Air cushioned vacuum blast head

A blast head has an air cushion device that directs a flow of relatively high velocity air inwardly and toward the surface being blasted from around the periphery of the blast head. This air flow creates a barrier preventing the escape of particles and debris from the blast head and also creates a so-called ground effect; i.e. the air flow between the head and the surface creates a force that lifts the head off the surface being blasted so that the head floats on cushion of air.

BACKGROUND OF THE INVENTION 
The present invention relates to vacuum blasting machines, and more 
particularly, to an improved blast head for use in vacuum-blasting 
machines. 
As will be appreciated by those skilled in the art, vacuum blasting refers 
to a technology for cleaning and preparing surfaces by blasting the 
surface with abrasive particles entrained in a relatively high velocity 
air stream and simultaneously vacuuming away abrasive particles and 
debris. Vacuum blasting machines use a vacuum blast head, with a small 
hood that surrounds a blasting nozzle, and a vacuum chamber within the 
hood, to suck up the abrasive particles and debris loosened from the 
surface being blasted. This abrasive particle and debris mixture passes 
through a hose to a separator, where the debris is separated from the 
abrasive particles. The separated debris is deposited in a waste chamber 
and abrasive particles, separated from the debris, may be returned to a 
hopper and used again in the blasting operation. Vacuum-blasting machines 
provide closed-system blasting by making the blast head in effect a small 
containment structure. In operation, the system is virtually dust free 
and, therefore, relatively safe for the operator and the environment, even 
when used to blast surfaces covered with potentially hazardous materials, 
such as lead and radioactive contaminants. 
FIG. 1 shows a prior art blast head, indicated generally by the reference 
numeral 1, in position to clean a planar surface 4, such as, for example a 
bridge, building, ship, or aircraft member. The blast head 1 includes a 
blast jacket 5, a nozzle 6 feeding abrasive particles 7 entrained in a 
stream of compressed air directed through the blast jacket so that the 
particles traverse the narrow gap 4a separating the surface 4 and the open 
end of the blast jacket 5 and strike the surface 4. A vacuum jacket 8 
surrounds the blast jacket, and a takeoff 9 in the vacuum jacket is 
connected to a vacuum source. The general trajectory of the particles 7 is 
indicated by the arrows 7a. A metal or plastic brush ring 8a is secured at 
a forward end of the vacuum jacket 8 for engaging a work surface and 
preventing the escape of abrasive particles 7. The "brush ring" does not 
directly prevent dispersal of abrasive and debris. Instead, it enables the 
economic maintenance of negative pressure, with respect to the ambient, in 
the blast area, and a vacuum air flow of sufficient velocity to transport 
the entrained particles. This ensures the recovery and prevents dispersal 
of abrasive and coating debris, and in particular, dust particles. The 
magnitude of the vacuum is greater than the magnitude of the pressure of 
the compressed air at the forward end of the blast head 1. As a result, 
there is a resultant vacuum which draws the blast head 1 into engagement 
with the work surface 4. 
There are problems related to the brush ring 8a. One problem is a 
requirement that the operator keep the blast head constantly perpendicular 
to the blast surface; any gap between the brush ring and the blast surface 
would allow the abrasive (and dust) to escape to outside the constant 
structure. Another problem is damage to the blast surface due to friction 
(scratching) between the brush and the surface. In addition, the brush 
needs frequent replacement. 
SUMMARY OF THE INVENTION 
An object of this invention is the provision of a vacuum blast head with a 
particle and debris barrier which allows some off-axis motion of the head, 
which is not subject to wear, and which will not mar the surface being 
blasted. 
Briefly, this invention contemplates the provision of a blast head with an 
air cushion device that directs a flow of relatively high velocity air 
inwardly and toward the surface being blasted from around the periphery of 
the blast head. This air flow creates a barrier preventing the escape of 
particles and debris from the blast head and also creates a so-called 
ground effect; i.e. the air flow between the head and the surface creates 
a force that lifts the head off the surface being blasted so that the head 
floats on cushion of air.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 2,3, and 4, in this specific embodiment of the 
invention, an air cushion device 18 for a blast head of the type generally 
illustrated in FIG. 1 is comprised of two members, a base 20 and a cap 22. 
Preferably, the blast head includes a pressure regulator 16 of the type 
disclosed in copending application Ser. No. 08/543,143 filed Oct. 13, 
1995, assigned to the assignee of this application, and incorporated 
herein by reference. 
As best shown in FIGS. 2 and 3, the base 20 is a generally cylindrical 
member, with an inner cylindrical surface 24 that fits over and engages 
the outer surface of the vacuum jacket 8. An annular groove 26 in the wall 
28 of the base 20 forms (in combination with the cap 22) a chamber or 
plenum 30, which is filled with air under pressure from a pressure hose 32 
screwed into an opening 34 in the top of the base. 
Referring now also to FIG. 4, the cap 22 is generally dish shaped with an 
upstanding side wall 36 which forms the outer wall of the chamber 30. The 
cap 22 has inwardly sloping surface 38. When the base and the cap are 
assembled, sloping surface 38 faces a sloping surface 40 on the base and 
cooperates with an inwardly sloping surface 40 of the base 20 to form a 
nozzle 44 which extends around the circumference of the head and directs 
air into the space or gap between the end of the blast jacket 5 and the 
surface 4. Sufficient air is directed by the nozzle 44 into the gap to 
create a ground effect which lifts the head (i.e. the cap) off the surface 
with a cushion of air flowing outwardly between the lower surface 47 of 
the cap and the surface 4. At the same time, the inward direction of the 
air flow creates a barrier to the outward flow of blast particles and 
debris from the surface 4. Bolts (not shown) fit through openings 50 
spaced around the cap 22 and are screwed into threaded bores in the base 
20) to secure the cap to the base. The base 20 can be secured to the 
vacuum jacket in any of a number of suitable ways, such as, for example, 
epoxy, or welding. The air cushion nozzle can be made by other methods. 
In operation, the nozzle 44 directs a stream of relatively fast moving air 
downwardly, toward the surface of the work piece, and inwardly, toward the 
particle stream (e.g. at an angle of about 45 degrees with respect to the 
longitudinal axis of the blast head). This creates an air barrier, which 
confines any stray particles or debris from the surface within the 
enclosure created by the vacuum jacket. The vacuum pressure, the particle 
stream pressure and barrier nozzle pressure are adjusted one relative to 
the other so that there is a positive pressure between the surface and the 
blast head creating sufficient pressure between the head and the surface 
to lift the head from the surface so air flows out between the peripheral 
surface 47 of the cap 22 and the surface 4, and the head rides on this air 
cushion. 
FIGS. 5 and 6 illustrate one piece designs for directing air into the gap 
between the blast head and the surface of the work piece in order to 
provide an air cushion for the blast head. In the design of FIG. 5, a 
circular tube 70 has a series of ports 72 to direct air into the gap and 
the air hose 32 connects to a fitting 69. The embodiment of FIG. 6 is 
similar to FIG. 5. Here, a one piece cast member 76 has a series of ports 
78 to direct air into the gap. The air hose 32 connects to a fitting 80. 
These and other modifications may be made to the specific embodiment of the 
invention without departing from. the spirit and scope of the invention.