CONNECTION DEVICE FOR SUPPORT ROD OF TENT BACKGROUND OF THE INVENTION 5 1. Field of the Invention The present invention relates to a connection device, and more particularly to a connection device for a support rod of a tent. 2. Description of the Prior Art A conventional small-sized tent comprises a plurality of support rods to support a 10 canopy. Each support rod of the tent is composed of a plurality of connected rod members. When unfolding the tent, the rod members of the support rod are unfolded in a curved shape to support the canopy. When folding the tent, the rod members are folded at their joints. In order to fix the rod members of the support rod when unfolding and to fold the rod 15 members, a connection member is provided at the joint. The connection member comprises a first connection portion and a second connection portion which are fixed to the rod members. The first and second connection portions are pivoted, and the two connection portions have a recess and a matching protrusion which are engaged with each other when the two connection portions are unfolded so as to fix the two rod members. 20 After the protrusion is disengaged from the recess, the first and second connection portions can be pivoted and bent with each other so as to fold the two rod members up. However, this connection is subject to the height of the protrusion, so the engaging area of the protrusion and recess is small. When a great external force is applied, the protrusion may be disengaged from the recess easily. Furthermore, because the two connection 25 portions are pivotally connected, the support rod will bend when the protrusion is disengaged from the recess and won't return back automatically, which results in collapse of the tent. In other words, the joint for folding is too weak against the wind. Besides, this connection member is adapted for a small-sized tent only, not for a large-size tent. Accordingly, the inventor of the present invention has devoted himself based on his many 30 years of practical experiences to solve these problems.
SUMMARY OF THE INVENTION The primary object of the present invention is to provide a connection device for a support rod of a tent so as to overcome the problems of the prior art, or at least to provide 5 the public with a useful choice. In order to pursue the aforesaid object, the present invention relates to a connection device for a support rod of a tent. The connection device is located between two abutted rod members of the support rod. The connection device comprises an outer sleeve, an inner sleeve and a locking mechanism. The inner sleeve is fitted in the outer sleeve. 10 The locking mechanism is located between the outer sleeve and the inner sleeve. One rod member of the two abutted rod members is inserted through the outer sleeve and secured to the inner sleeve, and the other rod member is inserted through the inner sleeve and secured to the outer sleeve. Preferably, the locking mechanism comprises a button and a spring. The outer 15 sleeve has an engaging trough at one side thereof. The inner sleeve has an accommodation trough corresponding to the engaging trough of the outer sleeve. The spring and the button are inserted in the accommodation trough in sequence. An outer end of the button is exposed out of the engaging trough. The outer sleeve and the inner sleeve of the connection device are engaged with each 20 other, not pivotally connected. The whole inner sleeve is inserted in the outer sleeve and then connected with the locking mechanism. When the two abutted rod members are collapsed, the two rod members are parallel to each other, not folded up. Through this connection relationship, the inner wall of the outer sleeve and the outer wall of the inner sleeve bear all forces, so the bearing area is larger and they won't disengage from each 25 other so as to enhance the strength of the tent against the wind. Besides, with this extension and retraction structure, the tent won't collapse at the joint even though the inner sleeve is disengaged from the outer sleeve. The present invention is especially adapted for a large-sized tent. 30 BRIEF DESCRIPTION OF THE DRAWINGS 2 Fig. 1 is a cross-sectional view according to a preferred embodiment of the present invention; Fig. 2 is a perspective view according to the preferred embodiment of the present 5 invention; and Figs. 3A through Fig. 3D are schematic views of the preferred embodiment of the present invention applied to a tent in a fully unfolded state, a semi-unfolded state, a semi-folded state, and a fully folded state. 10 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. As shown in Fig. 3A, the connection device 2 of the present invention is used to a 15 tent which can be a small-sized tent or a large-sized tent. The frame of the tent comprises a plurality of support rods 1 and a tent canopy on the support rods 1. Each support rod 1 is composed of at least two connected rod members. The connection device 2 is located between two abutted rod members 11, 12. As shown in Fig. 1 and Fig. 2, the connection device 2 comprises an outer sleeve 21, 20 an inner sleeve 22, and a locking mechanism 23. The inner sleeve 21 is fitted in the outer sleeve 22. The locking mechanism 23 is located between the outer sleeve 21 and the inner sleeve 22. The rod member 11 of the two abutted rod members 11, 12 is inserted through the outer sleeve 21 and secured to the inner sleeve 22. The other rod member 12 is inserted 25 through the inner sleeve 22 and secured to the outer sleeve 21. The locking mechanism 23 comprises a button 231 and a spring 232. The outer sleeve 21 has an engaging trough 211 at one side thereof. The inner sleeve 22 has an accommodation trough 221 corresponding to the engaging trough 211 of the outer sleeve 21. The spring 232 and the button 231 are inserted in the accommodation trough 221 in 3 sequence, and an outer end of the button 231 is exposed out of the engaging trough 211. When extending the tent, the inner sleeve 22 is inserted in the outer sleeve 21 and the button 231 on the inner sleeve 22 is biased by the spring 232 to be engaged in the engaging trough 211 of the outer sleeve 21, such that the outer sleeve 21 and the inner 5 sleeve 22 are engaged with each other. This can prevent the outer sleeve 21 and the inner sleeve 22 from disengagement, so that the two abutted rod members 11, 12 can be extended fully, as shown in Fig. 3A. Through this connection relationship, the inner wall of the outer sleeve 21 and the outer wall of the inner sleeve 22 bear all forces, so the bearing area is larger and they won't disengage from each other so as to enhance the 10 strength of the tent against the wind. The two abutted rod members 11, 12 can be collapsed by pressing the button 231 of the locking mechanism 23 to disengage from the engaging trough 211 of the outer sleeve 21, and then the inner sleeve 22 is pulled out from the outer sleeve 21, as shown in Fig. 5. After that, the outer sleeve 21 and the inner sleeve 22 are slid toward the two ends of the 15 rod members 11, 12, such that the two rod members 11, 12 are retracted side by side, as shown in Fig. 3B, Fig. 3C, and Fig. 3D, to collapse the tent. It is noted that the engagement of the outer sleeve and the inner sleeve is not limited to the button and the engaging trough, which can be achieved by other ways, such as a resilient protrusion and an engaging trough, or an engaging trough and a hook disposed at 20 two sides of the outer sleeve and the inner sleeve. Although particular embodiments of the present invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the present invention. Accordingly, the present invention is not to be limited except as by the appended claims. 4