Microwave oven energy stirrer

An energy stirrer for a microwave oven is disclosed wherein the stirrer is rotatably mounted in a manner to minimize frictional losses. The stirrer is caused to rotate by the passage of cooling and ventilating air across the blades, the air flow pattern being designed to control the direction and speed of rotation.

BACKGROUND OF THE INVENTION 
This invention relates to microwave ovens and specifically with apparatus 
and methods employed to improve the distribution of energy in the oven. 
It is well-known to use a rotatable stirrer in the path of microwave energy 
to vary energy standing wave patterns and improve energy distribution 
patterns. Such stirrers may be located in the waveguide, in the cooking 
cavity, or in a transition zone between the waveguide and the cooking 
cavity commonly referred to as a feed box. 
Stirrers are most frequently motor driven at fairly slow speeds of rotation 
on the order of 40-65 RPM. Substantially higher speeds of rotation have 
been found to be less effective in evenly dispersing microwave energy in a 
cavity than those of the slower speeds indicated. 
Several workers in the field have suggested motorless stirrers driven by 
the passage of air across the blades, the air being derived from that 
provided for oven component cooling. While air driven stirrers can 
represent a more economical arrangement than a motor driven stirrer, the 
air driven variety has been far less widely used because of problems in 
controlling rotation and arriving at the proper balance between the 
relative locations of the stirrer and the air source on the one hand and 
the proper speed of rotation on the other. The more remote the stirrer is 
located from the cooling air source the greater the requirement to provide 
a stirrer mechanism having very low rotational friction losses, hence 
requiring less driving power. 
SUMMARY OF THE INVENTION 
Many of the problems inherent in prior art designs of energy stirrers are 
overcome in the present invention in which a unique stirrer mounting 
arrangement is provided to attain extremely low rotational friction 
levels. The stirrer is positioned in a feed box outside of the cooking 
cavity by means of an easily installed mounting bar. The air is directed 
through the feed box to provide some control of the rotational speed.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 depicts a microwave oven 10 including a cooking cavity 11. Microwave 
energy is transmitted into a feed box 14 at the top of the cavity 11 and 
from there passes into the cavity. The feed box 14 is equipped with a 
stirrer 12, at least the blades of which are made from a microwave energy 
reflective material such as metal. The stirrer 12 is adapted to rotate 
within the feed box 14 in order to prevent or break up standing wave 
patterns that would otherwise exist in the oven and hence provide for more 
even distribution of energy within the cavity 11. 
The stirrer 12 is supported in the feed box 14 on a mounting bar 13 
attached to the surfaces 19 which form the top of the cavity 11 and the 
bottom of feed box 14. The mounting bar 13 may be attached in a number of 
different ways including welding or riveting. In the embodiment 
illustrated in FIG. 2, clips 15 are formed as an integral portion of the 
ends of the bar and may be attached directly to the edge portions of 
surfaces 19. 
A shaft 29 is mounted in the approximate center of the bar 14 by any 
suitable means, such as fasteners 22 and 23. The shaft 29 preferably is 
metal and it is expecially preferred to use chrome plated steel for the 
shaft because of its relatively smooth finish and resistance to rust. 
The blades of the stirrer 12 are attached to hub 16 is preferably made from 
a non-metal plastic material having high heat resistance characteristics. 
It is preferred to make the hub from Teflon, a synthetic fluoride resin. 
Such materials may include polytetrafluoroethylene (TFE) or fluorinated 
ethylene-propylene (FEP). A particularly preferred substance is an FEP 
sold under the brand name Teflon 110 by DuPont Chemical Company. A 
cylindrical bore 24 is formed in the hub sized to receive the shaft 20 in 
sufficiently loose engagement to allow free rotation but at the same time 
provide adequate lateral support for the stirrer so it will rotate in a 
horizontal orientation. 
The internal end of the bore 24 has an inverse conical shape shown at 17. 
The apex of the cone 17 faces the bore so as to engage the top surface 21 
of the shaft 20 when the stirrer 12 is in place. In this manner the area 
of contact between the hub 16 and the shaft 20 is minimized. The 
combination of minimum area of contact and the inherent lubricious nature 
of the Teflon material provides a structure having extremely low 
rotational friction losses. 
In the microwave oven illustrated in FIG. 1 the microwave power and control 
components are located generally on the right side of the oven adjacent 
the cavity. A fan is provided in the area to draw in outside air and blow 
it across the electrical components for cooling. The air is then routed or 
ducted at least in part to the feed box 14 which it enters through 
perforations 30 formed in one wall of the feed box and exits through 
similar perforations in the opposite wall. 
The perforations 30 are positioned so that the air flow, as shown by the 
arrows in FIG. 4 passes across the tips of the stirrer blades 12 on one 
side of the axis of rotation and exits the feed box on the same side of 
the axis of rotation. This insures a preselected rotational direction for 
the stirrer compatible with the pitch of the stirrer blades, and also 
provides for an essentially dead air space on the opposite side of the 
axis of rotation. This dead air space has been found to be of assistance 
in dampening the rotation of the stirrer 12 to assist in keeping its rate 
in the range of 20-70 RPM which is desired for optimum energy distribution 
pattern. 
The stirrer mounting arrangement disclosed herein provides the basis for an 
air driven stirrer system achieving excellent performance results. The 
conical shape of the bearing surface in the stirrer hub working in 
conjunction with the hard, smooth surface of the mounting shaft provides 
an almost frictionless rotational support. It is therefore unnecessary to 
provide a special fan to power the stirrer, or to alter the cooling air 
flow of the oven to accomodate the stirrer. Thus the cooling system can be 
constructed with the normal design considerations and the air stirrer does 
not add an additional design factor to be solved. Placing the stirrer in a 
feed box which can in effect operate as a fan should for purposes of the 
invention allow for optimum control of air flow across the stirrer and 
ultimately its speed of rotation.