ADJUSTABLE TENNIS TRAINING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a training device for exercise or recreation and more particularly, relates to a adjustable tennis training device. 2. The Related Art [0002] Many people love tennis since this kind of sport can sufficiently promote their strength by actions such as attack, defense and smash and the like. A training device in addition to necessary racket and ball is also important for conveniently taking tennis exercise. [0003] A conventional tennis training device includes a bracket supported on the ground. A pivot is provided on the bracket at suitable height, and a ball is connected to the pivot via a tension rope. A trainer strokes the ball with the tennis racquet, thus causing the ball flying and spinning along a predefined path with respect to the bracket. Then, the trainer beats the returned ball so as to practice and improve the skills of the trainer. However, the actual flying path of the ball is often uncertain, the above conventional tennis training device.can only generate a single tennis ball flying path and cannot comprehensively improve skills of the trainer such as angle of the ball hit and response competence.
[0004] Accordingly, it is strongly desired to provide an improved tennis training device to overcome drawbacks of the conventional device. SUMMARY OF THE INVENTION [0005] An object of the present invention is to provide an adjustable tennis training device which overcomes drawback of mono-training effect existing in conventional training device, hence the trainer being able to practice more comprehensive stroke exercise with multiple hit angles and hit positions. [0006] To arrive at the aforementioned object, the present invention provides an adjustable tennis training device which includes: a support pole, the lower end of which is fixed on the ground, while the upper end thereof is provided with an open ended hollow tube; a large tube, the lower end of which is inserted into the upper end of the support pole in a detachable manner, and the upper end of which has a thin insertion portion; a short tube slidably connected to the insertion portion of the large tube; a knob which is provided on the upper end of the short tube and which has a screw bolt formed centrally on its bottom surface, the screw bolt passing through the short tube and being connected with the upper end of the insertion portion; a tension spring, the upper end of which is coupled with the short tube, while the lower end thereof is coupled with the large tube, the middle portion of the spring being large in diameter and defining a helical path; and a training ball with a 2 tension cord tied thereto, the distal end of the cord being connected with a hook which is slidable along the helical path of the tension spring. [0007] The tennis ball spins in a helical manner along the helical path of the tension spring under the towing force produced by the hook and tension cord, when the ball is stroke by the trainer with the racquet. The flying height of the ball changes with its location at the helical path. Apparently, larger pitch of the helical path results in wider range of height change and larger speed of the tennis ball, thus the trainer's ball hit position and angle being changed greatly. Accordingly, the pitch of the helical path can be changed by changing the engaging distance between the large tube and short tube with the knob, hence providing the trainer with more comprehensive exercise with more angle and position changes. [0008] As an improvement, an inner threaded sleeve is fixed on the upper end of the insertion portion. The inner threaded sleeve serves to connect the insertion portion with the screw bolt. [0009] As an improvement, a guiding groove is defined axially on a sidewall of the short tube at the middle portion thereof. A locating-device is provided on th.e insertion portion of the large tube. The locating device can engage with the guiding groove and slide therein. The engagement between the locating device and guiding groove makes it possible for the short tube 3 to move only along axial direction when the knob is adjusted. As a result, the helical path will be subject to little changes in its diameter when the pitch is changed. [00010] Preferably, the locating device is a screw, one end of which is received in the guiding groove, while the other end thereof is connected in a threaded manner with the inner threaded sleeve. [00011] As an improvement, the both ends of the tension spring are secured on the short tube and large tube respectively via an upper rivet and a lower rivet. Two bearings, both of which are coaxial with the screw bolt, are disposed on the upper end of the screw bolt. One portion of the upper rivet extends into the short~tube and is secured between the two bearings. When the knob is adjusted, the upper rivet is pressed against to either the upper bearing or the lower bearing, thereby the relative distance between the knob and short tube being continuously maintained. [00012] As an improvement, a through hole is radially defined in the upper end of the screw bolt, and the knob is a plastic element integrally formed with the upper end of the bolt by injection-molding process. The knob has a plug which is inserted into the through hole to connect firmly with the bolt. This plug may assist in. production of the knob, and at the same time improve the structural strength. [00013] The support pole has at least one locating hole 4 defined axially on its hollow