purpose for which the test is being performed, to achieve high resistance to sliding or low resistance to sliding. Similarly, soft materials do not necessarily “stick” or resist sliding better than harder materials. 
   The apparatus of the present invention finds use in innumerable applications, most particularly in the determination of the frictional characteristics encountered when an aircraft lands on an aircraft carrier deck. Thus, in this determination the surface under evaluation  26  is the surface of the aircraft carrier deck and the material of spherical slider  24  should simulate the tire of a landing aircraft. 
   Thus, the key criterion for the selection of the material for spherical slider  24  is that it mimic closely the frictional characteristics or frictional behavior of the material that is going to contact the surface under evaluation  26  in a particular set of circumstances or a particular application. In the case of the aircraft carrier surface previously cited, for example, neoprene is the material of choice for spherical slider  24 , while in the case of the leather shoe on a stair step, a spherical leather material is appropriate for spherical slider  24 . What is most important in the selection of the material for spherical slider  24  is that its use achieves good correlation between the measurements taken with frictional testing device  10  and the behavior of the surface under evaluation  26  and the contacting surface in the particular application under study. 
   The shape of spherical slider  24  also plays and important role in the validity of the measurements taken and their correlation with the “real world” of the particular application. It has been found that flat plates do not perform entirely satisfactorily in most applications, hence the requirement for a spherical shape for spherical slider  24 . The validity or correlatablity of the results, however, also depend to some degree on the diameter of spherical slider  24 . Thus, it is desirable that the ideal diameter for spherical slider  24  be determined by trial and error for a particular application. 
   In addition to the foregoing essential elements of frictional testing device  10 , a number of other useful enhancements may be incorporated therein. These include: an angular position detector  38  to determine the relative angular location of measurement arm  22  along its circular path of travel during a friction measurement; a vertical deviation detector  40  to measure changes in height to simultaneously measure the topography of surface  26 ; a system lift mechanism  42  (depicted schematically in  FIG. 1 ) to raise and lower measurement arm  22  so as to bring spherical frictional slider  24  into and out of contact with surface  26 ; and a load assembly  44  that can be a separate load imparting mechanism as schematically depicted in the Figures or can be incorporated as a load inducing device into yoke  30  and an associated strain gauge arrangement  46  normal to the plane of surface  26  to measure an applied load. Of course any number of alternative force detecting devices could be substituted for this strain gauge arrangement  46 . 
   Other desirable enhancements include: a system housing  48  to protect frictional testing device  10  from environmental hazards or interferences and to provide a carrying case for device  10 ; a separate environmental shield located between motor  14  and measurement arm  22  to provide additional environmental protection; and a mechanism for leveling frictional testing device  10  such as the adjustable feet  52  shown in  FIG. 1 . Element  49  is connected to an output element from motor  14 . 
   As is apparent to the skilled artisan, frictional testing device  10  should also include as an integral part thereof or as a separate piece of equipment with appropriate connections being provided in device  10 , a data acquisition system  54  to collect, analyze and archive input from the various sensors, the tangential force detector  28 , the angular position detector  38 , the vertical deviation detector  46  and the load assembly  44  to accurately determine the coefficient of friction of spherical slider  24  relative to surface  26 . According to a highly preferred embodiment, frictional testing device  10  also incorporates a “Go/No-Go” acceptance system  56  that provides immediate feedback to a user on the acceptability of the detected coefficient of friction of a surface under analysis as defied by predetermined frictional standards. Such a “Go/No-Go” acceptance system  56  could, for example, comprise red, yellow and green lights where green indicates acceptability, red indicates an unacceptable measurement and yellow indicates a marginal measurement. 
   As the invention as been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.