1.  Field of the Invention
The present invention relates to a tape guide device for use in a video tape recorder or the like. 
2.  Description of the Prior Art
Tape guides for use in video tape recorders or the like are roughly classified into rotary tape guides and fixed tape guides. 
The rotary tape guides are advantageous in that they impose less resistance to tapes guided thereby.  However, the speeds of travel of the tapes guided by the rotary tape guides tend to reflect irregularities in the rotational speeds of bearings used in the rotary tape guides.  Furthermore, if the direction in which a tape travels when it is guided by a rotary tape guide is not perpendicular to the axis of rotation of the rotary tape guide, then the tape is subjected to a transverse force applied by the tape guide.  The latter drawback is aggravated when the tape is transversely shifted until an edge thereof is damaged by contact with a flange of the rotary tape guide, for example.  Therefore, rotary tape guides are required to be machined and assembled with high accuracy, and hence cannot be manufactured easily. 
The fixed tape guides allow tapes guided thereby to run stably, but present large resistance to the running tapes. 
There has been a demand for a fixed tape guide which imposes smaller resistance to a running tape.  One of such fixed tape guides that meet such a demand is an air tape guide for ejecting air from small holes defined in the surface of a guide body to float a tape off the guide body for thereby reducing the resistance applied to the tape.  The air tape guide is still problematic since a compressor is required as an air pressure source. 
To eliminate the drawbacks of the conventional tape guides, there has been proposed an ultrasonic vibration tape guide device as disclosed in Japanese patent application No.  02-103627.  The ultrasonic vibration tape guide device employs an ultrasonic energy to reduce resistance to a running tape while allowing the tape to run stably as with fixed tape guides.  The ultrasonic vibration tape guide device is adjustable in height.  The proposed ultrasonic vibration tape guide device will be described below with reference to FIG.  1 of the accompanying drawings. 
As shown in FIG.  1, the ultrasonic vibration tape guide device, generally designated by the reference numeral 1, includes a main shaft 5 mounted vertically on a base 18, and a laminated ultrasonic vibrator 3 fixed to a guide member 2 that is supported on support teeth 7b of a cylindrical support shaft 7 of brass. 
The ultrasonic vibrator 3 comprises a piezoelectric element having a number of piezoelectric ceramic plates with positive and negative electrodes interposed alternately therebetween.  The positive electrodes are electrically connected to a positive electrode plate on one side, and the negative electrodes are electrically connected to a negative electrode plate on another side.  An insulating member is attached to one end surface of the piezoelectric ceramic element, with the other end surface serving as a fixing surface. 
A positive lead 3a is connected to the positive electrode plate of the ultrasonic vibrator 3, whereas a negative lead 3b is connected to the negative electrode plate of the ultrasonic vibrator 3. 
The end surface as the fixing surface of the ultrasonic vibrator 3 is of a curved shape complementary to an outer circumferential surface of the guide member 2, and is bonded thereto. 
The ultrasonic vibrator 3 has a length of about 4.5 mm in the radial direction of the guide member 2 such that it can apply a sufficient amount of vibratory energy to the guide member 2 when expanded and contracted in response to electric energy supplied thereto over the leads 3a, 3b. 
Lower and upper flanges 9, 10 are disposed in abutment against lower and upper ends, respectively, of the support shaft 7, for guiding opposite edges of a tape wound around the guide member 2. 
The main shaft 5 extends through the lower and upper flanges 9, 10 and the support shaft 7.  A height adjustment screw 6 is fitted in an upper end of the support shaft 7, and threaded over a screw 23 on the upper end of the main shaft 5. 
The upper flange 10 is fastened to an upper end surface of an attachment 8 by a screw 15.  The lower flange 9 is fixed to a lower end surface of the attachment 8 by fixing pins 22, 24. 
The attachment 8 has an ultrasonic vibrator storage space 8a defined therein which houses the ultrasonic vibrator 3 therein.  As shown in FIG.  2 of the accompanying drawings, the ultrasonic vibrator storage space 8a is defined as a hole in the shape of a rectangular parallelepiped between side walls 8b having respective stopper insertion holes 8c defined therein. 
Disc-shaped stoppers 39 of rubber have engaging protrusions 39a fitted respectively in the stopper insertion holes 8c.  The ultrasonic vibrator 3 is sandwiched between the stoppers 39 to prevent the guide member 2 from rotating with respect to the attachment 8. 
The attachment 8 keeps the lower and upper flanges 9, 10 parallel to each other and spaced from each other by a distance that is about 0.1 mm larger than the length of the guide member 2. 
As shown in FIG.  1, the lower flange 9 is normally urged upwardly under the bias of a coil spring 35 disposed around the main shaft 5 between the lower flange 9 and the base 18.  The base 18 has a pin insertion hole 20 in which there is inserted an end of the fixing pin 22 that projects downwardly from the lower flange 9. 
When the height adjustment screw 6 is turned, the guide member 2 is adjusted in height under or against the bias of the coil spring 35. 
FIG.  3 of the accompanying drawings shows standing wave vibrations caused of the guide member 2 when an AC voltage is applied to the ultrasonic vibrator 3, the standing-wave vibrations being illustrated along line X--X. Dotted lines N represent nodes on the guide member 2 where the vibrations have zero amplitude.  The nodes N on the guide members 2 are axially spaced from the ends of the guide members 2 by a distance n, and the support teeth 7b are also axially spaced from the ends of the guide member 2 by the distance n, i.e., are positioned at the nodes N.
The ultrasonic vibrator 3 that is positioned substantially centrally in the longitudinal direction of the guide member 2 can be expanded and contracted in the radial direction of the guide member 2 for applying standing wave vibrations to the guide member 2. 
As described above, the length of the ultrasonic vibrator 3 in the radial direction of the guide member 2 is about 4.5 mm.  However, the ultrasonic vibrator 3 with the length of about 4.5 mm makes the tape guide device 1 relatively large in size when used in video tape recorders, and hence cannot meet a growing demand for smaller tape guide devices. 
The laminated ultrasonic vibrator 3 is of a complex structure including a number of piezoelectric ceramic plates with positive and negative electrodes interposed alternately therebetween, each of the piezoelectric ceramic plates having a thickness of 0.2 mm or smaller.  Since laminated ultrasonic vibrators of such a complex structure are generally highly expensive, the cost of tape guide devices and video tape recorders incorporating such laminated ultrasonic vibrators is relatively high.  Therefore, the conventional tape guide devices incorporating such laminated ultrasonic vibrators are one of the factors which make video tape recorders with the conventional tape guide devices relatively expensive on the market.