Abstract:
A vent cap has a long, narrow air communication passage covering the vent outlet of a sealed disk drive. The passage reduces diffusion of water vapor into the disk drive.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to sealed disk drives and means to reduce humidity inside the drive. 
     2. Brief Description of the Prior Art 
     The best prior art known to Applicant is shown in Ser. No. 646,537 filed 9/4/84, now U.S. Pat. No. 4,620,248 granted Oct. 28, 1986 and assigned to the present assignee. Applicant incorporates by reference the contents of this application. 
     U.S. Pat. No. 4,620,248 discloses and claims a relatively long narrow tube communicating between outside air and the interior of a sealed disk drive. The tube, so constructed, allows pressure equalization between the inside and outside of the housing during disk drive operation, but also retards water vapor diffusion into the housing. Humidity increases friction, particularly static friction, between the head and the magnetic surface, which undesirably reduces disk/head life. 
     SUMMARY OF THE INVENTION 
     The invention comprises a vent cap having, or forming, a long narrow communication passage between the vent and the outside air of a sealed disk drive. The long, narrow passage allows pressure equalization while retarding water vapor diffusion into the sealed drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the interior side of the cap. 
     FIG. 2 is a cross section of the cap along 2--2 of FIG. 1. 
     FIG. 3 is the vent outlet and filter of a sealed drive. 
     FIG. 4 is an alternative cap/vent mounting arrangemnt. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a plan view of the preferred vent cap 10. The cap includes ridges 20 which form a hollow groove 12 communicating between an inlet 14 and an outlet 16. The groove 12 comprises a half cylinder as can better be seen in FIG. 2. Mounted at the center of cap 10 is attachment boss 18, which is preferably threaded, for attaching cap 10 to vent outlet 30 (FIG. 3) via a correspondingly threaded bore 40. When mounted on the vent outlet 30, ridges 20 press against a gasket 32 mounted about the vent passageway 42 making an airtight seal therewith so that the only avenue for air to communicate between the outside air and the vent passageway 42 is through inlet 14, groove 12 and outlet 16. Also shown in FIG. 3 is a filter 44 for filtering dust out of the air which may enter vent passageway 42. Passageway 42 communicates to the interior of a sealed disk drive (not shown). 
     As can be seen from FIG. 1, groove 12 tracks the curvature of the circular cross sectional cap and doubles back on itself at elbow 22. This provides for a long path relative to the narrow cross section of the path between inlet 14 and outlet 16. This long, narrow path provides for pressure equalization during drive operation while retarding and reducing water vapor ingress. 
     In the following discussion, groove 12 is essentially a tube. It can be shown that the pressure differential through a tube is proportional to the length of the tube divided by the fourth power of its internal diameter (bore): 
     
         ΔP≈1/d.sup.4 
    
     On the other hand, the diffusion mass flow rate of water vapor through dry air in a tube is proportional to the square of the internal diameter (bore) of the tube divided by its length: 
     
         ΔQ≈d.sup.2 /1 
    
     It is desirable to maintain both the pressure differential, ΔP, and the mass flow rate of water vapor, ΔQ, as low as possible. It is evident that for a given length of tube, doubling the diameter will reduce the pressure differential by a factor of sixteen but will increase the mass flow rate of water vapor by a factor of only four. Conversely, lengthening a tube will increase the pressure differential and decrease the mass flow rate of water vapor in equal proportions. Therefore, for a given set of parameters associated with a given tube, if the length is increased by sixteen-fold and the diameter is doubled, the mass flow rate of water vapor by diffusion will be decreased by a factor of four while the pressure differential is unchanged. 
     It is evident that the tube should be as long as practical and have a diameter great enough as to not create a serious pressure differential. Further, it should have a bore large enough that the effects of surfaces tension and friction between air and water flowing in the tube and the surface of the tube are not significantly laminar, and the tube should be long enough as to minimize the mass flow rate of water vapor through the tube. The tube must not be a capillary tube. 
     Assuming a maximum rate of air expulsion of 5 cubic inches per hour due to thermal excursions of the disk drive, to maintain a pressure differential smaller than 0.1 psi, a tube 10 inches in length should have a bore no smaller than 0.011 inch; a tube 20 inches in length should have a bore no smaller than 0.013 inch. Tubes with bores greater than minimum may be used, recognizing that a sacrifice of water vapor diffusion will occur. 
     In the preferred embodiment, cap 10 is 0.875 inches in diameter. Groove 12 is 0.030 inches in diameter, and when ridges 20 are pressed against gasket 32, the resulting passageway is approximately 0.0006 square inches in cross section. The length of the groove is approximately 4.9 inches. 
     It is understood that a passage connecting the inlet 14 with outlet 16 and formed entirely within cap 10 is also within the scope of the present invention. 
     FIG. 4 shows an alternate preferred embodiment in which cap 10 is mounted on a top cover 50 of a sealed disk drive about the opening of a breather vent comprised of a plurality of inlet holes 60. In this embodiment, the boss 18 is inserted into a corresponding bore in the top cover 50 and heat staked to form an expanded end 52 which holds the cap 10 on the top cover 50. The airtight seal is maintained by circular adhesive tape 54, which is adhesive on both sides. The cap presses against the tape, forcing the tape to press against the top cover. The groove 12 permits air ingress and egress. The tape 54 extends inwardly to cover an outer circumferential portion of a circular filter 58 and held in against the top cover 50. The inner circumferential portion of this filter is held against cap 10 at boss 18 by O-ring 56. By this arrangement, outside air reaching the interior of cap 10 must pass through filter 58 to reach the air inlet holes 60 of the vent.