tube. At least one resilient locating pin is formed on the lower end of the large tube to engage the respective locating hole. The height of the large tube can be regulated by engagement of the locating hole with the locating pin, thereby making it possible for trainers of different age, height to conveniently adjust the height of the large tube, as well as further extend the kinds of the exercise. [00014] Preferably, the lower end of the support pole is tapered so as to be conveniently inserted into the ground of the training court. [00015] Preferably, the middle portion of the support pole has a laterally extended leg support against which legs of the trainer can be pressed with the purpose of quickly and easily inserting the support pole under the ground. This leg support may also function to enhance the stability of the support pole by engagement of the leg support with the ground after the insertion of the pole. [00016] Compared to prior art, the invention can change the pitch of the tension spring by regulating the engaging distance between the large tube and short tube by means of the knob, thus changing the flying height and speed of the tennis ball. By this way, the trainer is able to practice more comprehensive stroke exercise with multiple hit angles and hit positions. 5 BRIEF DESCRIPTION OF THE DRAWINGS [00017] The present invention will be apparent to those skilled in the art by reading the following description of a preferred embodiment thereof, with reference to the accompanying drawings, in which: [000181 FIG. 1 is a perspective view of a preferred embodiment of the invention; [00019] FIG. 2 is an exploded perspective view of a preferred embodiment of the invention; [00020] FIG. 3 shows the structural relationship among the tension spring, short tube and large tube; and [00021] FIG. 4 shows the structural relationship between the large tube and support pole. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [00022] The present invention is further explained with reference to the drawings and specification. [00023] The reference is made to figures 1-4. An adjustable tennis training device, which includes a support pole 1, the lower end of which is tapered and fixed on the ground, the middle portion of which has a laterally extended leg support 10, while the upper end thereof is provided with an open ended hollow tube. Two locating holes 11 are defined on the hollow tube of the support pole 1 at different height. 6 [00024] The device of the invention also includes a large tube2, the lower end of which is inserted into the upper end of the support pole 1 in a detachable manner. The lower end of the large tube 2 has a certain number of resilient locating pins 20 which engage the respective locating holes 11. The upper end of the large tube 2 has a thin insertion portion 21 into upper end of which an inner threaded sleeve 22 is placed. [000251 The device of the invention further includes a short tube 3 which is slidably connected to the insertion portion 21 of the large tube 2. A guiding groove 31 is defined axially on a sidewall of the short tube 3 at the middle portion thereof. A screw 23 is provided on the insertion portion 21 of the large tube 2. The screw 23 can engage with the guiding groove 31 and slide therein. One end of the screw 23 is received in the guiding groove 31, while the other end thereof is connected in a threaded manner with the inner threaded sleeve 22 along radial direction of the insertion portion 23. [00026] The device of the invention further includes a knob 4 which is provided on the upper end of the short tube 3 and which has a screw bolt 41 formed centrally on its bottom surface, the screw bolt 41 passing through the short tube 3 and being connected with the inner threaded sleeve 22 located at the upper end of the insertion portion. A through hole 411 is radially defined in the upper end of the screw bolt 41, and the knob 4 is a plastic element integrally formed with the upper end of the bolt 41 by injection-molding process. The knob 4 has 7 a plug 412 which is inserted into the through hole 411 to connect firmly with the bolt 41. [00027] The device of the invention further includes a tension spring 5. The both ends of the tension spring 5 are secured on the short tube 3 and large tube 2 respectively via an upper rivet 51 and a lower rivet 52. The middle portion of the spring 5 is large in diameter and defines therein a helical path 53. Two bearings 42, both of which are coaxial with the screw bolt 41, are disposed on the upper end of the screw bolt 41. One portion of the upper rivet 51 extends into the short tube 3 and is secured between the two bearings 42. [00028] The device of the invention further includes a training ball 6 with a tension cord 61 tied thereto. The distal end of the cord 61 is connected with a hook 62 which is slidable along the helical path 53 of the tension spring 5. [00029] Due to the improved construction of the adjustable tennis training device of the invention, the trainer is able to practice more comprehensive stroke exercise with multiple hit angles and hit positions, thereby making it possible for trainers of different age, height to conveniently adjust the height of the large tube, as well as further extend the kinds of the exercise. [00030] It is noted that the scope of the present invention is not intended to be limited to the structure of the above embodiments. Rather, the invention also compasses various modifications, variations, as well as equivalency thereof. 8